Manual of Petroleum Measurement Standards Chapter 20...approvals required to become an API Standard....

This document is not an API Standard; it is under consideration within an API technical committee but has not received all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction and API staff. Copyright API. All rights reserved.

Manual of Petroleum Measurement Standards Chapter 20.4 Phase Behavior Applications in Upstream Measurement First Edition


ii

Table of Contents

1 Scope ........................................................................................................................... 5 2 Normative References .................................................................................................. 5 3 Terms, Definitions, Abbreviations and Symbols ............................................................. 5 3.1 Terms and Definitions................................................................................................. 5 3.2 Abbreviations and Symbols ........................................................................................ 9 4 Overview of Phase Behavior in Upstream Measurement ............................................. 10 4.1 General .................................................................................................................... 10 4.2 Applicability .............................................................................................................. 11 4.3 Key Elements and Workflow ..................................................................................... 11 5 Theoretical Quantity Determination for Allocation Using PVT Fluid Properties ............. 13 5.1 Application Development .......................................................................................... 13 5.2 Application Implementation ....................................................................................... 21 6 Theoretical Quantity Determination for Allocation Using a Process Simulation Model

(PSM) ......................................................................................................................... 24 6.1 Application Development .......................................................................................... 24 6.2 Application Implementation ....................................................................................... 35 7 Multiphase and Wet Gas Flow Meter (MPFM) Configuration ........................................ 38 7.1 Application development .......................................................................................... 38 7.2 Application Implementation ....................................................................................... 45 8 Fluid Sample and PVT Fluid Property Quality Assurance (QA) .................................... 47 8.1 Application development .......................................................................................... 47 8.2 Application Implementation ....................................................................................... 49 9 Flow Modeling (Virtual Flow Metering) ........................................................................ 49 9.1 Application development .......................................................................................... 49 9.2 Application Implementation ....................................................................................... 56 10 PVT Fluid Property Interpolation to Alternate Process Conditions ................................ 58 10.1 Application Development .......................................................................................... 58 10.2 Application Implementation ....................................................................................... 61 11 Performance Management .......................................................................................... 62 11.1 General .................................................................................................................... 62 11.2 Functional Specification Review ............................................................................... 62 11.3 Functional Specification Modifications ...................................................................... 62 11.4 Validation and Reproducibility of Results .................................................................. 63 11.5 Performance Monitoring and Reporting ..................................................................... 63 11.6 Out-of-tolerance Performance Management ............................................................. 63 Annex A .............................................................................................................................. 64 Annex B .............................................................................................................................. 65


iii

Annex C .............................................................................................................................. 76 Annex D .............................................................................................................................. 80 Annex E .............................................................................................................................. 81 Annex F .............................................................................................................................. 85 Annex G .............................................................................................................................. 87 Annex H .............................................................................................................................. 93 Bibliography ........................................................................................................................ 97


iv

Introduction This document establishes a framework to develop, implement, and manage the application of hydrocarbon phase behavior in upstream measurement. The applied phase behavior modeling addressed in this document refers to a process simulation model (PSM) incorporating an equation of state (EOS) description of the phase behavior, or pressure, volume, temperature (PVT) properties, of the fluids within the modeled process. The intent of this document is to provide operators with a consistent and transparent approach for applying and managing EOS-based phase behavior applications within an upstream measurement system. It is not intended to prescribe a particular mathematical phase estimation (i.e. EOS), process simulation (i.e. PSM), measurement, or allocation method.

Additionally, guidance is provided regarding the application of the suite of API Manual of Petroleum Measurement Standards (MPMS) Chapter 20 documents applicable to phase behavior applications in upstream measurement:

— API MPMS Chapter 20.1, Production Measurement and Allocation Systems;

— API MPMS Chapter 20.2, Production Allocation Measurement Using Single-Phase Devices;

— API MPMS Chapter 20.3, Measurement of Multiphase Flow;

— API MPMS Chapter 20.5, Application of Production Well Testing in Measurement and Allocation.


5

Phase Behavior Applications in Upstream Measurement

1 Scope This document provides requirements and guidelines for the application of phase behavior (i.e., pressure, volume, temperature, or PVT fluid properties) in upstream measurement. The requirements and guidelines address the development, implementation, performance management, and reproducibility of phase behavior representation used to calculate PVT fluid properties applied in the following upstream oil and gas applications:

— Theoretical quantity determination for allocation using PVT fluid properties;

— Theoretical quantity determination for allocation using a process simulation model;

— Multiphase and wet gas flow meter configuration;

— Fluid sample and PVT fluid property quality assurance;

— Flow modelling (virtual flow metering);

— PVT fluid property interpolation to alternate process conditions.

The phase behavior representation guidelines include fit-for-purpose model selection, fluid component definition, and calculation validation.

2 Normative References The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document applies (including any addenda/errata).

API MPMS Chapter 11.1, Temperature and Pressure Volume Correction Factors for Generalized Crude Oils, Refined Products, and Lubricating Oils

API MPMS Chapter 14.1, Collecting and Handling of Natural Gas Samples for Custody Transfer

API MPMS Chapter 20.1, Production Measurement and Allocation Systems

API MPMS Chapter 20.2, Production Allocation Measurement Using Single-phase Devices

API MPMS Chapter 20.3, Measurement of Multiphase Flow

API MPMS Chapter 20.5, Application of Production Well Testing in Measurement and Allocation

API MPMS Chapter 21.1, Flow Measurement Using Electronic Metering Systems – Electronic Gas Measurement

3 Terms, Definitions, Abbreviations and Symbols

3.1 Terms and Definitions For the purposes of this document, the following terms and definitions apply.


6

3.1.1 acentric factor An EOS parameter that provides enhanced temperature dependence of the intermolecular potential of complex fluids from simple ideal fluids.

3.1.2 actual conditions Conditions of pressure and temperature of the fluid at the point where fluid properties (i.e. PVT) or flows are measured or calculated.

3.1.3 allocation The mathematical process of determining the proportion of produced fluids from individual entities (zones, wells, fields, leases, or producing units) when compared with the total production from the entire system (reservoir, production system, and gathering systems) in order to determine value or ownership to attribute to each entity.

3.1.4 binary interaction parameter BIP An EOS fitting parameter used to account for the nonideal interaction between molecules that addresses the difference between association terms in the EOS model compared with experimental data.

3.1.5 bubble point When the pressure is lowered on a liquid held at constant temperature, the pressure at which the first bubble of vapor forms is the bubble point.

3.1.6 critical pressure Pc The pressure at the critical point, where both liquid phase and gas phase specific volumes (densities) of a fluid are equal.

3.1.7 critical temperature Tc The temperature at the critical point, where both liquid phase and gas phase specific volumes (densities) of a fluid are equal.

3.1.8 critical volume Vc The volume at the critical point, where both liquid phase and gas phase specific volumes (densities) of a fluid are equal.

