Hot Dry Rock Geothermal Reservoir Research Articles

Numerical study on the effects of thermoelastic and poroelastic parameters on the geomechanical behaviour of Hot Dry Rock geothermal reservoirs

ABSTRACT During the last two decades, energy production has been directed towards renewable resources, one of the most important of which is geothermal energy. Despite the reliability of geothermal energy as a resource, its effects on the surrounding environment have not been investigated in detail. This study focuses on reservoir deformation as one of the environmental concerns of geothermal energy extraction. A coupled thermo-hydro-mechanical (THM) model is employed for simulating the reservoir deformations. The validity of the model results is examined by comparing the numerical results with the analytical solution of a benchmark THM problem. The model is then utilised to study the behaviour of a Hot Dry Rock reservoir consisting of two wells. The obtained results show that in some cases the reservoir deformation is significant, making it an important factor in design considerations. Also, it is found that the deformation caused by thermal volume change is up to 6 times more significant compared to that of poroelastic effects. The conducted parametric study demonstrated that the coefficient of thermal expansion of rock and the production rate severely influence the reservoir deformation, while rock elasticity modulus, porosity and Biot-Willis coefficient only affect the behaviour of the geothermal reservoir to some extent.

Coupled heat-fluid-solid numerical study on heat extraction potential of hot dry rocks based on discrete fracture network model

Fracture networks within hot dry rock (HDR) geothermal reservoirs are complex, and heat extraction via water injection is thus a coupled process of heat-fluid-solid multifield. In this paper, utilizing the theory of normally distributed random functions, we develop a corresponding pre-processing subprogram to establish a discrete network model of complex fracture distribution in HDR reservoirs; then construct a heat-fluid-solid finite element model for heat extraction via water injection and compare the numerical solution with the analytical solution of the one-dimensional non-isothermal consolidation problem for verification. The numerical simulation results show that the main factors affecting the heat extraction efficiency of HDR reservoirs include fracture width, fracture density, fracture permeability, and matrix permeability. When a HDR reservoir is injected with water for heat extraction, there is a certain threshold value of these influential parameters, beyond which the outlet temperature drops significantly, resulting in an obvious thermal breakthrough. When injecting water for heat extraction, the values of these parameters should be controlled and kept at a reasonable level, otherwise, the HDR reservoir may enter a thermal breakthrough stage in advance, which is not conducive for long-period heat extraction. Influenced by the random distribution of complex fractures, the leading edge of the cold front may present an irregular distribution. During the process of heat extraction, the stress gradually changes from a compressional state to a tensile state, which induces further damage to the HDR reservoir.

Seismic prediction and evaluation techniques for hot dry rock exploration and development

Abstract Hot dry rock is generally buried deep within the subsurface and exhibits high temperatures, hardness and density, with low porosity and a lack of permeable fluids. It is thus difficult to investigate and poses a high exploration risk. This paper emphasizes the role of seismic exploration in evaluating hot dry rock exploration by exploiting technical advantages, including strong penetration, high resolution and precise depth determination. Denoising and weak deep effective signal processing, unique processing methods (e.g. high-precision static correction, pre-stack noise attenuation, wide-angle reflection in weak signal areas and fine velocity analysis) are used for high-quality imaging of formations inside the hot dry rock geothermal reservoirs to address the difficulties of data static correction. The joint inversion/data fusion of seismic and other geophysical methods allow the realization of non-linear quantitative prediction and evaluation of hot dry rock reservoirs. The geothermal reservoir parameters, including lithology, physical properties and temperature, are predicted by integrating various geophysical attributes. Further, the development potential of the geothermal reservoir is accurately evaluated, and promising results achieved.

