Fractured Shale Research Articles

Analysis of Fracturing Expansion Law of Shale Reservoir by Supercritical CO2 Fracturing and Mechanism Revealing

The rapid expansion of reservoir fractures and the enlargement of the area affected by working fluids can be accomplished solely through fracturing operations of oilfield working fluids in geological reservoirs. Supercritical CO2 is regarded as an ideal medium for shale reservoir fracturing owing to the inherent advantages of environmental friendliness, excellent capacity, and high stability. However, CO2 gas channeling and complex propagation of fractures in shale reservoirs hindered the commercialization of Supercritical CO2 fracturing technology. Herein, a simulation method for Supercritical CO2 fracturing based on cohesive force units is proposed to investigate the crack propagation behavior of CO2 fracturing technology under different construction parameters. Furthermore, the shale fracture propagation mechanism of Supercritical CO2 fracturing fluid is elucidated. The results indicated that the propagation ability of reservoir fractures and Mises stress are influenced by the fracturing fluid viscosity, fracturing azimuth angle, and reservoir conditions (temperature and pressure). An azimuth angle of 30° can achieve a maximum Mises stress of 3.213 × 107 Pa and a crack width of 1.669 × 10−2 m. However, an apparent viscosity of 14 × 10−6 Pa·s results in a crack width of only 2.227 × 10−2 m and a maximum Mises stress of 4.459 × 107 Pa. Additionally, a weaker fracture propagation ability and reduced Mises stress are exhibited at the fracturing fluid injection rate. As a straightforward model to synergistically investigate the fracture propagation behavior of shale reservoirs, this work provides new insights and strategies for the efficient extraction of shale reservoirs.

Identification and parameter characterization of pores and fractures in shales based on multi-scale digital core data

Identification and parameter characterization of pores and fractures in shales based on multi-scale digital core data

Study on the influence of vertical stress difference coefficient on fracture characteristics of shale under high stress

AbstractTo study the effect of vertical stress difference coefficient on fracture characteristics of shale fracturing, high stress true triaxial hydraulic fracturing test was carried out. By analyzing the profile after fracturing, it was found that the area of hydraulic fracture increased with the increase of vertical stress difference coefficient, and the probability of shear fracture will increase when the stress difference coefficient was high. A high vertical stress differential coefficient exerts a strong control over the direction of crack propagation, while a low vertical stress difference coefficient is beneficial to improve the roughness of hydraulic fracture surface and promote the formation of complex fracture network. By analyzing the pump pressure curves of different tests, it was found that with the increase of vertical stress difference coefficient, the formation and expansion of hydraulic fractures were more difficult. The surface characteristics of hydraulic fractures were quantified based on three‐dimensional topography scanning technology, combined with fractal dimension and fracture area calculation method, the results showed that with the increase of vertical stress difference coefficient, the fractal dimension and fracture area decreased. Since shale is stratified, and the transformation of reservoir is mainly reflected in the enhancement of fracture complexity through tensile failure, Xsite discrete grid method was used to study the influence of fracture propagation behavior with different bedding strengths. The results showed that when the bedding tensile strength was high, hydraulic fractures were easy to pass through the bedding, and when the bedding tensile strength was low, hydraulic fractures were easy to be captured by natural fractures. In addition, tensile cracks were easy to form when the tensile strength of bedding was low, shear cracks were easy to form when the strength of bedding was high, and the fracture volume was larger when the strength of bedding was low. This study provides a theoretical basis for hydraulic fracturing in engineering.

Temporal evolution of fracture transport properties of fractured shales during long-term stress compaction

Understanding the temporal evolution of fracture transport properties in shales is of increasing importance for the long-term safe operation of many underground engineering projects. Here, a series of long-term (60 days each) water seepage and gas breakthrough experiments utilizing three shale samples, each with a single artificial fracture, was conducted to evaluate the time-dependent evolution of permeability and gas breakthrough pressure (GBP) under constant confining stress. Our results show continuous decreases in permeability and increases in GBP with time, and the rate of change in both is initially rapid but then slows. As the loading time increases, the permeabilities and GBPs become relatively constant. The inferred mechanisms are aperture reductions caused by mechanical creep and clay swelling of fractures. Pretest and posttest scans of fracture surfaces reveal some changes in asperity height, which demonstrates the occurrence of mechanical damage to the fracture during the experiments. Long-term stress compaction causes flow channels to narrow or even partially close over time. The fluid flow in confined fracture is controlled by some narrow apertures in the flow channels, which act as bottleneck points. The apertures at these points decrease to the micro-nano level under stress, significantly reducing fracture transport properties. While the fracture transport properties of fractured shale obviously decrease during long-term stress compaction, residual fracture voids still dominate the fluid flow over a longer period, and are especially prone to gas escape.

