Fractured Shale Research Articles

Distribution and effect of retained fracturing fluid in pores and fractures of shale: Experimental and field study

Large amounts of fracturing fluid retained in the multi-scale transport channel for shale gas after fracturing, such as the hydraulic fracture networks, natural fractures, matrix pores. Therefore, the distribution and occurrence state of retained fracturing fluids greatly affect shale gas production. The flowback rate of multi-scale pores and fractures of shale and the distribution characteristics of retained fracturing fluid in multi-scale pores and fractures of shale are revealed through the imbibition and flowback experiment of shale with variable fracture width and NMR test, the occurrence state of the retained fracturing fluid during the production process is clarified through flowback experiment with shale sample in parallel and microscopic observation, and suggestions for regulating the retained fracturing fluid under reservoir conditions are put forward through rich field data analysis. The pores with diameter larger than 100 nm are the main contribution space of the flowback fracturing fluid, which provide 74.33% of the flowback fluid. 82.96% retained fracturing fluid was retained in the mesopores (2 nm < pore diameter <50 nm), which are the main existing space for retained fracturing fluid. The main occurrence states of the retained fracturing fluid are liquid bridges, continuous water films or porphyritic water films. The suggestions for regulating the retained fracturing fluid under reservoir conditions are adding drainage aids to reduce the retention of fracturing fluid during fracturing process, reasonably determining the shut-in time to improve the flowback rate in pores and fractures of diameter >0.1 μm during shut-in process, and prolonging the duration of the dominant gas seepage channel and expanding the time of pure liquid seepage by increasing pressure difference during production process.

Laboratory investigation on cryogenically induced fractures in shale with beddings

Hydraulic fracturing is essential for effective production of shale oil and gas reservoirs. Liquid nitrogen (LN2) fracturing is promising to enhance oil and gas production. However, the cryogenically fracturing mechanisms of LN2 in shale are still not clear, which are analyzed through combining the laboratory experiments and numerical simulations in this work. Laboratory fracturing shales using the cryogenic LN2 are carried out with the digital image correlation (DIC) recording the fracturing processes. Three typical modes of induced fractures in shale with beddings are observed, including the splitter (mode-S), arrester (mode-A) and divider (mode-D). The mechanisms and detailed fractures of the three modes are analyzed, which strongly depend on the weak plane and thermal conductivity anisotropy. A numerical model is developed to simulate the thermal-mechanical coupling, based on which the applicability of the tensile strength criteria (TSC) and fracture toughness criterion (FTC) are analyzed. It shows that TSC and FTC determine the lower and upper bounds of the induced fracture lengths. TSC is suitable to specify the lengths of mode-A fractures, while FTC is applicable to specify the lengths of mode-S and mode-D fractures. The developed model is used to simulate the branch fracture propagations in a shale reservoir in Triassic Yanchang Formation under the in-situ temperature and stress conditions. The results suggest that mode-D fractures are mostly induced by the cryogenic LN2, which can form a complex fracture network in shale. The findings on fracture lengths and modes can provide references for the shale fracturing treatments with LN2.

Study on stress sensitivity of bedding fractures and sand-filled fractures in shale oil reservoirs

Shale oil reservoirs are characterized by low porosity and ultra-low permeability, and hydraulic fracturing technology is needed to realize industrial oil flow. The sand-filled fractures formed in the shale fracturing process and the bedding fractures developed in the reservoir itself interact to form a shale oil fracture network system, which is the main migration channel for shale oil production. Studies have shown that the higher the degree of fracture development, the stronger the stress sensitivity of the reservoir, and the stress sensitive damage is an important factor that causes the decline of reservoir seepage capacity during shale oil production. In this paper, based on the shale samples in the Jiyang Depression, the flow capacity experiments of bedding fractures and sand-filled fractures were carried out to analyze the stress sensitivity characteristics of shale bedding fractures and sand-filled fractures, and to clarify the seepage capacity decline mechanism of complex fractures in shale. Provide experimental data and theoretical support for productivity prediction of block shale reservoirs after fracturing.

