Fractured Shale Research Articles

Integrity assessment methodology of casing ovality deformation in shale gas wells

Casing deformation frequently occurs in shale gas wells during hydraulic fracturing. While the failure mechanism of casing ovality deformation is unclear due to the interweaving and compounding reasons from geological and engineering complexity. The multi-arm caliper tool can quantify the severity of casing deformation, and the spatial and temporal occurrence of casing deformations is analyzed. A finite element model of fractured shale squeezing casing is established to simulate the process of casing deformation. The simulated casing deformation is compared with logging data to validate the model with which the deformation mechanism would be revealed. Simulation results indicate that casing deformation in the Luzhou shale gas field belongs to compressive deformation. The length of casing deformation and the diameter shrinkage are local and small, and the deformation points are scattered along the horizontal well trajectory. The fracturing fluid pressure and fractured shale block generate a nonuniform load on the casing, which leads to stress yield and ovality deformation. The fluid pressure, formation degradation and cementing material are the key controlling factors to mitigate casing ovality deformation. This work provides a prompt & accurate assessment method and controlling measures for casing integrity in shale gas wells.

New microbiological insights from the Bowland shale highlight heterogeneity of the hydraulically fractured shale microbiome

BackgroundHydraulically fractured shales offer a window into the deep biosphere, where hydraulic fracturing creates new microbial ecosystems kilometers beneath the surface of the Earth. Studying the microbial communities from flowback fluids that are assumed to inhabit these environments provides insights into their ecophysiology, and in particular their ability to survive in these extreme environments as well as their influence on site operation e.g. via problematic biofouling processes and/or biocorrosion. Over the past decade, research on fractured shale microbiology has focused on wells in North America, with a few additional reported studies conducted in China. To extend the knowledge in this area, we characterized the geochemistry and microbial ecology of two exploratory shale gas wells in the Bowland Shale, UK. We then employed a meta-analysis approach to compare geochemical and 16S rRNA gene sequencing data from our study site with previously published research from geographically distinct formations spanning China, Canada and the USA.ResultsOur findings revealed that fluids recovered from exploratory wells in the Bowland are characterized by moderate salinity and high microbial diversity. The microbial community was dominated by lineages known to degrade hydrocarbons, including members of Shewanellaceae, Marinobacteraceae, Halomonadaceae and Pseudomonadaceae. Moreover, UK fractured shale communities lacked the usually dominant Halanaerobium lineages. From our meta-analysis, we infer that chloride concentrations play a dominant role in controlling microbial community composition. Spatio-temporal trends were also apparent, with different shale formations giving rise to communities of distinct diversity and composition.ConclusionsThese findings highlight an unexpected level of compositional heterogeneity across fractured shale formations, which is not only relevant to inform management practices but also provides insight into the ability of diverse microbial consortia to tolerate the extreme conditions characteristic of the engineered deep subsurface.

Digital-Rock Construction of Shale Oil Reservoir and Microscopic Flow Behavior Characterization

In shale oil reservoirs, nano-scale pores and micro-scale fractures serve as the primary fluid storage and migration space, while the associated flow mechanism remains vague and is hard to understand. In this research, a three-dimensional (3D) reconstruction of the shale core and micro-pore structure description technique is established; digital core technology for shale reservoirs was developed using X-ray computed tomography (X-CT), scanning electron microscope (SEM) and a focused ion beam scanning electron microscope (FIB-SEM). Microscopic oil–water two-phase flow is mimicked using the lattice Boltzmann method (LBM), a well-acknowledged approach to exploring nanoconfined fluid dynamics. In addition, coupled with digital cores, the flow characteristics of shale reservoirs are characterized. The total porosities of bedding fractures in shale and lamellar shale are 2.042% and 1.085%, respectively. The single-phase oil flow inside bedding fractures follows Darcy’s linear flow principle. This work can deepen the understanding of the microscopic flow characteristics of continental shale reservoirs and provide a reference for similar problems that may be encountered.