Coupling Upscaled Discrete Fracture Matrix and Apparent Permeability Modelling in DFNWORKS for Shale Reservoir Simulation

Modelling non-Darcy flow behaviour in shale rocks, composed of nanometer-sized pores and multi-scale fracture networks, is crucial for various subsurface energy applications. However, incorporating multiple physical mechanisms across numerous scales is not trivial. This work proposes an improved and practical upscaling workflow for coupling an Upscaled Discrete Fracture Matrix (UDFM) model and a pressure-dependent apparent permeability (Kapp) model to capture the effects of non-Darcy flow in multi-scale fractured shale reservoirs.First, a 3D DFN is upscaled into octree-refined continuum meshes, where equivalent rock parameters and rock-fluid functions are defined using the UDFM approach. Then, the flow simulation is coupled with a pressure-dependent Kapp updating scheme using an existing Kapp model and a multiple-restart technique. The effects of non-Darcy flow mechanisms (e.g., slip flow, transitional flow, Knudsen diffusion) are captured. The constructed models are then used to study the impacts of fracture network connectivity and pressure interference on production. The results of this new approach are compared against those obtained from another commercial package while preserving the advantages of DFNWORKS. Neglecting non-Darcy flow behaviours could significantly underestimate gas production and water recovery. It is illustrated that the nanoscale flow mechanisms help to enhance matrix-matrix and matrix-fracture flow. The constructed models are also utilized to study the effects of disconnected or isolated fractures, pressure interference, water retention, and shut-in durations on well performance. The proposed flexible strategies can be adopted in other commercial/open-source fractured-porous-media subsurface-flow simulation frameworks.

Study on the Fracture Extension Pattern of Shale Reservoir Fracturing under the Influence of Mineral Content.

This study aimed to investigate the effect of the microstructure of shale on fracture initiation and extension during hydraulic fracturing. The Longmaxi Formation shale reservoir in the Sichuan Basin was considered as the research object; its structure was modeled from a microscopic perspective, and a zero-thickness cohesive unit was embedded within the solid unit. Numerical simulations were performed to study the effect of mineral content on the microextension of the hydraulic fracture, extension behavior, and evolution law of shale. The results showed that changes in the mineral content resulted in changes in the forces between molecules within the minerals, which, in turn, affected the shale's brittleness. The percentages of brittle mineral content in the Long I, II, and III reservoir sections are 60.37, 47.60, and 53.56%, respectively. The fracture initiation pressures of the three reservoirs were 29.22, 31.42, and 30.22 MPa, respectively, and a linear correlation was found between the fracture initiation pressures and the brittle mineral contents of the reservoir sections. An increase in the reservoirs' percentage of brittle mineral content facilitated the fracture initiation, with a corresponding gradual decrease in the resistance to fracture initiation. The pore pressures of the fractures in the three reservoirs after fracture initiation were 0.90, 1.18, and 1.00 MPa, respectively. The larger the percentage of brittle minerals was, the lower was the fracture pore pressure. The greater the length, number, area, and width of the cracks were, the more likely they were to form longer and wider cracks. Hence, reservoirs with a high percentage of brittle minerals should be prioritized as the target formations for hydraulic fracturing operations. The results of this study reveal how the mineral content affects the extension of microscopic hydraulic fractures in shale reservoirs. As such, this work can provide a theoretical basis for rationally selecting a hydraulic fracturing operation layer in shale gas reservoirs.