Numerical Simulation of Hydraulic Fractures Intersecting Natural Fractures in Shale With Plastic Deformation

Numerical Simulation of Hydraulic Fractures Intersecting Natural Fractures in Shale With Plastic Deformation

Simulation of the Production Performances of Horizontal Wells with a Fractured Shale Gas Reservoir

The production performances of a well with a shale gas reservoir displaying a complex fracture network are simulated. In particular, a micro-seismic cloud diagram is used to describe the fracture network, and accordingly, a production model is introduced based on a multi-scale flow mechanism. A finite volume method is then exploited for the integration of the model equations. The effects of apparent permeability, conductivity, Langmuir volume, and bottom hole pressure on gas well production are studied accordingly. The simulation results show that ignoring the micro-scale flow mechanism of the shale gas leads to underestimating the well gas production. It is shown that after ten years of production, the cumulative gas production difference between the two scenarios with and without considering the micro-scale flow mechanisms is 19.5%. The greater the fracture conductivity, the higher the initial gas production of the gas well and the cumulative gas production. The larger the Langmuir volume, the higher the gas production rate and the cumulative gas production. With the reduction of the bottom hole pressure, the cumulative gas production increases, but the growth rate gradually decreases.

EVALUATION OF SHALE GAS EXPLORATION BY MICROSTRUCTURE BEHAVIOR AND SHALE PERMEABILITY BASED ON FRACTAL THEORY AND UNDER MULTI-FIELD EFFECTS

The key to shale gas exploration is the characterization of gas migration under the combination of multiple factors. To address the long-standing energy challenge of rapidly and accurately quantifying the behavior of natural fractures and matrix pores in shale at an engineering scale in interaction with gas migration. This study proposes an interdisciplinary model for shale gas extraction by adopting fractal theory. Five innovative microstructural parameters are developed to characterize the size and scale of natural matrix pores/fractures in shale, so as to investigate the contributions of fractal distributed pores and fractal power-law distributed fractures to shale gas extraction. The present results of the proposed model are consistent with the exploitation state of the UK Bowland Shale #114 well. The evolution of the shale microstructure will lead to changes in gas migration behavior throughout the reservoir and in turn affect shale stress, temperature and gas adsorption–desorption effect, and finally have a significant impact on permeability. It is found that in the present analysis of the entire Bowland Shale, the overall permeability changes by 10.8% with the evolution of fractal distributed pores and by 41.3% with the evolution of fractal power-law fractures. This work provides a new approach for rapidly exploring the behavior of shale fractures and matrix pores at engineering scales. This work also offers a new and practical baseline for shale gas extraction assessment and fossil energy management.

Study on stress-strain effect on gas production from shale formation

The extent of gas production from shale reservoirs depends heavily on the stress-strain impact on the reservoir. The reservoir's developed stress allows the rocks to fracture naturally. The developed fractures across the reservoir blocks enhance the gas flow from the reservoir towards the wellbore. This review paper provides a comprehensive overview of the technologies and engineering of shale formation permeability behavior and the impact of induced stress and strain. The impacts of stress and strain on the permeability of shale are also studied. This paper presents the effect of stress-strain analysis on gas recovery due to various gas injections, the dependence of gas shale fracture permeability on effective stress and reservoir pressure, analyzing the consequences of various gas injection rates on stress and strain, and the result of stress-strain elongation or variation on hydrocarbon recovery.

A semi-analytical approach to nonlinear multi-scale seepage problem in fractured shale gas reservoirs with uncertain permeability distributions

To investigate the influence of the inhomogeneity and uncertainty of porosity and permeability on gas production, a mathematical model for shale gas nonlinear seepage problem with random distributions of inhomogeneous equivalent porosity and permeability is established. Based on the statistical characteristic of the artificial fracture density distribution and incompletely controllable hydraulic fracturing technology, the random distribution models of continuous inhomogeneous equivalent porosity and permeability are proposed to simulate the uncertainty of the porosity and permeability distribution in SRV. Coupled with multi-scale flow and adsorption effects, a mathematical model of shale gas nonlinear seepage problem is presented to address the inhomogeneity and uncertainty of equivalent porosity and permeability. By Boltzmann transformation and local homogenization approximation, a semi-analytical method and the corresponding explicit iterative scheme are developed.Simulation results match well with field data, which verifies the validity of the presented mathematical model and approach. The morphology of the SRV has significant influence on gas production, followed by the uncertainly of the permeability distribution and the hydraulic fracture half-length. When the horizontal width reaches its maximum in the front part of the macro-fracture zone, shale gas production reaches its peak and increases by 60%. The uncertainty of the permeability distribution is resulted from the deviation of the hydraulic fracture. As the deviation angle increases from 0° to 10°, shale gas production drops by 16%. The greater the gas production, the bigger the drop rate. When the hydraulic fracture half-length is 50 m, gas production reaches its peak, however it drops by 13% if the hydraulic fracture half-length increases to 70 m.

