Pore Pressure Estimation

8/21/2019 Pore Pressure Estimation

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Investigation and Estimation of Changes in Pore Pressure and Effective

Stress on Sandstone Reservoir During Hydrocarbon Production

Kurniawan Adha1, Wan Ismail Wan Yusoff 1, Luluan Almanna Lubis1

1Petroleum Geoscience Department

Universiti Teknologi Petronas

Tronoh, Perak Darul Ridzuan - Malaysia

[email protected]

Abstract — Estimation of the correct pore pressure and

effective stress can affect the safety, economics and success in

production and recovery of oil and gas. From time to time the

oil and gas reservoir is always produced to meet energy

demands. But today, for answering oil and gas demands, the

recovery of mature oil field are often executed. Although, the

recovery mature oil field can be the solution of this problem,

consideration to change of reservoir characterization is quite

important. Since the production might be change the

characteristic of reservoir, especially the changes in pore

pressure and effective stress. In this study, the analysis consists

of investigation and estimation of changes in pore pressure and

effective stress through direct measurement and laboratory

analysis during the production of hydrocarbons. For

illustrating production history, there will be three scenario of

oil saturation (100%, 80% and 60%) that will be injected in to

the core (from real reservoirs). The investigation will

encompass the velocity (Vp and Vs) variations due to

modification of the confining pressure, pore pressure using

Auto Lab 500. Other parameters will be derived from velocity

data such as AI, EI and poisson ratio. The model will be

developed from laboratory results and will be verified by data

analysis. In consequence of various types of reservoirs and the

laboratory equipment (Auto Lab 500) can be arranged itsreservoirs condition, the estimate model from this study might

be applied universally in different type reservoir condition.

Keywords — pore pressure; effective stress; hydrocarbon

producti on; veloci ty; temperature

I. I NTRODUCTION

ore pressure at certain depth is of absolutely importance

in reservoir analysis, prior drilling and during

production while presence of pore pressure data can save the

money and avoid all unwanted event such as blowout andanother hazards. Generally, sandstone reservoir is one of

common reservoir in the world. Apparently, as the common

reservoir, from time to time sandstone reservoir has produced for fulfilling oil demands. Nevertheless, there are

still rooms of reservoir recovery for answering the request

of oil and gas demands.Most of the methods suggested are special and

unique to specify geological setting, depositional

environment, overpressure mechanism and many others.

Among of many methods, velocity method using seismic or

well or both data to determine pore pressure is most

frequently used [1]. The main model is velocity changes

through different confining pressure, temperature and pore

pressure. The model is characterized by the decreasing of

velocity from normal pressure to overpressure, caused by

the sound travel faster when the effective pressure higher.

The effective pressure (net pressure) is the difference

betwen overburden pressure and pore pressure. The

effective stress and velocity have a linear relationship [2].Pore pressure can be altered due to some process,

these processes are able to increase the pore pressure or

decrease the pore pressure. The process that decreases the

pore pressure is reservoir production (depletion) [3]. Along

production, the reduction in pore volume will increase

effective or inter granular stress [4]. During same time, thereduction of pore pressure will increase the volume of the

solid material or grains. The deformation of the reservoir

rock depends on the pore pressure and initial total stress, the

pore pressure will be reduced, the effective stress path and

the mobilization of the shear strength including failure

behavior.

Regarding problems associated with the

deformation and alteration of stress due to depleting of

reservoirs is important for many reasons. One of the reason,

in recovery mature reservoirs still encountered several

problems. In such, there are unsuccessful EOR activitieswhich is by the error in determining the characteristic of the

reservoir, including the changes of pore pressure [5].

Another challenge that we have to consider is the reservoir

has some types, starting from normal reservoir condition

until HPHT condition when the pore pressure over 70 Mpa

and temperature over than 1500C. Different reservoir

condition has different characteristic especially in pore

pressure and effective stress [4].

