Influence of Reservoir Stress Path on Deformation and Permeability of Weakly Cemented Sandstone Reservoirs

1999 ◽  
Vol 2 (03) ◽  
pp. 266-272 ◽  
Author(s):  
H. Ruistuen ◽  
L.W. Teufel ◽  
D. Rhett
Keyword(s):  
Pore Pressure ◽  
Stress Path ◽  
Stress Sensitivity ◽  
Uniaxial Strain ◽  
Matrix Permeability ◽  
Hydrostatic Loading ◽  
Stress Changes ◽  
Reservoir Conditions ◽  
Path Dependent ◽  
Rock Compressibility

Summary The influence of production-induced changes in reservoir pore pressure on compressibility and permeability of weakly cemented sandstones has been analyzed. Laboratory experiments simulating reservoir depletion have been conducted over a range of stress paths that a reservoir may follow. The results suggest that compressibility of weakly cemented sandstones is stress path dependent. Compressibility measured under uniaxial strain conditions, or a stress path defined by a lower ratio of the rate at which the effective horizontal to effective vertical stress were increased than the one associated with uniaxial strain, is more than twice the corresponding value found from the hydrostatic loading experiment. In contrast, matrix permeability measured in the maximum stress direction show no significant stress path dependence. Experimental results suggest that a better understanding of the stress-sensitive behavior of weakly cemented sandstones can only be gained by dealing more directly with the microstructure of the rock. The stress-path-dependent nonlinear behavior of weakly cemented sandstones is related to effects of shear-enhanced compaction. Increasing cementation has been experimentally shown to reduce stress sensitivity. The observed nonlinearity is attributed to dilatancy rather than shear-enhanced compaction, also reflected by permeability measurements made in the maximum stress direction. Introduction Reliable data on rock compressibility and matrix permeability are essential in reservoir engineering due to the significant impact these parameters have on reserves and productivity estimations. Laboratory measurements of rock compressibility are applied to production forecasts, reservoir pressure maintenance evaluations, as well as reservoir compaction and subsidence studies,1–4 while matrix permeability heavily influences reservoir productivity and injectivity and is essential in performance forecasting.4 Formation compressibility is defined as the in situ bulk volume strain that results from changes in reservoir pore pressure: c = − 1 V i d V d P . ( 1 ) By adopting this definition, formation compressibility is not related to specific stress conditions. Formation compressibility is simply defined as the bulk response of the reservoir rock to production-induced changes in pore pressure. The stress changes that result from changes in pore pressure are uniquely defined by reservoir characteristics such as boundary conditions, reservoir geometry, and the mechanical properties of the reservoir rocks and bounding formations. A common procedure within the oil industry has been to use the so-called uniaxial correction factor to correct the results obtained from the hydrostatic compressibility test (cb) to "formation compressibility:"5 c = 1 + μ 3 ( 1 − μ ) c b . ( 2 ) An inherent assumption in this expression is that the rock is elastic throughout its production-induced deformation history, which may not be the case for weakly cemented reservoir rocks. The validity of the procedure also relies on the assumption that the uniaxial strain model adequately simulates reservoir conditions during depletion. Recent in situ stress measurements have demonstrated that this assumption is not necessarily valid. Since the early 1950's a number of researchers have investigated the relationships between rock matrix permeability and applied external pressure. Early observations suggested that permeability declines approximately exponentially with increasing confining pressure6 and that a relatively greater permeability reduction should be expected for a lower permeability matrix.7 These results were obtained from tests conducted under hydrostatic loading conditions. More recently, permeability measurements have also been performed under triaxial stress conditions.8–10 Matrix permeability has been related to compressibility and thus to the fabric and mineralogy of rocks. Bruno, Bovberg, and Nakagawa9 have shown that mineralogy may play a significant role in high-porosity rocks. Both increasing clay content and decreasing cementation resulted in a larger reduction in permeability with increasing stress. Holt8 reported experimental results on stress sensitivity of matrix permeability of a Jurassic sandstone. Samples were loaded under both triaxial compression and extension. No major differences in permeability were found between deviatoric and hydrostatic loading prior to yielding. At the yield stress, a sharp decline in permeability was observed. Most of the permeability reduction took place in the range of 60% of 90% of the peak shear stress. Teufel and Rhett3 introduced the term "stress path" to quantify the actual stress changes that take place in the reservoir during pressure depletion. (In this work, stress path is denoted K (not K0) to avoid confusion with uniaxial strain conditions, which is commonly denoted K0 test conditions. Also note that stress path here describes a constant ratio of change in stress state, which implies that different stress paths do not approach a common point in stress space.) The term describes the constant ratio of change in effective minimum (horizontal) stress to effective maximum (vertical) stress from initial reservoir conditions: K = Δ σ m i n Δ σ m a x . ( 3 ) The changes in the reservoir stress state resulting from depletion along stress paths of K=0, 0.5, and 1 are illustrated in Fig. 1. The importance of the reservoir stress path is that the shear stress has a larger increase for a lower stress path.

