Fractured Rock Samples Research Articles

Fractal Characterization of Multimodal, Multiscale Images of Shale Rock Fracture Networks

An array of multimodal and multiscale images of fractured shale rock samples and analogs was collected with the aim of improving the numerical representation of fracture networks. 2D and 3D reconstructions of fractures and matrices span from 10−6 to 100 m. The origin of the fracture networks ranged from natural to thermal maturation to hydraulically induced maturation. Images were segmented to improve fracture identification. Then, the fractal dimension and length distribution of the fracture networks were calculated for each image dataset. The resulting network connectivity and scaling associations are discussed at length on the basis of scale, sample and order of magnitude. Fracture network origin plays an important role in the resulting fracture systems and their scaling. Rock analogs are also evaluated using these descriptive tools and are found to be faithful depictions of maturation-induced fracture networks.

Benchmarking conventional and machine learning segmentation techniques for digital rock physics analysis of fractured rocks

Image segmentation remains the most critical step in Digital Rock Physics (DRP) workflows, affecting the analysis of physical rock properties. Conventional segmentation techniques struggle with numerous image artifacts and user bias, which lead to considerable uncertainty. This study evaluates the advantages of using the random forest (RF) algorithm for the segmentation of fractured rocks. The segmentation quality is discussed and compared with two conventional image processing methods (thresholding-based and watershed algorithm) and an encoder–decoder network in the form of convolutional neural networks (CNNs). The segmented images of the RF method were used as the ground truth for CNN training. The images of two fractured rock samples are acquired by X-ray computed tomography scanning (XCT). The skeletonized 3D images are calculated, providing information about the mean mechanical aperture and roughness. The porosity, permeability, flow fields, and preferred flow paths of segmented images are analyzed by the DRP approach. Moreover, the breakthrough curves obtained from tracer injection experiments are used as ground truth to evaluate the segmentation quality of each method. The results show that the conventional methods overestimate the fracture aperture. Both machine learning approaches show promising segmentation results and handle all artifacts and complexities without any prior CT-image filtering. However, the RF implementation has superior inherent advantages over CNN. This method is resource-saving (e.g., quickly trained), does not need an extensive training dataset, and can provide the segmentation uncertainty as a measure for evaluating the segmentation quality. The considerable variation in computed rock properties highlights the importance of choosing an appropriate segmentation method.

Self-Organizing-Map Approach Characterizes Naturally Fractured Reservoir in a Mature Field

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 206027, “Naturally Fractured Basement Reservoir Characterization in a Mature Field,” by Muhammad Nur Ali Akbar, MOL Hungary. The paper has not been peer reviewed. The complete paper describes an alternative solution for identifying the presence of natural fractures, classifying them into fracture-quality-related flowability, and distributing them vertically within the well interval, and proposes a lateral distribution method for reservoir modeling. The proposed approach, using the machine-learning technique of self-organizing-map (SOM) clustering, effectively assists recognition of fracture presence and quality along the well-depth interval. Field Overview and Data Used The case study was conducted in an oil field discovered in the early 1980s in southwestern Hungary. Thirty-six wells penetrated the naturally fractured carbonate in the Triassic formation. The main lithology of this reservoir is limestone and dolomite associated with faults and exhumation breccia and marl/shale. The reservoir is saturated oil (with gas cap) with unlimited aquifer (strong water drive). The gas cap, however, is mainly composed of 85% carbon dioxide and up to 1,800 ppm of hydrogen sulfide. Generally, the oil is intermediate to heavy, with a gravity of approximately 20 °API. The reservoir rock properties of this case study are fully complex for both the pore system and its composition. In general, the matrix pore system does not significantly contribute to storativity or permeability. The effective porosity is approximately 4.2% on average. This value sometimes directs to the matrix porosity, but in this case study, high intensity of the microfracture presence behaves in a way similar to matrix porosity, a phenomenon the author terms “pseudomatrix porosity.” Moreover, the effective oil permeability value of the studied fractured reservoir is extremely high (up to 70 darcies per well-test interpretation). The permeability ranges from 1 to 2000 md in brecciated fractures or in naturally fractured rock samples. Two types of core samples were used in this study—fractured breccia and naturally fractured rock. Both types have similar behavior in terms of the porosity/permeability relationship. In this study, that relationship is not the one normally observed in clastic reservoir rock. Marl/shale content is one of the more- critical parameters, indicating low fracture quality in terms of permeability. More than 50% of the well-log data for this study were measured by Russian-type well logs, meaning that only simple electrical, gamma ray, and neutron- capture gamma logs were available. Other wells have standard triple-quad combination logs. Image logs are available only from two wells drilled in the 2000s. By considering these log-data limitations, cores, and production-test results, the study aimed to define the effective fracture locations and intervals.