Research on Rock Physics Modeling Methods for Fractured Shale Reservoirs

The Sichuan Basin is a significant region for exploration and development of shale gas in China, and it is essential to clarify the impact of deep shale gas reservoir parameters on cost-effective development at scale to ensure national energy security. Rock physics modeling is a significant means of communicating the physical and elastic parameters of rocks. A rock physics modeling method applicable to fractured shale gas reservoirs is proposed for the current situation of complex fluid relationships in shale gas reservoirs and unclear characteristics of gas identification seismic response. In this paper, based on the Self Consistent Approximation (SCA) model and the differential effective medium (DEM) model, the anisotropic source of shale is used as a starting point to add bound water, kerogen, clay, and brittle minerals, the Schoenberg linear slip theory is used to add fracture disturbance effects, and then the Brown–Korringa model is used to perform fluid replacement under anisotropic conditions. Finally a rock physics model applicable to fractured shale gas reservoirs is obtained, and the established rock physics model is used for analysis of elastic parameters, Thomsen parameters, and fracture weakness parameters. Rock physics tests were performed on shale in southern Sichuan as an example. The experimental results show that the model established by the process can accurately invert the longitudinal and transverse wave velocities of the shale, which can provide a conceptual basis for the study of fractured shale gas reservoirs.

Modeling the vulnerability of groundwater to pollution in a fractured shale aquifer in SE Nigeria using information entropy theory, geospatial, and statistical modeling approaches

Modeling the vulnerability of groundwater to pollution in a fractured shale aquifer in SE Nigeria using information entropy theory, geospatial, and statistical modeling approaches

Comparison of shale fracture toughness obtained from scratch test and nanoindentation test

Fracture toughness is an important parameter in the hydraulic fracturing design, which is the major tool in the development of unconventional resources. Laboratory techniques for fracture toughness measurements usually require intact core samples and time-consuming sample preparation. The objective of this study is to compare fracture toughness values obtained by two less conventional methods: nanoindentation test and scratch test, which could facilitate the evaluation of this important parameter on smaller samples and at different scales. A set of 5 Antrim shale samples characterized by different mineral compositions is used to test this approach. For the scratch test, the fracture toughness and hardness are linearly correlated and show the same changing trend along the scratch length on all tested samples. For the nanoindentation test, the fracture toughness also increases with the increasing hardness. Most importantly, the results show that the fracture toughness values derived from these two methods are very similar, despite the difference in the scale of the measurements. This study is the first to compare fracture toughness between scratch and nanoindentation tests. Our results suggest that these two methods can be used to quickly evaluate fracture toughness from the shale core intervals containing both intact and nonintact parts.

Effect of tectonic reworking on shale fracturing and gas preservation in the upper Yangtze block, south China

Lower Silurian shale, with a variable gas enrichment, in the Upper Yangtze Block is used to evaluate the response of shale fracturing and gas loss to differential tectonic reworking. The Meso-Cenozoic uplift process was reconstructed by apatite fission track, zircon fission track, apatite (U–Th)/He and zircon (U–Th)/He dating. During the Mesozoic, due to Pacific-Eurasian convergence the Upper Yangtze Block underwent northwestward progressive deformation, with a periodic uplift process characterized by rapid uplift during the Early-Late Cretaceous, followed by subsequent slow uplift. The Cenozoic uplift process was characterized by rapid uplift in the Sichuan Basin as a result of Indian-Asian collision and inherited slow uplift in the Hubei–Hunan–Guizhou fold belt in response to Pacific-Eurasia convergence. The pressure evolution of the Lower Silurian shale was reconstructed using fluid inclusions and variation in shale gas content was quantified. By comparing the differences in pressure reduction, fracturing, and shale gas loss we explore the effect of tectonic reworking on shale fracturing and gas preservation in relation to deformation, uplift and the maximum burial depth. Deformation increases fracture development and reduces the sealing capacity of the shale. Uplift amplitude is a major controlling factor. Large amounts of uplift resulted in clear fracturing and massive gas loss. Triaxial fracture tests show that the maximum burial depth is associated with shear fractures in the shale and thereby affects shale gas preservation during uplift. The differences of slip deformation, tectonic uplift amplitude and the maximum burial depth of shale resulted in the differential enrichment of the Lower Silurian shale gas.