A laboratory study on fracture initiation and propagation of granite under cyclic-injection hydraulic fracturing

The abundant geothermal energy in hot dry rock (HDR) geothermal reservoirs is an attractive renewable energy resource with great potential. Cyclic injection in hydraulic fracturing has been proved to be a suitable way for the geothermal energy exploitation. However, the fracture initiation and propagation regimes induced by cyclic injection have been inadequately studied, and the fracturing optimization for HDR is often by experience. For this reason, true tri-axial hydraulic-fracturing tests were conducted to study the initiation and propagation of hydraulic fractures, while the specimens were subjected to the cyclic injection with different cycle time duration and injection rate. The results revealed that the initiation of hydraulic fracture was displayed as three basic patterns, and these fracture-initiation patterns were developed into two fracture geometries: simple fracture with only 2–4 strands and complex fracture network with >4 strands. The main reason for this difference is that the initiation and propagation of hydraulic fracture is controlled by the way of fluid circulation, and the cycle time duration and injection rate each played a different role in hydraulic fracturing. At a low injection rate, the initiation and propagation of hydraulic fractures under the high-frequency cycle (cycle time duration = 10 and 20 s) are mainly controlled by the change of injection pressure. Here, the hydraulic energy mainly acts on the rock near the well. The frequent change of injection pressure promotes the initiation of micro-fractures and forms a complex but short hydraulic fracture network. However, when high injection rate is subjected, too high and too low cycle frequencies (cycle time duration = 5 and 40 s) both tend to form a simple fracture, but the fracture has strong extension ability. With that, the similarity criterion of physical phenomena between the on-site prototype and the experimental model were discussed, and these lab-scale results were translated into the fracturing site of HDR.

Upper Limit on Heat Extraction of a Super Long Gravity Heat Pipe in a Hot Dry Rock Geothermal Reservoir

Upper Limit on Heat Extraction of a Super Long Gravity Heat Pipe in a Hot Dry Rock Geothermal Reservoir

Investigation of the effect of different injection schemes on fracture network patterns in hot dry rocks - A numerical case study of the FORGE EGS site in Utah

Multi-stage stimulation using alternative injection has been successfully applied in low mobility hydrocarbon production. However, fracture initiation and growth induced by different injection schemes have been inadequately studied for hot dry rock (HDR) geothermal reservoirs. Here, the impact of injection schemes on hydraulic fracture (HF) propagation regimes was determined with PFC2D software. The results show that the propagation of natural fractures (NFs) created by cyclic injection are dominated by the mode of shear activation and direct penetration. However, cyclic injection with frequent starting and stopping can produce non-uniform stress and fatigue, resulting in crack initiation and more branched fractures growth. The stepped injection can activate NFs effectively, and the HF propagation are featured by a style of turning at the tip of NFs. However, the stepped injection often produces a single main fracture with few branches. Different injection methods can lead to different propagation regimes and ultimately result in variation of the fracture network. A numerical model of the FORGE site that contains relevant geological structure and a fracture network was established with 3DEC software, and the impact of NFs on the HF network formation was investigated systematically. Compared with cyclic injection, the fracture network formed by stepped injection is more susceptible to the distribution of the NFs. The value of stepped injection is about 1.31 times in surface area and 1.17 times in aperture than the cyclic injection. Cyclic injection is conducive to creating fractures in the matrix, while stepped injection is more inclined to activate the preexisting NFs. The method presented here can be adopted to other geologic settings to optimize the fracture growth regime and provide a scientific basis for Enhanced Geothermal System (EGS) multi-stage fracturing design.

A thermo-hydro-mechanical model of a hot dry rock geothermal reservoir

With sufficient data and the appropriate modelling tools, it is possible to replicate the real-life behaviour of geothermal systems. Modelling tools could guide geologists, engineers and decision-makers in developing an optimal design for these systems. In order to explore the effectiveness of the modelling techniques, this paper presents a new three-dimensional (3D) numerical model of a hot dry rock (HDR) geothermal reservoir using coupled thermo-hydro-mechanical (THM) processes. The model is implemented in the COMSOL Multiphysics Finite Element (FE) solver, and its reliability is confirmed by conducting a validation study using field measurements from the Fenton Hill Phase I HDR system in New Mexico. After confirming the model's reliability, two case studies are analysed to determine the different factors affecting reservoir performance during exploitation. The factors analysed for the two cases are wellbore alignment and placement, using single and multiple planar fracture HDR system configurations. The results show that wellbore alignment does not affect changes in fracture properties and reservoir productivity. Wellbore placement, however, is found to affect reservoir performance significantly. The simulation analysis performed shows that knowledge of injection/production wellbore placement could be a significant asset to reservoir engineers/managers during the planning, exploration, design, and exploitation stages.