Investigations of the mechanical behavior and failure mechanism of fractured rock samples

As excavation is often very deep with large unloading forces in the construction of a high-level radioactive waste (HLW) disposal repository, microcracks in the surrounding rock are unavoidable. In this work, a series of uniaxial compression tests were conducted on intact samples and samples with fractures at different inclination angles (30°, 45°, and 60°), to investigate the mechanical behavior and failure mechanism of fractured rock samples. According to the experimental results, a more inclined angle of the fractures would dramatically reduce the compressive strength of the sample. After the compression tests, the granite samples with different fracture inclinations have different failure modes and distribution characteristics of cracks on sample outside surfaces and the premade fracture planes. Higher dip angles of the premade fractures have less surface density of cracks on the sample outside surface after compression. The instability failure of granite samples with 60° fracture inclination is entirely due to shear deformation between the premade fractures. Based on this single failure mode, the compressive strength of a granite sample with 60° fracture inclination has a positive correlation with the difference in fractal dimensions induced by compression. The greater the difference in fractal dimensions, the higher the compressive strength.

Experimental Study on Water-Sand Seepage Characteristics in Fractured Rock Mass under Rheological Effect

This study investigates water-sand bursting disasters associated with fractured rock that affect safe mining in the mining areas of Western China. A broken rock water-sand seepage rheological test device was developed, and rheological tests were conducted on multiple groups of broken rock samples with single-stage axial loading and different load levels. When the rheology of each group of broken rock samples was stable, water-sand mixed fluid was injected into the samples at a certain pressure gradient to conduct water-sand seepage tests on broken rock masses. It was found that when the porosity of a fractured rock mass is within a certain range, the water-sand mixed fluid does not completely pass through the fractured rock mass and some sand particles are filtered by the fractured rock sample. There is an exponential relationship between the sand breaking ability and the sand filtration ability of fractured rock and its initial porosity, and the permeability of fractured rock decreases by a certain extent after sand filtration. However, for different load levels, when the flow through a fractured rock mass tends to be stable, the final porosity of the fractured rock mass decreases exponentially with axial compression. Based on the classical Kelvin rheological model and the basic theory of fractional calculus, a new fractional rheological model has been proposed and the rheological parameters under different load levels were fitted to the model. The new fractional rheological model is better able to describe the rheological characteristics of broken mudstone.

Mechanism study of preventing crack propagation of fractured rockunder dynamic loads

To deeply understanding dynamic fracture properties and preventing crack propagation of fractured rock mass under dynamic loads, impact experiments were conducted using TWSRC (tunnel with single radial crack) samples, and sandstone were selected as the raw material to manufacture fractured rock samples. The crack initiation, propagation and obstructing behavior were measured by using a drop hammer impact test device and crack propagation gauge measuring system. The mechanism of preventing crack propagation and failure behavior during dynamic fracturing process was focused, and then the corresponding numerical simulation was conducted by using the finite difference code, which can be used to accurately estimate the experiment result. The results indicate that the whole dynamic fracturing process of fractured rock under dynamic loads is composed of the cyclic process of crack initiation, high-speed crack propagation, slowly deceleration, preventing crack propagation. In addition, the period of crack obstruction was approximate the microsecond level. The ratio of transgranular (TG) fracture at the crack obstruction point of fractured rock was smaller than that of the crack initiation point, and the ratio of TG fracture of green sandstone during the dynamic fracturing process was larger than that of black sandstone. The fracture energy for crack initiation again after crack obstruction was much less than the fracture energy required for the initial initiation of pre-existing crack.