Experimental study on enhancing methane explosion characteristics by Al and KMnO4 powders under large pipelines

As an alternative to hydraulic fracturing, in-situ methane explosive fracturing technology holds great promise in exploiting shale gas and the difficulties of achieving shale fracturing lie in how to generate sufficient explosion overpressure, thus creating fractures and increasing the permeability of reservoirs. One of the efficient approaches to enhance explosion overpressure is to select optimum combustion improvers and initial parameters. In this work, the improved effects of aluminum (Al) and potassium permanganate (KMnO4) powders on explosion characteristics of CH4-O2-N2 mixtures were tested using a 500 L pipeline explosion setup. Meanwhile, the influences of intrinsic properties of KMnO4 including particle diameter, specific surface area and loading amount on explosion characteristics were explored, and the parameters were optimized. Eventually, the underlying improved mechanisms of Al and KMnO4 were discussed. Results indicate that methane rapidly enters the detonation stage after being ignited in high concentration of oxygen with the fastest pressure rise time of 0.5 ms. When both Al and pristine KMnO4 are sprayed, the explosion overpressure has been enhanced to 9.29 MPa with a pressure rise rate of 9.255 GPa/s. After KMnO4 has been ground into smaller particles with a diameter of 20 μm, 100 g KMnO4 can generate an explosion overpressure of 30.64 MPa with a pressure rise rate of 20.78 GPa/s. Regardless of pristine or ground KMnO4, it can enhance the explosion overpressure but cannot enhance the detonation speed of the methane. When Al and the pristine KMnO4 are separately sprayed into methane, the average detonation speed for the case of Al has been enhanced to 1.0 km/s whereas that for the case of KMnO4 almost remains unchanged.

Data-Driven Inversion-Free Workflow of Well Performance Forecast Under Uncertainty for Fractured Shale Gas Reservoirs

Abstract Well performance prediction and uncertainty quantification of fractured shale reservoir are crucial aspects of efficient development and economic management of unconventional oil and gas resources. The uncertainty related to the characterization of fracture topology is highly difficult to be quantified by the conventional model-based history matching procedure in practical applications. Data-space inversion (DSI) is a recently developed inversion-free and rapid forecast approach that directly samples the posterior distribution of quantities of interest using only prior model simulation results and historical data. This paper presents some comparative studies between a recent DSI implementation based on iterative ensemble smoother (DSI-IES), model-based history matching, and conventional decline curve analysis (DCA) for shale gas rate forecast. The DSI-IES method treats the shale gas production rate as target variables, which are directly predicted via conditioning to historical data. Dimensionality reduction is also used to regularize the time-series production data by low-order representation. This approach is tested on two examples with increasing complexity, e.g., a fractured vertical well and a multistage fractured horizontal well in the actual fractured Barnett shale reservoir. The results indicate that compared with the traditional history matching and DCA methods, the DSI-IES obtains high robustness with a high computational efficiency. The application of data-space inversion-free method can effectively tap the potential value directly from historical data, which provides theoretical guidance and technical support for rapid decision-making and risk assessment.

Full composition numerical simulation of CO2 utilization process in shale reservoir using projection-based embedded discrete fracture model (pEDFM) considering nano-confinement effect

Carbon dioxide (CO2) injection for shale oil production is a promising CCUS (Carbon dioxide Capture, Utilization and Storage) technique, which achieves the synergistic effect of oil displacement and CO2 storage . The conventional black oil model is difficult to accurately characterize the complex fractures, the phase state considering nanoconfinement effect (NC) and the distribution of CO2 in the oil phase and gas phase. In this work, a full-composition (FC), projection-based three-dimensional embedded discrete fracture numerical model (3D-pEDFM) considering nanoconfinement effects is proposed to simulate the complex mass transfer process of CO2 injected into shale oil reservoirs. The discrete equations of the model are obtained by finite volume method (FVM), and the full implicit equations is solved by Newton-Raphson iteration method. The proposed new model is effectively adapted to the actual situation of CO2 injection development of fractured shale oil reservoir. These results obtained by comparing this model with the commercial software Eclipse300 in dealing with simple cases show the reliability of the model. Moreover, the processes of depletion development and CO2 injection development are simulated depending on the case of a shale oil reservoir in China, which illustrates the importance of considering the impact of nano-confinement effects on simulation results.

