Fracture Propagation In Rock Research Articles

Experimental investigation on fracture propagation in shale fractured by high-temperature carbon dioxide

The productivity of shale reservoirs was significantly enhanced by the high-temperature CO2 fracturing technique. The injection of high-temperature CO2 into the formation induced rock fracture propagation, creating advantageous pathways for fluid flow. In this research, a self-developed in situ high-temperature convective heat simulation experimental apparatus was employed to systematically conduct simulated experiments on high-temperature CO2 fractured shale under different influencing factors. The experimental results demonstrated that the permeability of CO2 increased as the injection temperature increased. The rock fracture pressure was effectively reduced by high-temperature CO2 fractured shale. Higher complexity was observed in fracture propagation, accompanied by a substantial increase in microcracks and branching fractures. The shale fracture pressure increased with increasing triaxial stress and CO2 injection rate. The confining pressure restricted the further propagation of fractures under relatively high stress conditions, thereby reducing the width and density of fractures, lowering the fracture complexity. Nevertheless, the thermal shock effect of the fluid was exacerbated as the injection rate of high-temperature CO2 increased. The initiation of microcracks was facilitated by the intensification of local thermal stress in shale, inducing multiple curved fractures and forming a more complex fracture network. Compared to horizontal bedding shale, the fracture pressure of vertical bedding shale was relatively higher during high-temperature CO2 fracturing. In addition, the geometric morphology of fracture propagation was more complex, characterized by rougher fracture surfaces, leading to a greater improvement in reservoir reconstruction volume. This research contributed to the optimization of CO2 resource utilization, provided experimental evidence for the application of high-temperature convection fracturing technology in in situ shale conversion projects.

Influence of confining pressure on rock fracture propagation under particle impact

Revealing the influence of confining pressure on the propagation and formation mechanism of rock cracks under particle impact is significant to deep rock excavation. In this study, the three-dimensional fracture reconstruction of the rock after particle impact was carried out by CT scanning, and the stress and crack field evolution of the rock under particle impact were analyzed by PFC2D discrete element numerical simulation. The results demonstrate that after particles impact, a fracture zone and intergranular main crack propagation zone are formed in the rock. The shear stress and tensile stress caused by compressive stress are the main reasons for the formation of the fracture zone, while the formation of the intergranular main crack propagation zone is mainly due to tangential derived tensile stress. The confining pressure induces prestress between rock particles such that the derived tensile stress needs to overcome the initial compressive stress between the particles to form tensile fractures. And the increase in the confining pressure leads to increases in the proportion of shear cracks and friction effects between rock particles, resulting in an increase in energy consumption for the same number of cracks. From a macroscopic perspective, the confining pressure can effectively inhibit the generation of cracks.

Development of CASRock for modeling multi-fracture interactions in rocks under hydro-mechanical conditions

The interaction between multiple fractures is important in the analysis of rock fracture propagation, fracture network evolution and stability and integrity of rocks under hydro-mechanical (HM) coupling conditions. At present, modeling the mechanical behavior of multiple fractures is still challenging. Under the condition of multiple fractures, the opening, closing, sliding, propagation and penetration of fractures become more complicated. In order to simulate the HM coupling behavior of multi-fracture system, the paper presents a novel numerical scheme, including mesh reconstruction and topology generation algorithm, to efficiently and accurately represent fractures and their propagation process, and a potential function-based algorithm to address contact problem. The fracture contact algorithm does not need to set contact pairs and thus is suitable for complex contact situations from small to large deformations induced by HM loading. The topology of fracture interfaces is constructed by the dynamic adding algorithm, which makes the mesh reconstruction more rapid in the modeling of fracturing process, especially in the case of multiple fractures intersections. The numerical scheme is implemented in CASRock, a self-developed numerical code, to simulate the propagation process of rock fractures and the interaction of multiple fractures under the condition of HM coupling. To verify the suitability of the code, a series of tests were performed. The code was then applied to simulate hydraulic fracture propagation and fracture interactions caused by fluid injection. The ability of this method to study fracture propagation, multi-fracture interaction and fracture network evolution under hydro-mechanical coupling conditions is demonstrated.

