Hydraulic Fracturing Process Research Articles

Gravity Hydraulic Fracturing: A Method to Create Self‐Driven Fractures

AbstractIn this study, we investigate the possibility of using a high‐density fluid to induce downward fracture growth in a hydraulic fracturing process. We propose a mathematical model to calculate the minimum amount of a dense fluid required to trigger downward fracture propagation under gravity forces, and we verify the calculated minimum volume of the fluid through numerical simulations. Results show that when the injected fluid exceeds the minimum amount, a steady downward growth of the hydraulic fracture is obtained. The fracture propagation consists of two distinct responses: The first response can occur under either toughness‐dominated, viscosity‐dominated, or an intermediate hydraulic fracturing regime, depending on fluid rheology, rock properties, and injection scenario. The second response occurs mainly under the toughness‐dominated regime, meaning the predominant energy dissipation mechanism is to overcome the fracture toughness and break the rock. In the latter, the speed of the downward fracture growth depends on the viscosity and fluid weight.

Breakdown Pressure and Propagation Surface of a Hydraulically Pressurized Circular Notch Within a Rock Material

Rock masses contain pre-existing cracks. These cracks are not usually considered when predicting the maximum injection pressure, i.e. the breakdown pressure, in hydraulic fracture stimulations. The conventional approach to predict breakdown pressure is to use the maximum tensile stress failure criterion to calculate the pressure when a point on the borehole wall reaches the tensile strength of the rock. In addition, a pre-existing crack intersecting a hydraulically pressurized section of a borehole may produce a non-planar crack propagation surface. It is important to predict these non-planar crack propagation surfaces to design productive hydraulic fracturing stimulations and to mitigate risks associated with uncertainties of the resultant crack propagation. To gain a better understanding of this problem, a series of hydraulic fracturing experiments were conducted to investigate the breakdown pressures and crack propagation surfaces of a pressurized circular crack represented by a thin notch, subjected to different external triaxial stresses. The results show that the breakdown pressures under the shear stress conditions studied can be estimated using only the resultant normal stress on the plane of the crack. As the material properties of the experimental specimens are well defined and the crack propagation surfaces were mapped, the experimental results presented in this study provide a very useful measured dataset for the validation of various modelling approaches. The propagation surfaces from experiments were found to align closely to the computational predictions based on the maximum tangential stress criterion. Finally, this study gives evidence in three-dimensions that via the hydraulic fracturing process, the propagation of an initially arbitrarily oriented crack will eventually realign to be perpendicular to the minor principal stress direction.

A numerical study of fracture initiation under different loads during hydraulic fracturing

The fracture initiation behavior for hydraulic fracturing treatments highlighted the necessity of proposing fracture criteria that precisely predict the fracture initiation type and location during the hydraulic fracturing process. In the present study, a Mohr-Coulomb criterion with a tensile cut-off is incorporated into the finite element code to determine the fracture initiation type and location during the hydraulic fracturing process. This fracture criterion considers the effect of fracture inclination angle, the internal friction angle and the loading conditions on the distribution of stress field around the fracture tip. The results indicate that the internal friction angle resists the shear fracture initiation. Moreover, as the internal friction angle increases, greater external loads are required to maintain the hydraulic fracture extension. Due to the increased pressure of the injected water, the tensile fracture ultimately determines the fracture initiation type. However, the shear fracture preferentially occurs as the stress anisotropy coefficient increases. Both the maximum tensile stress and equivalent maximum shear stress decrease as the stress anisotropy coefficient increases, which indicates that the greater the stress anisotropy coefficient, the higher the external loading required to propagate a new fracture. The numerical results obtained in this paper provide theoretical supports for establishing basis on investigating of the hydraulic fracturing characteristics under different conditions.

