Changes In Effective Stress Research Articles

Issues With Reservoir Geomechanical Simulations of the SAGD Process

Abstract In the SAGD process, steam injection may result in pore pressure changes around the steam chamber in the reservoir. The magnitude of the pore pressure change depends on the steam injection pressure and the relative position of the steam chamber within the reservoir. Injected steam also increases the temperature within and adjacent to the steam chamber and thus introduces thermal expansion effects. The complex interaction of pore pressure and temperature throughout the reservoir result in varying degrees of reservoir geomechanical interactions that must be considered when contemplating reservoir geomechanical simulations of the SAGD process. The primary geomechanical influence on SAGD recovery is associated with the volume change of the sand matrix in response to effective stress changes induced by steam injection pressures and temperatures. Reservoir parameters and processes, such as compressibility, porosity (pore volume), absolute permeability, relative permeability, saturations, capillary pressure, enthalpy transmissibility, gas evolution, and thermal expansion effects, are all affected by bulk volume changes. In this paper, variations of these parameters due to geomechanical effect are analyzed and discussed based on related test results, calculation, and simulation. Their impacts on reservoir geomechanical simulations of the SAGD process are discussed. Introduction Reasonable prediction of SAGD performance by numerical simulation is an integral component in the design and management of a SAGD project. Conventional reservoir numerical simulation emphasizes multiphase flow in the porous media but generally does not take the interactions between fluid and solid into account. It applies elastic rock compressibility to characterize the coupling mechanism of multiphase flow and rock skeleton. The assumption of this treatment is that the boundary loads and temperature are constant, Δp = Δ σ'(1, 2). All analytical flow equations in petroleum engineering are based on this assumption. It is clear that the recovery process of conventional oil from sandstones and most carbonate rocks can roughly satisfy this assumption. For the SAGD process, however, volumetric deformations within the reservoir due to pore pressure and temperature changes result in variations of both in situ stress and strain. These stress and strain variations are functions of the in situ boundary conditions. Due to different directional deformation in response to in situ heating, the total stresses in vertical and horizontal directions may also vary. Moreover, the SAGD process is mainly applied in friable or uncemented (unconsolidated) oil sands that geomechanically behave differently to their cemented counterparts. Under these conditions, the assumption used in conventional numerical simulation is no longer effective because the total stress changes within the reservoir.

Compaction behavior of argillaceous sediments as function of diagenesis

The paper presents the results of laboratory compaction tests on samples of Kimmeridge clay taken from two different locations in the UK. The main objective of the laboratory tests is to determine how diagenesis affects the hydro-mechanical properties and compaction behavior of argillaceous sediments. The two Kimmeridge clays used in the tests are essentially from the same parent material but have undergone different degrees of mechanical and chemical diagenesis. The laboratory tests are designed to determine the effects of mechanical loading and chemical processes on changes in porosity, permeability, compressibility, strength and effective horizontal stress in argillaceous materials. Another objective of the study is to investigate the adequacy of soil mechanical constitutive models in predicting the response of argillaceous materials to conditions corresponding to burial depths in sedimentary basins. The tests on Kimmeridge clay are carried out to as high as 150 MPa effective vertical stress corresponding to about 9 km in burial. The results indicate that mechanical compaction and burial depth cannot explain the differences in the hydro-mechanical behavior of the tested materials. The differences in hydro-mechanical response are attributed to chemical diagenesis by precipitation of minerals in the pores of the sediment. Other observed effects of chemical diagenesis on hydro-mechanical behavior of lithified sediments are: (1) increase in apparent pre-consolidation stress; (2) decrease in compressibility; (3) decrease in permeability; and (4) decrease in effective horizontal stress. Implications of the results to fluid flow and overpressure development in sedimentary basins are discussed.

