Changes In Effective Stress Research Articles

Activated slip systems in bimodal Ti–6Al–4V plastically deformed at low and moderately high temperatures

Activated slip systems were investigated by analyzing slip traces and measuring the distribution of the Schmid factor to understand the controlling mechanism behind the change in effective stress seen in bimodal Ti–6Al–4V. It was ascertained that both basal and prismatic slips are dominant at 77 K and 225 K. It was suggested that the critical resolved shear stress (CRSS) for basal and prismatic slips are close to each other at 77 K and 225 K. The activation of basal alongside prismatic slips was unexpected, owing to previous reports which determined that the CRSS of prismatic slips is lower than that of basal slips in single-phase Ti-6.6Al. It is presumed that the difference arises between single phase Ti-6.6Al and bimodal Ti–6Al–4V from the boundary of α/β interfaces in bimodal Ti–6Al–4V, which acts as obstacles for prismatic and pyramidal slips. In addition to those, it was found that pyramidal slips are dominant at 550 K in this study. The reasons for the higher activation of pyramidal slips in comparison with basal and prismatic slips at 550 K are also discussed. Present study evidenced that the change in the dominant active slip systems is responsible for the change in the trend of temperature dependence interpreted in the effective stress.

Relating shaft friction of buried piles and CPT resistance in clayey sands

This paper examines factors controlling the ultimate shaft friction developed on buried piles in normally consolidated clay–sand mixtures and explores the relationship between this friction and the cone penetration test (CPT) end resistance. Results from tension load tests on buried piles in a laboratory pressure chamber and geotechnical centrifuge are presented. Variable rate CPTs combined with Rowe cell, undrained simple shear and interface direct shear tests are used to characterise the mixtures. It is shown that the measured drained CPT end resistances could be predicted using parameters assessed from the laboratory tests and a simple expression derived from cavity expansion theory in an elastic Mohr–Coulomb soil. The tension pile tests indicate how changes in radial effective stresses during loading can be inferred from interface shear tests and these combined with the cavity expansion analysis allow derivation of a relationship between shaft friction (τf) and CPT end resistance (qt). This expression reveals the significant effects on the qt/τf(= βc) ratio of soil stiffness, stress level, dilation and drainage during cone penetration and is shown to be consistent with a recently proposed empirical formula for bored piles.

Conditions for Triggering Seismic Ruptures and/or Slow Slip Events in the Framework of a Poro‐Plastic Fault Zone Model

AbstractActive fault zones are not simple frictional interfaces. They are complex, evolving 3‐D structures that contain pore fluids in drained or undrained conditions. They show stable or unstable slip, dilatant or contractant elastic or plastic local deformations in response to far‐field extensive or compressive displacements. Pore fluid pressure variations induce effective stress and fluid compressibility changes, possibly up to degassing. For these reasons, we choose to use an analytical model in which the fault zone is schematized by a mesoscale poroplastic interface surrounded by two symmetrical elastic pads to investigate the conditions that can lead to slow slip vs. seismic slip. The chosen end‐cap plastic failure criterion and associated flow rule account for hardening/softening regimes. The model computes stress paths, normal/tangential displacement, and fluid pressure variations of the fault zone with time. Depending on the initial stress, pore pressure conditions, far‐field loading rate, and fluid compressibility, scenarios emerge that link together stable plastic deformation regimes (months to years, small displacements) and unstable regimes (seconds, metric displacements). While some scenarios show the occurrence of instability preceded or not by slow slip events, others show slow events without instability (mainshock). Also investigated are the effect of a switch from undrained to drained condition and the role of the change in fluid compressibility due to dilatancy. Finally, we discuss the geodetic and geophysical data necessary to calibrate the parameters of the empirical relationships. Slow slip event areas are therefore not necessarily “safe” and might produce earthquakes if conditions change.