Recent years, there has been much study conducted

to identify and determine pore pressure. However, most of

pore pressure determination by data analysis (software) andrarely performs laboratory analysis or core direct

measurement. But this study focuses in creating model of

velocity and temperature changes due to changes in pore

pressure and effective stress using direct measurements

(laboratory measurements) by core samples as represent the

reservoir rock. In addition, oil injected into the core samplewhich is used for illustrating the production history and then

correlated the model with small stratigraphy sequence as

guide model for distribution. Furthermore, the equipment

used in this study can control the pressure (confining and

P



pore) and the temperature at reservoir condition with

expectations this study can address the problem or challengein pore pressure and effective stress estimation including the

different types of reservoir condition and characteristic.

Moreover, the model might be applied to investigate the

changes of pore pressure and effective stress due to

hydrocarbon production.

This study will characterize effective stress

changes, where effective stress represent the portion of thetotal stress carried by the rock grains or come from total

stress minus pore pressure [6]. This analysis through

laboratory testing and measurement for developing a model

about estimation of pore pressure and effective stresschanges due to production , then performs justification and

application of the model towards well and seismic data

analysis.

II. MATERIALS AND METHODS

A. Pore Pressure and Effective Stress

Pore pressure is the pressure which is formed by

the fluid contained in the pores space of rocks maintained atdepths. When there are not any processes, the pore pressure

is simply equal to the weight of the overlying fluid, under

the same circumstances the total of vertical stress is equal tothe weight of the overlying fluid and rock. This pressure is

often referred to as “hydrostatic pressure”. However, there

are several condition that can cause the pore pressure to be

different from hydrostatic pressure”. Pore pressure is

defined as the difference between overburden pressure (S)

to effective stress (σ), which can be written as follows[6].

P=S-σ

Since pore pressure and horizontal stress are

interrelated, changes in pore pressure also cause similar

changes in stress. The mathematical relationship betweenstress and pore pressure is defined in terms of the effectivestress. Implicitly, the effective stress is that portion of the

external load from total stress that is carried by the rock

itself. Effective stress (σ), controls sediment compaction

process, in the case the reduction of the effective pressure at

certain depth means a reduction in the yield of rock

compaction pressure [7]A new calculation method can accommodate the

normal compaction, under compaction and unloading

condition in a calculation formula which can be calibrated

with other data. This formula can be obtained by

determining Vo based on the normal compaction and

coefficients and exponents for the effective pressure [8].The method uses the assumption that the empirical

relationship between the effective pressure and velocity,

which can be formulated as follows.V= Vo + Aσ

B.

Where A and B are described as variations in

velocity to increase the effective pressure. When the data

rate is already known, the effective pressure estimated from

the following calculation.

σ = ((V-Vo) / A)1/b

The heat flow per unit temperature gradient and the

function of rock type, pressure, temperature and anisotropy

are referring to thermal conductivity. Assumed that the

thermal conductivity as rock property based on somereasons, although the thermal conductivity across a reservoir

rock is the total of thermal conductivity of rock and the

containing fluid. The thermal conductivity is function of

temperature. Although, the effect of the temperature for

sandstone is small compared to other minerals over the

range of reservoir temperature, temperature gradient can

still be used as a parameter for thermal conductivity. Theeffect of temperature is taken into consideration while the

effects of pressure are significant [9].

B. Core Sample

In order to accommodate the laboratory analysis for

investigating the changes of pore pressure and effective

stress due to hydrocarbon production, a number of reservoir

rock samples having different porosity and permeability are

selected for this study. All samples are sediment reservoir.

The samples are cut and shaped in to cylindrical form with 1

inch in diameter. The sample are from Miri outcrop and

berea sandstone. The whole of core samples are cleaned to

remove the remaining oil and/or salt before conducting

porosity and permeability analysis [table 1].

Table 1. Porosity and permeability data of core sample

C. Miri Field

This study used core sample from Miri Field. As the

birthplace of Malaysian petroleum industry the oil

exploration began in 1910 in Miri field. The Miri formation

consists of sand member that are very important in oil andgas reservoirs, especially in early production in Malaysia.