Study on the developmental characteristics and mechanism of shale FPZs

The fracture process and behavior of shale play a key role in the success of hydraulic fracturing stimulation in shale reservoirs. In this study, the central straight cracked Brazilian disc (CSCBD) splitting test in three fracture directions (arrester, divider, and short transverse) is conducted to reveal the propagation behavior and mechanism of closed symmetric fractures in laminated shales, with a focus on the fracture process zone (FPZ). The developmental characteristics and anisotropy of the FPZ near the crack tip of Mode I fracture of shale are identified and quantified based on the digital image correlation (DIC) method. By monitoring the crack tip opening displacement (CTOD) during the whole process, the critical CTOD, as one of the pivotal cohesive parameters, is obtained, and the development of the FPZ can be phased into three stages by CTOD evolution patterns. Based on double-K fracture criterion, the initiation fracture toughness and unstable fracture toughness of shale fractures at different laminar configurations are also analyzed. Additionally, the cohesive fracture model in the shale FPZ is derived based on the power function form using the energy release rate method.

Structure- and lithofacies-controlled natural fracture developments in shale: Implications for shale gas accumulation in the Wufeng-Longmaxi Formations, Fuling Field, Sichuan Basin, China

Descriptions, field emission-scanning electron microscopy and geochemical analyses on shale core samples taken from the fractured intervals of the Wufeng-Longmaxi Formations in three wells from the Fuling Field were used to understand the developmental properties of natural fractures and elucidate their controlling factors. Furthermore, the effects of geological fractures on shale gas accumulation were systematically analyzed. The macro-fractures observed in the shale cores are mainly non-tectonic bedding fractures; to a lesser extent, tectonic fractures, including oblique shear, vertical tension and tension-shear fractures, as well as bed-parallel slip fractures are visible. In contrast, micro-fractures are mainly interparticulate, intraparticulate, and grain marginal in nature. The extent of natural fracture development is controlled by external structural and internal lithofacies factors. For shales with similar lithofacies, the development degree of tectonic fractures increases with decreasing distance from the tectonically deformed and faulted zones. Within the same structural zone, the fractures are well developed in shales of organic matter-rich S-3 (i.e., argillaceous-rich siliceous) lithofacies. The study of fracture properties and their controlling factors has resulted in a model for the development of natural fractures in Wufeng-Longmaxi Shales generated by both structural and lithofacies effects. Overall, shale reservoirs in a “fractured but not broken” state are the most favorable ones for gas accumulation.

Seismic pre-stack inversion for physical and anisotropic parameters in fractured shale reservoirs

AbstractThe estimation of physical and anisotropic parameters is of great importance for the characterization of fractured reservoirs. Vertical fractures developing in a laterally isotropic (VTI) setting are equivalent to orthotropic anisotropic (OA) media common in stratified fractured shale reservoirs. To obtain independent anisotropic and physical information, a novel reflection coefficient approximation containing physical and anisotropic parameters is derived to improve the stability of the inversion for orthotropic media. To simplify the equation for the reflection coefficient, an approximate rock physics model is constructed using the approximate theory of rock modulus. The estimated parameters are reduced from nine to six. The accuracy analysis reveals that the new reflection coefficient is appropriate and suitable for inversion. In addition, a stepwise Bayesian amplitude versus angle and azimuth (AVAZ) inversion method with smooth background constraints is developed to estimate the anisotropic and physical parameters from the azimuthal seismic data. The smooth background constraint improves the robustness of the inversion. And the stepwise inversion strategy solves the problem that the contribution of the fracture parameter to the reflection coefficient is smaller than the other parameters. Synthetic cases show that the proposed stepwise Bayesian AVAZ inversion method is feasible in estimating the anisotropic parameters for OA media even when the signal-to-noise ratio is 2. The field cases show that the proposed inversion method has good stability and robustness in predicting shale reservoirs with vertical or near-vertical fractures.