A model for stress‐dependence of apparent permeability in nanopores of shale gas reservoirs

AbstractPredicting long‐term production from gas shale reservoirs is a challenging task due to changes in effective stress and permeability during gas production. Unlike coal, the variation of sorbing gas permeability with pore pressure in shale does not always feature a biphasic trend under a constant confining pressure. The present contribution demonstrates that the biphasic dependence of permeability on pore pressure depends on a number of physical and geometrical factors, each with a distinct impact on gas permeability. This includes pore size, adsorption isotherm, and the variation of gas viscosity with pore pressure. A single‐capillary model is proposed for the apparent permeability of real gas in shale. Results indicate that the biphasic relation between apparent permeability and pore pressure is prevalent when the sorbing gas flows in sufficiently small pores. In addition, the effects of sorption isotherm and internal resistance of nonideal gas to flow cannot be ignored.

The Effect of Mineral Dissolution on the Effective Stress Law for Permeability in a Tight Sandstone

AbstractWe present flow‐through experiments to delineate the processes involved in permeability changes driven by effective stress variations and mineral cement dissolution in porous rocks. CO2‐enriched brine is injected continuously into a tight sandstone under in situ reservoir conditions for 455 hrs. Due to the dolomite cement dissolution, the bulk permeability of the sandstone specimen significantly increases, and two dissolution passages are identified near the fluid inlet by X‐ray CT imaging. Prereaction and postreaction examinations of the effective stress law for permeability suggest that after reaction, the bulk permeability is more sensitive to pore pressure changes and less sensitive to effective stress changes. These observations are corroborated by Scanning Electron Microscopy and X‐ray CT observations. This study deepens our understanding of the effect of mineral dissolution on the effective stress law for permeability, with implications for characterizing subsurface mass and energy transport, particularly during fluid injection/production into/from geologic reservoirs.

Long-term deformations of monopile foundations for offshore wind turbines studied with a high-cycle accumulation model

A parametric study on the long-term deformations of monopile foundations for offshore wind turbines is presented. The finite element calculations of a monopile in fine sand were performed with a high-cycle accumulation (HCA) model. The results are analyzed with respect to the horizontal displacements, the tilting of the pile head, settlement of the ground surface and effective stress changes in the surrounding soil. The dimensions of the pile (diameter, length of embedding, wall thickness), the initial density of the sand and the cyclic loading (average value, amplitude, lever arm) have been varied. The influence of the drainage conditions (either fully drained or partially drained) is also discussed. The validity of Miner’s rule is checked for both types of drainage conditions. Furthermore, the effect of changes of the direction of the cycles and of a multidimensional cyclic loading is addressed. The interesting phenomenon of a “self-healing”, i.e. a re-erection of the deformed monopile after a storm event, is demonstrated based on simulations with small cycles following a large preloading. Simulations with homogeneous and stochastically fluctuating fields of void ratio are compared.

An effective stress analysis for predicting the evolution of SCR–seabed stiffness accounting for consolidation

Steel catenary risers (SCRs) are an efficient solution to transfer hydrocarbons from deep-water seabeds to floating facilities. SCR design requires an assessment of the fatigue life in the touchdown zone, where the riser interacts with the seabed, which relies on reliable estimates of the SCR–seabed stiffness over the design life. Current models for SCR–seabed stiffness consider only undrained conditions, neglecting the development and dissipation of excess pore pressures that occur over the life of the SCR. This consolidation process alters the seabed strength and consequently the SCR–seabed stiffness. This paper summarises experimental data that show that long-term cyclic vertical motion of an SCR at the touchdown zone leads to a reduction in seabed strength due to remoulding and water entrainment, but that this degradation is eclipsed by the regain in soil strength during consolidation. The main focus of this paper is on prediction of the temporal changes in seabed strength and stiffness due to long-term cyclic shearing and consolidation, to support calculations of SCR–seabed interaction. The predictions are obtained using a framework that considers the change in effective stress and hence soil strength using critical state concepts, and that considers the soil domain as a one-dimensional column of elements. The merit of the model is assessed by way of simulations of SCR centrifuge model tests with over 3000 cycles of repeated undrained vertical cycles in normally consolidated kaolin clay. Comparisons of the simulated and measured profiles of SCR penetration resistance reveal that the model can capture accurately the observed changes in SCR–seabed stiffness. Example simulations show the merit of the model as a tool to assess the timescale in field conditions over which this order of magnitude change in seabed stiffness occurs. It is concluded that current design practice may underestimate the seabed stiffness significantly, but the new approach allows rapid checking of this for particular combinations of SCR and soil conditions.