Induced Seismicity and Detailed Fracture Mapping as Tools for Evaluating HDR Reservoir Volume

The main goal of this paper was to estimate the heat exchange rock mass volume of a hot dry rock (HDR) geothermal reservoir based on microseismicity location. There are two types of recorded microseismicity: induced by flowing fluid (wet microseismicity) and induced by stress mechanisms (dry microseismicity). In this paper, an attempt was made to extract events associated with the injected fluid flow. The authors rejected dry microseismic events with no hydraulic connection with the stimulated fracture network so as to avoid overestimating the reservoir volume. The proposed algorithm, which includes the collapsing method, automatic cluster detection, and spatiotemporal cluster evolution from the injection well, was applied to the microseismic dataset recorded during stimulation of the Soultz-sous-Forets HDR field in September 1993. The stimulated reservoir volume obtained from wet seismicity using convex hulls is approximately five times smaller than the volume obtained from the primary cloud of located events.

Experimental and numerical studies on hydraulic fracturing characteristics with different injection flow rates in granite geothermal reservoir

AbstractGeothermal energy has been widely proposed as a potential renewable energy to replace traditional fossil fuel energy. Hot dry rock (HDR) reservoirs were the main geothermal energy resources and usually consist of low‐permeability hard granite without fluid. Developing HDR requires water cyclically flowing between injection and production wells to extract heat energy. Hydraulic fracturing, as a key reservoir stimulation technology, can create the path of fluid cyclically flowing. However, few studies have investigated hydraulic induced artificial fractures in HDR geothermal formations. This paper investigated HDR geothermal reservoir stimulation characteristics and fracture patterns under different injection flow rates. Numerical simulations were conducted to model the laboratory tests. Results showed they were in good agreement, and this indicated the possibility of numerical simulation to predict hydraulic fracturing behavior under different injection flow rates. With the increase of injection flow rate, the fracture initiation pressures and breakdown pressures increased, and the propagation times and postfracturing pressures decreased. The fracture geometries were observed and analyzed, mean injection power was proposed, and results showed that it could be used to roughly estimate the fracture total lengths. Moreover, the fracture permeabilities based on the pressure data were calculated. These results can provide some reasonable advice for implementing reservoir stimulations on application to field‐scale HDR operation.

An investigation on hydraulic fracturing characteristics in granite geothermal reservoir

Geothermal energy has been widely proposed as a potential renewable energy to replace traditional fossil fuel energy. Deep buried geothermal reservoirs are usually called Hot Dry Rock (HDR) or Enhanced Geothermal Systems (EGS). Hot dry rock (HDR) reservoirs are the main geothermal energy resources, and usually consist of low-permeability hard granite without fluid. Developing HDR requires water cyclically flowing between injection and production wells to extract heat energy. Hydraulic fracturing, as a key reservoir stimulation technology, can create the path of fluid cyclically flowing. Although hydraulic fracturing in HDR formations has attracted more and more attention these years, large-size HDR formation environment is difficult to establish in laboratory and large granite samples are difficult to crack because of their high toughness and strength, the reservoir stimulation characteristics in this hard granite have not been thoroughly addressed by the researchers and the hydraulic induced fracture morphology has yet remained unnoticed. To advance this technology development, a true triaxial hydraulic fracturing apparatus with high temperature heated system has been developed by Jilin University to simulate the real HDR environment, and this paper investigated HDR geothermal reservoir stimulation characteristics in large-size granite under different injection flow rates, temperatures and confining stresses conditions. The granite samples were collected form Gonghe Basin where the Chinese first EGS field operation will be built. Results showed that these laboratorial conditions affected hydraulic fracturing characteristics, breakdown pressures increased with the increase of injection flow rate and confining stresses, decreased with the increase of temperatures. Numerical simulations were conducted to model the laboratory tests. These results can provide some reasonable advice for implementing reservoir stimulations on the application of field-scale HDR operation.