Rock fracture propagation in a damage zone by a constitutive model coupling volume and cohesive elements

AbstractThis paper presents a method to simulate fracture propagation in a damage zone in 2D, whereby Cohesive Zone (CZ) elements are assigned the same Continuum Damage Mechanics (CDM) model as their adjacent finite elements. Material hardening is a result of damage‐induced stiffness reduction in the finite and cohesive elements, while softening is modeled by cohesive debonding only. The damage components calculated at the numerical integration points of each finite element are averaged and that average is assigned to the nodes of the adjacent CZ elements. CZ element debonding triggers when a critical damage threshold is reached, beyond which, a linear softening law is used in both mode I and mode II. The proposed model is calibrated against published triaxial compression tests conducted on shale. Single‐CZ element simulations confirm that, below the critical damage threshold, elastic deformation energy is stored and energy is dissipated by damage propagation and irreversible deformation. Passed the critical damage threshold, stored elastic energy is released in the form of cohesive debonding energy. Borehole breakout simulations indicate that the proposed CDM‐based CZ model can predict the formation of spiral‐shaped fractures around a free circular cavity under biaxial stress. Simulations of fracture propagation in textured materials suggest that cohesive debonding within and between grains is triggered by the coalescence of localized zones of continuum damage, and that the grain size distribution affects both the fracture pattern and the softening behavior of the specimen considered.

An insight into the effect of primary hidden microfissures on mechanical behaviors and failure characteristics of brittle basalt

The anisotropy of mechanical properties of hard rocks is usually affected by the internal primary hidden microfissure structures, which tend to induce secondary rock fracture propagation even surrounding rock instabilities in deep underground excavation. This research attempts to reveal the control effect of primary hidden microfissures on the mechanical behaviors and failure characteristics of cryptocrystalline basalt with hidden microfissures (CBWHM) through a series of laboratory tests. Firstly, a geometric feature extraction method for primary hidden microfissures of basalt rock has been proposed, by which 20 groups of three-dimensional (3D) visualization models of cylindrical basalt samples were established, and the distribution location and volume crack rate (KV) of hidden microfissures were simultaneously determined. On this basis, a series of the uniaxial compression tests of basalt samples with synchronous acoustic emission (AE) were carried out. Results show that the location and development degree of primary hidden microfissures significantly control the mechanical characteristics of basalt, with the relevant micro-fracture events occurring earlier and throughout the whole loading process compared with intact samples. In general, there exists a uniaxial polynomial fitting relationship between the strength of basalt samples and KV. The larger the KV, the lower the peak strength (σc) of basalt, which could decrease by 30–50% compared to that of the intact rock when KV is more than 5‰. However, when the hidden microfissures with large dip angles are located close to the central axis, the influence of KV on the rock strength becomes less, but the crack propagation and failure morphology are still obviously controlled by the hidden microfissures. The statistical analysis results of surface cracks and rock fragment characteristics of the damaged basalt samples further show that the basalt rock is easy to expand, penetrate and eventually fail along the hidden microfissures under loading, and it is tensile fracture that controls the failure of CBWHM. With an increase of KV, the mass percentage (W) of the coarse particle of the sample fragments increases gradually, and the fractal dimension (D) of the rock fragments and KV of the hidden microfisures show an obvious logarithmic curve relationship. The research could provide effective guidance for the interpretation of the meso-mechanism of hard rock failures.

Numerical Simulation Analysis of Fracture Propagation in Rock Based on Smooth Particle Hydrodynamics.

The mechanical properties of fractured rock have always been a focal point in the rock mechanics field. Based on previous research, this paper proposes improvements to the SPH method and applies it to the study of crack propagation in fractured rocks. By conducting uniaxial compression tests and simulating crack propagation on various specimens with different crack shapes, the characteristics of crack propagation were obtained. The comparison between the simulated results in this study and existing experimental and numerical simulation results confirms the validity of the SPH method employed in this paper. The present study utilizes the proposed methodology to analyze the influence of the crack angle, width, and orientation on crack propagation. The SPH method employed in this study effectively demonstrates the expansion process of fractured rock under uniaxial compression, providing valuable insights for the engineering applications of SPH.