Multi‐scale simulation of rock compaction through breakage models with microstructure evolution

Abstract. Regional subsidence due to fluid depletion includes the interaction among multiple physical processes. Specifically, rock compaction is governed by coupled hydro-mechanical feedbacks involving fluid flow, effective stress change and pore collapse. Although poroelastic models are often used to explain the delay between depletion and subsidence, recent evidence indicates that inelastic effects could alter the rock microstructure, thus exacerbating coupling effects. Here, a constitutive law built within the framework of Breakage Mechanics is proposed to account for the inherent connection between rock microstructure, hydraulic conductivity, and pore compaction. Furthermore, it is embedded into a 1-D hydromechanical coupled finite element analysis (FEA) to explore the effects of micro-structure rearrangement on the development of reservoir compaction. Numerical examples with the proposed model are compared with simulations under constant hydraulic conductivity to illustrate the model capability to capture the non-linear processes of reservoir compaction induced by fluid depletion.

Permeability Evolution of Shear Failing Chalk Cores under Thermochemical Influence.

Development of petroleum reservoirs, including primary depletion of the pore pressure and repressurization during water injection naturally, leads to changes in effective stresses of the formations. These changes impose mechanical deformation of the rock mass with subsequent altering of its petrophysical properties. Besides mechanical compaction, chalk reservoirs on the Norwegian Continental Shelf also seem susceptible to mineralogical and textural changes as an effect of the injecting fluid’s chemical composition and temperature. Understanding such chemical and thermal effects and how they interplay with the mechanical response to changes in effective stresses could contribute to improved prediction of permeability development during field life. This article presents results from mechanical testing of chalk cores of medium-porosity (32%) outcrop chalk (Niobrara Formation, Kansas) in triaxial cells. The experimental setup allows systematic combinations of fluctuating deviatoric stress, temperature (50 and 130 °C), and injecting fluid (calcite-equilibrated sodium chloride, calcite-equilibrated sodium sulfate, and reactive synthetic seawater) intended to replicate in situ processes, relevant to the North Sea chalk reservoirs. Deviatoric loading above yield resulted in a shear failure with a steeply dipping fracture of the core and a simultaneous increase in permeability. This occurred regardless of the brine composition. The second and third deviatoric loadings above yield did not have the same strong effect on permeability. During creep and unloading, the permeability changes were minor such that the end permeability remained higher than the initial values. However, sodium sulfate-injected cores retained most of the permeability gain after shear fracturing compared to sodium chloride and synthetic seawater series at both temperatures. Synthetic seawater-injected cores registered the most permeability loss compared to the other brines at 130 °C. The results indicate that repulsive forces generated by sulfate adsorption contribute to maintain the fracture permeability.

Cyclic response of a stiffened suction caisson anchor in calcareous silt

The offshore industry attempts to extend the application of suction caisson anchors in calcareous seabed sediments. However, very little data exists on the performance of caisson in calcareous soils. The aim of this paper is to provide insight into the behaviour of a stiffened caisson anchor under cyclic operational loading in lightly overconsolidated calcareous silt. A series of model tests were carried out in a beam centrifuge, varying the mudline load inclination at 0°, 20°, 40°. In each cyclic loading test, three episodes of loading were applied, varying mean load (average load in proportion to the corresponding monotonic capacity) as 30%, 50% and 70%, while the amplitude (defined as the maximum variation from the average load) and number of cycles for each episode were kept constant at 20% and 50 cycles, respectively, and then loaded monotonically. The accumulated displacement and negative excess pore pressure at the trailing side of the skirt tip increased, particularly from the second episode of cyclic loading, with decreasing mudline load inclination angle. The ratio of post-cyclic monotonic capacity to pure monotonic capacity reduced as 1.20, 1.01, and 0.90, with changes of excess pore pressure and effective stress dominating the behaviour for high load inclination angle and cyclic degradation for low load inclination angle. Comparing the results to the performance of other foundations and anchors, it was found that post-cyclic monotonic capacity depends heavily on the embedment depth of the object along with load/mooring inclination, cyclic load amplitude, and seabed material.