Mechanism of drilling rate improvement using high-pressure liquid nitrogen jet

Abstract To address the high rock strength and low drilling rate issues in deep oil/gas and geothermal exploitation, we performed mechanical property tests on three kinds of rock samples (granite, shale and sandstone) subjected to liquid nitrogen (LN2) cooling and conducted rock-breaking experiments using LN2 jet. Rock-breaking characteristics and mechanisms of LN2 jet, heat transfer features between LN2 and rock and thermal stress evolution in rock were analyzed. A novel high-pressure LN2 jet assisted drilling method was proposed accordingly. The study results show that LN2 thermal shock can significantly reduce uniaxial compression strength and elastic modulus of rock. Rock damage and corresponding mechanical deterioration become more pronounced with increasing rock temperature. The LN2 jet has merits of high rock-breaking efficiency and low threshold rock-breaking pressure. Rock failure under LN2 jet impact is characterized by large volume breakage and the rock-breaking performance becomes more significant with increase of rock temperature. Under the impact of LN2 jet, the damage of granite is the most remarkable among the three rock samples. Thus, this method works better for high temperature granite formations. It has a good application prospect in speeding up drilling rate in deep hot dry rock geothermal reservoirs.

Present challenge and prospects of drilling and hydraulic fracturing technology for hot dry rock geothermal reservoir

The hot dry rock geothermal energy is one kind of the clean energies which are contained in the China National Energy Strategy. This strategy is to encourage the development of the green and clean energy instead of the fossil energy. In the development of hot dry rock (HDR), the artificial geothermal reservoir is built through drilling and hydraulic fracturing. Two wells are drilled and connected by hydraulic fractures. Then, the geothermal energy can be extracted by the water circulation. The cold water is injected into one of the two wells and flowed along with the hydraulic fractures. The water becomes gas when it contacts the formation rock because of the high temperature. The phase change since the temperature and pressure are reduced at the wellhead of the another well can provide power to generate electricity. Therefore, the key technology to build artificial geothermal reservoir is drilling and hydraulic fracturing. However, there is few experiences in drilling and hydraulic fracturing in the HDR in China. This paper summarized and analyzed the research and field cases around the world to investigate the challenges and key points during drilling and hydraulic fracturing in the HDR. The investigation was based on geology condition, rock physics and chemistry, and field operation during the HDR development. The summary provided the common problems and experience of the drilling and hydraulic fracturing. In addition, the further research directions of related technologies were expounded. Finally, this paper could be referenced for the drilling and hydraulic fracturing in the development of HDR in China for effective utilization.

Thermomechanical Behavior of Late Indo-Chinese Granodiorite under High Temperature and Pressure

This study investigates the influence of temperature, effective stress, and rock fracture on the bulk modulus and Biot’s coefficient of granodiorite from a hot dry rock geothermal reservoir using the triaxial compression test. Three types of representative granodiorite samples were chosen for comparative experiments. The experiments were conducted with 0–55 MPa effective stress under cyclic loading. Results show that bulk modulus can continuously increase with the increase in effective stress at a constant temperature. The influencing law on Biot’s coefficient is opposite that on bulk modulus. Interestingly, the temperature effects on the drained bulk modulus and Biot’s coefficient depend on the effective stress. With regard to rock fractures, temperature and effective stress exert similar effects on the Biot’s coefficients and bulk moduli of the samples compared with those of intact rock. The data of this experiment have a wide range of applications because most of the reservoir rocks in dry-hot-rock geothermal system have lithology of granite or granodiorite. The change law of rock modulus and Biot’s coefficient with the temperature and pressure in this experiment provide the data basis for the future simulation calculation making the considered factors more comprehensive and the results closer to the real situation.