Integrating in-situ hydraulic conductivity measurements and vertical electrical sounding for groundwater exploration in fractured shales within Alex Ekwueme Federal University Ndufu Alike (AE-FUNAI), South Eastern Nigeria

Integrating in-situ hydraulic conductivity measurements and vertical electrical sounding for groundwater exploration in fractured shales within Alex Ekwueme Federal University Ndufu Alike (AE-FUNAI), South Eastern Nigeria

Beyond the Surface: Non-Invasive Low-Field NMR Analysis of Microbially-Induced Calcium Carbonate Precipitation in Shale Fractures

Beyond the Surface: Non-Invasive Low-Field NMR Analysis of Microbially-Induced Calcium Carbonate Precipitation in Shale Fractures

The effects of three-dimensional bedding on shale fracture behavior: Insights from experimental and numerical investigations

The three-dimensional (3D) structural characteristics of the bedding planes exhibit strong anisotropy in the fracture behavior of shale. Previous studies on the anisotropy of shale fracture characteristics focused on 2D bedding effects, while 3D bedding effects that comprehensively consider bedding angles α and β have been rarely investigated. Therefore, the fracture behavior of shale with 3D bedding planes was studied based on laboratory experiments and numerical simulation. The results indicated that the apparent fracture toughness (AFT) of shale decreased with increasing bedding angle α. The fracture toughness of shale with a bedding angle of α = 0° was 2.26 MPa·m0.5 (β = 0°). However, when the loading angle α increased to 90°, the AFT decreased by 67.70 %. The increase in bedding angle β could improve the AFT significantly, and the higher the loading angle α, the more significant the improvement. The improvement amplitude at α = 90° reached 156.65 %. Due to the enhanced ability of shale to resist crack propagation by the bedding angle β, the trend of dissipation energy under the influence of 3D bedding effects was consistent with the AFT, and the two were positively correlated. Affected by the 3D bedding effect, three typical failure modes of shale have been observed: bedding failure, matrix failure, and mixed failure mode. Based on the calculation of the fractal dimension of the fracture surface, the roughness of the fracture surface under different failure modes was mixed failure > matrix failure > bedding failure. In addition, a 3D numerical model was established based on the DEM method, and the influence of new cracks on fracture toughness was considered after verifying the effectiveness of the numerical model. The length of newly formed cracks at the tip of shale prefabricated cracks exceeded 4.0 mm under critical load. Ignoring the length of new cracks will underestimate the fracture toughness of shale by about 30 %. Finally, the influence of 3D bedding planes on the crack propagation path was discussed.

CT imaging to study meso-structure evolution of fractured oil shale during in-situ pyrolysis by high-temperature water vapor injection

Preceding the in-situ pyrolysis of oil shale, it is imperative to undertake reservoir fracturing and modification. The intricate evolution of meso-structures in both the fractured fissures and the surrounding rock matrix, constrained by in-situ stress during the pyrolysis process, plays a significant role in influencing the efficiency of heat transfer and the extraction effectiveness of produced materials.This article is based on the Micro-CT scanning technology to study the microscopic structural evolution patterns of high-temperature water vapor in-situ pyrolysis in oil shale with fractured fissures.The main research findings are as follows: Firstly, pre-existing fracturing fissures facilitate the formation and development of vertical bedding fractures.The vertical bedding fractures first appear at 300 °C, with a relative frequency of 1.35%, occurring earlier than the temperature at which such fractures emerge in intact oil shale.Secondly, at 450 °C,there is a pivotal transition point in the evolution of pore-fracture systems, shifting from a predominant increase in length to a predominant increase in aperture.Below 450 °C, changes in pore-fractures are primarily manifested in terms of quantity and length. However, after 450 °C, there is a noticeable increase in the aperture of pore-fracture systems.Thirdly,at temperatures within the range of 300 °C–350 °C, a closure phenomenon occurs in fractures, predominantly involving micro and medium fractures. This closure predominantly affects smaller fractures, providing only a moderate retardation effect on the development of larger fractures. Moreover, in the temperature range of 500 °C–550 °C, there is interconnection observed between large and medium fractures. Pre-existing multiple large fractures are consolidated into a singular interconnected large fracture.Finally, at 450 °C, there is a significant increase in the connectivity of pore-fracture networks. Below 450 °C, the primary connected flow pathways consist of pre-existing fracturing fissures and newly formed parallel bedding fractures. Beyond 450 °C, a complex network of pores takes over as the main interconnected flow channel.