An Improved Coal Permeability Model with Variable Cleat Width and Klinkenberg Coefficient

The evolution of permeability in coal seams plays an important role in the exploitation of coalbed methane, and the Klinkenberg effect is an important factor affecting the permeability. In most studies, the Klinkenberg coefficient is considered a constant although it is closely related to the cleat width which changes dynamically due to the competing processes of change in effective stress and adsorption induced deformation. In this work, based on the double strain spring model and the effective stress principle, a new stress–strain relationship is proposed for cleat space. In addition, the influence of the adsorption induced swelling effect on coal deformation is considered. By considering both stress-induced strain and adsorption-induced strain, the cleat width model and Klinkenberg coefficient model are established. Finally, we develop an improved permeability model with variable cleat width and Klinkenberg coefficient based on cubic law. This improved permeability model is compared with experimental data and several classical models under conditions of constant effective stress and constant confining stress. Furthermore, the relationship between Klinkenberg coefficient and pore pressure under different conditions is analyzed. Our results indicate that the improved apparent permeability model matches the experimental data well and has a wider range of applications.

Stress response during in-situ gas depletion and its impact on permeability and stability of CBM reservoir

Coalbed methane (CBM) reservoir is generally believed to be under the uniaxial strain condition. Because of this stress/strain controlled boundary, the horizontal stress is passively changed with gas depletion. This dynamic stress evolution has great impact on permeability and stability of the CBM reservoir. Even though it is generally known that the stress depletion is both poromechanics and desorption controlled process, however, how the gas pressure and the gas type affect the horizontal stress profile and its induced coal failure is still unclear. In this work, we experimentally simulated the uniaxial strain condition in the lab. Both the horizontal stress and permeability were continuously monitored during of different gases (He, N2, CH4 and CO2) depletion. The results show that regardless of gas type, the horizontal stress decreases with the reduction of gas pressure. And gas with a higher adsorption capacity corresponds to a greater horizontal stress loss. During the depletion, the effective vertical stress rises, while effective horizontal stress show various trends for different gases. The effective horizontal stress increases during helium depletion, which leads to a reduction of the permeability. And it decreases during methane and carbon dioxide depletion, which corresponds to a significant increase of the permeability. While for nitrogen, the permeability decreases first and followed by a slight uptick. Both the lab test and modeling results show that a significant non-linearity exists in effective horizontal stress change during adsorptive gas depletion, which is expected to affect the reservoir stability. To evaluate the reservoir stability during depletion, a stability factor (SF) was put forward, and the influencing factors on this parameter were analyzed. With this parameter, the critical reservoir pressure indicating reservoir failure can be determined with appropriate geomechanical parameters input. This research will advance the understanding of the geomechanics change during CBM production.

The Influence of Coupled Thermomechanical Processes on the Pressure and Temperature due to Cold Water Injection into Multiple Fracture Zones in Deep Rock Formation

