Fundamentals of Formation Evaluation

Fundamentals Of Formation Evaluation

Elhadi AbusinSchlumberger

Sudan

Outlines

Introduction

Reservoir Properties

Logging Measurement and Applications

Tools and Applications

OH Logging and Quick Look Analysis

Introduction

• Permeable rockstores the hydrocarbon

• Source rockproduces hydrocarbon

• Impermeable rocktraps hydrocarbon

• Trapcaptures fluids

Introduction

Measurement

Open Hole Logging

• Lithology (reservoir rock?)• Resistivity (HC,water,both?)• Porosity (how much HC?)• What type of HC• Formation mech. properties• Permeability / cap pressure• Shape of the structure• Geological information• Geothermal• Unconventional applications

Why we log

Introduction

The life of a well

Regional Studies – Gravity / Other Geophysical Studies. Identification of main sedimentary basins.

Regional Seismic – Large 2D surveys over sedimentary basins. Identification of main subsurface

structures that can allow HC presence.

Local 3D Seismic – Detailed seismic survey over a smaller area to provide a more detailed

description on a prominent structure. In this stage a well is located to be drilled.

Well Drilling – After previous studies a well is located. During drilling: mud logging/cores and Logging

While Drilling allows for geological and preliminary petrophysical info.

Well Logging – Detailed subsurface petrophysical/geophysical measurements are obtained in order

to better characterize the formation on interest and determine its hydrocarbon potential. At this

stage, intervals are selected to be put on production.

Well Completion – The well is completed. Casing and Cement are applied. Preliminary Cased Hole

Logging can be run. Perforation is performed in the selected intervals. Production string is set and the

well is put on production.

Well Production – During this phase, monitoring the well production is performed using cased hole

techniques and data obtained from other wells in the field.

Introduction

During the drilling operations, the fluid is continuously circulated down the inside of the drill pipe, through the bottom of the bit, and back.

Mud Logging

Introduction

Mud Logging

The rock fragments are collected at regular intervals to provide information on the lithology, texture and presence of hydrocarbons.

Size of Cuttings 1x3mm

Introduction

Mud Logging

Weight on Bit (WOB), Drill string rotation speed (RPM), Mud pump speed (SPM), Mud pump pressure (SPP), Drilling Rate or Rate of Penetration (ROP), Lithology and texture of cutting, Percentage of gas in air and/or gas composition, Hydrocarbon staining on the cuttings and information such as bit diameter (D) and Drill-string configuration.


Hydrocarbon-in-Place =Area * Net Pay Thickness * Porosity * Hydrocarbon Saturation

where

Area - Area of hydrocarbon accumulation

Net Pay - Thickness of net hydrocarbon producing reservoir

Porosity - Void space in rock, occupied by fluids

Saturation - Proportion of pore space occupied by hydrocarbon

Three of these four key formation properties are derived from well data

For Volumetrics


Thickness Terminology

∑Net Pay <= ∑ Net Reservoir <= Gross Reservoir

Claystone

Porous & Permeable Sst

Claystone

Claystone

Claystone



Wa

ter

Tight Cemented Sst / Lst

OWC

Net Pay

Net Pay

Net Pay

Net Pay

Net Reservoir

Net Reservoir

Net Reservoir

Net Reservoir

Non-Reservoir

Non-Reservoir

Gro

ss Re

servo

ir

Aquifer

Oil

Oil

Oil

Oil

Non-Reservoir


Proportion of total rock volume that is void space (fluid filled) – Units

% or v/v

Total Porosity ØT

Effective Porosity ØE

Matrix/Primary Porosity

Secondary Porosity

Porosity

Porosity


Pore Space / Total Rock Volume

1 -

Rock Matrix

Porosity indicates how much fluid a rock can hold.

Most of all oil and gas produced today comes from the accumulations in the pore spaces of reservoir rocks.

Primary porosity refers to the porosity characteristics that have existed in a rock formation since it was first deposited.

Secondary porosity refers to porosity characteristics as they have been changed.