The invasion law of drilling fluid along bedding fractures of shale

In the process of drilling, the drilling fluid will invade into the bedding plane of shale under the action of pressure difference that will cause hydration collapse and wellbore instability. In order to ensure the wellbore stability during shale oil and gas drilling, it is necessary to clarify the invasion law of drilling fluid along bedding fractures during the drilling process. The immersion experiment method is often used to study the invasion law of drilling fluid, which is quite different from the actual invasion process of drilling fluid underground. In this paper, the depth of drilling fluid invasion into shale under different confining pressures and displacement times is intuitively and accurately determined by the displacement experiment and NMR scanning first. Also, then the mathematical relationships between drilling fluid invasion depth and invasion time, invasion pressure difference, confining pressure, bedding angle, and drilling fluid viscosity were established. The errors between the calculated values of the drilling fluid invasion depth and the experimental values were less than 15%, and the calculation accuracy was high. In addition to the influence of invasion time, formation pressure difference and confining pressure on invasion depth were researched through the method of numerical simulation. The results showed that the liquid invasion depth increased logarithmically with the increase of invasion time and formation pressure difference, but it grew slowly in the later period and tended to be stable; the invasion depth decreased exponentially with the increase of confining pressure, bedding plane angle, and drilling fluid viscosity. The results in the paper provide a basis for the subsequent determination of the collapse pressure and collapse period of bedding shale.

Experimental CO2 interactions with fractured Utica and Marcellus Shale samples at elevated pressure

Experiments showed that 5% KI carbonated brine injection into fractured Marcellus and Utica Shale cores at room temperature (∼20 °C) and at 1700 psi (MPa) confining pressure resulted in dissolution along fractures, together with an increase in the total core permeability of the fractured cores. Dissolution in the more calcium-rich Utica Shale concentrated along larger aperture zones that likely channeled the bulk of the flow through the core. In contrast, dissolution in the Marcellus Shale sample was more uniform across the fracture plane and accompanied by the development of a porous, permeable zone of partial dissolution within the shale matrix that developed throughout the sample and decreased in volume along core length. X-ray fluorescence measurements in both cores revealed depletion in calcium, and in the Marcellus core the composition of the porous reaction rind was non-reactive and silica rich. The partial dissolution of the Marcellus Shale matrix adjacent to the fracture resulted in a permeable zone that contributed to the sample's increased transmissivity, as shown by fracture aperture mapping, high-resolution microtomography, and numerical simulations of flow through the fracture. Shale lithology and mineralogy were combined with the initial fracture morphology to determine where, how, and to what degree reactive fluids caused changes to fractured shales. Although the experimentally determined macroscopic permeability of both cores increased approximately one order of magnitude, the different compositions of the rocks resulted in distinct microscopic dissolution patterns. The umbrella term of “shale” encompasses a variety of lithological compositions with variable reactive potential. Our analysis demonstrates that even small lithological variations in carbonate and silicate mineral content and distribution can have an outsized impact on shale fracture behavior during reactive flow. Any field-scale assessment of the utility of shales as sealing formations or reservoirs in the presence of reactive brines must entail a thorough description of the given lithological unit and its susceptibility to reactive flow.

Experimental study on fracture propagation and induced earthquake reduction by pulse hydraulic fracturing in shale reservoirs