Effects of in-situ temperature in deep reservoirs on shale fracture properties

The development of deep shale reservoirs is a major strategic demand to meet national energy requirements. However, the high-temperature characteristics of in-situ reservoirs make it difficult to understand the fracture properties of deep shale clearly, which restricts the development of reservoir stimulation technologies. Based on digital image correlation method and fracture mechanics theory, this study investigates the issue of rock fracture parameters testing in high-temperature environments. This study builds an in-situ high-temperature environment rock fracture visualization test system that can dynamically capture crack-tip initiation and propagation. Further, the influence law of temperature on shale fracture behaviour has been analysed. The results have shown that, under the condition of no confining pressure, with increases in the temperature, the macroscopic material stiffness and fracture toughness of shale increase; this shows that high temperatures inhibit crack initiation and propagation. Further, with increases in the temperature, the temperature-dependent CTOD is positively correlated with the fracture toughness, the simultaneous increase of these two factors leads to the increase of local plasticity in the crack tip region. When considering high-temperature conditions alone, the fracture tip still experiences a small-scale yield state; the research conclusions can provide a reference for studies on deep reservoir fractures.

Guar Gum Stimulates Biogenic Sulfide Production in Microbial Communities Derived from UK Fractured Shale Production Fluids.

Shale gas production fluids offer a window into the engineered deep biosphere. Here, for the first time, we report on the geochemistry and microbiology of production fluids from a UK shale gas well in the Bowland shale formation. The composition of input fluids used to fracture this well were comparatively lean, consisting only of water, sand, and polyacrylamide. This formation therefore represents an interesting comparison to previously explored fractured shales in which more additives were used in the input fluids. Here, we combine cultivation and molecular ecology techniques to explore the microbial community composition of hydraulic fracturing production fluids, with a focus on the potential for common viscosity modifiers to stimulate microbial growth and biogenic sulfide production. Production fluids from a Bowland Shale exploratory well were used as inocula in substrate utilization experiments to test the potential for polyacrylamide and guar gum to stimulate microbial metabolism. We identified a consortium of thiosulfate-reducing bacteria capable of utilizing guar gum (but not polyacrylamide), resulting in the production of corrosive and toxic hydrogen sulfide. Results from this study indicate polyacrylamide is less likely than guar gum to stimulate biogenic sulfide production during shale gas extraction and may guide planning of future hydraulic fracturing operations. IMPORTANCE Shale gas exploitation relies on hydraulic fracturing, which often involves a range of chemical additives in the injection fluid. However, relatively little is known about how these additives influence fractured shale microbial communities. This work offers a first look into the microbial community composition of shale gas production fluids obtained from an exploratory well in the Bowland Shale, United Kingdom. It also seeks to establish the impact of two commonly used viscosity modifiers, polyacrylamide and guar gum, on microbial community dynamics and the potential for microbial sulfide production. Not only does this work offer fascinating insights into the engineered deep biosphere, it could also help guide future hydraulic fracturing operations that seek to minimize the risk of biogenic sulfide production, which could reduce efficiency and increase environmental impacts of shale gas extraction.

New insight into fracturing characterization of shale under cyclic soft stimulation: A lab-scale investigation