The formation consists of siliciclastic sequence of a

succession of clay-sand packages that are coarsening

upwards. The age of formation is middle miocene and

exposed around Miri city, Serawak, Malaysia which uplifted

part of subsurface, oil-bearing sedimentary strata of the Miri

Field and possibly also for the offshore fields [10]

D. Hydrocarbon Production and Pore Pressure History

During production, the change in reservoir

condition is a natural thing to happen. With Exceptionallythe reducing in volume of oil, transformation of

hydrocarbon contact (Oil water contact (OWC), Gas Oil

contact (GOC) ), pressure gradient and many others. As we

know, today more of the global reserves of petroleum

industry are added by increasing the recovery in existing

fields rather than by discovering new fields.Pressure gradient represent the reservoirs fluids

increments in pressure in relation to a given increase in

SamplesParameters

Porosity Permeability

Miri 1 23.59 378.224

Miri 2 22.74 346.315

Miri 3 23.40 287.403

Berea clean sandstone 17.48 176.74



depth. During production, there are changes in pressure

gradients due to out of fluid contain. It is possible to ensurewhether the reservoir has lost energy and to identify the

acting of drive mechanism. If along depletion, the line

shows the pore pressure gradient of the hydrocarbons moves

parallel, but unchanged in water zone, it sign that aquifer is

a very active (Figure 1). However, when the pore pressure

profiles of both the water and the hydrocarbon zones are

purely offset parallel to the original ones, the reservoir isvolumetric type and the horizontal offset of the pore

pressure profile indicates a loss of energy from the system

(Figure 2). In this case, the aquifer is not active, or only

slightly so, and the depth of contact will remain practicallyunchanged [11]

Knowing the pressure history is quite important;

especially the changes of pore pressure during depletion for

succeeding in the oil recovery and cannot despite the fact

that pore pressure has large impact for this recovery. In

addition, a reservoir may be overpressured, giving it the potential to flow to surface. If the wells are not managed

properly a blow out or other unwanted event may occur.

Figure 1 . Pore pressure profile changes with change in OWC

Figure 2. Pore pressure profile changes with change in OWC

E. General Experiment Procedure

There are several stages in the procedures that will be

conducted in this experiment. The stages involve collecting

data, preparation of samples include coring and cutting of

samples, determination of porosity and permeability of

sample (core sample), pore pressure investigation used

direct measurement, and developed model and justification

the model.The main parameters in this study are temperature

changes and velocity changes due to changes in pore

pressure and effective stress during production. From

velocity changes, the next step is determining others

parameters such as Vp/Vs, Poisson ratio, AI and EI. On the

other side, other parameters including temperature gradient

and heat flow can be determined from temperature changes.

The first procedure in this study is collecting data

where in this process involve field study, collecting sample

and other data necessary for the analysis. After that, corring

and cutting sample in 1 inch diamater. The samples are

cleaned to remove the remaining oil and/or salt before

performs porosity and permeability analysisBoth of velocity and temperature changes are the main

parameters that used for determining and identifying pore

pressure and effective stress change during production. To

obtain parameters that are needed to build the model, thereare several methods need to be performed. Direct

measurement or laboratory analysis divided in two main

scope. First, velocity analysis before to some action to the

sample used SONIC OYO . The velocity result from this

analysis used for velocity picking guide to next analysis.

Second, Auto Lab 500 is mandatory equipment that usedfor this study. To describe production history in this study,

there are three scenario of oil saturation conducted. Firstly

the sample inject by crude oil into 100 % oil saturation, thenthe reduce the oil contain to 80% and lastly in 60 % oil

contains using core flooding.The main results from this analysis are velocity

variations caused by changes of some parameters. Consist of

changes in temperature, confining pressure and pore

pressure. Velocity variations derived from alter values of

temperature, pore pressure and confining pressure for

estimated and identify the correlation between them.Confining pressure is set from 1000 to 10000 psi, pore

pressure from 1000 to 5000 psi and temperature from 100c

to 1000c

.