Large-scale and complex cracks have always been the eternal goal of hydraulic fracturing (HF) technology in shale reservoirs, but there has been the public concern that the HF could trigger earthquake disasters. It is urgent to develop the new HF technology that can generate a large number of complex cracks and reduce the risk of induced earthquake. Herein, the fracture propagation, fluid pressure distribution and acoustic emission (AE) energy of four types of hydraulic fracturing are analyzed through laboratory experiment and numerical simulation in this study. The new rectangular pulse HF is proposed. It is concluded that the pulse HF with higher fluid pressure gradient takes a shorter time to fracture shale. It is easier to form complex cracks. Relying on reciprocating impact and strong “Water Hammer Effect”, the pulse HF can form a large number of micro-cracks and reduce the energy released at the moment of complete failure of shale. The research shows that the pulse HF with the larger pressure gradient performs better in fracturing shale and reducing the risk of induced earthquake. The rectangular pulse HF has more significant advantages than the sine and triangle pulse HF. It relies on low instantaneous rock rupture energy, long rupture time and widely distributed low energy rupture events to reduce induced earthquakes. These can help to develop the pulse HF technology and reduce the risk of induced earthquakes for shale gas exploitation.

Application of a multi-layer feedforward neural network to predict fracture density in shale oil, Junggar Basin, China

Fengcheng Formation in the Mabei Slope of Junggar Basin has low porosity and permeability. However, fractures are well developed, representing an effective storage space for shale oil. Core and experimental data show that the shale oil reservoir of Fengcheng Formation positively correlates with oil content and fractures. And the fracture density has a good quantitatively positive correlation with crude oil production from the production data. Fengcheng Formation has been significantly enriched and accumulated with shale oil due to fractures serving as reservoirs and seepage channels. Therefore, quantitative prediction of fractures is the key to finding high production areas of shale oil in the Fengcheng Formation. The purpose of this study is to extract the seismic attributes that are sensitive to shale oil reservoir fractures. These attributes include curvature, deep learning fracture detection, maximum likelihood, eigenvalue coherence, and variance cube. Furthermore, a seismic multi-attribute fracture density prediction model is trained at the well point using a feedforward neural network method, and the spatial distribution of fracture density is predicted. The results show that the predicted fracture density is consistent with the formation micro imaging logs in the area. Simultaneously, combined with the understanding of the quantitative relationship between fracture density and shale oil production, quantitative prediction results of fracture density could provide the basis for determining the distribution and optimal location of high-quality shale oil wells in the study area. This study will serve as a benchmark for identifying fractures in shale oil reservoirs worldwide.

Experimental study on mechanical properties and fracture characteristics of shale layered samples with different mineral components under cyclic loading

In shale plays, the in-situ effective stress is continuously being perturbed because of the production which impacts field operations. Therefore, mechanical properties of the formation and its response to stress field changes, with respect to mineral variations, is of great significance for the proper design of horizontal drilling and hydraulic fracturing. In this paper, clay-rich, calcareous and siliceous black shales are classified based on their XRD analysis, from the Wufeng and Longmaxi Formations in the Sichuan Basin of the Upper Yangtze Plate. Uniaxial quasi-static and cyclic loading tests, acoustic emissions, and CT scanning are combined to study mechanical behavior and fracturing mechanisms of the shale layer samples that own different mineral components. The results show that the strength and deformation response of the samples is a function of their mineral composition. The damage from cyclic loading is gradually accumulated in the samples which ultimately leads to fatigue and sample failure. The compressive strength of the samples under cyclic loading is lower than in the uniaxial quasi-static conditions. Furthermore, the fracturing of rock under uniaxial quasi-static loading mainly occurs after the peak strength is achieved while in cyclic loading, when a certain stress level is reached, fracturing begins, the damage accumulates up to the peak stress and the rock suddenly fails. Based on CT images from the samples after rupture, failure modes of the clay-rich, calcareous and siliceous black shales mainly constitute extensional and shear fractures, with some conjugate shearing. In comparison with uniaxial quasi-static loading, the fracture volume of the clay-rich, calcareous and siliceous black shales after cyclic loading increases by 12.2%, 25.9% and 32.9%, respectively. Overall, this study explains how cyclic loading that can be applied in the field via changing the injection pressure and other stimulation parameters, can effectively improve the complexity of the shale fracture network and lead to higher productivity of the formation.