Strain localization in hydraulic fracturing of granite specimens based on DIC method

Hydraulic fracturing is a crucial and widely-used technology in shale gas exploitation and enhanced geothermal systems, while the fundamental mechanisms of hydraulic fracturing are not comprehensively understood. This paper aims to promote the understanding of strain localization in hydraulic fracturing of granite specimens based on digital image correlation (DIC) method. Through a specially designed pressure enclosure, the hydraulic fracturing processes were captured by a high-speed camera. The strain localization phenomenon, evolution, and coalescence of SLZs in hydraulic fracturing were analyzed. Results show that obvious strain localization occurs before the specimen failure, which gradually develops into a continuous strain localization zone. With the evolution of SLZs, the specimen will be unable to bear the liquid pressure, and macro cracks occur at SLZs. The statistical data in the query region shows that the strain localization in hydraulic fracturing occurs earlier than previously thought, and the evolution of SLZs can be divided into steady, soaring, and fracture stages according to the changing trend. The coalescence of the patchy distribution SLZs occurs frequently throughout the evolution process of SLZs, and the coalescence of SLZs may be the cause of the increasing speed of evolution.

Temperature-dependent factors on hydraulic fracturing of hot dry rock (HDR): An experimental investigation

Hot dry rock (HDR) is rich in geothermal energy and renewable and does not cause pollution; therefore, it has attracted considerable research attention. Previous studies have extensively discussed the process of HDR hydraulic fracturing based on the crack initiation mechanism, initiation pressure, and crack propagation law. However, the temperature-dependent factors (thermal damage and thermal shock) affecting the HDR hydraulic fracturing process have not been specifically discussed. Therefore, HDR hydraulic fracturing tests with different sample temperatures and injection rates were designed in this study. Further, various factors affecting the hydraulic fracturing related to temperature, including the thermal damage, thermal shock, and injection rate, were analyzed. The test results indicate that under the condition of variable temperature parameters, thermal damage and thermal shock caused the initiation pressure of the reservoir to decrease with the increasing temperature. The degree of weakening increased with the increasing temperature. After fracturing, the initial fracture in HDR continued to expand under the action of thermal shock and residual pressure, and the fracture expansion scale increased with the increasing temperature. Under the condition of variable fracturing parameters, microfractures with large areas were more likely to form when fracturing occurred at a lower injection rate, which is beneficial to improve the heat exchange efficiency of the reservoir and avoid short circuits. These research results provide a scientific explanation for the influencing factors of HDR hydraulic fracturing and offer geological and engineering guidance for optimizing the hydraulic fracturing in an enhanced geothermal system.

Multi-Basin Real-Time Perforation Assessment Using Tube Wave Measurements

This article addresses a means to improve hydraulic fracturing operations by measuring the perforation effectiveness on a stage-by-stage basis before the hydraulic fracturing process begins. Observations lead to the conclusion that pre-frac measurements provide meaningful insight about well-to-reservoir opening, allowing operators to identify the difficult-to-treat stages before the treatment begins. Based on the results of a case study from a North American unconventional basin, the limitations and conditions for employing pre-frac measurements are discussed. Background Often, perforation operations fail to initiate a well-established hydraulic communication between the wellbore and the reservoir rock. When hydraulic fracturing operations begin, a lack of wellbore to reservoir connectivity may cause unexpected difficulties such as unusually high pumping pressures, difficulty injecting the design volumes of fluid and proppant, and/or the inability to achieve design pumping rate or screenouts. The hydraulic fracturing industry is increasingly looking for an efficient, nonintrusive method for identifying poor wellbore to reservoir connectivity. Currently, step-rate tests are commonly used (Massaras et al. 2007) to give the operator an estimate of open perforations taking fluid, as well as frictional losses within the wellbore and near-field region. However, these tests are costly, time-consuming, and rarely accurately confirm the number of poor perforations (Cramer et al. 2019). Fiber-optic technologies can improve these uncertainties but are costly and susceptible to damage during installation and/or operation. How the Method Works Low-frequency hydraulic tube waves, or Stoneley waves (Stoneley 1924), are induced at the surface by equipment connected to the wellhead. Tube waves travel rapidly through the fluid within the wellbore and reflect by any changes within the hydraulically connected parts of the wellbore. In this case, the reflection off the bottom of the well influences the tube-wave properties - dispersion and attenuation of the normal modes (Hsu, Kostek, and Johnson 1997), depending on the condition and number of the perforations, the quality of their connection with the near-field region, and the quality of the near-field region itself (e.g., a region already naturally fractured) [Dunham et al. 2017].