3.1.9 discrete component An EOS component representing a single molecule structure, e.g. Methane, Ethane, i-Butane.


7

3.1.10 end point(s) Allocation end quantity(ies) physical location and associated thermodynamic state in the PMAS.

3.1.11 end point conditions Conditions of pressure and temperature at the end point.

3.1.12 equation of state EOS Thermodynamic equation describing the state of matter under a given set of physical conditions.

NOTE An EOS provides a mathematical relationship among the state variables pressure, temperature, and molar volume.

3.1.13 EOS tuning An adjustment of EOS parameters (e.g. Tc, Pc, binary interaction parameters) to minimize the difference between EOS-predicted PVT values and measured PVT values within the PSM domain.

3.1.14 flash gas factor FF Ratio of evolved hydrocarbon gas quantity at standard conditions (evolved from hydrocarbon liquid as it transitions from measurement point conditions to end point conditions) to the hydrocarbon liquid quantity at standard conditions.

NOTE 1 Both the evolved hydrocarbon gas and hydrocarbon liquid quantities are adjusted from end point conditions to standard conditions.

NOTE 2 For volume calculations, the flash gas factor is in units of mscf/bbl (103 m3/m3). For mass calculations, the flash gas factor is in units of lbm/lbm (kg/kg). For molar calculations, the flash gas factor is in units of mol/mol.

3.1.15 gas correction factor Bg Ratio of hydrocarbon gas quantity at measurement point conditions to the hydrocarbon gas quantity at standard conditions. NOTE For volume calculations, the gas correction factor is in units of acf/scf (m3/m3). For mass calculations, the gas correction factor is in units of lbm/lbm (kg/kg). For molar calculations, the gas correction factor is in units of mol/mol.

3.1.16 hydrocarbon dew point A temperature at a given pressure at which hydrocarbon vapor condensation begins.

3.1.17 individual theoretical quantity The quantity represented by an individual contributing meter or measurement point after conversion to a theoretical value by applying an EOS or other correction factor, usually done in order to adjust the measured quantity for comparison at the same pressure and temperature base as the master quantity.


8

3.1.18 measurement point conditions Conditions of pressure and temperature at the measurement point.

3.1.19 non-discrete component An EOS component that represents multiple molecule structures, e.g. Hexanes, Cn+, PsC.

3.1.20 oil correction factor Bo Ratio of hydrocarbon liquid quantity at measurement point conditions to the hydrocarbon liquid quantity at standard conditions. The oil correction factor is the inverse of the oil shrinkage factor, SF.

NOTE For volume calculations, the oil correction factor is in units of bbl/bbl (m3/m3). For mass calculations, the oil correction factor is in units of lbm/lbm (kg/kg). For molar calculations, the oil correction factor is in units of mol/mol.

3.1.21 oil shrinkage factor SF Ratio of hydrocarbon liquid quantity at standard conditions to the hydrocarbon liquid quantity at measurement point conditions. The oil shrinkage factor is the inverse of the oil correction factor, Bo.

NOTE For volume calculations, the oil shrinkage factor is in units of bbl/bbl (m3/m3). For mass calculations, the oil shrinkage factor is in units of lbm/lbm (kg/kg). For molar calculations, the oil shrinkage factor is in units of mol/mol.

3.1.22 pressure, volume, temperature PVT The phase behavior and description of hydrocarbon fluid physical properties for a given set of composition, pressure, and temperature.

NOTE Physical properties of interest include relative phase fraction, GOR, bubble point and hydrocarbon dew point, density, correction factors, compressibility, and viscosity.

3.1.23 process simulation model PSM A computer-based model representing physical and chemical processes (mass and energy balances) to predict process conditions (e.g. pressures, temperatures, flows, compositions) as well as thermophysical properties (e.g. density, viscosity, heat capacity).

3.1.24 pseudo-component PsC A grouping of several compositional components as a single component in an EOS phase description model.

3.1.25 solution condensate-gas ratio rs


9

Ratio of condensed hydrocarbon liquid quantity at standard conditions (condensed from hydrocarbon gas as it transitions from measurement point conditions to end point conditions) to the hydrocarbon gas quantity at standard conditions.

NOTE 1 Both the evolved hydrocarbon gas and hydrocarbon liquid quantities are adjusted from end point conditions to standard conditions.

NOTE 2 For volume calculations, the solution condensate-gas ratio is in units of bbl/mscf (m3/103 m3). For mass calculations, the solution condensate-gas ratio is in units of lbm/lbm (kg/kg). For molar calculations, the solution condensate-gas ratio is in units of mol/mol.

3.1.26 surrogate component A single discrete component used to represent a group of components with similar properties.

NOTE 1 The surrogate differs from the PsC by having the well-defined properties of a discrete component but limited in the range of molecules it covers. An example of a surrogate is Ethylbenzene used to represent C8 aromatics.

3.1.27 standard conditions Pressure and temperature conditions used to normalize quantities for allocation and/or sales (e.g. 14.696 psia, 60°F; 101.325 kPa, 15°C).

3.1.28 vapor-liquid equilibrium VLE A state where vapor and liquid coexist at the same pressure, temperature, and total volume with no net mass transfer between phases.

3.1.29 volume shift A constant correction term that refers to the translation of the repulsive hard sphere volume in a cubic EOS by the corresponding states approach, used to adjust an EOS-calculated molar volume to improve liquid density calculations.

3.1.30 water correction factor Bw Ratio of water quantity at measurement point conditions to water quantity at standard conditions.

NOTE For volume calculations, the water correction factor is in units of bbl/bbl (m3/m3). For mass calculations, the water correction factor is in units of lbm/lbm (kg/kg). For molar calculations, the water correction factor is in units of mol/mol.

3.2 Abbreviations and Symbols For the purposes of this document, the following abbreviations and symbols apply.

acf actual cubic feet

bbl barrel

Bg gas correction factor

Bo oil correction factor


10

Bw water correction factor

EOS equation of state

FF flash gas factor

FWKO free water knockout

GOR gas-oil ratio

kg kilogram

KPI key performance indicator

lbm pounds mass

m3 cubic meter

mscf thousand standard cubic feet

MW molecular weight

Pc critical pressure

PFD process flow diagram

PMAS production measurement and allocation system

PsC pseudo-component

PSM process simulation model

PVT pressure, volume, temperature

rs solution condensate-gas ratio

scf standard cubic feet

SF oil shrinkage factor

Tb normal boiling point temperature

Tc critical temperature

Vc critical volume

VLE vapor-liquid equilibrium

VRU vapor recovery unit

4 Overview of Phase Behavior in Upstream Measurement

4.1 General Reservoir fluids undergo physical changes upon movement through a production process prior to becoming stabilized hydrocarbon liquids and dry hydrocarbon gas. These physical changes are due to the inherent variation in pressure and temperature over the entire production process and are characterized by the VLE or PVT properties of the fluids, otherwise referred to as phase behavior. The phase behavior of the various produced fluids throughout a production process is a distinctive but integral


11

part of upstream measurement. In particular, phase behavior is applied to upstream measurement applications including theoretical quantity determination for allocation, configuration of multiphase and wet gas flow meters, fluid sample quality assurance, flow modeling (virtual metering), and interpolation of PVT properties among alternate process conditions.