Chemical stimulation on the hydraulic properties of artificially fractured granite for enhanced geothermal system

Hydraulic fracturing is often conducted to create artificial fractures as flow paths in Hot Dry Rock (HDR) geothermal reservoirs. However, the fractures could be closed due to the high geo-stress conditions, resulting in low efficiency of geothermal productivity. Hence, maintaining a high hydraulic permeability of the created fractures is a key point for the success of Enhanced Geothermal Systems (EGS).In this study, chemical stimulation was carried out to improve the hydraulic properties of fractured granite. The effects on the hydraulic properties of two sets of samples were examined by laboratory flowing tests. Results show that chemical stimulation solution with mixture of 12%HCl+5%HF improved efficiently permeability. The permeability was increased by four orders of magnitudes with an optimized reaction time. The mud acid was found to react fast with minerals such as feldspars and biotite, but rather inactive with quartz as revealed by the microscopic structure and ionic detection of the fluid and rocks. Thereby, the quartz particles can be used as fracturing proppant to support opening of aperture. The findings obtained from this study indicated that a hydraulic fractured granite based HDR geothermal reservoir can be effectively improved by chemical stimulation.

Numerical Simulation of Hydraulic Fracturing Process in an Enhanced Geothermal Reservoir Using a Continuum Homogenized Approach

The current study presents a numerical modeling approach for three-dimensional simulation of hydro-shearing in jointed rocks for the generation of a man-made, multi-fracture heat exchanger in Hot Dry Rock (HDR) Geothermal reservoir. Various literatures have suggested the presence of in-situ fractured rock mass even in massive granite formations. Thereby, 3D numerical modelling is essential, since the prediction of fracture growth in 3D is key to investigation of different fracture designs and furthermore various operational parameters in order to optimize the heat exchanger design and the resulting energy production. The numerical approach incorporates the Dynardo's approach of homogenized continuum method to simulate the hydro-shearing process in jointed rocks unlike vast majority of commercial and scientific approaches which use the discrete modelling technique. The main motivation of the continuum approach is the numerical efficiency of the 3D coupled hydraulic-mechanical simulations of hydraulic fracturing process in comparison to other alternatives. The input parameters of the numerical model from the best available well log and reservoir data are calibrated from diagnostics measurements such as Micro-seismic events, Bottom Hole Pressure, etc to assign the correct level of forecast quality to the important mechanisms of hydraulic fracturing. The numerical procedure is applied to a prospective Granite reservoir in Thüringen, Germany within the purview of a German joint research project – optiRiss. The integrated approach involves ANSYS as a pre-processor and solver, Dynardo's fracturing simulator on top of ANSYS, Tamino – a post-processing tool and optiSLang – an environment for optimization and uncertainty quantification.

An enhanced stochastic optimization in fracture network modelling conditional on seismic events