Compaction behavior of argillaceous sediments as function of diagenesis

The paper presents the results of laboratory compaction tests on samples of Kimmeridge clay taken from two different locations in the UK. The main objective of the laboratory tests is to determine how diagenesis affects the hydro-mechanical properties and compaction behavior of argillaceous sediments. The two Kimmeridge clays used in the tests are essentially from the same parent material but have undergone different degrees of mechanical and chemical diagenesis. The laboratory tests are designed to determine the effects of mechanical loading and chemical processes on changes in porosity, permeability, compressibility, strength and effective horizontal stress in argillaceous materials. Another objective of the study is to investigate the adequacy of soil mechanical constitutive models in predicting the response of argillaceous materials to conditions corresponding to burial depths in sedimentary basins. The tests on Kimmeridge clay are carried out to as high as 150 MPa effective vertical stress corresponding to about 9 km in burial. The results indicate that mechanical compaction and burial depth cannot explain the differences in the hydro-mechanical behavior of the tested materials. The differences in hydro-mechanical response are attributed to chemical diagenesis by precipitation of minerals in the pores of the sediment. Other observed effects of chemical diagenesis on hydro-mechanical behavior of lithified sediments are: (1) increase in apparent pre-consolidation stress; (2) decrease in compressibility; (3) decrease in permeability; and (4) decrease in effective horizontal stress. Implications of the results to fluid flow and overpressure development in sedimentary basins are discussed.

Evaluation of particulate materials using wave-based techniques

Elastic and electromagnetic waves provide important information about particulate materials. In order to facilitate the application of wave-based techniques to soil characterization, fundamental soil properties are first reviewed, and then experiments are performed for both elastic and electromagnetic waves. The first application is related to characterization of particulate materials using shear wave, concentrated on the change of effective stress in consolidation process, multi-phase phenomenon with relation to capillarity, and microscale characteristics of grain particles. The second application is relevant to the electromagnetic wave, focused on stratigraphy detection in layered soils, estimation of void ratio and its spatial distribution, and conduction in unsaturated soils. Experimental results suggest that the shear wave measurements allow studying the evolution of effective stress in unsaturated soils as well as in saturated soils while the electromagnetic wave measurements give an insight into conduction process and allow estimating void ratio and its spatial distribution.

ISRM Suggested Methods for rock stress estimation—Part 4: Quality control of rock stress estimation

Hydraulic fracturing is an effective method for producing coalbed methane. To better understand the evolution of the fracture and pore structure of coals that contain macro-fractures during hydraulic fracturing, we have performed a series of in-situ compression experiments under continuous injection liquid conditions and dynamically monitored the samples using nuclear magnetic resonance methods. The mechanism of fracture and pore structure dynamic evolution is identified in terms of spherical expansion theory. The total porosity increases dramatically within 10–120 min of the initial injection stage, and pore channels form. The number of micro-pores (<10 nm) and transition pores (10–100 nm) continues to increase during the injection pressure reduction stage or increasing confining pressure stage. Total porosity tends to initially decrease and then increase when the confining pressure remains either constant or increases. Continuous liquid injection leads to a constant change of coal pore pressure and effective stress, such that pores break, close, and continuously reorganize. The results presented here provide a reference for permeability calculations, coalbed methane productivity predictions, and improved hydraulic fracturing techniques.

Soil disturbance of Shanghai silty clay during EPB tunnelling

The disturbance of Shanghai silty clay during earth pressure balance (EPB) tunnelling has been studied through field monitoring, field measurement and laboratory test. The soil disturbance during tunnelling consists of two parts: stress disturbance, which is the change of effective stress; and strain disturbance, which is caused by the soil movement. The definition of stress disturbance degree of Shanghai silty clay is given by the change in the in situ effective stress before and just after tunnelling at the same site. According to the changes in the static cone penetrometer resistance, the extent of stress disturbance in a transverse section is determined. The relationships between the mechanical properties and stress disturbance degree are also studied.