Frictional process, AE characteristics, and permeability evolution of rough fractures on Longmaxi shale with specific roughness under water injection

Studying the frictional process, acoustic emission (AE) characteristics and permeability evolution of fractures under water injection is fundamental for proposing theories and methods to control the stability of geological structures. In this work, the shear-flow test was conducted on Longmaxi shale fractures with different specific roughness to analyze the frictional deformation and mechanical properties of shale fractures during the frictional slip process, and the influence of effective normal stress and surface morphology was investigated. AE monitoring technology was also utilized to reveal the AE evolution during the frictional slip process. Based on the AF/RA values, AE signals were subjected to cluster analysis using the Gaussian Mixture Model (GMM) to distinguish and quantify the crack types and proportions during frictional slip. Combining the development of cracks and considering the ratio of worn area, a theoretical analytical expression was established to reasonably describe the friction-damage evolution, classifying the friction-damage during the frictional slip process into tensile and shear damage, respectively. Meanwhile, the permeability evolution of Longmaxi shale fractures was also studied in detail. A theoretical model capable of describing and predicting the permeability evolution was proposed under the condition that the permeability evolution and damage process of Longmaxi shale fractures correspond to each other. Finally, the interaction between asperities on shale fractures with different surface morphologies during the frictional slip process was discussed, and the influence mechanism of surface morphology on permeability evolution was revealed.

Development characteristics and controlling mechanism of different microfracture combinations in shale reservoir: A case study of silurian longmaxi formation in Weiyuan area

Fractures in organic-rich shale are important reservoir spaces and seepage channels of shale gas, and they are closely related to the gas-bearing properties of shale. The development characteristics and laws of fractures are of great significance in the exploration and development of shale oil and gas. This study examines organic-rich shales of the Wufeng–Longmaxi Formation in the Weiyuan area of the Sichuan Basin. On the basis of two-dimensional large-area multi-scale combination electron microscopy characterization and digital core platform technology, the development degree and distribution of different fractures are quantitatively characterized. The results show the following. (1) The shale of the Wufeng and Longmaxi formations developed a variety of fractures with different occurrences, sizes, and origins. According to the number and combination relationship between fractures of different occurrences, the shale can be divided into four fracture combination types: horizontal bedding fractures; vein fractures; reticular fractures; and ring fractures. Of these, the horizontal bedding fracture group has the largest number of samples and a higher average fracture surface porosity. (2) The degree of fracture development in the shale is affected by many factors, such as the laminar type, mineral composition, mineral particle size, mineral distribution, and total organic carbon, and the controlling mechanisms of different fracture combination types differ. Factors such as horizontal stratification, high clay mineral content, and uneven mineral particle size are conducive to the development of horizontal bedding joints. (3) Differences in the sedimentary environment affect the variation laws of the vertical fracture combination types and density. The total organic carbon and organic quartz content of the Long111 layer with deeper sedimentary water is higher, and the vein fracture formation is more developed than in other small layers, while the clay mineral content of the Long112 and Long114 layers with shallower sedimentary water is higher and the horizontal layer is more developed; the fracture combination type is dominated by the horizontal bedding fracture combination. At the same time, the fractures at the junction of each layer of the Long11 sub-member are the most developed because sea level rise and fall make the mineral particle size heterogeneity most prominent at the junction of the small layer.

Research on Shale Fracture Characteristics and Developmental Controlling Factors

Research on Shale Fracture Characteristics and Developmental Controlling Factors

Experimental study on the flow capacity of shale fractures with different degrees of sand filling

Experimental study on the flow capacity of shale fractures with different degrees of sand filling

ShaleSeg: Deep-learning dataset and models for practical fracture segmentation of large-scale shale CT images