Investigation of hydraulic fracturing mechanism by using a coupled continuous-discontinuous hydromechanical model

Fluid-induced fracture nucleation, propagation, and interaction are the essentials for a better understanding of the hydraulic fracturing process in unconventional reservoirs. In this study, a coupled continuous-discontinuous hydromechanical model was established to investigate the hydraulic fracturing propagation under varied conditions. The interactions between induced fractures and natural fractures are investigated and discussed by a series of hydraulic fracturing simulations. Also considered are the influences of bedding joints, in-situ stress ratios, and fluid injection rates on the patterns of hydraulic fractures and the stress field. It was found that hydraulic fracture propagation is controlled by both in-situ stress state and strength anisotropy of the reservoir rock. The simulations indicate that an increase in the fluid injection rate is favorable to the formation of a complex fracture network. More hydraulic fractures were developed when fracture fluids were injected into rock specimens with a faster pressurization rate than a quasi-static pressurization rate. However, higher fluid injection rates could result in higher breakdown pressures for fracture initiation and propagation. In addition, hydraulic fractures tend to extend along the direction of the maximum principal stress or approach this preferred path. Bedding joints are preferred locations and orientations for fracture initiation and propagation in laminated shale reservoirs.

An experimental investigation into the influence of pre-existing natural fracture on the behavior and length of propagating hydraulic fracture

Propagation of hydraulic fracture (HF) in the presence of natural fracture (NF) fundamentally differs with fracture propagation in reservoirs without natural fracture. Natural fracture can significantly affect the behavior and length of propagating hydraulic fracture. In this study, a series of true triaxial hydraulic fracturing tests on large size synthetic rock specimens with a single pre-existing natural fracture was conducted in the laboratory scale in order to investigate the propagation behavior and length of hydraulic fracture in naturally fractured reservoirs. The parameters of laboratory experiments were scaled based on the scaling analysis to simulate hydraulic fracturing process at the field conditions. For the purpose of controlling the hydraulic fracture propagation behavior, different values for approaching angle, θ (30°–90°) and horizontal differential stress, Δσ (1.5–10 MPa) were considered. Four modes of interaction between hydraulic fracture and pre-existing natural fracture were found including (1) net-crossing, (2) net-opening, (3) crossing-opening and (4) opening-crossing. Modes of crossing-opening and opening-crossing were introduced as two new behaviors of hydraulic fracture propagation. These interaction modes play a significant role on the length of growing hydraulic fracture. It was observed that by increasing the approaching angle and differential stress, the hydraulic fracture propagation behavior change from opening (net-opening and opening-crossing) to crossing (crossing-opening and net-crossing) and, consequently, the length of hydraulic fracture increases from a minimum of 9.10 cm to a maximum of 14.30 cm. The fracture initiation (break-down) pressure decreases with the increase of differential stress from 1.5 MPa to 10 MPa. Besides, increasing the differential stress and approaching angle results in a decrease in the pressure fluctuations of hydraulic fracture propagation.