Porosity


Primary Porosity Secondary Porosity

RockMatrix

Pores formed during originaldeposition

RockMatrix

Pores formed after originaldeposition

Porosity Types


Matrix Pore System with predominantlyEffective Intergranular Porosity

Predominantly Non-effective Porositywith Effective Secondary fracture

Porosity


Proportion of void space that is occupied by a specific fluid - Units

% or v/v

Total Fluid Saturation SwT

Effective Fluid SaturationSwE

Moveable Fluid Saturation Shmov

Residual Fluid SaturationShr

Irreducible Fluid Saturation Swirr

Saturation

Saturation


SH = Hydrocarbon saturation (oil + gas)

SW = Water Saturation

SH + SW = 1

The saturation of the formation is the fraction of its pore volume occupied by a specified fluid. Water saturation is the fraction of the pore volume that contains formation water. If only water exists in the pores, the formation has a water saturation of 100%.

1 -

SH

SW

Rock Matrix


Capacity to permit fluid flow – Units D or mD

Darcy (D) (1 D = 1000 mD)

1 Darcy is that permeability that will permit a fluid of 1 cp

viscosity to flow at a rate of 1 cc per second through a cross-

sectional area of 1 sq cm when the pressure gradient is 1

atmosphere per cm.

Permeability

Permeability

Permeability is a measure of how easy a fluid flows through the connecting pore spaces. The unit of measurement is the millidarcy (mD).


Rock Matrix Pore Space

FluidPath

Intermediate fluid flow path

Tortous fluid flow path

FluidPath

Easy fluid flow path

FluidPath

Fluid Flow in various formations with same porosity but different grain size, structure and packing

Logging Measurement and

Applications

A full range of measurements are acquired, either via Wireline or LWD, then interpreted to reveal the rock and fluid properties of the formation such as:

Applications of Open Hole Logging

• Lithology • Porosity• Permeability• Hydrocarbon Identification• Hydrocarbon Quantification• Reservoir Evaluation


Applications

Applications of

Open Hole Logging


Applications

Measurement

Open Hole Logging

Open hole logging provides the most important source of information for well evaluation. It can be performed:1) After the well has been drilled by lowering a set of sensors on an electrical cable (Wireline Logging); 2) While the well is being drilled by placing the sensors in the drill string (Logging While Drilling).

Wireline Logging Logging While Drilling


Applications

Measurement

Open Hole Logging

How do we get a log

A tool string containing 4 tools


Applications

Measurement

Open Hole Logging

How do we get a log

Tool string is moving at a certain logging speed, and data are recorded at certain intervals called sampling rate.


Applications

Measurement

Open Hole Logging

How do we get a log


Applications

Measurement

Open Hole Logging

How do we get a log

And we get a group of wiggly lines called a “log”

Lithology

Some naturally occurring elements(Potassium, Thorium & Uranium) in the earth’s crust undergo radioactive decay spontaneously. Alpha and beta particles and gamma rays are emitted as their atomic nuclei disintegrate.

The Gamma Ray is the only one that can penetrate the rock for an appreciable distance. A Gamma Ray (GR) log measures this radiation.


Radioactive DecayGamma ray Logging

Rock Formation Rock Formation

In sedimentary formations, the Gamma Ray (GR) log reflects the clay or shale content.

Clean formations usually have a very low level of radioactivity and consequently a low GR log reading.

Lithology

Tools and ApplicationsTypical Gamma-Ray Responses

CleanSand

ShalySand

Vsh = (GR – GRmin) / (GRmax – GRmin)

The spontaneous potential is a measurement against depth of the borehole the difference between the voltage in the wellbore and an electrode on the surface.

Differences in potential are developed due to the differences in salinity between the mud filtrate and the formation water.

Lithology


Borehole

Electrode

IsolatedCable

Ground onsurface

Spontaneous Potential Measurement

The SP log cannot be recorded when a well is drilled with oil based mud, or when Rmf = Rw, or in a cased hole.