The formation mechanism of dynamic water and mud inrush geohazard triggered by deep-buried tunnel crossing active fault: Insights from the geomechanical model test

Water and mud inrush geohazard is one of the most important geohazards for underground engineering. Understanding the formation mechanism of water and mud inrush is critical for directing the prevention and control of this geohazard. As a result, a large number of methods have provided insight into this topic, such as the geomechanical model test. However, this geohazard triggered by deep-buried tunnel crossing active fault has not yet been explored. Thus, this study develops a geomechanical model test system to simulate the dynamic water and mud inrush caused by Kangding No.2 tunnel crossing Yunongxi active fault. The results imply that seismic loading can result in fluctuating changes in seepage pressure and stress in the tunnel rock mass. Increasing seismic intensity broadens the fluctuation range, together with a dramatic rise in peak seepage pressure. The coupled effect of earthquake loading and high seepage pressure makes the water-resisting rock mass extremely vulnerable to fracturing, increasing the probability of water and mud inrush. High seismic intensity leads to rock fracture propagation, ulteriorly giving rise to stress releasing, eventually provoking the fracturing of rock mass. Precursor of water and mud inrush can be observed before the geohazard occurs, reflected by the gradual rise of seepage pressure and intermittent fluctuation of stress. Following the increasing earthquake intensity, peak acceleration occurs in a random position of water-resisting rock mass, but mostly appears at the tunnel contour of surrounding rock. The free face of the tunnel exerts an amplifying effect on seismic acceleration. Seismicity affects the formation of water and mud inrush from active fault, which ulteriorly determines the minimum safety thickness of water-resisting rock mass. Monitoring the seepage pressure and stress in water-resisting rock mass is effective for the early warning of inrush hazard.

Comparison of subcritical crack growth and dynamic fracture propagation in rocks under double-torsion tests

Relaxation and rapid displacement loading tests were performed on five types of rocks to gain a deeper understanding of the differences between subcritical crack growth (SCG) and dynamic fractures. The critical surface energy was obtained by fitting a hyperbolic sine function to the SCG velocities and mechanical energy release rates of the five rocks. The specific SCG range and the dimensions of the fracture process zone (FPZ) were determined via the displacement field of the sample bottom surface provided by digital image correlation. The FPZ lengths of the double-torsion rock samples range from 21.03 mm to 34.10 mm, and the lengths increase with the increasing maximum grain size of the rock type. The FPZ width is between 1.63 mm and 3.46 mm and exhibits a similar variation to that of the FPZ length. A three-dimensional optical scanner was used to obtain the morphologies of the subcritical crack surfaces and the dynamic fracture surfaces. The results show that the roughness of the subcritical crack surface is larger than that of the dynamic fracture surface, and both are positively correlated with the maximum grain size of the rock. The scanning electron microscope images demonstrate the change in the roughness of the crack surface; the subcritical cracks preferentially propagate along the grain boundaries while the dynamic fractures directly cut across crystals with some intergranular fractures.

Effects of rock ductility on the fracability of ductile reservoirs: An experimental evaluation