Shale gas plays a crucial role as an exploration resource for human life, and its contribution to energy production has been steadily increasing. The success of hydraulic fracturing operations is highly dependent on the understanding of shale fracture propagation at large scales. Currently, the segmentation of large-scale shale fractures is primarily carried out through time-consuming manual or semi-automated methods. The identification of shale fractures is similar to image segmentation, in which deep-learning holds the potential to achieve accurate performance close to human experts. However, its application encounters challenges when dealing with large-scale shale images due to factors such as a lack of datasets, severe artifacts, and limited sample sizes. In this study, we compiled the first publicly available large-scale shale CT dataset, which consists of a total of 4313 scanned images, with over 700 images coming from scanning experiments on 300 cm3 cubic shale samples. We also proposed a new data augmentation method for shale CT images to overcome challenges posed by severe artifacts and small-sample sizes. This method involves simulating image artifacts and reducing high edge gradients along the sample edges, which has proven to be effective in facilitating the efficient training of deep-learning models. We've also conducted several ablation studies to determine the best model design, namely ShaleSeg. ShaleSeg was able to accurately identify multi-scale fractures, achieving a precision of 0.84 and an F-score of 0.81, which is close to the manual results. In the case of minor fractures, precision increased by an increase significantly from 0.191 to 0.513, compared to the traditional algorithm. ShaleSeg significantly improves the accuracy and efficiency of fracture segmentation for large-scale shale images. This advancement will be crucial for a thorough comprehension of the fracture mechanism in shale.

Natural fractures and their attributes in organic-rich shales: Insights from the Paleozoic Wufeng-Longmaxi formation, southeastern Sichuan Basin

Fractures in organic-rich shale affect the evolution of permeability and control shale gas preservation. We characterize fracture attributes in the Qiyue-Huaying Fold-Thrust belt in the southeastern Sichuan Basin, revealing the distribution, origin and factors controlling fracture localization through investigation of cores, image logs, and thin section petrography. We found that the deformation intensity, organic matter content and lithology are the major factors for controlling fracture occurrence and location in the Wufeng-Longmaxi deep shale. The major fracture pattern in the Fuling Block is characterized by abundant inclined shear fractures, bed-parallel shear fractures, and bed-normal extension fractures, while bed-parallel veins prevail in the Luzhou Block. In general, fracture density and size in the Fuling Block are larger than those in the Luzhou Block. The competent layers (siliceous shale with high TOC) have the highest fracture density, and noticeably, organic matter content controls bed-parallel vein localization. Based on the distribution of fractures in two blocks, we suggest that the dominant origin of fractures in organic-rich shale gradually changes from tectonic events to fluid pressure changes due to organic maturation (organic events), from the Fuling Block to the Luzhou Block.

Production of bio-crude from co-pyrolysis of oil shale and peanut shell over semi-coke supported Ni catalyst

Bio-crude producted from the co-pyrolysis of oil shale and peanut shell over a Ni-based catalyst supported on semi-coke was investigated in a vertical double-temperature zone fixed bed reactor. The effect of Ni species loading on the characteristics of bio-crude was evaluated. Results indicated that the effective H/C and calorific value of bio-crude gradually increased and the average carbon chain length gradually decreased with the increase of Ni species loading. The average carbon chain length decreased from 17.54 to 12.61 when the Ni loading was 9 %. An analysis of liquid product components indicated that the presence of the catalyst promoted the combination of small molecular side chains generated by oil shale fractures and active molecules generated by peanut shell thermal decomposition, increasing the generation of phenolic compounds. Furthermore, the phenolic compounds were further converted to aromatic hydrocarbons by promoting the alkylation transfer of the methoxy group to the benzene ring, demethoxylation, and dehydration reactions.

Fracture characteristics and energy dissipation law of shale under water-based impact loads

Shale fracture and energy dissipation are the basis of reservoir fracturing effect evaluation. The damage and fracture mechanism of shale under dynamic impact load is the key scientific concern of reservoir fracturing. To study the law of fracture, fracture propagation and energy dissipation of shale under dynamic impact, dynamic tests of water-based impact load under different impact velocities were carried out using impact rock breaking test platform based on air cannon. Topological and fractal theory, Weibull distribution model, space vector geometry method and Griffith fracture mechanics theory were used to systematically study the effects of impact velocity on fracture development and expansion, fragment mass and size distribution, damage degree of abutment residual and energy dissipation transformation. Scanning electron microscopy (SEM) was used to observe the micromorphology of the impact fracture surface. The results demonstrated that an increase in impact velocity led to the formation of a network fracture featuring main fractures, cavity fissure, bedding fractures, and crushing zones, thereby enhancing the fracture’s capability to penetrate bedding. The apparent fracture network connectivity, topology structure and fractal dimension were positively correlated with impact velocity. The residual height of the shale decreased, resulting in an escalation of the damage degree from mild (0.43) to severe (0.75), while the average penetration angle decreases from 81.00° to 63.52°. The area of new surface experiences rapid initial growth followed by a more gradual increase, with dissipative energy showing a more pronounced increase compared to crushing energy. The micro-fracture mode underwent an evolution from brittleness to toughness and a transition from low energy consumption to high energy consumption. The propagation, reflection, and superposition of stress waves within the rock mass gave rise to a complex stress zone, ultimately causing the rock mass to break. These findings lay the groundwork for developing a stress-damage-energy-failure combined model for shale subjected to dynamic impact loads, thereby enhancing the development of intricate networks in shale reservoir and improving the effective extraction of shale gas.