The Effect of Geological and Geomechanical Parameters on Reservoir Stress Path and Its Importance in Studying Permeability Anisotropy

2000 ◽  
Vol 3 (05) ◽  
pp. 394-400 ◽  
Author(s):  
M. Khan ◽  
L.W. Teufel
Keyword(s):  
Boundary Condition ◽  
Pore Pressure ◽  
Effective Stress ◽  
Stress Path ◽  
Horizontal Stress ◽  
Uniaxial Strain ◽  
Reservoir Conditions ◽  
Permeability Anisotropy ◽  
Sandstone Reservoirs

Summary Reservoir stress path is defined as the ratio of change in effective horizontal stress to the change in effective vertical stress from initial reservoir conditions during pore-pressure drawdown. Measured stress paths of carbonate and sandstone reservoirs are always less than the total stress boundary condition (isotropic loading) and are either greater or less than the stress path predicted by the uniaxial strain boundary condition. Clearly, these two boundary-condition models that are commonly used by the petroleum industry to calculate changes in effective stresses in a reservoir and to measure reservoir properties in the laboratory are inaccurate and can be misleading if applied to reservoir management problems. A geomechanical model that incorporates geologic and geomechanical parameters was developed to more accurately predict the reservoir stress path. Numerical results show that reservoir stress path is dependent on the size and geometry of the reservoir and on elastic properties of the reservoir rock and bounding formations. In general, stress paths become lower as the aspect ratio of reservoir length to thickness increases. Lenticular sandstone reservoirs have a higher stress path than blanket sandstone reservoirs that are continuous across a basin. This effect is enhanced when the bounding formations have a lower elastic modulus than the reservoir and when the reservoir is transversely isotropic. In addition, laboratory experiments simulating reservoir depletion for different stress path conditions demonstrate that stress-induced permeability anisotropy evolves during pore-pressure drawdown. The maximum permeability direction is parallel to the maximum principal stress and the magnitude of permeability anisotropy increases at lower stress paths. Introduction Matrix permeability and pore volume compressibility are fundamentally important characteristics of hydrocarbon reservoirs because they provide measures of reservoir volume and reservoir producibility. Laboratory studies have shown that these properties are stress sensitive and are usually measured under hydrostatic (isotropic) loads that do not truly reflect the anisotropic stress state that exists in most reservoirs and do not adequately simulate the evolution of deviatoric stresses in a reservoir as the reservoir is produced. Recent laboratory studies1–3 have shown that permeability and compressibility are dependent on the deviatoric stress and change significantly with reservoir stress path. In-situ stress measurements in carbonate and clastic reservoirs indicate that the reservoir stress path is not isotropic loading (equal to 1.0) and can range from 0.14 to 0.76. 