This paper presents an approach to modelling fracture networks in hot dry rock geothermal reservoirs. A detailed understanding of the fracture network within a geothermal reservoir is critically important for assessments of reservoir potential and optimal production design. One important step in fracture network modelling is to estimate the fracture density and the fracture geometries, particularly the size and orientation of fractures. As fracture networks in these reservoirs can never be directly observed there is significant uncertainty about their true nature and the only feasible approach to modelling is a stochastic one. We propose a global optimization approach using simulated annealing which is an extension of our previous work. The fracture model consists of a number of individual fractures represented by ellipses passing through the micro-seismic points detected during the fracture stimulation process, i.e. the fracture model is conditioned on the seismic points. The distances of the seismic points from fitted fracture planes (ellipses) are, therefore, important in assessing the goodness-of-fit of the model. Our aims in the proposed approach are to formulate an appropriate objective function for the optimal fitting of a set of fracture planes to the micro-seismic data and to derive an efficient modification scheme to update the model parameters. The proposed objective function consists of three components: orthogonal projection distances of the seismic points from the nearest fitted fractures, the amount of fracturing (fitted fracture areas) and the volumes of the convex hull of the associated points of fitted fractures. The functions used in the model update scheme allow the model to achieve an acceptable fit to the points and to converge to acceptable fitted fracture sizes. These functions include two groups of proposals: one for updating fracture parameters and the other for determining the size of the fracture network. To increase the efficiency of the optimization, a spatial clustering approach, the Distance-Directional Transform, was developed to generate parameters for newly proposed fractures. A simulated dataset was used as an example to evaluate our approach and we compared the results to those derived using our previously published algorithm on a real dataset from the Habanero geothermal field in the Cooper Basin, South Australia. In a real application, such as the Habanero dataset, it is difficult to determine definitively which algorithm performs better due to the many uncertainties but the number of association points, the number of final fractures and the error are three important factors that quantify the effectiveness of our algorithm.

A thermo‐hydro‐mechanical coupled model in local thermal non‐equilibrium for fractured HDR reservoir with double porosity

The constitutive thermo‐hydro‐mechanical equations of fractured media are embodied in the theory of mixtures applied to three‐phase poroelastic media. The solid skeleton contains two distinct cavities filled with the same fluid. Each of the three phases is endowed with its own temperature. The constitutive relations governing the thermomechanical behavior, generalized diffusion and transfer are structured by, and satisfy, the dissipation inequality. The cavities exchange both mass and energy. Mass exchanges are driven by the jump in scaled chemical potential, and energy exchanges by the jump in coldness. The finite element approximation uses the displacement vector, the two fluid pressures and the three temperatures as primary variables. It is used to analyze a generic hot dry rock geothermal reservoir. Three parameters of the model are calibrated from the thermal outputs of Fenton Hill and Rosemanowes HDR reservoirs. The calibrated model is next applied to simulate circulation tests at the Fenton Hill HDR reservoir. The finer thermo‐hydro‐mechanical response provided by the dual porosity model with respect to a single porosity model is highlighted in a parameter analysis. Emphasis is put on the influence of the fracture spacing, on the effective stress response and on the permeation of the fluid into the porous blocks. The dual porosity model yields a thermally induced effective stress that is less tensile compared with the single porosity response. This effect becomes significant for large fracture spacings. In agreement with field data, fluid loss is observed to be high initially and to decrease with time.

Development program of hot dry rock geothermal resource in the Yangbajing Basin of China

Geothermal energy from hot dry rock (HDR), considered an almost inexhaustible source of “green” energy, was first developed and tested in the 1970s, leading to installations in America, Japan, Britain, France and other countries. In the present work, a liquating rock mass at a depth of 5–15 km in the Tibet Yangbajing region in China was subjected to detailed analysis. The temperature distribution of the geothermal field in the region was determined by the finite element method. The results estimate that the HDR geothermal resource of the Yangbajing region is 5.4 × 10 9 MW a, representing a huge potential source of HDR geothermal energy for China. Based on detailed research into the continental dynamics of the environment forming the HDR geothermal field of Tibet, along with the tectonic characteristics of the southern slope of Tanggula Mountain and the Dangxiong–Yangbajing Basin, and the magnitude and orientation of the in situ stresses in the region, the design of an arrangement for extracting these HDR geothermal resources is proposed: taking the fault zone nearest the high-temperature liquating rock region as the location of an artificial reservoir, a vertical injection well could be drilled at a low point on the downdip side of the fault, and two dipping production wells drilled higher up. In this way, an artificial reservoir 3 × 10 11 m 3 in volume would be created: 360 times the volume of the HDR geothermal reservoir in Cornwall, UK, which uses hydrofracturing. An investigation of the reservoir features, including seepage analysis of the heat exchange area, project implementation and investment analysis, indicates that a 10 4 MW capacity power station with a projected operating life of approximately 100 years could be constructed. An analysis of a geothermal extraction system comprising one injection well and two production wells suggest that a power station of 1000 MW installed capacity could be constructed initially to provide electricity production of 8.64 × 10 9 kWh per year.