Effect of dry-wet alternations on the mechanical strength of deep tight sandstone

The working fluid used for gas drilling and the excessive differences in production pressure during the gas production process causes repeated fluctuations between increasing and decreasing water saturation of the rock. These repeated fluctuations result in dry-wet state alternations of the rock, which weakens the rock strength. Weakening rock strength will lead to sand shedding, which damages reservoirs because of particle migration, solid blockage, and sand production. This will also change the stress sensitivity of deep tight sandstone, thus affecting gas well production. Therefore, this study conducted both a dry-wet alternation experiment and a stress sensitivity experiment after dry-wet alternation using deep tight sandstone. During the experiment, the changes of the rock dynamic Young's modulus, Poisson's ratio, and stress sensitivity coefficient were monitored. The experimental results show that the dynamic Young's modulus and Poisson's ratio of rock samples decreased after dry-wet alternations. Furthermore, these alternations increase the stress sensitivity of the rock matrix and weaken the stress sensitivity of fractured rock samples. • The mechanism of weakening the strength of deep tight sandstone by dry-wet alternation is analyzed. • The influence of dry-wet alternation on stress sensitivity of deep tight sandstone is analyzed. • Dry-wet alternation may be one of the reasons for sand production in deep tight sandstone.

Experimental investigation on rock breaking performance of cutter assisted with hydraulic fracturing

To improve the efficiency of cutting hard rocks in roadway tunneling, a rock breaking method using a cutter assisted with hydraulic fracturing is proposed. In addition, the influence of the circumferential distribution of holes on the crack propagation was studied through a hydraulic fracturing experiment. On this basis, an experiment of rock breaking by a cutter assisted with hydraulic fracturing was conducted, and the effect of the hydraulic fracturing on the cutting performance was analyzed. The results show that the main cracks’ initial direction and propagation path were influenced by the location and number of circumferentially distributed holes, around which the secondary cracks appeared. During the fracturing process, the main cracks expanded to the circumferentially distributed holes and developed into a crack network; the more empty holes there were, the denser the crack network was. Compared with the original rock samples, the cutting torque and propelling force of the non-directionally and directionally perforated fractured rock samples were significantly lower, and the cutting performance of the directionally perforated fractured rock samples was better. The research provides a possibility for the rapid excavation of extremely hard and abrasive rock.

How can biogeomechanical alterations in shales impact caprock integrity and CO2 storage?

Shales have been a major focus of the energy industry over the past few decades. Recently, there is a paradigm shift in the energy industry to low-carbon solutions, such as carbon capture and storage (CCS), to mitigate global warming caused by carbon footprint. The problem of long-term safe and efficient geological CO2 storage (GCS) and caprock integrity are some of the major challenges impeding large-scale CCS application. Here, we investigated how localized and bulk biogeomechanical alterations could potentially impact caprock integrity and CO2 storage in depleted shale reservoirs. We cultivated the shale core samples (containing both artificial-induced and pre-existing natural fractures) with a cultured microbial solution at specific temperature, time, and growth conditions. Subsequently, we obtain the properties of the fractured shale rock samples impacted by this microbial process. We investigate the impact of the mechanical responses due to the microbial process, on the long-term integrity and storage potentials of CO2 in shale reservoirs. Our results suggest that in Eagle Ford, Marcellus, and Niobrara shale formations, microbially-altered local and bulk mechanical properties can enhance the long-term caprock integrity and CO2 storage security by: (1.) Increasing the localized (+19% unconfined compressive strength, −20% Poisson’s ratio, +35% fracture toughness) and bulk (+50% unconfined compressive strength, −13% Poisson’s ratio) mechanical integrity; (2.) Decreasing permeability (−93%) and porosity (−38%); (3.) Altering the clay mineral content (−56%), calcite content (+21%), and morphology; (4.) Occluding microfractures; and (5.) Mitigating any potential leakage to the atmosphere through the caprock. This study considers the heterogeneity of shales, and provide valuable insights and viable assessment in solving the long-term GCS application in depleted hydrocarbon reservoirs.