4 The measured reservoir stress paths are also inconsistent with the elastic uniaxial strain model5 commonly used to calculate horizontal stress and changes in horizontal stress with pore-pressure drawdown. The calculated uniaxial strain stress path can be significantly less or greater than the measured stress path.4 Knowledge of the stress path that reservoir rock will follow during production and how this stress path will affect reservoir properties is critical for reservoir management decisions necessary to increase reservoir producibility. However, in-situ stress measurements needed to determine reservoir stress path are difficult and expensive to conduct, and may take several years to collect. Various analytical models have been proposed to calculate in-situ horizontal stresses and they could be applied to the prediction of reservoir stress path during pore-pressure drawdown.5–9 However, none of these models addresses all of the essential geological and geomechanical factors that influence reservoir stress path, such as reservoir size and geometry or the coupled mechanical interaction between the reservoir and the bounding formations. Accordingly, a geomechanical model was developed to more accurately predict reservoir stress path. The model incorporates essential geological and geomechanical factors that may control reservoir stress path during production. In addition, laboratory results showing the effect of reservoir stress path on permeability and permeability anisotropy in a low-permeability sandstone are also presented. These experiments clearly demonstrate that during pore-pressure drawdown permeability decreases and that permeability parallel and perpendicular to the maximum stress direction decreases at different rates. The smallest reduction in permeability is parallel to the maximum principal stress. Consequently, stress-induced permeability anisotropy evolves with pore-pressure drawdown and the magnitude of permeability anisotropy increases at lower stress paths. Field Measurements of Stress Path in Lenticular Sandstone Reservoirs Salz10 presented hydraulic fracture stress data and pore-pressure measurements from reservoir pressure build-up tests in low-permeability, lenticular, gas sandstones of the Vicksburg formation in the McAllen Ranch field, Texas (Table 1). This work was one of the first studies to clearly show that the total minimum horizontal stress is dependent on the pore pressure. Hydraulic fractures were completed in underpressured and overpressured sandstone intervals from approximately 3100 to 3800 m. Some of the sandstones (9A, 10A, 11A, 12A, 13A, and 14A) were later hydraulically fractured a second time to improve oil productivity after several years of production. For initial reservoir conditions before production, the total minimum horizontal stress shows a decrease with decreasing pore pressure for different sandstone reservoirs. The effective stress can also be determined from these data. Following Rice and Cleary11 effective stress is defined by σ = S − α P , ( 1 ) where ? is the effective stress, S is the total stress, ? is a poroelastic parameter, and P is the pore pressure. For this study ? is assumed to equal unity. A linear regression analysis of the minimum horizontal and vertical effective stress data shows that at initial reservoir conditions the ratio of change in minimum effective horizontal stress to the change in effective vertical stress with increasing depth and pore pressure is 0.50.

Experimental Investigation of Reservoir Stress Path Hysteresis During Production-Injection Periods

2014 ◽  
Author(s):  
Mojtaba P. Shahri ◽  
Stefan Z. Miska
Keyword(s):  
Pore Pressure ◽  
Poisson’S Ratio ◽  
Stress Path ◽  
Horizontal Stress ◽  
Poisson's Ratio ◽  
Injection Process ◽  
Fracture Pressure ◽  
Industry Standards ◽  
Depletion Period ◽  
Stress Changes