In the shale zones, the SP curve usually defines a more or less straight line on the log.

In sands and more permeable formations, the curves show excursions from the straight line.

Lithology


Shale A

Sandstone B

Rmf > Rw

Shale C

Sandstone D

Rmf < Rw

SP LOG

- 42

+ 44

For a quick evaluation at the well-site, the SP log is used in a similar way as the GR log.

Baselines for 100% sandstone and 100% shale are established at the maximum and minimum SP excursions.

Lithology


SP Log Example

Vsh = (SP – SPcl) / (SPsh – SPcl)

The Litho-Density tool (LDT) consists of a chemical gamma ray (-ray) emitting source and -ray detectors. The -rays emitted from the source interact with the formation in various ways and are scattered back to detectors.

One of the ways in which the -rays interact with the formation is known as photo-electric absorption. A parameter called PEF is defined by measuring the number of g-rays that are absorbed by photo-electric absorption.

Lithology


Density LoggingPhotoelectric Absorption

The PEF is a good matrix indicator.

PEF responds mainly to the lithology and has very little effect due to changes in or fluid content.

Lithology

Tools and ApplicationsLithology Identification using PEF

PEF as a function of fluid content and lithology

Porosity

Sonic

A simple sonic tool consists of:1) A transmitter which emits a sound pulse.

2) And a receiver which picks up and records the pulse as its passes the receiver.

For eliminate the influences of tool tilting and borehole size, modern sonic tools has several receivers.


Wellbore FormationReceiver

Compressional Shear Stoneley

The Sonic log records the time required for a sound wave to travel 1 ft of the formation. Commonly referred to as interval transit time (t) or slowness, it is the reciprocal of the velocity of the sound wave and is measured in micro-seconds per foot (s/ft).

Porosity

Sonic

Tools and ApplicationsTwo Transmitters/

Four Receivers

Porosity

Sonic


Willie Time Average Equation

Uncompacted Sands Raymer-Hunt Equation

In sedimentary formations sound velocity primarily depends on the rock matrix and the distributed porosity.

For clean consolidated formations with uniformly distributed small pores, a relationship between porosity and transit time was proposed by Willie.

Density

The majority of the -ray interactions are due to Compton scattering. As a result of this interaction, -rays lose energy as they interact with electrons in the formation.

The more electrons in the formation, the more likely -rays are to undergo Compton scattering. By measuring the -rays and their energy levels at detectors, the electron density of the formation (proportional to bulk density) can be predicted.

Porosity

Tools and ApplicationsDensity Logging Compton Scattering

Density log response is a combination of the matrix density and the density of fluids in their relative proportions.

The porosity can be computed from the density log for a known matrix and saturating fluid densities.

Density

Porosity


The density log consists of three curves. RHOB - Formation bulk density, DRHO -Bulk density correction – depends on caliper. CALI - Caliper - the density tool is a pad type tool. PEF – Photo - electric factor is also a part of the density tool recording.

Density

Porosity


Neutron

Porosity

Tools and ApplicationsThermal Neutron AbsorptionNeutron Logging Thermal Neutron Absorption

The neutron tool consists of a chemical or an electronic high energy neutron emitter and neutron detectors.

The high energy neutrons slow down to thermal levels via elastic scattering, mainly due to the collision with the hydrogen atoms in the formation. Assuming that the majority of the hydrogen atoms are present in the water or hydrocarbons in the pore spaces, the formation porosity can be determined.

The Hydrogen atoms in the pore fluid of the formation act as tiny bar magnets. The Nuclear Magnetic Resonance measurement is obtained by manipulating the hydrogen nuclei with magnetic field.