A fully coupled seepage–heat transfer model including a dynamic heat transfer coefficient in fractured rock sample with a single fissure

Conventional seepage–heat transfer models in simulating the heat transfer between fluid and rock in fractures mainly involve one-way coupling and do not consider the influence of temperature on the seepage. Moreover, it is an enormous challenge to define parameters of the explicit heat transfer between the rock and fluid. In order to resolve these shortcomings, a two-way fully coupled model of the seepage–heat transfer in the fractured rock was established in the present study. Based on the original geometric structure of the experimental device, combined with the actual engineering scale, the local dynamic heat transfer coefficient of the fractured rock was established, which is related to the fracture aperture, flow velocity and thermal parameters. Then, the proposed model was verified through the experiment and excellent agreement was achieved in this regard. It was found that the dynamic heat transfer coefficient changes the temperature distribution of fluid and rock in the original static heat transfer coefficient fracture system. The proposed model simplifies the parameters required for the calculation of the heat transfer coefficient. These parameters are related to characteristic variables, such as velocity and rock temperature, and can be simply obtained from standard laboratory tests.

Shear Induced Seepage and Heat Transfer Evolution in a Single-Fractured Hot-Dry-Rock

In the enhanced geothermal system (EGS), the injected fluid will induce shear sliding of rock fractures (i.e., hydroshearing), which consequently, would increase the fracture aperture and improve the heat transfer efficiency of the geothermal reservoir. In this study, theoretical analysis, experimental research and numerical simulation were performed to uncover the permeability and heat transfer enhancement mechanism of the Hot-Dry-Rock (HDR) mass under the impact of shearing. By conducting the direct shear test with the fractured rock samples, the evolution process of fracture aperture during the shearing tests was observed, during which process, cubic law was adopted to depict the rock fracture permeability. To investigate the seepage characteristics and temperature distribution of the fractured HDR under the influence of shearing, a simulation study of shear-seepage-heat transfer in a fractured rock mass has been conducted to validate the observed shear-induced fracture dilation during the direct shear test. The results demonstrate that (1) the hydroshearing increases the dilation of granite fracture and enhances the permeability of the HDR rock mass, while the temperature around the HDR fracture will reduce. (2) Fracture roughness is of vital importance to enhance the permeability during the shearing tests. To be more specific, a rougher fracture always implies a higher permeability and a greater heat extraction efficiency. (3) The shear induced heat extracting efficiency is dominated by the increased fluid flux in the earlier period of the EGS reservoir, and this efficiency is controlled by the outlet water temperature since the fluid flux becomes stable after the shearing test. Therefore, balancing the hydroshearing enhanced heat extraction efficiency and EGS reservoir lifespan would be significant to the sustainable development and utilization of geothermal energy