4.2 Applicability This document applies to upstream measurement in general, and specifically is intended to address phase behavior within the suite of API MPMS Ch. 20 documents in the determination and assurance of equitable allocation of production quantities. It is not the intent of this document to specify or prescribe meter types, allocation methodologies, flow modeling techniques or specific simulation software packages. Nor is it the intent of this document to encourage the use of one approach over another.

Referenced single-phase flow metering should follow API MPMS Ch. 20.2

Referenced multiphase and wet gas flow metering should follow API MPMS Ch. 20.3.

Allocation of production quantities should follow API MPMS Ch. 20.1.

Production well testing and the application of well flow modeling (virtual metering) should follow API MPMS Ch. 20.5.

4.3 Key Elements and Workflow Applying phase behavior in upstream measurement applications shall incorporate development, implementation, and performance management activities that ensure the application is capable of meeting measurement and/or allocation requirements. Key elements that shall be performed to establish and maintain an application of phase behavior include:

— application development (specific to the intended application, refer to Sections 5.1, 6.1, 7.1, 8.1, 9.1, 10.1), incorporating:

— relevant phase behavior parameter definition (i.e. the parameters of interest to the application);

— functional specification (i.e. fluid characterization, process description, determination of input/outputs, simulation tools, and validation and acceptance criteria);

— application implementation (specific to the intended application, refer to Sections 5.2, 6.2, 7.2, 8.2, 9.2, 10.2), incorporating:

— validation of fluid characterizations, simulation tools, and input data;

— determination and validation of output results;

— performance management (refer to Section 11).

Figure 1 outlines the key elements, the associated workflow and supporting activities used to establish and maintain an application of phase behavior in upstream measurement.


12

Figure 1 – Phase Behavior Application Key Elements and Workflow


13

5 Theoretical Quantity Determination for Allocation Using PVT Fluid Properties

5.1 Application Development

5.1.1 General The operator shall be responsible for the development of a PVT fluid properties application when PVT fluid properties are used in a PMAS. The PVT application or PVT tool incorporates an EOS and is used to develop PVT factors applied in the PMAS. The factors account for volumetric and mass changes due to phase conversions and can provide a more rigorous theoretical quantity determination than laboratory generated factors. Where laboratory factors are generally products of single stage flash, blind to actual processing conditions, the PVT tool can capture the effects of multistage processing with heat input and removal. The PVT tool does not address the effects of commingling PMAS source streams (refer to Section 6).

5.1.2 Phase behavior parameter definition: required outputs

5.1.2.1 Oil shrinkage factor, SF and flash gas factor, FF The PVT tool shall include a process that emulates the hydrocarbon liquid stabilization process of the source liquid to end conditions. The PVT tool shall be valid for the range of temperatures and pressures that include allocation source conditions, conditions at points of separation, and end point conditions. The output of the process shall be a SF and an FF, (refer to API MPMS Ch. 20.1), from source to end point conditions.

5.1.2.2 Gas correction factor, Bg and solution condensate-gas ratio, rs The PVT tool shall include a process that emulates the hydrocarbon gas process of the source gas to end conditions. The PVT tool shall be valid for the range of temperatures and pressures that include allocation source conditions, conditions at points of separation, conditions at compression and cooling, and endpoint conditions. The output of the process shall be a Bg and a rs, (refer to API MPMS Ch. 20.1), from source to endpoint conditions.

5.1.2.3 Other parameters The PVT tool may include other processes and outputs. In this case, the PVT tool shall be valid for the range of temperature and pressure conditions to which it is applied. The output of the process shall be a in accordance with API MPMS Ch. 20.1, if applicable.

5.1.3 Fluid Characterization

5.1.3.1 General A fluid characterization process should be included in the PVT tool however, the fluid characterization may be performed as a separate process or omitted from the process if laboratory analytical data is demonstrated to yield adequate results.

5.1.3.2 Measurement The following is a list of field data that may be used in source fluid characterization:

— liquid bubble point pressure (separation pressure);

— gas dew point temperature (separation temperature);

— liquid flowing density (separation densitometer);


14

— gas oil ratio (oil meter and gas meter);

The characterization process may include some or all these data. Other field data may also be used. Data source equipment shall be identified and documented.

The following is a list of laboratory analysis data for use in source fluid characterization:

— liquid composition;

— gas composition;

— non-discrete component MW and liquid density;

— fluid MW and liquid density;

— liquid shrinkage factor;

— liquid flash gas factor;

— liquid flash gas composition;

— residual oil composition.

The characterization process may include some or all these data. Other laboratory data may also be used.

5.1.3.3 Component Set 5.1.3.3.1 General This Standard does not prescribe a specific component set for the PVT tool. The selection of a component set should be based on factors including range and types of fluids, variation in composition, and available sampling and laboratory services. The component set may include surrogate components and PsC (refer to Annex B). The following properties and parameters should be defined for each component in the set:

— molecular weight (MW),

— Pc (critical pressure),

— Tc (critical temperature),

— Vc (critical volume),

— Tb (normal boiling temperature),

— standard density,

— acentric factor,

— binary interaction parameters, and

— volume translation when applicable.


15

The component set shall be structured to adequately represent all process streams within the PMAS and shall align with a laboratory analysis (refer to 5.1.4.7). The EOS shall use a single component set for the allocation source streams.

User developed properties may be assigned to components. Some useful component properties are not included in the applied software library or the values in the library are not consistent with the values agreed to or required by permit.

EXAMPLE Gross Heating Values (GHV) found in GPA-2145 can be assigned to individual components such that the GHV of endpoint streams can be calculated directly by the PVT tool and in accordance with GPA-2172.

NOTE In general, an extensive well-defined component list can minimize the amount of tuning required. Abbreviated lists tend to require more tuning and are applicable over a more limited range of source operating conditions.

The component set should not include water unless it can be demonstrated that the inclusion of water has a material improvement on the allocation results.

5.1.3.3.2 Discrete Components The PVT tool shall include the following discrete components:

— Nitrogen

— Carbon Dioxide

— Methane

— Ethane

— Propane

— iso-Butane

— n-Butane

— iso-Pentane

— n-Pentane

Other discrete components may be included when there are corresponding source fluid sample analyses containing the discrete component. Discrete components may be removed from the set if no source fluid sample analyses contain the component.

Discrete components may be included to act as surrogate components for defined component groups (refer to Annex B).

Discrete components shall not be used to represent carbon number groups.