NMR

Porosity


NMR PrinciplesH Protons as magnets

RandomAlignments

Inherent Spin

NMR

Porosity


Each H-atom has a nuclear (proton)spin

No magnetic Field Present:

Nuclear Spins not aligned

Static Magnetic Field Present B0:

Spins aligned by static magnet

Apply RF pulse B1 (B1 I B0)

Spins swept to new direction

Spins decay to old static field

direction, giving off RF signal

NMR

Porosity


Measured decay of signal with time

Calculate from the decay curve

T2 relaxation time distribution

T2 distribution

Normalized signalAmplitude

T2

signal

time

Number of pores

Pore size

The initial signal amplitude of the exponentially decaying curve is proportional to the hydrogen index of the pore fluids. In water saturated rocks this signal amplitude can be calibrated to give a porosity, without specifying mineralogy or matrix properties.

NMR

Porosity


from NMR

Water in test tubeT2 = 3700 msec

Water in pore space of rockT2 = 10 to 500 msec

CMR porosity

CMR porosity = 100%

Time (T2)

Signalamplitude

Saturation

Resistivity

Invasion affects all logs. Lithology and porosity do not change significantly in the invaded zone.

However, saturation has to be computed for the uninvaded zone, since invasion changes fluid saturation in the invaded zone.

The main tools for determining hydrocarbon saturation in open hole are the Resistivity tools.


Inv

ad

ed

Zo

ne


• Measure: Electrical resistivity / conductivity of formation

• Units: Resistivity - ohm-m, Conductivity – mmho

• Most log measurements presented as resistivity

• Resistivity = 1000/Conductivity

• Electrical currents primarily conducted through water. Rock matrix and hydrocarbons are non-conductive.

• For a current to flow the conducting medium (water) must provide a continuous network through the formation

• Water conductivity is dependent on salinity and temperature

• Formation conductivity is dependent on the water conductivity, the porosity (volume of water) and the pore geometry

• Hydrocarbon in the reservoir reduces the volume of conducting water and so reduces the conductive capability of the formation

Saturation

Resistivity

The formation lithology and do not alter much due to invasion.

The Rt changes significantly in the invaded zone and the resistivity

tools need to measure deep into the virgin zone to compute the Sw.

ResistivityBasics


HydrocarbonSaturation

Invasion

Invaded Transition Annulus Virgin

Rmf100%

BoreholeWall

0%

ResistivityBasics



Depth of Investigationof Resistivity Logs

Rt determination Deep Log Medium Log

Very ShallowLog

Invaded Zone

Virgin Zone

Rxo = f (Very Shallow Log) Sxo = 1/ (Rmf/Rxo)1/2

Rt = f (Very Shallow Log,Medium Log,Deep Log) Sw = 1/ (Rw/Rt)1/2

The resistivity tools are designed to read at various depths of investigation: The shallow reading is used to measure the Sxo.

The deep reading is affected by the invasion and is often not the R t.

The medium reading, together with the shallow reading, is used to correct the deep reading to obtain the the Rt.


• Invasion by mud filtrate displaces reservoir fluids

away from the borehole

• Resistivity tools designed to read at different depths

of investigation with deepest reading ‘virgin’

formation properties

• Resistivity tools also designed for different mud

systems:

– Laterologs for conductive muds (saline WBM)

– Induction logs for non-conductive muds

(Oil/Fresh Water based)

Saturation

Resistivity

The objective of the shallow resistivity tools is to determine the invaded zone resistivity. The tools consist of an arrangement of electrodes on a pad that is pressed against the formation and minimizes borehole effect.

ResistivityMicro



Rxo Measurement

Laterolog tools function in conductive drilling mud environments.

They send focused current into the formation to measure the voltage in a specific volume of the formation.

This voltage is related to the resistivity of the formation.

Saturation

Resistivity


Dual Lateralog MeasurementDeep Measurements Shallow measurement

Volume ofInvestigation

Saturation

Resistivity


In non-conductive mud environments, lateralog tools do not work and induction-principles based tools are used.

The principles of an induction tool:1) The transmitter coil creates a primary magnetic field;2) The primary magnetic field induces current loop in the formation;3) The current loop creates an secondary magnetic field;4) The secondary magnetic field induces a current in the receiver coil. The current tensity is related to the formation conductivity (or resistivity).