CO2 Foam and CO2 Polymer Enhanced Foam for Heavy Oil Recovery and CO2 Storage

Enhanced oil recovery (EOR) from heavy oil reservoirs is challenging. High oil viscosity, high mobility ratio, inadequate sweep, and reservoir heterogeneity adds more challenges and severe difficulties during any EOR method. Foam injection showed potential as an EOR method for challenging and heterogeneous reservoirs containing light oil. However, the foams and especially polymer enhanced foams (PEF) for heavy oil recovery have been less studied. This study aims to evaluate the performance of CO2 foam and CO2 PEF for heavy oil recovery and CO2 storage by analyzing flow through porous media pressure profile, oil recovery, and CO2 gas production. Foam bulk stability tests showed higher stability of PEF compared to that of surfactant-based foam both in the absence and presence of heavy crude oil. The addition of polymer to surfactant-based foam significantly improved its dynamic stability during foam flow experiments. CO2 PEF propagated faster with higher apparent viscosity and resulted in more oil recovery compared to that of CO2 foam injection. The visual observation of glass column demonstrated stable frontal displacement and higher sweep efficiency of PEF compared to that of conventional foam. In the fractured rock sample, additional heavy oil recovery was obtained by liquid diversion into the matrix area rather than gas diversion. Aside from oil production, the higher stability of PEF resulted in more gas storage compared to conventional foam. This study shows that CO2 PEF could significantly improve heavy oil recovery and CO2 storage.

Natural analogue evidence for controls on radionuclide uptake by fractured crystalline rock

Fractured Crystalline Rocks (FCR) are being considered in several countries as hosts for radioactive waste repositories. In FCR, radionuclides may be transported relatively rapidly by bulk groundwater flow through open fractures, but much more slowly by diffusion through porewater in the rock matrices. Rock matrix diffusion (RMD) is the diffusion of radionuclides in the aqueous phase, between open fractures and rock matrices. Sorption or co-precipitation on the fracture surfaces and walls of the matrix pores causes further radionuclide retardation. RMD may be important in a repository's safety case and has been investigated by many published short-term (to a few years) laboratory and in-situ experiments. To improve understanding over longer timescales, we investigated evidence for RMD of several natural radioelements, and radioelement analogues, in five exemplar fractured crystalline rock (FCR) samples aged between c. 70 Ma and c. 455 Ma. The sample suite consisted of two samples of Borrowdale Volcanic Group (BVG) meta-tuff from northwest England, a sample of Carnmenellis Granite from southwest England and two samples of Toki Granite from central Japan. Uptake or loss of the studied elements is limited to an altered damage zone in each sample, coupled to mineral alteration processes. These zones are most extensive (a few tens of millimetres) in the Toki Granite samples. We also found unstable primary igneous minerals to persist in the immediate wallrocks of fractures in studied granite samples, suggesting that pores were not permanently water saturated in these samples. Although only a small sample suite was studied, the results show that while RMD may be important in some kinds of FCR, in others it may be negligible. Site-specific information is therefore needed to determine how much reliance can be placed on RMD when developing a safety case.

Experimental Investigation on Permeability Evolution of Granite Samples Containing a Grout Infilled Fracture under Triaxial Compression

Grouting has been widely used in rock engineering to improve the stability and strength of fractured rock mass, as well as to decrease the permeability of fractured rock mass. Fractures infilled with grout may endure different in-situ stress conditions during the construction and operation life of tunnels and other underground spaces in fractured rocks. To study the stress induced changes in permeability of fractures infilled with grout, we carried out triaxial compression tests on the fractured rock samples infilled with grout, during which the permeability of fractured rock samples was measured. Both planar and rough fractures with different apertures were tested. The results showed that the permeability of infilled fracture samples increases with increasing grout width, but the strength of infilled fracture samples decreases with increasing grout width. The evolution of permeability is highly related with the volumetric strain. The permeability decreases slightly with increasing deviatoric stress from 0 to crack damage stress, followed by drastic enhancement in permeability with further increasing deviatoric stress. The sample’s permeability is enhanced by about 5~10 times at the peak deviatoric stress compared with the initial value at 5MPa deviatoric stress.