Fracture Hits in Unconventional Reservoirs: A Critical Review

Summary“Fracture hit” was initially coined to refer to the phenomenon of an infill-well fracture interacting with an adjacent well during the hydraulic-fracturing process. However, over time, its use has been extended to any type of well interference or interaction in unconventional reservoirs. In this study, an exhaustive literature survey was performed on fracture hits to identify key factors affecting the fracture hits and suggest different strategies to manage fracture hits. The impact of fracture hits is dictated by a complex interplay of petrophysical properties (high-permeability streaks, mineralogy, matrix permeability, natural fractures), geomechanical properties (near-field and far-field stresses, tensile strength, Young’s modulus, Poisson’s ratio), completion parameters (stage length, cluster spacing, pumping rate, fluid and proppant amount), and development decisions (well spacing, well scheduling, fracture sequencing). It is difficult to predict the impact of fracture hits, and they affect both parent and child wells. The impact on the child wells is predominantly negative, whereas the effect on parent wells can be either positive or negative. The “child wells” in this context refer to the wells drilled with pre-existing active/inactive well(s) around. The “parent well” refers to any well drilled without any pre-existing well around. Overall, fracture hits tend to negatively affect both the production and economics of lease development. The optimal approach rests in identifying the reservoir properties and accordingly making field-development decisions that minimize the negative impact of fracture hits. The different strategies proposed to minimize the negative impact of fracture hits are simultaneous lease development, thus avoiding parent/child wells (i.e., rolling-, tank-, and cube-development methods); repressuring or refracturing parent wells; using far-field diverters and high-permeability plugging agents in the child-well fracturing fluid; and optimizing stage and cluster spacing through modeling studies and field tests. Finally, the study concludes with a recommended approach to manage fracture hits. There is no silver bullet, and the problem of fracture hits in each shale play is unique, but by using the available data and published knowledge to understand how fractures propagate downhole, measures can be taken to minimize or even completely avoid fracture hits.

Study on Breakdown Pressure in Hydraulic Fracturing Process of Hard Rock Based on Numerical Simulation

Pre-conditioning by hydraulic fracturing is a vital work in block caving mining of hard-rock metal mine, and breakdown pressure is one of the important parameters in rock hydraulic fracturing process, which determines the equipment selection and job-parameter control. Based on numerical simulation method, a pre-holed cylindrical hard rock sample was built for simulation of hydraulic fracturing process to investigate the effect of stress condition, rock properties and water injection rate on breakdown pressure. The results show that the breakdown pressure is increased with the confining stress, and the breakdown pressure is 2.28 times confining stress based on rock tension strength. The variation of rock tension strength affects the breakdown pressure and a positive linear relation exists. Pre-formed notch could obviously reduce the breakdown pressure and control the orientation of hydraulic fracture. And the borehole diameter has a reverse influence on rock breakdown pressure.

Disclosing Influence: Hydraulic fracturing, interest groups, and state policy processes in the United States

This paper examines copy-and-paste regulating in hydraulic fracturing (HF) fluid disclosure regulation across US states. Using text analysis, cluster analysis and document coding, we compare HF regulations of twenty-nine states and two “model bills” drafted by the conservative American Legislative Exchange Council (ALEC) and the Environmental Defense Fund (EDF, an environmental NGO). In contrast to recent studies that have documented ALEC’s widespread influence across policy domains, we find limited evidence of ALEC influence in HF fluid disclosure regulations. Instead, elements of the EDF bill are more prevalent across state regulations. Yet, text similarity scores between states are higher than similarity scores between states and the EDF bill. In particular, Colorado and to a lesser extent Pennsylvania functioned as leader states for other states to follow. This indicates that state-to-state communication was a more influential channel of policy diffusion than interest group model bills in this instance. Future research should better examine processes of information sharing amongst state oil and gas regulators as regulatory text is but one channel of policy diffusion. The cluster analysis also reveals that contiguous states, often within the same shale basins, frequently have different regulations. This finding calls for a reconsideration of the current state-led environmental regulatory framework for HF, which has resulted in a patchwork of regulations across the US. Finally, through the use of novel text analysis tools, this paper adds methodological diversity to the study of policy diffusion within energy policy.

Multifractal Analysis of Acoustic Emissions during Hydraulic Fracturing Experiments under Uniaxial Loading Conditions: A Niutitang Shale Example