Research on the Effect of Shale Core Mechanical Behavior on Casing Deformation

As an unconventional, high-quality, efficient, and clean low-carbon energy, shale gas has become a new bright spot in the exploration and development of global oil and gas resources. However, with the increasing development of shale gas in recent years, the anisotropic load of the shale reservoir during the mining process has caused the casing to be deformed or damaged more and more seriously. In this paper, the mechanical behavior of shale core shear, triaxial and radial compression are studied using rock true compression tests, shear tests and nuclear magnetic resonance (NMR) technology. The process of macroscopic and microscopic changes of shale fractures during the tests were analyzed to predict the effect of the fracture-state changes and stress-state changes of different shale reservoirs on the casing deformation. The results show that after the shale core is damaged, the overall pore structure changes, resulting in the decrease or increase in shale porosity. During the process of triaxial pressurization, as the pressure continues to increase, there will be a critical pressure value from elastic deformation to plastic deformation. When the pressure value exceeds the critical pressure value, the shale reservoir will have strong stress sensitivity, which can easily cause wellbore collapse. The research results have important guiding significance for determining the casing deformation under shale reservoir load and preventing casing deformation failure.

Data and code/Seismic Wave Scattering and Dissipation in Fractured Shales

data and code/Seismic Wave Scattering and Dissipation in Fractured Shales

Experiment data/Seismic Wave Scattering and Dissipation in Fractured Shales

Experiment data/Seismic Wave Scattering and Dissipation in Fractured Shales

Experiment data/Seismic Wave Scattering and Dissipation in Fractured Shales

Experiment data/Seismic Wave Scattering and Dissipation in Fractured Shales

Propagation behavior of hydraulic fractures in shale under triaxial compression considering the influence of sandstone layers

The shale-sandstone interbedded reservoirs are widely distributed in China and the natural gas reserves are huge, and the particularity of this type of reservoir has brought great challenges to hydraulic fracturing design. In this respect, laboratory tests modelling the hydraulic fracturing process in the horizontal well under true triaxial compression were carried out based on real rock specimens. Considering factors such as stress state, sandstone proportion, and natural fractures, the spatial morphology and influence mechanism of hydraulic fractures were studied. Besides, based on acoustic emission (AE) monitoring and real-time monitoring of water pressure, the influence of sandstone layers on the propagation behavior of hydraulic fractures was further investigated. The results show that, the fracture morphology in sandstone is relatively simple, while the fracture morphology and orientation in shale are more complex due to serious bedding interference. A large vertical stress difference is the guarantee for realizing hydraulic fractures penetrating multiple lithologic layers, and the long distance between wellbore and shale-sandstone interface reduces the probability of the hydraulic fracture reaching the sandstone layer. Whether hydraulic fractures passed through the shale-sandstone interface can be judged from the fluctuation characteristics of the water pressure curve and the AE signal. The research results are expected to provide theoretical support for the efficient exploitation of shale-sandstone interbedded reservoirs.

Optimizing construction parameters for fractured horizontal wells in shale oil

Shale oil is mainly extracted by fracturing. However, it is difficult to determine the optimum construction parameters to obtain maximum productivity. In this paper, a fuzzy comprehensive production evaluation model for fractured shale oil horizontal wells based on random forest algorithm and coordinated principal component analysis is proposed. The fracturing parameters of the target wells are optimized by combining this model with an orthogonal experimental design. The random forest algorithm was used to calculate the importance of data sample factors. The main controlling factors of the production of fractured horizontal wells in shale oil were obtained. To reduce the noise of the sample data, principal component analysis was used to reduce the dimensions of the main control factors. Furthermore, the random forest algorithm was used to determine the weight of the principal components after reducing the dimensionality. The membership function of the main control factors after reducing dimensionality was established by combining the fuzzy statistics and assignment methods. In addition, the membership matrix of the effect prediction of fractured horizontal wells in shale oil was determined. The fuzzy comprehensive evaluation method is used to score and evaluate the effect of fractured horizontal wells. Combined with the orthogonal experimental design method, the optimized parameter design of a fractured horizontal well considering the comprehensive action of multiple parameters is realized. After construction according to the optimized parameters, production following fracturing increases significantly. This verifies the rationality of the optimization method that is proposed in this paper.