A technique to produce geothermal energy from deep rock formations at elevated temperatures consists of drilling two parallel deep boreholes, the second of which is directed so as to intersect a series of fractures produced by hydraulic fracturing in the first borehole. Then, the first borehole is used for injection of cold water and the second used to produce water that has been heated by the deep rock formation. Some very useful analytical solutions have been applied for a quick estimate of the water outlet temperature and injection/production pressures in this enhanced geothermal system (EGS), but they do not take into account the influence of thermomechanical and hydromechanical effects on the time evolutions of the pressure and temperature. This paper provided help for the engineering design of the EGS based on these analytical solutions, by evaluating the separate influences of the thermal (T), hydromechanical (HM), thermo-hydro-mechanical (THM) effects on the fluid pore pressure and temperature. A thermo-hydro-mechanical (THM) model was developed to simulate the heat extraction from multiple preexisting fracture zones in the hot rock formation, by considering permeability changes due to the injection pressure as a function of changes in the mean effective stress. It was found that the thermal effects (without coupling with mechanical effects) led to a decrease of the transmissivity of the fracture zones and a consequent increase in the injection pressure, by a maximum factor of 2. When the temperature is constant, the influence of the hydromechanical effects on the fluid pore pressure was found to be negligible, because in such scenario, the variation of the mean effective stress was 3 MPa, which was associated with a maximum increase in the initial permeability of the fracture zone only by a factor of 1.2. Thermo-hydro-mechanical effects led to a maximum increase in the permeability of the fracture zones of approximately 10 times the initial value, which was associated with a decrease in the fluid pore pressure by a maximum factor of 1.25 and 2, when hydrological and thermohydrological effects were considered, respectively. Changes in temperature were found not to be affected significantly by the thermomechanical and hydromechanical effects, but by the flow rate in the fracture zones. A sensitivity analysis was conducted to study the influence of the number, the initial permeability, the elastic modulus and the residual porosity of the fracture zones, and the elastic modulus of the confining intact rock, on the simulation results. The results were found to be the most sensitive to the number and the initial permeability of the fracture zones.

Modelling of oedometer tests on pellet-powder bentonite mixtures to support mock-up test analysis

Bentonite mixtures of MX-80 (80% of high-density pellets and 20% of bentonite powder on a mass basis) have been recently proposed as a candidate material for sealing deep geological disposals of high-level radioactive waste. A loading/unloading oedometer test at constant water content has been performed on this mixture, which has been modelled using the finite element Code_Bright. The constitutive model used to represent the mechanical response is the Barcelona Expansive Model (BExM), since a multi-modal pore size distribution characterises the pore network of the mixture. During compression at constant water content, an increase in the degree of saturation and a consequent reduction of suction is induced. Consequently, two competing effects occur at different pore-size scales: (a) compression due to mean net stress increase; and (b) expansion on induced suction reduction that mainly affects the micro-porosity level inside aggregates. A sensitivity analysis has been performed to explore these effects, in which the elastic compressibility parameter at the micro-porosity scale for changes in mean effective stress plays an important role.

Numerical Simulation of Pipeline‐Pavement Damage Caused by Explosion of Leakage Gas in Buried PE Pipelines

In order to investigate the damage influence of the leakage explosion in urban gas pipeline on the surrounding environment, the numerical models of buried PE (polyethylene) pipes under urban pavement were established by using ANSYS/LS‐DYNA in this study. The reliability of the numerical models was verified on the basis of the explosion experiments. According to the amount of gas leakage, the TNT explosive equivalent was determined. The gas leakage explosion process of buried PE pipes was studied, and the pressure and stress changes of pipes and pavements under different explosive equivalents and buried depths were analyzed; at last, the deformation law of pipes and pavements were discussed. The results show that the PE pipes are fractured during the leakage explosion and a spherical explosion cavity is formed in the soil. The pavement above the explosion point bulges upward and forms a circle. The maximum pressure of pipe near the explosion point increases linearly with the increase of explosive equivalent, and a proportional relation is observed between the fracture width of pipe and the explosive equivalent. The degree and duration of pavement deformation increase significantly with the increase of explosive equivalents. The dynamic response of the pipes is rarely affected by the buried depth, and the change of maximum effective stress is no more than 7%. However, the buried depth is of great influence on the damage degree of pavement. When the buried depth increases from 0.9 m to 1.5 m, the pavement deformation can be reduced effectively. The variation rule of pavement deformation is similar to the change rule of maximum overpressure and effective plastic stress; they change in the form of concave functions with the increase of buried depth. The results can provide theoretical basis for municipal pipeline construction design and urban safety planning and provide references for the risk assessment of gas explosion in buried pipelines.