Heat transfer in a 3D rough rock fracture with heterogeneous apertures

A clear understanding of the convective heat transfer characteristics in the three-dimensional (3D) rough rock fracture is important for evaluating heat recovery in fractured reservoirs. A granite sample containing a Brazilian-induced artificial fracture, which was tested at increasing normal compressive stresses, was adopted to build 3D numerical models for rough rock fractures. At each stress level, flow-through simulation tests with different injection velocities were performed to examine the effect of fracture geometrical alterations induced by changing stresses on the heat transfer processes within hot fractured rock samples. Meanwhile, a hypothetical rough fracture and a parallel plate fracture, with an equivalent mechanical aperture to the artificial fracture, were constructed for the same heat transfer simulations as references. The results show that the flow and heat transfer behaviors in three types of fractures are significantly different, and the difference becomes more obvious as the normal stress and flow velocity increase. The alterations in asperity contacts and void spaces due to stress changes increase the heterogeneities of the distributions of streamlines and water temperatures in the artificial fracture. The tortuosity induced by the global fluctuation of the surface roughness in the hypothetical fracture cannot describe the flow and heat transport in the artificial fracture. The heat transfer coefficient in three types of fractures decreases non-linearly as the hydraulic aperture increases. Under the same hydraulic aperture, the heat transfer coefficient is larger in the hypothetical fracture than in the other two fractures. The channeling flow within the artificial fracture under high normal stresses weakens the heat transfer, resulting in the smallest heat transfer coefficient compared to the other two fractures. An empirical model was developed to describe the relationship between the normalized heat transfer coefficient and the hydraulic aperture, and this model was validated well using the published heated flow-through experimental results.

Scaling of Imbibition Front Dynamics in Heterogeneous Porous Media

AbstractWe report a quantitative study on the dynamics of countercurrent imbibition in fractured rock samples with the application of CO2 geosequestration, as the rock matrix is saturated with CO2 and brine exists in the adjacent microfracture. Direct pore‐scale simulations of spontaneous and forced imbibition in a three‐dimensional heterogeneous porous matrix enable us to capture the effect of microscopic properties on the imbibition dynamics as these pore‐level descriptions are ignored in the existing coarse‐grained models. We have established new scaling classes for the propagation of the center of mass of the imbibition front and fluid‐fluid interface broadening. It has become evident that the wettability is a major factor which leads to anomalies in these scaling laws. While the governing capillarity under the water‐wet condition induces fast roughening dynamics of the interface, the adverse effect of capillary pressure under the intermediate‐wet condition leads to a damping effect on the imbibition front fluctuations.

Matrix permeability measurement from fractured unconventional source-rock samples: Method and application

Shale matrix permeability is one of the most important parameters for characterizing a source rock reservoir and for predicting hydrocarbon production. The low permeability value and the presence of induced fractures during core retrieval and transportation make the accurate measurement of the true permeability values for source rocks a significant challenge for the industry. The steady state flow method and the transient pressure pulse decay method on core plug samples mainly measure the permeability of fractures when fractures are present. While the Gas Research Institute (GRI) method that uses pressure decay on crushed rock samples was designed to overcome this difficulty associated with the induced fractures, its measurement results are reported to be sensitive to the particle size of crushed rock samples and also need correction of Knudsen diffusion effect. Moreover, the GRI method is limited to the unconfined stress condition.This work develops a practical method to measure the matrix permeability values from fractured source rock samples by extending the commonly used pressure pulse decay method. A source rock sample with fractures can be more accurately described by a dual-continuum system consisting of a fracture continuum and a matrix continuum. During the pulse decay test, the initial flow across the source rock sample is dominated by the fracture continuum because it has much higher permeability values than those for the rock matrix. Thus, the initial gas pressure signals from the test are used to estimate the fracture permeability. During the late-stage of a pulse decay test, the flow process within the rock sample is controlled by the rock matrix. The observed pressure signals at this stage are used for estimating matrix permeability. The method is based on the analytical solution to gas flow in the fractured rock sample and relatively simple to apply in practice. Both fracture and matrix permeability's dependence on the effective stress can be assessed with this method.