EXAMPLE N-Hexane should not be used to represent hexanes, n-Heptane to represent heptanes, or n-Octane to represent octanes.

5.1.3.3.3 Pseudo-Components (PsC) PsC shall be developed from laboratory supplied data including molecular weight and liquid density of the non-discrete components. Additional information such as boiling temperature may be used.


16

PsC properties and parameters not provided by the laboratory may be developed by applying correlations to the laboratory supplied data or by allowing the applied simulation software to determine them.

PsC properties, parameters, and fractions may be developed by lumping or de-lumping laboratory supplied component properties and fractions.

The fewest number of PsC should be used while meeting the PVT tool acceptance criteria. Both PsC that are applicable across multiple allocation sources and PsC that are source specific may be used.

5.1.3.4 EOS This Standard does not prescribe a specific EOS for use in a PVT tool. The EOS applied shall be valid for the range of fluids and process conditions applicable to the PMAS.

When fluid characterization is applied, an EOS tuning process should incorporate the following:

— Methodology;

— target parameters;

— fitting parameters;

— tuning parameters;

— validation methods and criteria.

Table 1 below includes a list of typical target, fitting, and tuning parameters.

Table 1: Target, fitting, and tuning parameters

Target Parameters Fitting Parameters Tuning Parameters

liquid sample bubble point

gas sample dew point

shrinkage factor

flash factor

separator GOR

gas sample composition relative to equilibrium gas of liquid sample

flash gas composition

binary interaction parameters (BIP)

volume shift

PsC MW

PsC Density

PsC boiling temperature

PsC critical properties

PsC acentricity

NOTE Target parameters are typically measured or observed fluid properties and factors that are essential parameters to the EOS results. Fitting parameters are not considered fluid properties and exist for the purpose of fitting the EOS to observed properties. Tuning parameters are nonessential fluid properties that are adjusted such that the EOS matches the observed for essential properties.

The number and type of target parameters used shall be established as part of a tuning methodology development process. Limitations on fitting and tuning parameters shall be established as part of this process.


17

The EOS shall be tunable with each new composition. The tuning process may be automated to facilitate the timeliness, repeatability, and auditability of the process.

When the PsC is fully defined prior to tuning, the tuning method should include tuning the EOS using fitting parameters. The tuning parameters should be used only when a fit is not achieved with fitting parameters. The fitting parameters can be used to concentrate on specific target parameters while tuning parameters tend to impact multiple target parameters. The initial fitting parameters listed in Table 2 should be used.

Table 2: Fitting parameter for target parameter

Target Parameter Fitting Parameter

bubble point BIP, starting with methane - largest PsC

dew point BIP, starting with methane - largest PsC

shrinkage factor volume translation, starting with PsC

flash factor BIP, starting with largest discrete component - largest PsC

separator GOR volume translation, starting with PsC

gas sample composition BIP for largest PsC

flash gas composition BIP for largest PsC

EOS tuning methods, target parameters, fitting parameters, and tuning parameters shall be documented in the functional specification. Citations to published papers which document the EOS tuning may be included. Tuning methods shall be reproducible and auditable

5.1.3.5 Correlations Correlations may be used in place of the EOS to determine tuning and fitting parameter values for use in the PVT tool. The correlation used shall not be proprietary and shall be documented in the functional specification.

5.1.4 Functional Specification

5.1.4.1 General A functional specification shall be developed and agreed to by the affected parties prior to placing the PSM in service. The functional specification shall include key assumptions, descriptions, and design considerations for the determination of the configuration parameter values. The functional specification shall also include validation methods and acceptance criteria, and supporting information on EOS tuning, if applicable.

5.1.4.2 Process Description The production process description shall be developed and maintained as a process flow diagram (PFD) prior to implementation and documented as part of the functional specification.

The PFD shall include all:

— allocation sources;

— other contributing hydrocarbon inlet streams;


18

— points of liquid and gas separation;

— stages of pumping and compression;

— points of heat addition and removal;

— fluid flow paths;

— recycle streams of condensed liquids from compression;

— sampling points;

— allocation endpoints.

Annex A contains an example of an allocation PFD. It shows the process path, points of phase separation, measurement, pumping, compression, and recirculation.

5.1.4.3 Simulation Tools The functional specification shall document the type and precise version of the simulation software used including EOS, outside correlations and methods and all applied settings. Proprietary software not commercially available shall not be used.

The PVT tool may contain a fluid characterization process. The PVT tool can be split into a liquid composition tool and a gas composition tool. Figue1 below contains an example liquid PVT tool while Figure 2 contains an example gas PVT tool.

Figure 2: Process environment view of an example liquid PVT tool

NOTE1 For this tool, liquid composition and conditions along with the pressure and temperature of three stages of separation. The output is a SF and FF for the liquid composition. This SF and FF will typically not match lab generated SF and FF because the laboratory factors would not account for the heat input and the multiple stages of separation. NOTE2 Figure 2 and Figure 3 show the example tools as viewed in the process simulation environment. Another aspect to the tools is the fluid property environment. It is in this environment that the PVT fluid properties are defined.


19

Figure 3: Process environment view of an example gas PVT tool

NOTE3 For gas PVT tool, gas composition and conditions along with the pressure and temperature of three points of separation are the input. The output is a Bg and rs for the gas composition. This Bg and rs will typically not match lab generated Bg and rs because the laboratory factors would not account for the separation, compression, and cooling.

The PVT tool shall be updated along with the functional specification PFD when process changes are made that can impact the PVT factor results.

5.1.4.4 Input Data Requirements 5.1.4.4.1 Fluid characterization input requirements When a source fluid characterization or tuning process is used, the initial input into the PVT tool may include:

— The source gas stream composition,

— The source liquid stream composition,

— non-discrete component MW and Liq. Density,

— Separation pressure at the time of the sampling,

— Separation temperature at the time of the sampling, and

— Laboratory determined parameters to use as target parameters.

5.1.4.4.2 PVT tool input requirements When the allocation process is run independently of the fluid characterization process, the allocation input into the PVT tool may include:

— Source pressures and temperatures,

— Endpoint pressures and temperatures,

— Pressure and temperature at separation points, and


20

— Pressure and temperature at points of enthalpy change.

5.1.4.4.3 PVT tool (without characterization) input requirements When there is no fluid characterization process, the allocation input into the PVT tool may include:

— The source gas stream composition,

— The source liquid stream composition,

— non-discrete component MW and Liq. Density,

— Source pressures and temperatures,

— Endpoint pressures and temperatures,

— Pressure and temperature at separation points, and


5.1.4.4.4 Sampling and laboratory analysis requirements The compositional input shall be traceable to a laboratory analysis. All laboratory obtained input data shall be obtained in accordance with a prescribed applicable industry standard. Proprietary lab analyses shall not be used. Where no standard is indicated, a full description of the method used to determine the composition, property, or factor shall be documented and included in the functional specification.