Typical dual laterolog (HALS) and micro-spherically focused log (MSFL) log. The Rt can be computed from the three curves using correction charts.

Saturation

Resistivity


In oil based mud, only induction based

resistivity logs can be recorded. The Rt

can be computed from the deep curve.

Saturation

Resistivity



• Formation resistivity a function of

– Formation Water Salinity & Temp.

– Porosity

– Pore Geometry

– Hydrocarbon Saturation

• Higher resistivity when:

– Fresh water

– Low Porosity

– Low permeability

– High hydrocarbon saturations

• Lower Resistivity when:

– Highly saline water

– High porosity

– High permeability

– Low or zero hydrocarbon saturations

POTA%0 0.1

0POTA

THORPPM0 20

URANPPM0 20

0THOR

0URAN

GRGAPI0 150

CALIIN6 16

BSIN6 16

0 GR

DEPTHFT

RHOBG/CC1.95 2.95

DRHOG/CC-0.75 0.25

NPHIv/v0.45 -0.15

PEFB/E0 20

DTUS/F140 40

LLDOHMM0.2 2000

LLSOHMM0.2 2000

MSFLOHMM0.2 2000

5850

5900

5950

6000

6050

6100

6150

6200

6250

6300

6350

6400

6450

6500

Saturation Resistivity

Saturation

The saturation of a formation represents the amount of a given fluid present in the

pore space.

The porosity logs react to the pore space.

The resistivity logs react to the fluids in the pore space.

The combination of the two measurements gives the saturation

Matrix

water

oil

Basics 1

Rw = resistivity of water in the pore space.

Define Ro = resistivity of a rock totally filled with water.

F: Fn Factor.

At constant porosity F is constant.

As porosity increases, Ro decreases and F decreases.

Experiments have shown that F is inversely proportional to m.

m: is called the "cementation exponent".

a: is called the "lithology" constant.

F R0

Rw

F a

m

Basics 2

Saturation can be expressed as a ratio of the resistivities:

where n is the "saturation exponent", an empirical constant.

Substituting for Ro:

Substituting for F:

Swn

R0

R t

Swn

FRw

R t

w

nS a

m

RwRt

Saturation Equation

The Archie equation is hence very simple. It links porosity and resistivity with the amount of water present,

Sw.

Increasing porosity, will reduce the saturation for the same Rt.

Increasing Rt for the same porosity will have the same effect.

w

nS a

m

RwRt

Type

Density-Neutron

Gas in formation causes the apparent porosity from density log to increase and porosity from neutron log to decrease, causing a density-neutron separation.


The tester can obtain vertical pressure profiles which are used to define fluid type within the reservoir and locate fluid contacts in medium to high permeability virgin reservoirs.

Formation Testing

Type


MicroResistivity

Separation between the two curves from microlog resistivity can indicate permeability.

The log does not provide the absolute permeability value and is only used as an indicator for permeable formations.


Permeability

Permeability from Microlog


Rock

Reservoir

Non-Reservoir

Hydrocarbon

Bearing

Water

Bearing

Gas Bearing - Evaluate

Oil Bearing - Evaluate

1) Locate reservoir

2) Detect Hydrocarbon

3) Hydrocarbon Type

4) Evaluate , Sw

Basic Steps for

QuickLook Evaluation

Before start interpretation:

•Review the quality of the logs : check headers, depths, scales, calibrations. Read the remarks section.

•Inspect the mud log for intervals with reservoir rock, hydrocarbon shows and mud gains.

•Use logs from surrounding wells, if available, to identify any obvious anomalies in the data. Have a look on the local geology.



GASGAS

OILOIL

WATERWATER

sandstone gradessandstone gradesto clay sedimentto clay sediment

sandstonesandstonepinch outpinch out

lithological

Local geology


Descriptor Measurement Functional Behavior

Clean/Shaly SP

GR

Vshale SP Vshale GR

Porosity() Density

Neutron

Sonic

b n t

Hydrocarbon Rt Sw Rt So Rt Rt

Recoverable/

Moveable

Rxo vs. Rt

(shallow vs. deep)

Rxo=Rt no invasion

Rxo/RtRmf/Rw Moved fluid

This is a summary of Qualitative Interpretation

ReservoirRocks

To discriminate potential reservoir rock from non-permeable rock GR or SP log can be used.