Effects of inclination angle and confining pressure on triaxial unloading-induced slip behaviors of shale fractures

To explore the effects of fracture inclination angle θ and confining pressure σ3t on the slip behaviors and friction properties of fractures, the triaxial unloading-induced slip experiments were performed on the shale fractures. The results show that the σ3t controls the slip modes of fractures, while the θ affects the occurrence of the stick-slip events during the quasi-static slip stage. With the increase in σ3t, the main slip modes of fracture transform from the stable-slip to stick-slip, and eventually to the creep-slip. The increase in θ facilitated the occurrence of stick-slip events. As the θ increased from 30° to 50°, the number of stick-slip events increased from 0 to 3 and from 2 to 4 for σ3t = 10 MPa and 20 MPa, respectively. For σ3t = 40 MPa, no stick-slip event occurred in the slipping process. The θ and σ3t have great effects on interaction modes between asperities, which directly affected the friction properties of fractures. With increasing σ3t, the void spaces between the asperities were further compacted, resulting in the transition of asperity interaction from overriding mode to shear-off mode. The transition of asperity interaction model therefore brought about the weakening of friction coefficient at the activation point and the onset of dynamic slip stage. There is a competitive relationship between the θ and σ3t for the evolution of the friction properties of fractures. As the θ increase from 30° to 50°, for σ3t = 10 MPa, the mean sheared-off thickness decreased from 0.502 mm to 0.433 mm, while for σ3t = 40 MPa, the mean sheared-off thickness decreased from 0.505 mm to 0.319 mm. With the increment of θ, the anisotropy of joint roughness coefficient was weakened. We suggested that by adjusting the fracturing angle of hydro-fracturing, the earthquakes with large seismic moments may be effectively mitigated.

Heat front propagation in shale oil reservoirs during air injection: Experimental and numerical studies

Air injection technique for developing shale oil has gained significant attention. However, the ability of the heat front to consistently propagate within the shale during air injection remains uncertain. To address this, we investigated the heat front propagation within oil–detritus mixtures, shale cores, and fractured shale cores using a self-designed combustion tube (CT) and experimental schemes. By integrating the results obtained from high-pressure differential scanning calorimetry and CT, we developed a comprehensive reaction kinetics model to accurately analyze the main factors influencing the heat front propagation within fractured shale. The findings revealed that in the absence of additional fractures, the heat front failed to propagate within the tight shale. The flow of gases and liquids towards the shale core was impeded, resulting in the formation of a high-pressure zone at the front region of the shale. This pressure buildup significantly hindered air injection, leading to inadequate oxygen supply and the extinguishment of the heat front. However, the study demonstrated the stable propagation of the heat front within the oil–detritus mixtures, indicating the good combustion activity of the shale oil. Furthermore, the heat front successfully propagated within the fractured shale, generating a substantial amount of heat that facilitated the creation of fractures and enhanced gas injection and shale oil flow. It was important to note that after the heat front passed through the shale, the combustion intensity decreased. The simulation results indicated that injecting air into the main fracturing layers of the shale oil reservoir enabled the establishment of a stable heat front. Increasing the reservoir temperature (from 63 to 143 °C) and oxygen concentration in the injected gas (from 11% to 21%) promoted notable heat front propagation and increased the average temperature of the heat front. It was concluded that temperature and oxygen concentration had the most important influence on the heat front propagation, followed by pressure and oil saturation.