Composite compositional analyses shall not be used for an allocation PVT tool unless it can be demonstrated that the composite sampling period matches the allocation period. The periods are considered matching if they start on the same day and end on the same day. If an allocation source has a material change in composition, a sample representing the new composition shall be taken and processed for inclusion in the allocation. If an allocation source conditions change, the existing valid composition may be conditioned in accordance with Section 10 or a new sample may be taken and processed for inclusion in the allocation. Factors that define a material change shall be established and documented.

The source liquid and gas compositions shall be determined from samples taken at the same time. The separation pressure and temperature shall be recorded from separator instrumentation when possible. Sampling pressure and temperature shall not be used if different than separation pressure.

The PSM shall include a conditioning function in accordance with Section10 to ensure that source fluid compositions are representative of the current allocation period.

5.1.4.4.5 Field data requirements A list of inputs along with the units of measurement and associated devices shall be included in the functional specification. Table 3 below is an example input listing.

Table 3: Example input list

Input Units of Measurement Associated Device

Separator A Pressure psig PIT-123

Separator A Temperature °F TIT-123


21

The minimum requirements for input data, including the methods and processes utilized to obtain the input data and the manipulation of data such as flow weighting, shall be and documented and included in the functional specification.

5.1.4.5 PSM Outputs The output parameters shall match the PMAS requirements (refer to API MPMS Ch. 20.1).

The unit of measure and number of significant figures for each output parameter shall be included.

The uncertainty for calculated output parameters should be provided. Uncertainty determination methodology and requirements for the output parameters should be documented.

5.1.4.6 Validation and Acceptance Criteria The PVT tool validation methods, validation frequency, and acceptance criteria shall be established prior to implementation and documented in the functional specification.

All processes used to aid PVT tool validation, including methodologies for EOS tuning, shall be established prior to implementation and documented in the functional specification.

The input composition validation methods and acceptance criteria shall be in accordance with Section 47 and established prior to implementation and documented in the functional specification.

5.2 Application Implementation

5.2.1 General The operator shall be responsible for the implementation of a PVT tool when a PVT tool is used to describe phase behavior for a PMAS. Maintaining data quality are key to the PVT tool performance while a systematic application of the PVT tool facilitates repeatability and auditability. Implementation of the PVT tool shall be in accordance with the implementation of the PMAS (refer to API MPMS Chap. 20.1).

5.2.2 Fluid characterization validation Fluid quality validation shall be conducted in accordance with the fluid quality confirmation documentation (refer to Section 5.1.4.5).

A log of rejected sample pairs along with the basis for rejection shall be maintained. A review and reconciliation of the rejections listed in this log shall be conducted on a periodic basis. The frequency, criteria, and remediation plans, if any, shall be included in the fluid quality confirmation documentation.

Often in fluid characterization, the tuning process will mask compositional and methodology issues as the target parameters are driven to match observed conditions and properties. The acceptance criteria associated with fluid characterization should be based on multiple factors to ensure that quality issues are addressed. Factors that may be used include:

— Basic compositional logic (a single component fraction out of bounds);

— Thermodynamic relationship between liquid and gas compositions of the sample pair (equilibrium);

— Tuning objective met (EOS match observed);

— Degree to which fitting and tuning parameters are adjusted (limitations).


22

5.2.3 Simulation tool validation

5.2.3.1 Overall Material and Energy Balance Validation An overall material and energy balance (refer to Annex H) shall be performed for each allocation period where a PVT tool is used. Errors between individual theoretical quantities, energy content, and sales data indicate problems related to measurement, fluid characterization, or EOS. A material and energy balance is a tool that can facilitate troubleshooting and problem resolution.

5.2.4 Input data validation

5.2.4.1 Composition input data validation Laboratory oversight and auditing shall be conducted on a periodic basis and as part of the operation of the PMAS. (refer to API MPMS Chap. 20.1)

The reported compositional analyses’ conditions shall match the source conditions at the time the sample was taken. For samples taken at a liquid/gas separation point, the source conditions shall mean the pressure and temperature of the separator.

MW and liquid density of the discrete components contained in the laboratory analysis shall align with the EOS MW and liquid density of the same components. Differences found between values should be evaluated for impact to the allocation results and reconciled, as necessary.

5.2.4.2 Process input data validation Instrument calibration shall be monitored as part of the operation of the PMAS. (refer to API MPMS Chap. 20.1)

Input process data for PVT tool implementation include temperatures, pressures, and flow volume measurements as documented in the functional specification of the PVT tool. The specified input process data shall be reviewed for accuracy prior to each run of the PVT tool.

NOTE Temperature, pressure, and flow measurements might exhibit high uncertainties that can adversely affect the input process data for the PVT tool.

In reviewing the input process data:

— the operator shall verify that the input flow measurement equipment is installed, operated, and maintained per API MPMS Chapter 20.2 for single-phase measurement devices and API MPMS Chapter 20.3 for multiphase measurement devices.

— the operator should verify that the temperature and pressure transmitters are installed per API Recommended Practice 551;

— the operator should verify and calibrate temperature and pressure transmitters per API MPMS Chapter 21.1.

— The operator shall input only data that have been screened. As a minimum, screened process data shall have:

— temperature and pressure data recorded during upset or shut-in periods removed,

— pressure and temperature data evaluated for continuity.

— The operator shall document any change or omission of data from the PVT tool.


23

5.2.5 Output result determination PVT tool output determination should be a documented stepwise process, following the progression outlined in Figure 4.

— The compositional and process data for the allocation period shall be validated as described in 5.2.4.

— Validated compositional data should be characterized in accordance 6.2.5.1 and conditioned in accordance in accordance with Section 10.

— Fluid characterization should be validated in accordance with 5.2.2.

— If the characterization validation fails, the sample shall be rejected.

— A second material balance should be performed in accordance with 5.2.3.

— When the material balance passes, the PVT tool may be executed.

— If the material balance fails, the source of the imbalance shall be identified and corrected.

— The output of the PVT tool is passed to the PMAS.

Figure 4: PVT development process


24

5.2.6 Output result validation The output of the PVT tool is the source theoretical quantities used by the PMAS to allocate end point quantities. The output of the PVT tool shall be validated with a material and energy balance with an emphasis on the mass balance of the phases (refer to Annex H).

PSM performance monitoring and reporting shall be incorporated into the PMAS performance monitoring and reporting program. (refer to API MPMS Chap. 20.1)

6 Theoretical Quantity Determination for Allocation Using a Process Simulation Model (PSM)

6.1 Application Development

6.1.1 General The operator shall be responsible for the development of a PSM when a PSM is used to describe phase behavior for a PMAS. An allocation PSM is a model, incorporating an EOS, used to apply phase behavior in the determination of theoretical quantities for allocation (refer to API MPMS Ch. 20.1). A PSM is incorporated into a PMAS when there is a need to mitigate bias beyond what is achievable when addressing phase behavior as described in Chap 20.1 alone. This need is often driven by a disparity in fluids or difference in ownership.