GPS-measured land subsidence in Ojiya City, Niigata Prefecture, Japan

Land subsidence caused by compression of clay layers in Ojiya City, Japan was measured by global positioning system (GPS) between 1 April 1996 and 31 December 1998.Three baselines were selected in and around the city, and height difference on a WGS-84 ellipsoid was measured by GPS on each baseline. The ground at the GPS station in the city subsides and rebounds 7 cm every winter and spring, respectively. Measurement accuracy was 9.5 mm standard deviation. Ground water level was observed at a well near the GPS station. Regression analysis between total strain, calculated as ratio of the height difference displacement to the total thickness of the clay layers, and the layers' effective stress change with ground water level change gave good correlation. The slope of regression line 7.0×10−11 m2/N was obtained as an average apparent coefficient of volume compressibility of the layers.

Vibratory compaction of coarse-grained soils

The variation of the coefficient of earth pressure in normally consolidated and overconsolidated soil and the effect of soil compaction on the change of the horizontal effective stress are discussed based on cone penetration test (CPT) data. A method is outlined for estimating the increase in the effective earth pressure based on sleeve friction measurements. Soil compaction increases not only soil density, but also horizontal effective stress. Since the cone stress is influenced by the vertical and horizontal effective stress, particularly at shallow depths, the cone stress needs to be adjusted for effective mean stress. A relation is presented for determining the soil compressibility from the adjusted cone stress. A case history is presented where a 10 m thick sand fill was compacted using vibratory compaction. Cone penetration tests indicated a significant increase in cone stress and sleeve friction and a decrease in compressibility (increase in modulus number) due to compaction. The friction ratio was unchanged. It was concluded that the earth pressure about doubled corresponding to an increase in the overconsolidation ratio of at least 5. The results of settlement calculations based on the Janbu method demonstrate the importance of considering the preconsolidation effect in the analyses.Key words: sand, CPTU, vibratory compaction, earth pressure, overconsolidation, modulus number, settlement.

A modeling approach for analysis of coupled multiphase fluid flow, heat transfer, and deformation in fractured porous rock

Abstract This paper presents the methodology in which two computer codes—TOUGH2 and FLAC3D—are linked and jointly executed for coupled thermal–hydrologic–mechanical (THM) analysis of multiphase fluid flow, heat transfer, and deformation in fractured and porous rock. TOUGH2 is a well-established code for geohydrological analysis with multiphase, multicomponent fluid flow and heat transport, while FLAC3D is a widely used commercial code that is designed for rock and soil mechanics with thermomechanical and hydromechanical interactions. In this study, the codes are sequentially executed and linked through external coupling modules: one that dictates changes in effective stress as a function of multi-phase pore pressure and thermal expansion, and one that corrects porosity, permeability, and capillary pressure for changes in stress. The capability of a linked TOUGH-FLAC simulator is demonstrated on two complex coupled problems related to injection and storage of carbon dioxide in aquifers and to disposal of nuclear waste in unsaturated fractured porous media.

A study of caprock hydromechanical changes associated with CO2-injection into a brine formation

A numerical study of hydromechanical changes during a deep underground injection of supercritical CO2 in a hypothetical brine aquifer/caprock system is conducted. The injection process is simulated using a newly developed computer model for multi-phase analysis of CO2 and brine water flow, coupled with heat transfer and rock deformations. In this modeling, CO2 is injected at a constant rate over a 10-year period at a depth of 1,300–1,500 m. The injection zone is overlain by a 100-m-thick caprock, located at 1,200–1,300 m, which in one of the studied cases is intersected by a vertical fault. The hydraulic, mechanical as well as hydromechanical responses caused by the injection are studied. This includes the spread of the CO2 plume, effective stress changes, ground surface uplift, stress-induced permeability changes, and mechanical failure analysis. The analysis shows that most hydromechanical changes are induced in the lower part of the caprock near its contact with the injection zone, whereas the sealing mechanism of the upper part may remain intact, despite an injection pressure close to the lithostratic stress value.