Deep hydrocarbon resources below 4000 m are an important replacement for shallow-depth resources; however, as depth increases, the difficulty and cost of development increase sharply. One of the possible reasons is that deep reservoir rocks exhibit strong ductility, which causes difficulties to stimulate a well-developed fracture network in rocks. To date, the mechanism by which rock ductility affects reservoir fracturing remains unclear, and there is a lack of quantitative evaluation models for reservoir fracability. To overcome the problems, in this study, artificial rock specimens with different ductility were fabricated using similar materials based on the geological data and petrophysical properties of the target deep reservoir; the specimens were classified using the proposed ductility index BIf. The hydrofracturing and CT scanning experiments of the artificial specimens with different BIf values under various horizontal stress differences Kh were conducted. The spatial fractal index Df was proposed to characterize the three-dimensional morphology and development of the fracturing network. The fracability evaluation function of ductile rock correlating the indices BIf and Kh was established. It was shown that ductile rocks tended to form non-planar hydraulic fractures with obvious tortuosity and the degree of fracture network development was lower than that of brittle rocks. The maximum breakdown pressure was approximately twice that of the brittle specimen, while the corresponding injection energy was approximately 5–10 times and the energy dissipation of fracture propagation was approximately 4–6 times those of the brittle rock. When Kh ≥ 0.3 the maximum horizontal principal stress played a major role in controlling the hydraulic fracture propagation in ductile rocks. If Kh < 0.3, the fracture propagation in rocks was mainly governed by material ductility rather than the maximum horizontal principal stress. This study provides a new way to explore and quantify the effect of rock ductility on the fracability of deep reservoirs.

Modeling land subsidence near the shaft pillars at the Upper Kama potassiummagnesium salt deposit based on the geological and geomechanical model

Introduction. Land subsidence is a visible consequence of deformation processes in the rock mass as a result mining. Deformation processes originate at producing horizons and propagate in the rock mass, usually upwards. The presence of natural physical and mechanically weakened zones in the rock mass complicates the situation creating risks of rock fracture propagation. The main problem is that subsidence of the earth's surface can lead to negative consequences for facilities within the undermining zone. Research objective is to carry out geomechanical modeling to adequately display the current situation on the earth's surface in accordance with real actual observational data. The created geological and geomechanical model can later be used to analyze and predict potential fracture zones in the rock within the rated section. Methods of research. The geological and geomechanical model is based on strata geological description, salt and oil wells geophysical exploration data, current position of workings, as well as physical and mechanical properties of rocks building up the rock mass. Results. The paper presents the results of geomechanical modeling of land subsidence at a site of the Upper Kama potassium-magnesium salt deposit in the vicinity of shaft pillars. Calculations were based on a geomechanical model of salt rock deformation and the developed geological and geomechanical model of an orefield site, including shaft barrier pillars regulated by the relevant reference documents. Timely measures for preventing negative consequences are recommended.

Microstructural Controls on Mixed Mode Dynamic Fracture Propagation in Crystalline and Porous Granular Rocks

AbstractBrittle fracture propagation in rocks is a complex process due to significant grain‐scale heterogeneity and evolving stress states under dynamic loading conditions. In this work, we use digital image correlation and linear elastic fracture mechanics to make instantaneous measurements of the opening (mode I) and in plane shear (mode II) components of the stress intensity field during dynamic mixed mode crack initiation and propagation in crystalline and granular rocks. Both rock types display some similar fracture behaviors as observed in engineered materials, including rate dependent fracture initiation toughness and a direct relationship between propagation toughness and crack velocity; however, measured propagation toughness is higher than quasi‐static values at crack velocities well below the branching velocity in both rocks. Additionally, due to grain scale controls on the fracture process, mixed mode crack propagation is fundamentally different between these two rock types. Mixed mode propagation is energetically more favorable than pure opening mode propagation in sandstone, while the opposite is true in granite. Furthermore, following initiation, propagation in granite occurs so as to minimize the mode II contribution, irrespective of the initiation conditions, while fractures in sandstone maintain a non‐negligible mode II contribution during propagation across the sample.