Large wave flume tests on wave-induced response of sandy seabed adjacent a water intake

The response of near-field seabed under wave loads is important to the safety of subsea structures and foundations. This study investigates the wave-induced response of sandy seabed adjacent a water intake through a series of large scale model tests. The tests are conducted at currently the world's largest wave flume. The spatial and temporal distribution of dynamic sandy seabed response under regular and irregular waves is investigated. The change in excess pore water pressure and effective stress within the sandy seabed is studied through direct test measurement and combination of measurement and analytical solutions. The influences of wave height, wave period, and overlaid armor stones on the response of soil close to the water intake are analyzed. Attenuation and phase lag of oscillatory pore pressure in the seabed with respect to depth is observed in the tests, and the presence of the water intake amplifies the excess pore pressure in the upstream direction. Analysis of the effective stress shows that the principal axes of stress in the soil is subjected to continuous rotation under wave loading. Residual excess pore water pressure is generated in the tests of irregular waves with relatively large height and regular waves with relatively high frequency.

Resolution limits of fluid overpressures from mineralogy, porosity, and sonic velocity variations in North Sea mudrocks

Pore pressure identification in mudrocks largely relies on indirect methods, including analyses of sonic velocity data. The magnitude of pore pressure changes that can be resolved from such data depends on the impact of fluid pressures on well log responses and on the velocity responses to lithological variations that could be mistaken for indications of pore pressure changes. Analysis of relationships between sonic velocity, mineralogy, and porosity from 11 North Sea wells demonstrated porosity variations of up to 5% from the mean value in mudrocks at any given depth between 1 and 3.5 km (3300–11,500 ft). Comparisons of these wells with seven wells from the Norwegian Sea demonstrated that the sonic velocity and density variations are influenced by the content of silica with a biogenic origin, but that they can otherwise not, in general, be related to mineralogy and pore pressure variations. The sonic velocity deviations from the mean value are of the same magnitude as those that would be caused by porosity changes of ±8% in mudrocks of uniform lithology. Furthermore, the deviations are not significantly reduced when the velocity data are compensated for changes in density (porosity). Detection limits for mechanically compacting mudrocks that are at their maximum effective stress are equivalent to static mud density changes of 0.1–0.15 g/cm3. These numbers apply to mudrocks with similar lithological variability to those analyzed in our data set but should be much higher when significant amounts of silica with a biogenic origin are present. The impact of effective stress changes on the sonic velocities of chemically compacting as well as of mechanically unloading rocks is much less because small or no porosity changes follow from the effective stress changes in such cases. The velocity responses for mechanically unloading and stress-insensitive chemical compaction can only be identified from four-dimensional (time-lapse) seismic data unless the rocks are close to the fracturing limit. These observations imply that reasonably accurate velocity-based pore pressure identification in North Sea mudrocks requires a knowledge of local mudrock heterogeneity and compaction that cannot easily be obtained. As a result, sonic velocity data should be treated with considerable caution in pore pressure evaluation.

Leak detection on water pipelines in unsaturated ground by discrete fibre optic sensing

The article describes a study using discrete fibre optic sensing as a means of leak detection on water distribution pipes installed in unsaturated ground. A short length of pipe fitted with artificial leak points was installed, to which a fibre optic cable with fibre Bragg gratings was attached. An optical fibre with fibre Bragg gratings was also installed in the ground parallel to but separate from the pipe. Thermistors were installed at selected locations to measure temperature changes independent of strain. It was found that a simulated water leak resulted in clearly detectable temperature changes and thermally induced fibre Bragg grating wavelength changes in the ground around the pipe. However, significantly larger deformation-induced fibre Bragg grating wavelength changes were measured on the pipe walls and also in the initially unsaturated ground in response to leaks. A wetting front originating from a water leak propagating through unsaturated soil is associated with significant effective stress changes because the infiltrating water alters the ambient matric suction in the soil. This effective stress change is associated with significant ground deformation resulting in a fibre Bragg grating response which significantly exceeds the thermal response associated with (usually) colder water leaking into unsaturated soil. The study illustrates advantages of measuring ground deformation-induced fibre Bragg grating wavelength changes over pure temperature changes as an efficient means of leak detection in unsaturated ground. However, due to the limited number of fibre Bragg gratings that can be monitored along a single optical fibre, a leak detection system suitable for practical implementation should be based on distributed fibre optic strain sensing, an aspect requiring further research.