There has been an increasing consciousness regarding stress changes associated with reservoir depletion as the industry moves towards more challenging jobs in deep-water or depleted reservoirs. These stress changes play a significant role in the design of wells in this condition. Therefore, accurate prediction of reservoir stress path, i.e., change in horizontal stresses with pore pressure, is of vital importance. In this study, the current stress path formulation is investigated using a Tri-axial Rock Mechanics Testing Facility. The reservoir depletion scenario is simulated through experiments and provides a better perspective on the currently used formulation and how it’s applicable during production and injection periods. The effect of fluid re-injection into reservoirs on the horizontal stress is also analyzed using core samples. According to the results, formation fracture pressure would not be equal to its initial value if pressure builds up using re-injection. The irrecoverable formation fracture pressure has a power law relation with pore pressure drawdown range. In order to avoid higher permanent fracture pressure reduction, it’s recommended to start the injection process as soon as possible during the production life of reservoirs. According to the experimental results, rocks behave differently during production and injection periods. Poisson’s ratio is greater during pressure build-up as compared to the depletion period. According to the current industry standards, Poisson’s ratio is usually obtained using fracturing data; i.e., leak-off test or mini-fracture test, or well logging methods. However, we are not able to use the same Poisson’s ratio for both pressure drawdown and build-up scenarios according to the experimental data. Corresponding to Poisson’s ratio values, the change in horizontal stress with pore pressure during drawdown (production) is higher than during build-up (injection) period. The outcomes of this study can significantly contribute to well planning and design of challenging wells over the life of reservoirs.

Spatio-Temporal Stress Path Prediction Under Different Deformational Conditions

2017 ◽  
Author(s):  
Saeed Rafieepour ◽  
Stefan Z. Miska
Keyword(s):  
Boundary Conditions ◽  
Flow Regime ◽  
Plane Strain ◽  
Stress Path ◽  
In Situ Stress ◽  
Uniaxial Strain ◽  
Fracture Pressure ◽  
Stress Changes ◽  
Path Prediction

Drilling new infill wells in depleted reservoirs is extremely problematic and costly due to low formation fracture pressure and narrow mud window resulting from in-situ stress changes due to fluid extraction. This is of paramount importance especially for drilling operations in deep-water reservoirs, which requires precise prediction of formation fracture pressure. In turn, this entails accurate prediction of reservoir stress changes with pore pressure depletion, i.e., the stress path. Currently-used models assume a transient flow regime with reservoir depletion. However, flow regime in depleted reservoirs is dominantly pseudo-steady state (PSS). Shahri and Miska (2013) proposed a model under plane-strain assumption. However, subsea subsidence measurements confirm that depletion-induced reservoir deformation mainly occurs in axial direction. We provide analytical solutions for stress path prediction under different deformational conditions namely, plane strain-traction and displacement boundary conditions, generalized-plane-stress, generalized uniaxial strain, and uniaxial-strain. For this purpose, constitutive relations of poroelasticity are combined with equilibrium equations, and pore pressure profile is described by PSS flow regime. In a numerical example, we examine the effects of different deformational conditions on depletion-induced in-situ stress changes. Interestingly, results indicates that stress path in reservoir is significantly affected by reservoir’s boundary conditions. The stress path under plane strain-displacement assumption overestimates the stress path predicted under uniaxial strain state by almost a factor of two. However, the generalized plane stress and traction plane strain conditions underestimates the results of uniaxial strain assumption. The order of stress path values for different boundary conditions can be summarized as: SPps-disp > SPuniaxial > SPps-trac > SPgps.

Production-Induced Compaction of a Sandstone Reservoir: The Strong Influence of Stress Path

2000 ◽  
Vol 3 (04) ◽  
pp. 342-347 ◽  
Author(s):  
M.H.H. Hettema ◽  
P.M.T.M. Schutjens ◽  
B.J.M. Verboom ◽  
H.J. Gussinklo
Keyword(s):  
Pore Pressure ◽  
Field Data ◽  
Laboratory Experiments ◽  
Strong Influence ◽  
Stress Path ◽  
Gas Field ◽  
Reservoir Compaction ◽  
Compaction Behavior ◽  
Stress Changes ◽  
Groningen Gas Field