Pore pressure and poroelasticity effects in Coulomb stress analysis of earthquake interactions

Pore pressure changes are rigorously included in Coulomb stress calculations for fault interaction studies. These are considered changes under undrained conditions for analyzing very short term postseismic response. The assumption that pore pressure is proportional to fault‐normal stress leads to the widely used concept of an effective friction coefficient. We provide an exact expression for undrained fault zone pore pressure changes to evaluate the validity of that concept. A narrow fault zone is considered whose poroelastic parameters are different from those in the surrounding medium, which is assumed to be elastically isotropic. We use conditions for mechanical equilibrium of stress and geometric compatibility of strain to express the effective normal stress change within the fault as a weighted linear combination of mean stress and fault‐normal stress changes in the surroundings. Pore pressure changes are determined by fault‐normal stress changes when the shear modulus within the fault zone is significantly smaller than in the surroundings but by mean stress changes when the elastic mismatch is small. We also consider an anisotropic fault zone, introducing a Skempton tensor for pore pressure changes. If the anisotropy is extreme, such that fluid pressurization under constant stress would cause expansion only in the fault‐normal direction, then the effective friction coefficient concept applies exactly. We finally consider moderately longer timescales than those for undrained response. A sufficiently permeable fault may come to local pressure equilibrium with its surroundings even while that surrounding region may still be undrained, leading to pore pressure change determined by mean stress changes in those surroundings.

SEISMIC EXPRESSION OF ABNORMAL GEO-PRESSURE IN THE BARROW SUB-BASIN

Drilling uncertainties related to abnormal geopressure are common in the Barrow and Dampier Sub-basins of the North West Shelf. These uncertainties contribute to increased drilling risk and costs. There have been a number of published studies in this area which have been directed towards understanding the mechanisms and modelling of the expected pressures. These studies, however, have been in general isolated and have concentrated on non-seismic related methods. This paper provides an empirical analysis of the seismic response in an area with known variation of overpressure, and critically is integrated with a comprehensive research effort looking at aspects of overpressure from a laboratory, empirical and theoretical perspective.The study was conducted using data taken from permit WA-25-P (P25) in the Barrow Sub-basin. These data included 3D surface seismic and VSP, well logs, mud weight and pressure data from the wells. The results of a mineralogical analysis conducted on core samples and basin-wide geological modelling studies were also incorporated into the study. The Muderong Shale, which comprises the principal seal in the area and is believed to be overpressured was selected as the prime target for the analysis. Initially we assess the potential of existing methods, such as velocitybased methods, for remote prediction of excessive pore pressure in the area P25. This is extended to amplitude related effects involving an analysis of reflectivity in the presence of a velocity transition zone over the overpressured interval. Finally the relationship of well data, VSP and surface seismic derived attributes is described.The available data in the P25 area was sparse and consequently we could not rely on statistical based associations. Current industry methods that rely on a limited number of calibration points suggest that the application of either velocity or AVO based methods may produce unreliable predictions of pore pressure. Ambiguities in inferring overpressure introduced by a variable mineral composition of shales and the presence of a strong velocity gradient, which distorts the wave shape, reduces the reliability of these methods.A detailed analysis using VSP data acquired in a highly overpressured well was found to be crucial for understanding the response of various seismic attributes to changes in effective stress. This enabled us to propose a new qualitative, but efficient approach for remote prediction of overpressure, particularly suited for underexplored areas such as P25. The applicability of the method, which uses single and combined seismic sequence and trace multi-attributes to predict overpressured zones, is demonstrated with the Venture-Carey 3D data recorded in the Barrow Sub-basin.

PROGRESSIVE LIQUEFACTION PROCESS OF LOOSELY DEPOSITED SAND BED UNDER OSCILLATING WATER PRESSURE ON ITS SURFACE

In this paper, the progressive liquefaction process of the loosely deposited sand bed under oscillating water pressure on its surface is investigated theoretically by using the vertically one-dimensional model, in which the elasto-plasticity of the sand bed skeleton is taken into account. The changes of pore water pressure, effective stress and liquefaction depth in the sand bed with time are simulated during the initial time period after the cyclic pressure loading. The experimental results verified that the proposed analytical model expressed fairly well the progressive liquefaction process of the sand bed.