Fracture characterization is essential for estimating the stimulated reservoir volume and guiding subsequent hydraulic fracturing stimulations in shale reservoirs. Laboratory fracturing experiments can help provide theoretical and technical guidance for field operations. In this study, hydraulic fracturing experiments on the shale samples from Niutitang Formation in Hunan Province (China) under a uniaxial loading condition are conducted. The multifractal method is used to analyze the acoustic emission (AE) signals and characterize fracture initiation and propagation. The hydraulic fracturing process can be divided into three stages based on the characteristics of AE signals: the initial stage, the quite stage, and the fracturing stage. The multifractal analysis results showed that: (1) the value of the spectrum width, Δα, continues to increase as the energy accumulates until the fracturing stage starts; and (2) the difference in the multifractal spectrum values, Δf, reflects the relationship between small and large signal frequencies and can quantify the fracture scale, i.e., the lower the Δf, the larger the fracture scale and vice versa. The results were further verified using a time-frequency analysis of the AE signals and micro-CT scanning of the samples. This study demonstrates that the multifractal method is feasible for quantitatively characterizing hydraulic fractures and can aid field hydraulic fracturing operations.

Impacts of natural fractures on hydraulic fracturing treatment in all asymptotic propagation regimes

Hydraulic fracturing is a technique in which pressurized fluid is pumped into the well to induce fracture propagation in the rock formation. The treatment aims at enhancing permeability and well-reservoir connectivity. However, the presence of natural fractures can impact the hydraulic fracture propagation, increasing the complexity of the hydraulic fracturing treatment, and affect the final configuration of the fracture network. Furthermore, different propagation regimes can develop depending on field conditions, properties of the porous matrix, fractures, the injection fluid, and time. This work introduces a robust fully coupled hydro-mechanical approach to investigate the impacts of natural fractures on hydraulic fracturing in four limiting propagation regimes: toughness-storage, toughness-leak-off, viscosity-storage, and viscosity-leak-off dominated. The proposed approach is based on the finite element method and incorporates the coupling of pore pressure/stress within the permeable rock formation and fracture propagation. An innovative mesh fragmentation technique with an intrinsic pore-cohesive zone approach is implemented in the in-house multiphysics framework to simulate fracture propagation with complex crack patterns. Cohesive Zone Model (CZM) represents the initiation and propagation of hydraulic fractures while a contact model with the Mohr–Coulomb criterion is used to represent the normal closure/opening and friction/shear dilation of natural fractures. The results of the new approach are compared against analytical and numerical solutions. Moreover, the influence of parameters such as rock permeability, fluid viscosity, initial stress state, and intercepting angle on the hydraulic and natural fracture is also investigated. The robustness of the presented methodology is demonstrated by simulating crossing with an offset, branching, fracture propagation from the tip of a natural crack, and interaction of multiple cracks. These results can provide guidance for a better understanding of the complex process of hydraulic fracturing.

Experimental study on the permeability of methane hydrate-bearing sediments during triaxial loading

The permeability of methane hydrate (MH)-bearing sediments is closely related to the exploitation efficiency of MH deposits. During the exploitation of MHs, the stress state of hydrate-bearing reservoirs is complex, and stratum deformation may occur simultaneously. Both the change in the stress state and stratum deformation may influence the permeability of hydrate-bearing sediments. In this paper, a series of permeability measurement tests was conducted to study the influence of the effective confining pressure and triaxial compression process on the permeability of MH-bearing samples. The test results indicated that the permeability of hydrate-bearing samples was influenced by the effective confining pressure. With increasing effective confining pressure, the permeability of the pure sand sample gradually decreased. For the hydrate-bearing samples, the permeability was also influenced by the effective confining pressure. When the effective confining pressure reached a specific value, a sharp drop in the permeability of the hydrate-bearing samples occurred. The change in permeability might be caused by the increase in fine hydrate particles in the pore space. Based on the test results, the permeability of the hydrate-bearing samples was influenced by the triaxial compression process. During the shearing process, the permeability greatly increased when the axial strain reached a specific value. This phenomenon might be caused by the formation of a shear band. During the production of hydrates, controlling the change in stress applied to the hydrate particles could possibly promote production. Furthermore, the appearance of shear bands or fissures might also increase production by certain methods, such as the hydraulic fracturing process.