Development and Application of Rock Fracture Propagation Numerical Simulation System

Development and Application of Rock Fracture Propagation Numerical Simulation System

A novel LBM-DEM based pore-scale thermal-hydro-mechanical model for the fracture propagation process

Fracture propagation in rocks is a complex thermal-hydro-mechanical (THM) phenomenon present in geotechnics, such as enhanced geothermal systems, CO2 sequestration, and shale gas exploitation. Numerical simulation is an essential way to study its mechanism. In this study, a pore-scale model combined with the lattice Boltzmann method (LBM) and the discrete element method (DEM) is developed, in which the comprehensive THM coupling phenomena are considered, consisting of convective heat transport, conjugate heat transfer, hydrodynamic force, and thermal strain. In addition, to couple the temperature and anisotropic thermal expansion of rock in LBM-DEM, a novel method based on the deformation of sphero-triangle particle is presented, where the deformation is calculated using the principle of minimum potential energy. The main features of the proposed THM model are that it can accurately calculate the fluid flow in fractures and the temperature in both fractures and DEM particles, and it takes full consideration of multiple anisotropic properties in rock. After validations, the THM fracturing mechanism is preliminarily investigated using the proposed model.

Interaction between Hydraulic Fracture and Pre-Existing Fracture under Pulse Hydraulic Fracturing

Summary Pulse hydraulic fracturing technology can greatly improve the effect of fracture propagation in rock and form complex fracture networks in reservoirs. The interaction mechanism between hydraulic fractures and pre-existing fractures under pulse hydraulic pressure is unclear. The induced laws of pre-existing fractures on the propagation direction of hydraulic fractures under different pulse frequencies and pulse hydraulic pressures are revealed in this work. We have carried out traditional hydraulic fracturing (THF) tests and pulse hydraulic fracturing tests with rock-like specimens. We compared the interaction between hydraulic fractures and pre-existing fractures in the two hydraulic fracturing tests. Acoustic emission (AE) characteristics of the interaction between hydraulic fractures and pre-existing fractures during pulse hydraulic fracturing are analyzed. The results show that pre-existing fractures in the rock-like specimen can induce the direction of propagation of hydraulic fractures. The influence of pre-existing fracture tips on hydraulic fracture propagation is greater with low pulse frequencies than with traditional hydraulic pressures and high pulse frequencies. When the pulse frequency is 1 Hz, hydraulic fractures are easily induced by pre-existing fracture tips. With increasing pulse frequency, the hydraulic fracture propagation direction gradually moves away from the pre-existing fracture tips and extends perpendicularly to the direction of the minimum principal stress. Under pulse hydraulic loading, more hydraulic fractures are generated around the wellbore than under THF and extend to the pre-existing fracture, and more hydraulic fractures around the wellbore are created with low-frequency pulse loading than with high-frequency pulse loading. Compared with traditional hydraulic pressures, hydraulic fracture propagation with low pulse frequencies (1 and 3 Hz) is more complex than hydraulic fracture propagation with traditional hydraulic pressures and high pulse frequencies (5 Hz). Under high pulse hydraulic pressure and pulse frequency, hydraulic fractures easily extend along the direction perpendicular to the direction of the minimum principal stress like propagation under traditional hydraulic pressure. The study of the interaction mechanism between hydraulic fractures and natural fractures under pulsating hydraulic pressure can provide a method for the formation of fracture network systems in large-scale fracturing and may improve the fracturing efficiency.

Optimization of Criterion for Rock Crack Propagation under Acid Izing Treatment

Since most of the conventional oil and gas resources have been intensively investigated, unconventional oil and gas resources such as deep marine carbonate reservoirs and heavy oil reservoirs have become the primary focus in the exploration and development of oil and gas. The oil and gas reservoirs with complex fracture geometries account for nearly 45% of the global oil and gas reserves, and the production of such complex fractured oil and gas reservoirs accounts for nearly 60% of the total natural gas production. Consequently, fractured oil and gas reservoirs have received considerable research attention in recent years. Further, fractures determine the permeability and productivity of petroliferous reservoirs. In this paper, we have investigated the current status of research on petroliferous rock under the action of acid etching. At the same time, we have summarized the existing rock fracture propagation criterion and crack propagation criterion theory, and selected the most common crack propagation criterion for setting simulation parameters. Subsequently, the Brazilian split test and the rock acidification sensitivity experiment are combined, and the comparative experiment method is used to obtain the hydrochloric acid concentration and acidification treatment time that can meet the DIC optical test. Through the DIC method, the expansion of rock cracks from the moment of loading of experimental species to the moment when the completion of splitting is recorded, the strain and displacement of each point on the rock surface during the Brazilian splitting test are obtained. Finally, through the ABAQUS software, different fracture propagation criteria are used to simulate the fracture propagation process in the rock samples in the Brazilian fracturing test, and the results are recorded. Then, the expansion model is selected that best fits the carbonate fracture before and after acid etching. The in-situ stress field of the fault zone in the Shunbei area is exceedingly complex. Based on the Shunbei area, the model is set up. Furthermore, the features of plane geomechanics are studied, and the fracture propagation laws of acid salt reaction are analyzed. The results provide the basis for optimizing the acid fracturing technology and construction parameters in the Shunbei area.