The effect of clay dehydration on land subsidence in the Yun-Lin coastal area, Taiwan

The smectite dehydration theory developed by Ransom and Helgeson was applied for simulation of land subsidence in the Yun-Lin coastal area, Taiwan. The volumetric reduction of smectite clay at equilibrium state was computed by assuming that the dehydration of interlayer water in smectite clay can be described with a regular solid solution reaction. By using the in situ stratigraphic data collected from the subsidence monitoring wells in the simulated area, the amounts of land subsidence caused by smectite dehydration in three scenarios with pressure variation were calculated. The results indicate that significant amounts of land subsidence can be attributed to smectite dehydration. This finding reveals that smectite dehydration is of importance for assessment and prediction of land subsidence. Additionally, the results also indicate the overburden weight has a larger effect on clay dehydration than the effective stress change resulting from over-pumping, although both of them induce relatively minor variations on land subsidence.

The change in effective stress associated with shrinkage from gas desorption in coal

Volumetric shrinkage associated with gas desorption from the coal matrix has a significant influence on the stress field. This is of particular practical importance around headings in underground coal mining, as it controls the permeability to gases and will affect the strength of the strata. In this paper, the volumetric changes of the coal matrix were monitored for four different gases (methane, nitrogen, carbon dioxide and helium) on a sample from the South Island, New Zealand. The specimen was initially saturated with the gas to a pressure of 4 MPa. The volumetric strains were recorded during the adsorption and desorption processes. The overall shrinkage coefficient, C m, of coal matrix was found to be 1.2×10 −3 MPa −1 for methane and 5.2×10 −3 MPa −1 for carbon dioxide desorption. A series of loading/unloading cycles applied by gas pressure were performed on the specimen using a non-adsorbable gas (helium). The results showed elastic behaviour of the sample with some effective stress recovery associated with the incremental change in pressure. The elastic properties were determined from a uniaxial test on the sample. From this data and using Terzaghi's equation, a relationship was established between the effective stress and gas pressure. It was also found that the slope of volumetric strain rate and gas pressure is a function of coal permeability, which could be used to estimate permeability.

Poroelastic coupling between the 1992 Landers and Big Bear Earthquakes

A three‐dimensional finite element model was constructed to investigate the significance of poroelastic coupling between the 1992 Landers and Big Bear earthquakes in southern California. The homogeneous poroelastic model predicted a maximum increase in left lateral slip potential (change in shear stress less the change in effective fault normal stress scaled by a coefficient of friction) along the southwest part of the Big Bear fault, consistent with the epicentral location. In contrast, slip potential calculated for a weak fault zone in a state of isotropic stress for drained conditions, indicated a maximum increase along the northeast part of the Big Bear fault.

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

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.

Sampling Disturbance Effects in Reconstituted Coastal Soils

The effects of tube sampling disturbances on undrained shear properties of reconstituted samples of coastal soils were investigated. The values of undrained shear strength (s u ), initial tangent modulus ( E i ) and secant modulus ( E 50 ) were reduced while axial strain at peak deviator stress (ɛ p ) increased because of disturbance caused by the penetration of samplers. Disturbances also considerably reduced initial effective stress ( σ i ') and pore pressure changes. The changes in soil parameters between the “in-situ” and “tube” samples depended markedly on the characteristics of the tube samplers. Values of s u , E i E 50 and σ i ' decreased while the values of ɛ p increased with increasing area ratio (or decreasing external diameter to thickness ratio) of the samplers. It was also found that the reductions in the values of s u , ɛ p , E i , E 50 and σ i ' increased with decreasing plasticity of the soils. A number of reconsolidation procedures, both isotropic and anisotropic including SHANSEP methods, were adopted in order to assess the suitability of reconsolidation of “tube” samples to recover the “in-situ” properties of the soils. It appeared that reconsolidation using SHANSEP procedures may not be applicable to these coastal soils. However, K 0 -reconsolidation to “in-situ” stresses provided a better estimate of “in-situ” properties than other reconsolidation techniques.