The cyclic soft stimulation (CSS) is a new method of reservoir reforming for which the mechanism of fracturing crack propagation is ambiguous with regard to the alternating fluid pressure. This study aims to provide a comprehensive understanding of the fracturing mechanical characterizations of CSS under different magnitudes and amplitudes of the alternating fluid pressure. Acoustic emission (AE) is recorded to investigate the damage evolution under CSS based on the b value analysis of AE. Experimental results reveal the difference of pressure in a crack under different cyclic fluid pressure conditions. The AE results show that the maximum radiated energy under CSS tends to be reduced with the increase in the amplitude and magnitude of the alternating fluid pressure. The finishing crucial touch is that the crack extending criterion under CSS is proposed, which combines the injection parameters, the rock properties and in-situ stress. According to the crack extending criterion, the fluctuation fluid pressure causes the reduction of a critical crack extending pressure, and the CSS causes the crack to initiate and propagate under low fluid pressure. Under a higher-value magnitude of alternating fluid pressure, the cyclic times of CSS is less for the crack initiation. In supplement to the crack extending criterion, a distinct relationship between the radiated energy and the cyclic fluid pressure also is established based on the energy dissipation criterion. These new findings provide an insight into the determination of crack extending criterion under CSS for efficiently implementing shale fracturing.

Efficiency and limitation of vertical electrical sounding in evaluation of groundwater potential in fractured shale terrain: a case study of Abakaliki Area Lower Benue Trough Nigeria.

Water supply in the Abakaliki Area of Southeastern Nigeria and its environs has been a source of worry. The area is marked by a series of abortive boreholes, sparsely productive boreholes, and failed water wells. These problems are a result of the direct and indirect function(s) of the prevailing geology of the area which is underlain by aquiclude (Abakaliki Shale, lenses of siltstone and sandstone) dominantly indurated shale. Sixteen (16) vertical electric sounding (VES) were acquired using a resistivity meter. The acquired data was plotted on a bi-log graph using INTERPEX and SURFER. Quantitative and qualitative interpretations were used in preparing geoelectric layers and layer parameter maps. Three (3) to five (5) geoelectric layers were delineated. The first layer is a compacted lateritic overburden. VES 1, 3, 4, 6, 11, and 15 were observed to have thin layers of clay with very low apparent resistivity values with an average of 2.642 Ωm and average thickness of 1.08m, and depth generally less than 10m. Fractured and non-fractured/baked shale layers were dominant in all points of the study area. The non-fractured consolidated shale has a high apparent resistivity value ranging from 180.3 Ωm to 770.42 Ωm with an average of about 339.53 Ωm while the fractured shale has an average resistivity value of about 21.36-68.79 Ωm and an average of 38 Ωm. The confinement of the aquifer implies that the indurated shale generally underly the Abakaliki Area. The low resistivity value of the fractured shale is an indication of the presence of fluid, possibly water. The results of this research implied that the application of exploration method(s) that could delineate the subsurface fractures should precede groundwater development in the area. Also, there is no specific depth to the water table in the area; however, the area has a generally shallow depth to groundwater on average.

Salinity and hydraulic retention time induce membrane phospholipid acyl chain remodeling in Halanaerobium congolense WG10 and mixed cultures from hydraulically fractured shale wells.

Bacteria remodel their plasma membrane lipidome to maintain key biophysical attributes in response to ecological disturbances. For Halanaerobium and other anaerobic halotolerant taxa that persist in hydraulically fractured deep subsurface shale reservoirs, salinity, and hydraulic retention time (HRT) are important perturbants of cell membrane structure, yet their effects remain poorly understood. Membrane-linked activities underlie in situ microbial growth kinetics and physiologies which drive biogeochemical reactions in engineered subsurface systems. Hence, we used gas chromatography-mass spectrometry (GC-MS) to investigate the effects of salinity and HRT on the phospholipid fatty acid composition of H. congolense WG10 and mixed enrichment cultures from hydraulically fractured shale wells. We also coupled acyl chain remodeling to membrane mechanics by measuring bilayer elasticity using atomic force microscopy (AFM). For these experiments, cultures were grown in a chemostat vessel operated in continuous flow mode under strict anoxia and constant stirring. Our findings show that salinity and HRT induce significant changes in membrane fatty acid chemistry of H. congolense WG10 in distinct and complementary ways. Notably, under nonoptimal salt concentrations (7% and 20% NaCl), H. congolense WG10 elevates the portion of polyunsaturated fatty acids (PUFAs) in its membrane, and this results in an apparent increase in fluidity (homeoviscous adaptation principle) and thickness. Double bond index (DBI) and mean chain length (MCL) were used as proxies for membrane fluidity and thickness, respectively. These results provide new insight into our understanding of how environmental and engineered factors might disrupt the physical and biogeochemical equilibria of fractured shale by inducing physiologically relevant changes in the membrane fatty acid chemistry of persistent microbial taxa. GRAPHICAL ABSTRACTSalinity significantly alters membrane bilayer fluidity and thickness in Halanaerobium congolense WG10.