Sampling and preparation of c.200 mm diameter cylindrical rock samples for geomechanical experiments

Experimental investigation of rock mechanical properties of real and artificial samples often requires much care and attention to detail during sample preparation. This especially applies to high fidelity state of the art complex experimental apparatuses where sample tolerance is low due to the complexity of the measuring and stress control devices as well as the nature of the experiments to be conducted. Although sometimes mundane, the sample preparation methodology is as equally important as the experimental apparatus itself, and can require several new technological developments. The methodology and technical developments required to prepare realistic heterogeneous, fractured and natural reservoir analogue rock samples for coupled thermo-hydro-mechanical-chemical process experimental investigation is described here. We present the sample recovery and preparation procedures for large (c.200 mm diameter), cylindrical samples of 200 mm ± 5 mm length, with variable composition and mechanical properties e.g. rock strength, existing fractures/fracture networks, macro-porosity, or lithic fragments. Although the technology demonstrated is for a specific application, the procedures developed, equipment and methodology are applicable to multiple sizes of sample requirements.

Micromechanical analysis of hydraulic fracturing in the toughness-dominated regime: implications to supercritical carbon dioxide fracturing

Supercritical carbon dioxide (SC-CO2) is considered as an ideal non-aqueous fracturing fluid due to its superior properties of liquid-like density, gas-like viscosity, high compressibility, and diffusivity. This study aims to investigate the micromechanical behavior of SC-CO2 fracturing in both intact and fractured rock samples by using a coupled fluid-solid discrete element method (DEM) model. A new numerical algorithm for hydraulic fracturing in the toughness-dominated regime is developed by assuming that the pressure in the whole fracture is uniform. This new numerical algorithm could achieve a much higher computational efficiency compared with the conventional hydromechanical scheme in DEM. Hydraulic fracturing cases using high-viscosity fracturing fluid are also performed for comparison. The results indicate that the fracture propagation induced by SC-CO2 tends to be less smooth and continuous, more asymmetric, and tortuous compared to that induced by viscous fluid. Besides, the low-viscosity fluid like SC-CO2 can lead to a lower breakdown pressure, and the fluid leak-off into the rock matrix can result in a lower breakdown pressure and higher fracture propagation pressure. The simulations also illustrate that SC-CO2 fracturing tends to create a more complex and productive fracture network if the pre-existing natural fractures are involved. As a result, we can conclude that SC-CO2 could be an alternative fracturing fluid to induce a more effective fracture network for hydrocarbon production.

Upscaling 3D coupled hydro mechanical properties of fractured porous rocks

A methodology for upscaling 3D coupled Hydro-Mechanical (HM) properties of fractured porous rocks is developed theoretically, tested on synthetic fractured rock samples, and applied to a real site. The upscaled HM equations take into account the HM coupling in the dual matrix/fracture medium, comprising the cracks system as well as the intact porous matrix, yielding the equivalent stiffness tensor and two different sets of equivalent tensorial Biot coefficients Bij(I) and Bij(II) and moduli M(I) and M(II) for the upscaled system (I and II become identical only under certain hypotheses). We provide detailed theoretical expressions in the general tensorial case,and in the particular case of statistically homogeneous and isotropic cracks. The real site application is performed in a damaged and fractured claystone around the GMR gallery (Meuse/Haute-Marne Underground Research Laboratory). The geometric structure of the fracture set around the "Excavation Damage Zone" of the GMR gallery is described by a hybrid model comprising: (i) a set of 10 000 fissures with radially inhomogeneous statistics (size, thickness and density increasing towards the wall) and; (ii) a deterministic set of large curved fractures, periodically spaced along the axis of the gallery according to a 3D chevron pattern. Both "3D" and "2D transverse" distributions of the upscaled coefficients are calculated, and displayed using spheres or ellipsoids. Global tensorial coefficients are also obtained by upscaling the entire annular fractured zone. Equivalent isotropic coefficients are extracted from these tensors: Young modulus E, bulk modulus K, Lamé shear modulus μ, Poisson ratio ν, and HM coupling coefficients: the Biot coefficient B and the Biot modulus M. In all cases considered, we discuss the impact of the degree of fissuring and fracturing on the upscaled stiffness and hydro-mechanical coefficients.