Geomechanical Response of the Shale in the Colorado Group Near a Cased Wellbore Due to Heating

Abstract Steam-stimulation is a viable in situ thermal recovery technique for heavy oil and oil sand reservoirs. This thermal recovery process introduces many complex geomechanical problems in oil sand and overburden formations. This paper addresses the geomechanical response of Colorado shale near a cased wellbore due to heating. The temperature, pore pressure, and stress response in Colorado shale near a cased wellbore due to heating are analyzed using a coupled thermal-mechanical-hydraulic solution. The possibility of failure or fracturing of the shale due to heating is assessed. Introduction Steam-stimulation processes are currently the most viable techniques for oil recovery from oil sand reservoirs. These thermal recovery processes cause many complex geomechanical effects in the oil sand layer and the geologic overburden strata due to elevated temperatures and pressures involved in the steam injection. The study in this paper focuses on the effect of heating on shale near a cased well. Analytical solutions developed by Booker and Savvidou(1) are used to analyze the distributions of induced pore pressure, temperature, and stress near a heated wellbore. These results will be used to investigate the potential occurrence of any tensile fracture in the heated shale. Conclusions will be drawn, along with the limitations of the analysis, and recommendations made for further studies. Problem Description Figure 1 defines the problem. Steam is injected into a cased well such that the inside temperature is elevated to a constant level. Heat is continually conducted from the well to its metal casing, cement annulus, and the shale in the formation. Heating of shale causes thermal expansion of the shale structural matrix and pore fluid. Expansion of the structural matrix induces total stress changes. Expansion of the pore fluid increases the pore pressure, thereby generating pore pressure gradients and the potential for pore fluid flow. Hence, the heating process results in a thermalhydraulic-mechanical coupled process. Calculation of the changes in temperature, pore pressure, and stress changes near the heated well will require solution to this complex coupled problem. FIGURE 1: Problem description. (Available in full paper) Analytical Technique Equilibrium Equation and Constitutive Law For a saturated thermo-elastic geological medium, the equilibrium equations are: Equation (1.a) (Available In Full Paper) Equation (1.b) (Available In Full Paper) Equation (1.c) (Available In Full Paper) where σxx, σyy,..... σyz are the increase of total stress components (compressive stresses and strains are considered to be positive). In the present study, the stiffness of the solids and pore fluid are assumed to be infinite in comparison to the shale skeleton stiffness. Thus, volume changes are due to changes in temperature and effective stresses. For an isotropic thermo-linear elastic material, the stress-strain relations are given by: Equation (2.a) (Available In Full Paper) Equation (2.b) (Available In Full Paper) Equation (2.c) (Available In Full Paper) Equation (2.d) (Available In Full Paper) Equation (2.e) (Available In Full Paper) Equation (2.f) (Available In Full Paper) where the primed symbols represent effective stresses, E and ν are the drained Young's modulus and Poisson's ratio, G is the shear modulus, Ψ is the increase in temperature, and a' is the drained coefficient of volumetric thermal expansion of the geologic me

Disturbances due to Tube Sampling in Coastal Soils

This study investigated the effects of sampling disturbances on undrained shear characteristics of reconstituted samples of three coastal soils. The values of undrained shear strength (s sub u) and initial tangent modulus (E sub i) were reduced, whereas axial strain at peak deviator stress (epsilon sub p) increased because of a disturbance caused by penetration of samplers. Disturbances also reduced initial effective stress (sigma' sub i) and pore pressure changes considerably. The changes soil parameters between the in and tube samples depended markedly on the geometry of samplers used to retrieve samples. Values of s sub u, E sub i, and sigma' sub i of samples were reduced, whereas the values of epsilon sub p increased with an increasing area ratio (or decreasing external diameter to thickness ratio) and an increasing outside cutting edge angle of the samplers. Different reconsolidation procedures were adopted to assess the suitability of reconsolidation of samples to recover situ properties. It was found that the coefficient of earth pressure at rest-reconsolidation of samples to vertical effective stresses equal to 1.5 and 2.5 times the situ vertical effective stress produced the best overall estimate of situ characteristics.