6.1.2 Phase behavior parameter definition: required outputs

6.1.2.1 PSM functionality The PSM should function both as a source fluid characterization model and as an allocation model. See Figure 5. This dual functionality is driven by the need to define fluid properties for multiple independent and diverse sources (characterization) while combining these source streams into a single model simulating the production process. The result being theoretical source quantities at the allocation end points (allocation).

Figure 5: Dual function PSM

The characterization function emulates the field conditions and laboratory analyses of the individual source streams while the allocation function emulates the hydrocarbon stabilization process of the combined streams. Because the fluid characterization is not performed using the combined streams, the


25

PSM may be divided into separate models to achieve this functionality. If the characterization function is deemed unnecessary, it need not be included.

6.1.2.2 Characterization function The PSM should include a process where source fluid data is fully characterized using known conditions, component properties, and laboratory determined factors. The results of the process serve as an EOS tuning process and an intermediate output of the PSM. This intermediate output includes a representation of a full separator stream composition for each source and the properties and parameters of source specific PsC.

Because this process is performed without the use of allocation conditions, it may be structured to perform independently of an allocation period. Since the source stream characterization is performed independently of the other source streams, it may be structured to perform on a source by source basis.

The characterization function may be omitted when a source fluid sampling and analysis regime is developed that can achieve allocation results that meet the acceptance criteria. When using this approach, the applied methodology and validation method and criteria shall be documented as part of the functional specification.

6.1.2.3 Allocation function The PSM shall include a process that emulates the hydrocarbon stabilization process of the combined source streams. The PSM shall be valid for the range of temperature and pressure conditions that include allocation source conditions, end point conditions, and intermediate conditions such as compressor discharge. The PSM shall be capable of constant updating of volumes, temperatures, and pressures as part of the periodic allocation process.

The PSM output shall include source theoretical quantities of gas and oil at standard conditions (refer to API MPMS Ch. 20.1). Other outputs may include end point compositions, component quantities, retrograde condensate volume and composition, intermediate stream compositions, and energy consumed in compression or pumping.

6.1.3 Fluid Characterization

6.1.3.1 Measurement The following is a list of field data that may be used in source fluid characterization:

— liquid bubble point pressure (separation pressure);

— gas dew point temperature (separation temperature);

— liquid flowing density (separation densitometer);

— gas oil ratio (oil meter and gas meter).

The characterization process may include some or all these data. Other field data may also be used. Data source equipment shall be identified and documented.

The following is a list of laboratory analysis data for use in source fluid characterization:

— liquid composition;

— gas composition;

— non-discrete component MW and liquid density;


26

— fluid MW and liquid density;

— liquid shrinkage factor;

— liquid flash gas factor;

— liquid flash gas composition;

— residual oil composition.

The characterization process may include some or all these data. Other laboratory data may also be used.

6.1.3.2 Component Set 6.1.3.2.1 General This Standard does not prescribe a specific component set for the PSM. The selection of a component set should be based on factors including range and types of fluids, variation in composition, and available sampling and laboratory services. The component set may include surrogate components and PsC (refer to Annex B). The following properties and parameters should be defined for each component in the set:

— molecular weight (MW),

— Pc (critical pressure),

— Tc (critical temperature),

— Vc (critical volume),

— Tb (normal boiling temperature),

— standard density,

— acentric factor,

— binary interaction parameters, and

— volume translation when applicable.

The component set shall be structured to adequately represent all process streams within the PMAS and shall align with a laboratory analysis (refer to 6.1.4.8). The EOS shall use a single component set for the allocation source streams.

User developed properties may be assigned to components. Some useful component properties are not included in the applied software library or the values in the library are not consistent with the values agreed to or required by permit.

EXAMPLE Gross Heating Values (GHV) found in GPA-2145 can be assigned to individual components such that the GHV of end point streams can be calculated directly by the PSM and in accordance with GPA-2172.

NOTE In general, an extensive well-defined component list can minimize the amount of tuning required. Abbreviated lists tend to require more tuning and are applicable over a more limited range of source operating conditions.


27

The component set should not include water unless it can be demonstrated that the inclusion of water has a material improvement on the allocation results.

6.1.3.2.2 Discrete Components The PSM shall include the following discrete components:

— Nitrogen

— Carbon Dioxide

— Methane

— Ethane

— Propane

— iso-Butane

— n-Butane

— iso-Pentane

— n-Pentane

Other discrete components may be included when there are corresponding source fluid sample analyses containing the discrete component. Discrete components may be removed from the set if no source fluid sample analyses contain the component.

Discrete components may be included to act as surrogate components for defined component groups (refer to Annex B).

Discrete components shall not be used to represent carbon number groups.

EXAMPLE N-Hexane should not be used to represent hexanes, n-Heptane to represent heptanes, or n-Octane to represent octanes.

6.1.3.2.3 Pseudo-Components (PsC) PsC shall be developed from laboratory supplied data including molecular weight and liquid density of the non-discrete components. Additional information such as boiling temperature may be used.

PsC properties and parameters not provided by the laboratory may be developed by applying correlations to the laboratory supplied data or by allowing the applied simulation software to determine them.

PsC properties, parameters, and fractions may be developed by lumping or de-lumping laboratory supplied component properties and fractions.

The fewest number of PsC should be used while meeting the PSM acceptance criteria. Both PsC that are applicable across multiple allocation sources and PsC that are source specific may be used.

6.1.3.3 EOS This Standard does not prescribe a specific EOS for use in the PSM. The EOS shall be valid for the range of fluids and process conditions applicable to the PMAS.

When fluid characterization is applied, an EOS tuning process should incorporate the following:

— Methodology;


28

— target parameters;

— fitting parameters;

— tuning parameters;

— validation methods and criteria.

Table 4 below includes a list of typical target, fitting, and tuning parameters.

Table 4: Target, fitting, and tuning parameters

Target Parameters Fitting Parameters Tuning Parameters

liquid sample bubble point

gas sample dew point

shrinkage factor

flash factor

separator GOR

gas sample composition relative to equilibrium gas of liquid sample

flash gas composition

binary interaction parameters (BIP)

volume shift

PsC MW

PsC Density

PsC boiling temperature

PsC critical properties

PsC acentricity

NOTE Target parameters are typically measured or observed fluid properties and factors that are essential parameters to the EOS results. Fitting parameters are not considered fluid properties and exist for the purpose of fitting the EOS to observed properties. Tuning parameters are nonessential fluid properties that are adjusted such that the EOS matches the observed for essential properties.

The number and type of target parameters used shall be established as part of a tuning methodology development process. Limitations on fitting and tuning parameters shall be established as part of this process.

The EOS shall be tunable with each new composition. The tuning process may be automated to facilitate the timeliness, repeatability, and auditability of the process.