Laboratory study on the propagation of edge cracks in rock-like material and its implication in coal mining

Rock fracture propagation is a major hazard for mining and tunnel excavation in fractured rock masses or coal seams. A longwall mining panel with a typical dimension of 200m (width)×1000m (length)×3m (height) can be considered as an open edge crack. The fracturing processes in the vicinity of the edge crack (or the longwall panel) particularly in the roof and floor are critically important for the safety of mining operation because fracturing can lead to water inrush and dynamic loading on the working face. It’s therefore important to understand and predict the pre-existing edge crack initiation and propagation in rock masses. This paper describes a study investigating the mechanisms and pathways of rock fracture under uniaxial compression. In this study, a rock-like material which consists of model gypsum, water and diatomaceous earth at a mass ratio of 165:75:2 was used. The uniaxial compression strength of the material decreased with the increase of the length of pre-existing edge crack. During the tests, wing (tensile) cracks were first observed at the tip of the pre-existing edge crack. This was followed by secondary cracks as the loading increased. The final failure of the specimens however was dominated by tensile cracks throughout the specimens. Due to the sudden crack initiations in the specimens, the loading stress in the specimen varies stepwise, and acoustic emission (AE) energy and amplitude showed abrupt changes when crack initiated. When the crack initiation occurred, the loading stress of the specimens showed a notable retreat in the stress-strain curve, and the recorded AE energy and amplitude showed a sharp spike. These findings from this experimental study have been applied to the underground longwall mining to explain the failure mechanisms in the floor of the mining panel. The fracturing process associated with the pre-existing edge crack resembles the formation of flow channels for water inrush during longwall mining.

Modeling Rock Fracture Propagation and Water Inrush Mechanisms in Underground Coal Mine

Water inrush in underground mines is a major safety threat for mining personnel, and it can also cause major damage to mining equipment and result in severe production losses. Water inrush can be attributed to the coalescence of rock fractures and the formation of water channel in rock mass due to the interaction of fractures, hydraulic flow, and stress field. Hence, predicting the fracturing process is the key for investigating the water inrush mechanisms for safe mining. A new coupling method is designed in FRACOD to investigate the mechanisms of water inrush disaster (known as “Luotuoshan accident”) which occurred in China in 2010 in which 32 people died. In order to investigate the evolution processes and mechanisms of water inrush accident in Luotuoshan coal mine, this study applies the recently developed fracture-hydraulic (F-H) flow coupling function to FRACOD and focuses on the rock fracturing processes in a karst collapse column which is a geologically altered zone linking several rock strata vertically formed by the long-term dissolution of the flowing groundwater. The numerical simulation of water inrush is conducted based on the actual geological conditions of Luotuoshan mining area, and various materials with actual geological characteristics were used to simulate the rocks surrounding the coal seam. The influences of several key factors, such as in situ stresses, fractures on the formation, and development of water inrush channels, are investigated. The results indicate that the water inrush source is the Ordovician limestone aquifer, which is connected by the karst collapse column to No. 16 coal seam; the fracturing zone that led to a water inrush occurs in front of the roadway excavation face where new fractures coalesced with the main fractured zone in the karst collapse column.