Study on the method of step-up hydraulic fracturing infiltration for soft coal seam

In view of the dilemma of the current hydraulic fracturing effect of soft coal seams, based on the existing hydraulic fracturing technology, a model corresponding to the jump pressure and the length of fracture propagation during the stepwise hydraulic fracturing process was constructed, the influence of the elastic modulus of coal and rock mass and the step-up pressure on the crack extension length during crack propagation was analyzed, and a step-up hydraulic fracturing infiltration method suitable for soft coal seam was formed. The research results show that the coal seam has strong stress sensitivity, and in the step-by-step step-up fracturing process, the coal seam cracks expand as the energy potential increases step by step, and they always extend along the direction of the maximum principal stress. The effect of infiltration is very obvious. In the process of fracture propagation, the elastic modulus of coal and rock mass, and the pressure difference of gradual jump have a great influence on the length of fracture propagation, and the initial crack width and crack height have a certain effect on the effect of fracture expansion and infiltration of the gradual jump hydraulic fracturing. The newly proposed step-up hydraulic fracturing and infiltration method can effectively extend the soft coal seam rock mass gradually when the cracks are fully developed, and has a good application prospect.

Hybrid Multiobjective Optimization Using Deterministic and Metaheuristic Techniques for Flowback Water Reusing in Hydraulic Fracturing Processes

This paper presents a new optimization framework that reconciles deterministic and metaheuristic optimization in order to solve nonconvex models. In this way, it is possible to solve the convex part in a software specialized in deterministic optimization and the nonconvex part using metaheuristic tools. The chosen software for the deterministic optimization part was general algebraic modeling system (GAMS), and the metaheuristic optimization algorithm is the improved multiobjective differential evolution (I-MODE) coded in visual basic for applications (VBA). Likewise, a linking strategy is proposed through VBA code to execute the GAMS solver. The new solution strategy consists in manipulating the data that serve as parameters in the problem to be solved in the GAMS platform and evaluating the performance of the optimal values of the objective functions in the I-MODE algorithm with updated values for the involved parameters. The proposed methodology is applied to the hydraulic fracturing (HF) process to obtain shale gas. The problem related to HF process corresponds to the use of fresh water necessary for the extraction of shale gas, as well as the type of technology that can be used in the treatment of flowback water. The mathematical model that optimizes the return water management problem is solved using deterministic optimization, while the scheduling is fixed using evolutionary algorithms of metaheuristic optimization strategies. The obtained results offer attractive alternatives for the specified objective functions in an acceptable computation time.

Hydraulic fracturing: New uncertainty based modeling approach for process design using Monte Carlo simulation technique.

Hydraulic fracturing is a key method used in completion of shale gas wells as well as in well stimulation. There are a lot of factors affecting the hydraulic fracturing treatment; i.e. formation in-situ stresses, fracturing fluid properties, proppant, pumping rate, reservoir fluid and rock properties…etc. For predictive modeling, these factors are associated with a lot of uncertainties, since most of them are laboratory measured, calculated or subjectively estimated. Moreover, the precise contribution of each factor on the final fracturing result is unknown for each individual case. Therefore, for better treatment performance and in order to find the best range of designing parameters, a hydraulic fracturing predictive model that involves these uncertainties is required specially for newly exploited shale gas reservoir. In this paper a new uncertainty-based approach is described for hydraulic fracturing processes. It is based on assigning probability distribution for some variables and parameters used in the designing process. These probability distributions are used as input data for analytical equations that describe the fracturing processes. Monte Carlo Simulation technique is used to apply uncertainty-based values on the designing analytical formulas. A hypothetical hydraulic fracturing example is used to simulate the effect of different variables and designing parameters on the entire fracturing process. The simulation results are illustrated into probability distribution curves and variance-based sensitivity analysis is performed to assess the contribution and the correlation between different variables and outcomes. Fracture geometry is almost controlled by the injection fluid’s viscosity, in case of constant injection rate; while rock properties have insignificant effect on the fracture width compares to fracturing fluid’s effect. Therefore more emphases shall be directed to rheological modeling of the fracturing fluid. It is found also that fracture height, which is difficult to be estimated, is the most crucial parameter in the calculation of treatment size or the injected fluid’s volume. Proppant porosity, injected fluid viscosity and formation strength are slightly affecting propped fracture width, while proppant final concentration plays the main role of determining the calculated propped fracture width. It is observed from the simulation results that the initial formation permeability will extremely affect the post fracturing skin factor while other formation rock properties have almost no effect on the skin factor. Throughout the implementation of the uncertainty-based modeling approach for hydraulic fracturing process design, it is found that uncertainties in the value of many variables and parameters are slightly affecting the process outcomes. However, injected fluid viscosity, shale formation permeability and proppant final concentration are found to be the most influencing factors in the entire process. Therefore, it is highly recommended to perform in-depth study for these factors prior conducting any designing process of hydraulic fracturing.