When the PsC is fully defined prior to tuning, the tuning method should include tuning the EOS using fitting parameters. The tuning parameters should be used only when a fit is not achieved with fitting parameters. The fitting parameters can be used to concentrate on specific target parameters while tuning parameters tend to impact multiple target parameters. The initial fitting parameters listed in Table 5 should be used.


29

Table 5: Fitting parameter for target parameter

Target Parameter Fitting Parameter

bubble point BIP, starting with methane - largest PsC

dew point BIP, starting with methane - largest PsC

shrinkage factor volume translation, starting with PsC

flash factor BIP, starting with largest discrete component - largest PsC

separator GOR volume translation, starting with PsC

gas sample composition BIP for largest PsC

flash gas composition BIP for largest PsC

EOS tuning methods, target parameters, fitting parameters, and tuning parameters shall be documented in the functional specification. Citations to published papers which document the EOS tuning may be included. Tuning methods shall be reproducible and auditable.

6.1.3.4 Correlations Correlations may be used in place of the EOS to determine tuning and fitting parameter values for use in the PSM. The correlations used shall not be proprietary and shall be documented in the functional specification.

6.1.4 Functional Specification

6.1.4.1 General A functional specification shall be developed and agreed to by the affected parties prior to placing the PSM in service. The functional specification shall include key assumptions, descriptions, and design considerations for the determination of the configuration parameter values. The functional specification shall also include validation methods and acceptance criteria, and supporting information on EOS tuning, if applicable.

6.1.4.2 Process Description The production process description shall be developed and maintained as a process flow diagram (PFD) prior to implementation and documented as part of the functional specification.

The PFD shall include all:

— allocation sources;

— other contributing hydrocarbon inlet streams;

— points of liquid and gas separation;

— stages of pumping and compression;

— points of heat addition and removal;

— fluid flow paths;

— recycle streams of condensed liquids from compression;


30

— sampling points;

— allocation endpoints.

Annex A contains an example of an allocation PFD. It shows the process path, points of phase separation, measurement, pumping, compression, and recirculation.

6.1.4.3 Simulation Tools The type and precise version of the simulation software used, including EOS, correlations and methods and all applied settings shall be documented in the functional specification. Proprietary software not commercially available shall not be used.

The PSM may be split into a fluid characterization tool and an allocation tool.

The fluid characterization tool simulates the first stage or inlet separation in the field and the laboratory shrink determination. Source analyses may be processed one at a time or in groups. The output of this tool is, in part, the input into the allocation tool. Figure 5 below contains an example fluid characterization tool.

Figure 6: Process environment view of an example fluid characterization tool NOTE1 For this tool, oil composition, GOR, and temperature are initial inputs while separator pressure, gas composition, and shrinkage factor are the target parameters. The fitting parameters are PsC BIP and volume shift. The BIP are adjusted to match the oil composition stream’s bubble point pressure to the separator pressure and the equilibrium gas composition to the gas composition stream’s composition. The volume shift is adjusted to match the ratio of the shrink stream volume / oil composition volume to the lab shrinkage factor. The Full Stream composition is made up of the oil composition stream and gas equilibrium stream compositions combined at GOR proportions.

NOTE2 Figure 6 shows the example tool as viewed in the process simulation environment. Another aspect to the tool is the fluid property environment. It is in this environment that the PsC fitting parameters are adjusted to reach target parameters.

The allocation tool simulates the production process of the combined source streams. The process layout of the tool should represent the PFD (refer to 6.1.4.2). The allocation PSM tool shall be updated along with the PFD when process changes are made that can impact the allocation results. Process equipment groups may be modeled as a single piece of equipment when operating in parallel or when operating in series with no intermediate side streams.


31

The allocation PSM tool can represent the entire PSM when fluid characterization is not performed. The characterization tool and allocation tool may be combined into a single simulation model.

6.1.4.4 Input Data Requirements 6.1.4.4.1 Fluid characterization input requirements When a source fluid characterization or tuning process is used, the initial input into the PSM may include:

— The source gas stream composition;

— The source liquid stream composition;

— The ratio of gas volume to liquid volume at the time of the samples;

— non-discrete component MW and Liq. Density;

— Separation pressure at the time of the sampling;

— Separation temperature at the time of the sampling;

— Laboratory determined parameters to use as target parameters:

— Shrinkage factor;

— Flash factor.

6.1.4.4.2 Allocation process input requirements When the allocation process is run independently of the fluid characterization process, the allocation input into the PSM may include:

— Source gas quantities;

— Source liquid quantities;

— Fuel and flare quantities;

— Circulated quantities;

— Intermediate measured quantities;

— Source pressures and temperatures;

— Endpoint pressures and temperatures;

— Pressure and temperature at separation points;


6.1.4.4.3 Allocation process only input requirements When there is no fluid characterization process, the allocation input into the PSM may include:

— The source gas stream composition;

— The source liquid stream composition;


32

— non-discrete component MW and Liq. Density;

— Source gas quantities;

— Source liquid quantities;

— Fuel and flare quantities;

— Circulated quantities;

— Intermediate measured quantities;

— Source pressures and temperatures;

— Endpoint pressures and temperatures;

— Pressure and temperature at separation points;


6.1.4.4.4 Sampling and laboratory analysis requirements The compositional input shall be traceable to a laboratory analysis. All laboratory input data shall be obtained in accordance with a prescribed applicable industry standard. Proprietary lab analyses shall not be used. Where no standard is indicated, a full description of the method used to determine the composition, property, or factor shall be documented and included in the functional specification.

Composite compositional analyses shall not be used for an allocation PSM unless it can be demonstrated that the composite sampling period matches the allocation period. The periods are considered matching if they start on the same day and end on the same day.

The source liquid and gas compositions shall be determined from samples taken at the same time. The separation pressure and temperature shall be recorded from separator instrumentation when possible. Sampling pressure and temperature shall not be used if different than separation pressure.

The PSM shall include a conditioning function (refer to Section 10) to ensure that source fluid compositions are representative of the current allocation period source fluids.

6.1.4.4.5 Field data requirements A list of inputs along with the units of measurement and associated device shall be included in the functional specification. Table 6 below is an example input listing.

Table 6: PSM field data input list

Input Units of Measurement Associated Device

Separator A Pressure psig PIT-123

Separator A Temperature °F TIT-123

Separator A Oil Volume actual barrels FIT-123L

Separator A Gas volume mscf FIT-123G

The minimum requirements for the input data, including the methods and processes utilized to obtain the input data and the manipulation of data such as flow weighting, shall be documented and included in the functional specification.


33

6.1.4.5 PSM Output result requirements 6.1.4.5.1 Intermediate outputs (fluid characterization) When characterization and allocation are separate functions in the PSM, the fluid characterization parameters should be saved as an intermediate output from the PSM. This output should include all component specific properties and parameters in a form that can be readily incorporated into the allocation function of the PSM. Table 7 contains an example intermediate output listing.