Effect of microcapsule layout on crack repair

Similar rock specimens without microcapsules were prepared on the basis of similarity principle, and uniaxial compression tests were carried out to obtain the general law of rock fracture propagation. Microcapsules were mixed into rock specimens in three ways. The first way is uniform mixing. The second method is to add microcapsules at different levels. The third way is to add microcapsules along the crack propagation angle. The uniaxial preloading tests were carried out to obtain the compressive strength of the microcapsules. After the curing of 7d and 14d, three specimens were subjected to compressive test again and the strength recovery rate was calculated. Draw the following conclusions: after pre-compression repair, the compressive strength increases, indicating that microcapsules play a repair role; adding microcapsules along the average angle of crack propagation is the best way to repair the crack; the stress-strain curves show good elastic-plastic, the pressure relief curve is stable, and the residual strength is stable.

Predicting mode-I fracture toughness of rocks using soft computing and multiple regression

Mode-I fracture toughness (FT) is an important property to model fracture propagation in rocks and it has wide application in a plethora of rock mechanics problems such as hydraulic fracturing design, tunneling, geothermal energy extraction, CO2 sequestration, blasting and drilling activities, well-bore stability analysis etc. Determination of fracture toughness from different individual index geomechanical properties is an ongoing research area. Multiple past research have established simple regression relations between FT and individual mechanical parameters with various degree of success. But these were found to be rock and site specific and lack wide acceptance. In this paper, multiple regression analysis, and three types of soft computing methods (e.g. artificial neural network, fuzzy inference system, and adaptive neuro-fuzzy inference system) have been employed to predict mode-I fracture toughness from a set of common geomechanical properties. Based on Pearson’s correlation coefficient, three index geomechanical properties, namely, tensile strength, P-wave velocity, and S-wave velocity were chosen to estimate the mode-I FT. As the results suggest, a single model is sufficient to estimate FT in all the sedimentary and crystalline rocks. Such model is capable of predicting fracture toughness irrespective of strength or compositional heterogeneity that is encountered in field. Statistical analysis of the predictions using correlation coefficient (R2), root mean square error (RMSE), mean absolute percentage error (MAPE), and variance accounted for (VAF) show that all the soft computing methods performed better than the multiple regression models. However, among the soft computing methods, adaptive neuro-fuzzy inference system, which incorporates elements of both the artificial neural network and fuzzy inference system, constructs the best model for prediction. This is the first ever reported instance, where soft computing has been used to predict mode-I fracture toughness of dry rocks from the mechanical properties, and the results confirmed that this is an easier, efficient but highly accurate tool for prediction.

Thermal shale fracturing simulation using the Cohesive Zone Method (CZM)

Extensive research has been conducted over the past two decades to improve hydraulic fracturing methods used for hydrocarbon recovery from tight reservoir rocks such as shales. Our focus in this paper is on thermal fracturing of such tight rocks to enhance hydraulic fracturing efficiency. Thermal fracturing is effective in generating small fractures in the near-wellbore zone - or in the vicinity of natural or induced fractures - that may act as initiation points for larger fractures. Previous analytical and numerical results indicate that thermal fracturing in tight rock significantly enhances rock permeability, thereby enhancing hydrocarbon recovery. Here, we present a more powerful way of simulating the initiation and propagation of thermally induced fractures in tight formations using the Cohesive Zone Method (CZM). The advantages of CZM are: 1) CZM simulation is fast compared to similar models which are based on the spring-mass particle method or Discrete Element Method (DEM); 2) unlike DEM, rock material complexities such as scale-dependent failure behavior can be incorporated in a CZM simulation; 3) CZM is capable of predicting the extent of fracture propagation in rock, which is more difficult to determine in a classic finite element approach. We demonstrate that CZM delivers results for the challenging fracture propagation problem of similar accuracy to the eXtended Finite Element Method (XFEM) while reducing complexity and computational effort. Simulation results for thermal fracturing in the near-wellbore zone show the effect of stress anisotropy in fracture propagation in the direction of the maximum horizontal stress. It is shown that CZM can be used to readily obtain the extent and the pattern of induced thermal fractures.