Stress sensitivity in naturally fractured reservoirs: a case study of the Lower Cretaceous Xiagou Formation, Qingxi Oilfield, Jiuxi Basin, northwestern China

Stress sensitivity is the variation of rock petrophysical parameters resulting from changes in effective stress. In fractured reservoirs, experimental methods exhibit a certain amount of error in the quantitative analysis of reservoir rock stress sensitivity. In addition, fracture-bearing experimental rock samples are difficult to obtain and prepare. Therefore, in the present study, reservoir rock stress sensitivity in naturally fractured reservoirs was investigated based on geomechanical modeling using a case study of the Lower Cretaceous Xiagou Formation in the Qingxi Oilfield. The results indicate that the Xiagou fractured reservoir experiences strong stress sensitivity with a fracture permeability damage rate reaching 94.38%. Natural fractures influence reservoir rock stress sensitivity. The degree of filling and type of filled minerals within natural fractures have great effects on the permeability damage rate. A higher permeability damage rate suggests stronger rock stress sensitivity. Generally, I) for reservoir rocks with unfilled natural fractures, the permeability damage rate is extremely high; II) for reservoir rocks with partially filled natural fractures, the permeability damage rate is high, and if the minerals within the natural fractures are insoluble, the permeability damage rate is slightly higher than if the materials are soluble; III) for reservoir rocks with completely filled natural fractures, the permeability damage rate is extremely low if the minerals within the natural fractures are insoluble; however, if the materials are soluble, the permeability becomes slightly higher with the increase of effective stress. Most importantly, this study provides a practical method for analyzing stress sensitivity in naturally fractured reservoirs.

Relation between coalbed permeability and in-situ stress magnitude for coalbed methane exploration in Jharia and Raniganj coalfields, India

India is among the top five countries in the world in terms of proven coal reserves and coal production. As such, significant potential exists for commercial recovery of coalbed methane (CBM). Two coalfields, Jharia and Raniganj, located in eastern India are currently under development for CBM. This paper describes work done to determine coal seam properties, ambient stress conditions, and effects of depletion at these coalfields that influence CBM production. Coalbed permeability is a parameter that has a major influence on CBM production. Other influences include in-situ stress direction, gas content, and the application of suitable stimulation techniques. A robust methodology is required to determine both initial coalbed permeability and its relation to in-situ horizontal stress magnitudes. Coalbed permeability at the Jharia and Raniganj coalfields was estimated from porosity and known cleat spacing. Initial permeability of major coalbeds was correlated with effective horizontal stress, yielding satisfactory to very good exponential fit using data from Raniganj and Jharia wells. Acoustic televiewer image-logging tool measurements in a single well in the Jharia coalfield were used to infer a maximum horizontal stress orientation between N25°W and N25°E. Reservoir-pressure-dependent permeability models are presented for coalbeds under uniaxial strain condition. The coalbed permeability is dominated by the existing effective horizontal stresses normal to the cleats. Two prospective coal seams from Jharia have been identified through assessment of the response of horizontal stress to the decline of CBM reservoir pressure. Coalbed permeability increases with the drawdown of reservoir pressure and is exponentially related to the change of effective horizontal stress during reservoir depletion. The results of this study are to be used for production history matching for wells in Jharia and to determine optimal horizontal drilling directions for increased CBM production.