Fractal concept in numerical simulation of hydraulic fracturing of the hot dry rock geothermal reservoir

AbstractHydraulic fracturing or hydraulic stimulation is one of the most effective methods of enhancing hot dry rock (HDR) geothermal system productivity. The 3D structure of the fractured rock is approximated by the network models of ‘fractal geometry’. The models of fracture networks are generated by distributing fractures randomly in space and assuming the fractal correlation Nr = Cr−D that incorporates the number of fractures Nr, fractal length r, fractal dimension D and fracture density within the rock mass C. This procedure makes it possible to characterize the geothermal reservoirs using parameters measured from field data. Based on this approach, a mathematical model of the hydraulic rock fracturing is proposed. The model incorporates approximations of the fracture mechanical behaviour drawn from the rock mechanics literature, a very simplified analysis of the operative physical processes and mapping of the connectivity of fracture network to a cubic regular grid. Along with the fractal‐type distribution of the fracture lengths, the fracture surfaces are also assumed to follow fractal geometry. The latter allows numerical simulation of the natural rock fracture dilation caused by fracture shear offset. The other problem that can be resolved by fracture surface modelling is the apparent limitation on the number of fractures that can be analysed experimentally. In this respect, the suggested mathematical model can be used to simulate fractal surfaces identical to fractures found in the natural rocks. Taken together, these tools permit the approximate engineering resolution of the multi‐parametric, highly complex mechanical problem. The reliability of the developed model was validated by comparisons with the experimentally determined data for the Hijiori deep reservoir. This implicitly justifies the numerical results and conclusions drawn in the present research. In particular, a series of computations indicate that the connectivity of the fracture network is greatly affected by the fractal dimension of the fracture network. The strong effect of the fractal dimension on the reservoir's size is also observed. The numerical results illustrate the controlling effect of the pressure and flow rate in the stimulating well for reservoir growth. Copyright © 2003 John Wiley & Sons, Ltd.

A shear‐dilation‐based model for evaluation of hydraulically stimulated naturally fractured reservoirs

AbstractThe role of shear dilation as a mechanism of enhancing fluid flow permeability in naturally fractured reservoirs was mainly recognized in the context of hot dry rock (HDR) geothermal reservoir stimulation. Simplified models based on shear slippage only were developed and their applications to evaluate HDR geothermal reservoir stimulation were reported. Research attention is recently focused to adjust this stimulation mechanism for naturally fractured oil and gas reservoirs which reserve vast resources worldwide. This paper develops the overall framework and basic formulations of this stimulation model for oil and gas reservoirs. Major computational modules include: natural fracture simulation, response analysis of stimulated fractures, average permeability estimation for the stimulated reservoir and prediction of an average flow direction. Natural fractures are simulated stochastically by implementing ‘fractal dimension’ concept. Natural fracture propagation and shear displacements are formulated by following computationally efficient approximate approaches interrelating in situ stresses, natural fracture parameters and stimulation pressure developed by fluid injection inside fractures. The average permeability of the stimulated reservoir is formulated as a function of discretized gridblock permeabilities by applying cubic law of fluid flow. The average reservoir elongation, or the flow direction, is expressed as a function of reservoir aspect ratio induced by directional permeability contributions. The natural fracture simulation module is verified by comparing its results with observed microseismic clouds in actual naturally fractured reservoirs. Permeability enhancement and reservoir growth are characterized with respect to stimulation pressure, in situ stresses and natural fracture density applying the model to two example reservoirs. Copyright © 2002 John Wiley & Sons, Ltd.