Developing and verifying the model is the end

result of this study. Trend derived from laboratorymeasurements and analysis will be developed into a modeland performed justification with data analysis. In this study,

data analysis of well and seismic are supporting data for

support the final model.

III. EXPECTED RESULTS

The pore pressure will be reduced due to production ordepletion. Reservoir depletion is one of processes that

decrease pore pressure and reduction in volume will bring

down the pore pressure over time. During oil and/or gas

production, the pore pressure within the reservoir is

reduced. The rates of pore pressure drop are controlled by



the rate from the surrounding rocks. The trend of pore

pressure line will be reduced due to production like examplemodel as shown example result at figure 3.

Combination of several parameters will give proper

determination in pore pressure estimation. Based on the

limitations of the existing methods which generally uses

velocity as the main parameters in the determination of pore

pressure, the model resulted from the combination several

methods such us velocity, temperature and poisson ratio thatcan estimate the pore pressure and effective stress more

properly.

Reservoir has some types, starting from normal

reservoir condition until HPHT condition when the pore pressure over 70 Mpa and temperature over than 150

0C.

Different reservoir condition has different characteristic

especially in pore pressure and effective stress. This is

becasue the reservoir condition can be arranged by the

equipment used in this study. The estimate model from this

study might be applied universally in different typereservoir condition.

Figure 3. Example estimate model of pore pressure for this studyfor got the point of velocity

IV. CONCLUSIONS

Determining pore pressure is quite important for the

petroleum industry, because drilling into overpressure zone

is very risky and hazardous. The success in drilling,

production and recovery of the mature field is a main reason

for paying attention in pore pressure estimation. The presence of pore pressure data can affect safety, reservoir

depletion history.Today, oil and gas industry is an industry still one of

critical industry in the world, since oil and gas energy still

needed by the human civilization. After such a long time

hydrocarbon field production, the production rate willalways decline due production of oil. Improve in oil

recovery and exploration are the main activity which is

conducted in the world.

The main contribution of this study is improving our

understanding of the changes in pore pressure and effective

stress during hydrocarbon production.

V. R EFERENCES

1. Bell, D.W., 2002, Velocity Estimation For Pore

Pressure Prediction, in Huffman, A. L. and Bowers,

G.L., eds, Pressures Regimes In Sedimentary Basin

and Their Prediction, AAPG Memoir 76, p. 177-

215.

2. Dutta, N and Khazanehdari, J., 2006, Estimation of

Formation Fluid Pressure using High-ResolutionVelocity From Inversion of Seismic Data and a

Rock Physics Model Based on Compaction andBurial Diagenesis of Shales, The Leading Edge,

December 2006, p.1528-1538.

3. Dutta, N.C., 2002, Geopressure Prediction using

Seismic Data: Current Status and The Road Ahead,

Geophysics, Vol. 67, No. 6, p. 2012-2041.

4. E. Skomdel, 2002, Effect of Pore Pressure and

stress path on Rock Mechanical Properties for

HPHT Application, SPE/ISRM 78152, October

5. Petrowiki,subsurface stress and pore presure,

SPE.http://petrowiki.org/Subsurface_stress_and_po

re_pressure6. Sayers, C.M., 2006, An Introduction to Velocity-

Based Pore Pressure Estimation, The Leading

Edge, December 2006, p.1496-1500.

7. Bowers, G.L., 1995, Pore Pressure Estimation from

Velocity Data: Accounting for pore pressure

mechanism beside undercompaction, SPE Drillingand Completion, 10, p. 89-95.

8. Tosaya, C. A. 1982, Acoustical properties of clay-

bearing rocks : PhD, thesis, Standford

9. Terzaghi, K., 1943, Theoritical Soil Mechanics,

John Wiley & Sons, Inc.

10. Robinson, K., 1985, Assessment of undiscovered

conventionally recoverable petroleum resources inTertiary sedimentary basins of Malaysia and

Brunei: Geol. Soc. Malaysia, Bull. 18, p. 119-131

11. Verga . 2009, Drive Mechanism and Displacment

Prosesses

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