Reservoir Rocks

Vsh = (GR – GRmin) / (GRmax – GRmin)or

Shale Index

GR (API) GR Interpretation

Shale

Shale

Shale

Sand

Sand

Shaly Sand ???

ReservoirRocks


Other Shale Volume ComputationFrom SI (or Shale Index)

SP log can be used in a similar way to identify the reservoir rock and compute the shale volume.


ReservoirRocks

Vsh = (SP – SPcl) / (SPsh – SPcl)

SP Log Interpretation

Rmf > Rw


ReservoirRocks

The sand and shale line are selected on the GR log as indicated by the respective lines and the cut-off line drawn.

The zones where the GR is below the cut-off line can be considered to be reservoir quality.


ReservoirRocks

HydrocarbonZones

From the Archie’s equation Rt increases when the water is replaced

by oil with and lithology remaining constant. This feature is used to differentiate oil and water zones in the reservoir intervals.

OH Logging and Quick Look AnalysisIdentify Hydrocarbon Zones

OilZone

WaterZone

Archie’s Equation

Rt = Rw / (m Swn)

Cle

an s

and

s w

ith

co

nst

ant

Resistivity Log

Water bearing intervals are characterized by low resistivity and density and resistivity with same behavior.

When water is replaced by oil in formation with the same porosity, density decreases while resistivity increases.

HydrocarbonZones


Gamma-Ray Density Resistivity

Anti-Correlation

WA

TE

RO

IL

Correlation

Once the reservoir intervals have been identified, next step is to identify the hydrocarbon bearing zones. Zones with high resistivity are the candidates to confirm the presence of hydrocarbons.

HydrocarbonZones


Oil/GasDifferentiation

Gas in formation causes the apparent porosity from Density log to increase and porosity from Neutron log to decrease, causing a high density-neutron separation (cross-over).


Porosity,

Formation porosity can be obtained from:•Density Log alone : matrix density and fluid density has to be known•Density-Neutron Log : Compute the density and neutron porosity assuming the same matrix and use the relevant equation.•The computed will be a good approximation whatever the matrix.


Here matrix is sandstone, then density log alone is used to compute the formation porosity.


Porosity,

To compute the RW from the log, the interpreter must find a clean water bearing

zone from the log and read the resistivity in this zone from a deep resistivity tool. Using the Archie’s equation for SW=1, the RW can be computed as above.


Rw

QuickLook Archie:

Rt = Rw / (m Swn)

a=1, m=n=2

Saturation in the hydrocarbon bearing clean zones can be found using the generalized Archie’s equation when the values of the a, m and n are known. If they are not known, for a quicklook evaluation, use a = 1 & m = n = 2.


Water Saturation (Sw )

Using the Archie’s equation the SW can be computed in the

hydrocarbon zone using the values of RW and determined earlier.




Summary


Thank You

Other computations that can be done:

For each major reservoir and fluid type:

Total net sand count (Net / Gross)

Average porosity

Average hydrocarbon saturation

Fluid Contact Levels

Petrophysical parameters used (ma, Rw, m, n...)


Practical

1

2

3

4

5

Questions 1-5: Which answer best describes the zones 1-5 below. (A,B,C or D)

Question 6: Which zone below likely contains Oil or Gas

A- Permeable, low resistivity and porous.

B- Permeable, high resistivity and porous.

D- Permeable, high resistivity and tight.

C-Impermeable shale.

Answers

B- Permeable, high resistivity and porous.


D-High resistivity and tight.


1

2

3

4

5

C-Impermeable shale

6 – Zone 1 is probably Oil productive

Fundamentals of Formation Evaluation

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