Shale Microstructure Characteristics under the Action of Supercritical Carbon Dioxide (Sc-CO2)

Supercritical carbon dioxide (SC-CO2) is suitable to extract low-polar organics and to assist in the dissolution of pores and fractures in shale. In this work, we investigate the effect of temperature on the structure of five shale samples via high pressure reaction assisted with SC-CO2. Shale samples were analyzed using X-ray diffraction, field emission scanning electron microscopy, and ImageJ software. Due to the extraction of CO2, after Sc-CO2 treatment, carbonate and clay content decreased, while quartz and plagioclase increased slightly, which improved gas and oil flow in microscopic pores and shale cracks. Shale samples showed an increase in surface fracture area as experimental temperature increased. Since Sc-CO2 fluid density and solubility increase with temperature, more organics can be extracted from shale pores and fractures, resulting in newly formed pores and fractures. As a result, the threshold temperature for shale high-temperature Sc-CO2 cracking was confirmed to be 400 °C, and the fracture area increased by more than 45% at this temperature. Based on the findings of this study, Sc-CO2 technology can be used to potentially recover low-maturity shale oil efficiently.

Experimental Study on the Effect of Mineral Composition on Shrinkage Fractures: Implications for the Origin of the Diagenetic Shrinkage Fractures in Marine–Continental Transitional Shales

Natural fractures in shale have been commonly recognized as a vital factor in shale gas production. Diagenetic shrinkage fracture is an important type of natural fracture; however, its formation mechanism and prediction remain largely unexplored. Given the similarity between diagenetic shrinkage fractures and desiccation cracks, the influence of smectite, kaolin, quartz, and feldspar on shrinkage fractures were investigated using the physical simulation experiment of desiccation in this study. Here, water evaporation, surface cracks initiation and propagation processes were monitored during the whole desiccation. The morphological characteristics of cracks patterns were quantitatively described. Diagenetic shrinkage fractures of transitional shale with different minerals were observed using SEM. The results show that the development and morphology of cracks are affected by the mineral composition, and the sediments with higher clay content tend to form more complex fracture networks. We further propose a morphological prediction model for cracks and compare this model with diagenetic shrinkage fractures under SEM. We found that the effects of mineral composition on both were similar, with more well-developed diagenetic shrinkage fractures in those clay-rich samples. Taken together, this study implies the feasibility of using physical simulation experiment of desiccation cracks to invert diagenetic shrinkage fractures and provides new insights into the mechanism and development regularity of diagenetic shrinkage fractures.

Fractal Analysis for Wave Propagation in Combustion–Explosion Fracturing Shale Reservoir

The in-situ combustion–explosion fracturing technology in shale reservoirs can promote continuous fracture expansion with a radial detonation wave first converging into a shock wave and then decaying into an elastic wave. The transformation scale of the shale reservoir is determined by the range of wave propagation during combustion–explosion. As wave propagation paths are usually tortuous and fractal, the previous integer wave models are not competent to describe the wave propagation and estimate the impact range of the combustion–explosion fracturing process. This study develops two fractional wave propagation models and seeks analytical solutions. Firstly, a novel fractional wave model of rotation angle is proposed to describe the process of detonation waves converting into shock waves in a bifurcated structure. The radial displacement gradient of the detonation wave is represented by the internal expansion and rotation deformation of the shale. Secondly, another fractional wave propagation model of radial displacement is proposed to show the process of a shock wave decaying into an elastic wave. Thirdly, the proposed models are analytically solved through the fractional variable separation method and variational iteration method, respectively. Analytical solutions for rotation angle and radial displacement with fractal time and space are obtained. Finally, the impacts of the branching parameter of the detonation wave converge bifurcation system, aggregation order of detonation compression wave, and different types of explosives on the rotation angle of the shock wave are investigated. The propagation mechanism of the primary wave (P-wave) with time and space is analyzed. The analytical solutions can well describe the wave propagation process in fractured shales. The proposed fractional wave propagation models can promote the research of wave propagation in the combustion–explosion fracturing process of shale reservoirs.