Summary The decrease of pore pressure during hydrocarbon production (depletion) leads to compaction of the reservoir, which in turn changes the stresses acting on the reservoir. The prediction of reservoir compaction and its consequences is usually based on laboratory experiments performed under uniaxial strain conditions, i.e., allowing no lateral strain during depletion. Field data of the Groningen gas field (The Netherlands) indicate that the stress development of the field deviates significantly from the stress path under uniaxial strain conditions. Laboratory experiments show that the applied stress path has a strong influence on the depletion-induced compaction behavior. We discuss the consequences of these results for the field compaction behavior by considering the responsible deformation mechanisms active in reservoir and experiment. The new Groningen field data, in combination with our experimental results, provide an explanation for the difference between the prediction of compaction and subsidence based on uniaxial experiments and the measurement of compaction and subsidence in the Groningen field. With the use of the new stress path, the predicted and measured compaction and subsidence are in agreement. Introduction The prediction of the amount of depletion-induced reservoir compaction and its adverse consequences (such as subsidence, casing deformation, and seismicity) requires three types of input parameters: The mechanical behavior of the reservoir rock and the rock surrounding the reservoir, the reservoir stress path induced by the depletion, and the dimension and depth of reservoir and overburden formations. Also, a model is required to upscale the laboratory experiments to predict reservoir compaction and the associated surface or seabed subsidence during and after depletion. The first two types of input parameters (mechanical behavior and stress path) are actually linked: The depletion leads to compaction and deformation of the reservoir, which in turn changes the total stresses acting on the reservoir. It is the combination of pore pressure change and total stress change, which alters (and generally increases) the effective normal and shear stresses acting on the load-bearing grain framework. This results in elastic (recoverable) and inelastic (permanent) deformation which, in turn, has a time-independent component, usually referred to as plasticity, and a time-dependent component, referred to as creep. The bulk rock compaction is the result of the various micro mechanisms activated by the depletion, and their dependence on stress path and stress rate (typically, a few MPa per year), stress level (<100 MPa), and temperature (<200°C) and possibly also pore fluid composition.1–3 Ideally, the laboratory experiments are performed along the same stress path that the reservoir undergoes during depletion. However, the reservoir stress path is not known before depletion starts, and analytical or numerical models for the stress development in depleting reservoirs are very sensitive to the input parameters mentioned earlier. To make things worse, field data describing depletion-induced changes in total stress are very scarce, so only a few case studies are available to guide the design of laboratory experiments. In most studies it is assumed that the reservoir compacts uniaxially; that is, there is only vertical compaction and no horizontal deformation. During uniaxial compaction of sandstone with 10 to 30% porosity, the ratio of change in total horizontal stress per change in pore pressure is typically in the range 0.7 to 0.9.3 For the Groningen gas reservoir (The Netherlands) a similar strategy was followed, and a large amount of uniaxial compaction experiments were performed, partly published.3 The tested rock types ranged from low-porosity (5 to 10%) conglomerates to highly porous (25 to 30%) coarse sandstone. However, the compaction and subsidence prediction based on these uniaxial strain experiments is larger than the measured compaction and subsidence in the Groningen field, and the reason for this is still unknown. This paper describes the important role of stress path in compaction prediction and offers a new explanation for the difference in predicted and measured compaction and subsidence in the Groningen field. We start with an analysis of the changes of the total stresses during reservoir compaction, using basic rock mechanics theory. Then, new field stress data are presented and analyzed to estimate the production-induced stress path of the Groningen gas field. Next, the results of triaxial compaction experiments on Groningen core samples are shown, indicating a strong influence of stress path on compaction. Finally, we discuss the experimental results and the consequences of the stress path to the compaction behavior by considering the underlying compaction mechanisms. Although we discuss only field data and core measurements from the Groningen gas field, we think that our conclusions can be generalized, and may be of value to other studies aimed at the prediction of depletion-induced reservoir compaction. Reservoir Stress Changes During Production Prior to production, the Earth's stress field determines the state of stress in the reservoir. Production causes a decrease of the fluid and/or gas pressure in the pores. These pressure changes also result in changes in the total vertical and horizontal stresses acting on the reservoir. Strong evidence for this comes from the occurrence of seismic events inside and close to compacting reservoirs.4,5 Geertsma6 developed a theory of the subsidence and stress changes associated with reservoir compaction, based on linear poroelastic rock behavior. Regarding the total vertical stress, the depletion-induced stress changes at the axis just above a disk-shaped compacting reservoir can be written as6 Δ σ V = h Δ p r ( 1 − 2 ν 2 − 2 ν ) f ( d r ) . ( 1 )