Numerical simulation of hydraulic fracturing based on two-dimensional surface fracture morphology reconstruction and combined finite-discrete element method

Hydraulic fracturing is an important technology widely applied in engineering practice for the exploitation of unconventional oil and gas resources. Further research on its mechanism has the potential to advance the development of unconventional reservoir exploitation. In this study, previously acquired experimental data, including surface fracture image and fluid pressure curve, were used for modelling and comparison. Employing digital core reconstruction and the combined finite-discrete element method, A process of modelling crack geological model was proposed based on the rock surface sketch after fracturing. Two-dimensional models were established according to the surface fracture morphology of specimens. The feasibility of these numerical models was simultaneously verified by fluid pressure curves and the surface fracture morphology obtained in physical experiments. The evolution of fracture morphology during hydraulic fracturing was analysed. Simulation results indicate that: (1) the simulation method of this study is more suitable for simulation of coal hydraulic fracturing than for shale hydraulic fracturing; (2) the rapid rise of fluid pressure might be accompanied with the initiation and development of micro-fractures, leading to the occurrence of larger fractures; (3) the hydraulic fracturing process on the experimental scale could induce significantly larger maximum total area and wider fractures in coal specimens than in shale specimens. This indicates that coal hydraulic fracturing forms more complex fractures than in shale; (4) when the rock elastic modulus increases, the maximum fracture width and fracture total area decreases, and fracture length is extended. (5) The initiation and propagation pressures are positively related to the elastic modulus in the fluid pressure evolution. The crack geological model proposed in this study provides direct guidance for crack reconstruction in simulations. Moreover, the research results may provide a guidance for studies on the evolution of hydraulic fracture under the influence of rock heterogeneity.

An integrated approach to characterize hydraulic fracturing-induced seismicity in shale reservoirs

There has been a remarkable increase in induced seismicity in Western Canada and the majority of these events can be related to hydraulic fracturing operations applied to develop unconventional reservoirs. The underlying mechanism of such induced seismicity is still unclear and needs to be investigated to mitigate risks of future events. In this paper, we propose a new integrated approach to characterize hydraulic fracturing-induced seismicity. Inferred faults in the target area were first identified by the ant-tracking approach and hydraulic fractures were characterized by simulating hydraulic fracturing processes. The coupled flow-geomechanics modeling was then performed to compute the Coulomb Failure Stress and determine the reactivation of pre-existing faults. A case study was finally utilized to demonstrate the applicability of the integrated method. Four north-south-oriented faults were interpreted and half-length of hydraulic fractures were calculated from 84 to 127 m. The calculated results of the flow-geomechanics model were in good agreement with the actual induced seismicity spatially and temporally. Three types of triggering mechanisms were accounted for HF-induced seismicity, including hydraulic fractures propagation, the connection between hydraulic fractures and a fault and connections of hydraulic fractures with the natural fractures around the fault. The injection rate could be decreased to mitigate risks of future seismic events.

Hydroshearing poorly connected preexisting fractures in the presence of stress anisotropy as a random percolation process

This work shows that the hydraulic fracturing process in shale formations, where natural fractures are abundant but are poorly connected, is universal. The authors demonstrate that the correlation length of the natural fractures and the anisotropy in the stress field do not influence fluid propagation and the universality class of the fracturing process is the same as that of opening bonds randomly in a diluted bond lattice.