In addition to the component properties and parameters, a full stream composition for each source stream may also be generated. This output can be conditioned to the current allocation period conditions and can be used for multiple allocations when only the operating conditions change from period to period but not the compositional makeup of the source stream. Table 8 contains an example compositional output listing.

Table 7: Intermediate PSM output consisting of component properties and parameters for use in the PSM

Source A PsC Source B PsC Source C PsC

Binary Interaction Parameters component-PsC

N2 X X X

CO2 X X X

C1 X X X

C2 X X X

C3 X X X

iC4 X X X

NC4 X X X

iC5 X X X

NC5 X X X

surrogate1 X X X

surrogate2 X X X

surrogate3 X X X

Properties & Parameters

PsC MW X X X

PsC Liq. Density X X X

PsC Vol. Shift X X X

NOTE Properties and parameters for the discrete and surrogate components are contained within the allocation model and do not change from allocation to allocation. PsC properties and parameters not included in Table 7 are calculated in the allocation model from the PsC MW and Density.


34

Table 8: PSM fluid characterization compositional output

Source A Source B Source C

Mole Fractions

N2 X X X

CO2 X X X

C1 X X X

C2 X X X

C3 X X X

iC4 X X X

NC4 X X X

iC5 X X X

NC5 X X X

surrogate1 X X X

surrogate2 X X X

surrogate3 X X X

source A PsC X - -

source B PsC - X -

source C PsC - - X

The PSM output shall include source theoretical quantities of gas and oil at standard conditions (refer to API MPMS Ch. 20.1). Other outputs may include end point compositions, component quantities, retrograde condensate volume and composition, intermediate stream compositions, and energy consumed in compression or pumping.

A list of outputs along with the units of measurement and associated device shall be included in the functional specification. Table 9 below is an example output listing that may be used.

Table 9: Example PSM output list

Output Units of Measurement Value

Separator A theoretical oil volume standard barrels

Separator A theoretical gas volume mscf

Separator A theoretical gas energy mmBtu

Separator A theoretical energy consumption mmBtu

6.1.4.6 Validation and Acceptance Criteria The PSM validation methods, validation frequency, and acceptance criteria shall be established prior to implementation, and documented in the functional specification.

All processes used to aid PSM validation, including methodologies for EOS tuning, shall be established prior to implementation, and documented in the functional specification.

Fluid quality confirmation documentation that describes all PMAS fluid quality related information and activities shall be provided. This documentation should clearly identify the frequency of activities, all


35

specifically cited reference standards, along with fluid quality validation parameters and associated acceptance criteria (refer to API MPMS Chap. 20.1).

6.2 Application Implementation

6.2.1 General The operator shall be responsible for the implementation of a PSM when a PSM is used to describe phase behavior for a PMAS. Maintaining data quality are key to the PSM performance while a systematic application of the PSM facilitates repeatability and auditability. Implementation of the PSM shall be in accordance with the implementation of the PMAS (refer to API MPMS Chap. 20.1).

6.2.2 Fluid characterization validation Fluid quality validation shall be conducted in accordance with the fluid quality confirmation documentation (refer to Section 6.1.4.6).

A log of rejected sample pairs along with the basis for rejection shall be maintained. A review and reconciliation of the rejections listed in this log shall be conducted on a periodic basis. The frequency, criteria, and remediation plans, if any, shall be included in the fluid quality confirmation documentation.

Often in fluid characterization, the tuning process will mask compositional and methodology issues as the target parameters are driven to match observed conditions and properties. The acceptance criteria associated with fluid characterization should be based on multiple factors to ensure that quality issues are addressed. Factors that may be used include:

— basic compositional logic (a single component fraction out of bounds);

— thermodynamic relationship between liquid and gas compositions of the sample pair (equilibrium);

— tuning objective met (EOS match observed);

— degree to which fitting and tuning parameters are adjusted (limitations).

6.2.3 Simulation tool validation

6.2.3.1 Overall Material and Energy Balance Validation An overall material and energy balance (refer to Annex H) shall be performed for each allocation period where a PSM is used. Errors between individual theoretical quantities, energy content, and sales data indicate problems related to measurement, fluid characterization, or EOS. A material and energy balance is a tool that can facilitate troubleshooting and problem resolution.

6.2.3.2 Internal Balances Validation The comparison of theoretical PSM data to actual data collected in the production process may be used to validate the tool. These data are typically measured flash gas or intermediate compositional data used to determine if the PSM is accurate from start to finish (refer to Annex H). Often overall material and energy balances can be acceptable while internal balances are not. This can occur when the PSM is not correctly predicting the phase behavior of a fluid or combination of fluids.

6.2.4 Input data validation

6.2.4.1 Composition input data validation Laboratory oversight and auditing shall be conducted on a periodic basis and as part of the operation of the PMAS. (refer to API MPMS Chap. 20.1)


36

The reported compositional analyses’ conditions shall match the source conditions at the time the sample was taken. For samples taken at a liquid/gas separation point, the source conditions shall mean the pressure and temperature of the separator.

MW and liquid density of the discrete components contained in the laboratory analysis shall align with the EOS MW and liquid density of the same components. Differences found between values should be evaluated for impact to the allocation results and reconciled, as necessary.

6.2.4.2 Process input data validation Instrument calibration shall be monitored as part of the operation of the PMAS (refer to API MPMS Chap. 20.1).

Input process data for PSM implementation include temperatures, pressures, and flow volume measurements as documented in the functional specification of the PSM. The specified input process data shall be reviewed for accuracy prior to each allocation run of the PSM.

NOTE Temperature, pressure, and flow measurements might exhibit high uncertainties that can adversely affect the input process data for the PSM.

In reviewing the input process data:

— the operator shall verify that the input flow measurement equipment is installed, operated, and maintained per API MPMS Chapter 20.2 for single-phase measurement devices and API MPMS Chapter 20.3 for multiphase measurement devices.

— the operator should verify that the temperature and pressure transmitters are installed per API Recommended Practice 551;

— the operator should verify and calibrate temperature and pressure transmitters per API MPMS Chapter 21.1.

— The operator shall input only data that have been screened. As a minimum, screened process data shall have:

— temperature and pressure data recorded during upset or shut-in periods removed,

— recirculated flows reconciled,

— pressure and temperature data evaluated for continuity.

— The operator shall document any change or omission of data from the PSM.

6.2.5 Output result determination

6.2.5.1 Determination of intermediate outputs (fluid characterization) Because fluid characterization is performed without the use of allocation conditions, it may be performed independently of an allocation period and the allocation process. Because the source stream characterization is performed independently of the other source streams, this process may be performed on a source by source basis.

The fluid characterization process sh

Manual of Petroleum Measurement Standards Chapter 20...approvals required to become an API Standard....

Documents

Transcript of Manual of Petroleum Measurement Standards Chapter 20...approvals required to become an API Standard....