Automatic substepping integration of the subloading tij model with stress path dependent hardening

2009 ◽  
Vol 36 (4) ◽  
pp. 537-548 ◽  
Author(s):  
Márcio M. Farias ◽  
Dorival M. Pedroso ◽  
Teruo Nakai
Keyword(s):  
Stress Path ◽  
Path Dependent

Effects of Stress Path and Stress History on the Stiffness of Reconstituted London Clay

1986 ◽  
Vol 2 (1) ◽  
pp. 139-144 ◽  
Author(s):  
J. H. Atkinson ◽  
J. S. Evans ◽  
D. Richardson
Keyword(s):  
Stress Path ◽  
Triaxial Tests ◽  
Soil Parameters ◽  
Stress History ◽  
Strain Behaviour ◽  
Reconstituted Soil ◽  
Path Dependent ◽  
London Clay ◽  
Soil Behaviour

AbstractSoil behaviour is stress history dependent and stress path dependent and soil parameters, particularly those for stress-strain behaviour, measured in conventional triaxial tests may not represent the behaviour of soil in many civil engineering works.To obtain more realistic parameters it may be necessary to conduct laboratory tests which more closely represent in situ conditions before and during construction.The paper describes equipment developed at The City University to carry out stress path tests simply and economically. A series of CU triaxial tests and stress path tests on reconstituted soil illustrate the dependence of measured soil parameters on stress history and stress path.

Stress Path Analysis of the Depleted Middle Miocene Clastic Reservoirs in the Badri Field, Gulf of Suez Rift Basin, Egypt

2021 ◽  
Author(s):  
Ahmed E. Radwan ◽  
Souvik Sen
Keyword(s):  
Path Analysis ◽  
Pore Pressure ◽  
Middle Miocene ◽  
Stress Path ◽  
Horizontal Stress ◽  
Gulf Of Suez ◽  
Normal Faulting ◽  
Future Production ◽  
Uniaxial Compaction ◽  
Rate Optimization

Abstract The purpose of this study is to evaluate the reservoir geomechanics and stress path values of the depleted Miocene sandstone reservoirs of the Badri field, Gulf of Suez Basin, in order to understand the production-induced normal faulting potential in these depleted reservoirs. We interpreted the magnitudes of pore pressure (PP), vertical stress (Sv), and minimum horizontal stress (Shmin) of the syn-rift and post-rift sedimentary sequences encountered in the studied field, as well as we validated the geomechanical characteristics with subsurface measurements (i.e. leak-off test (LOT), and modular dynamic tests) (MDT). Stress path (ΔPP/ΔShmin) was modeled considering a pore pressure-horizontal stress coupling in an uniaxial compaction environment. Due to prolonged production, The Middle Miocene Hammam Faraun (HF) and Kareem reservoirs have been depleted by 950-1000 PSI and 1070-1200 PSI, respectively, with current 0.27-0.30 PSI/feet PP gradients as interpreted from initial and latest downhole measurements. Following the poroelastic approach, reduction in Shmin is assessed and reservoir stress paths values of 0.54 and 0.59 are inferred in the HF and Kareem sandstones, respectively. As a result, the current rate of depletion for both Miocene reservoirs indicates that reservoir conditions are stable in terms of production-induced normal faulting. Although future production years should be paid more attention. Accelerated depletion rate could have compelled the reservoirs stress path values to the critical level, resulting in depletion-induced reservoir instability. The operator could benefit from stress path analysis in future planning of infill well drilling and production rate optimization without causing reservoir damage or instability.

Sign in / Sign up

Export Citation Format

Share Document