Hydraulic Fracture Initiation Research Articles

Experimental study on the influencing factors of hydraulic fracture initiation from prefabricated crack tips

In this work, large-scale true triaxial tests were conducted with sandstone specimens to investigate several factors that determine whether hydraulic fractures can propagate from the tips of axial prefabricated cracks (pre-cracks). Specifically, we investigated the influence of pre-crack inclination angles on hydraulic fracture initiation locations and how the critical pre-crack angle varies with different confining stresses, fluid viscosities, and leak-off effects. The experimental results show that (1) the critical pre-crack angle declines with an increase in the horizontal stress difference. When the viscosity of the fracturing fluid is maintained at 70 mPa·s, as the horizontal stress difference increases from 2 MPa to 3 MPa to 4 MPa, the critical pre-crack angle decreases from 90° to 75° to 70°. (2) The critical pre-crack angle increases with an increase in the fracturing fluid viscosity. With a 3 MPa horizontal stress difference, the critical pre-crack angle increases from 60° to 70° to 75° as the viscosity of the fracturing fluid increases from 1 mPa·s to 40 mPa·s to 70 mPa·s. Under the condition that the horizontal stress difference is set to 4 MPa, as the fracturing fluid viscosity increases from 1 mPa·s to 40 mPa·s to 70 mPa·s, the critical pre-crack angle increases from 50° to 65° to 70°. (3) After waterproof treatment of the boreholes and pre-cracks, the critical pre-crack angle increases from 75° to 80° to 85° as the viscosity of the fracturing fluid increases from 1 mPa·s to 40 mPa·s to 70 mPa·s under the condition that the horizontal stress difference is fixed at 4 MPa.

Numerical simulation on the effect of pore pressure gradient on the rules of hydraulic fracture propagation

To clarify the influence of pore pressure gradient on hydraulic fracture propagation, the stress distribution in and around the borehole is explained by theoretical analysis method in this paper. A mechanical model of hydraulic fracture initiation under the action of pore pressure gradient is established. Then coupled seepage-stress-damage software is used to simulate the initiation and propagation of hydraulic fractures in rock samples under the action of pore pressure gradient. Finally, the influence of the number and spatial position of the induction holes on the initiation and propagation of hydraulic fractures is analyzed. It is shown that: (1) Pore pressure gradient can effectively reduce the initiation pressure of hydraulic fractures. (2) The greater the pore pressure gradient is, the easier the hydraulic fracture is to spread to the region with high pore pressure. (3) With the action of pore pressure gradient, the hydraulic fracture is shaped as ‘丨’, ‘丿’ and ‘S’ types and can be represented by the four abstract conceptual models.

Multiple Fracture Growth in Modified Zipper Fracturing

Operational parameters, such as multiple-cluster and well spacing, flow rates, and injection time, play a vital role in the deviation and growth of multiple-cluster hydraulic fractures during the implementation of modified zipper fracturing technology. Fundamental analysis of the dynamic zipper fracture growth because of changing flow rates, well spacing, and injection time does not appear to have been studied in the literature, although it is of critical importance for fracturing optimization. In this study, a fully coupled two-dimensional (2D) extended finite-element method (XFEM)–cohesive zone model (CZM) in combination with a phantom-node method (CPNM) was used in ABAQUS to investigate the initiation and propagation of multiple hydraulic fractures with 10 injection clusters that are distributed in two horizontal boreholes. This study aims to assist fracturing design to achieve minimal deviation in multiple fracture propagation and create deeper and much wider fractures between two horizontal boreholes of a modified zipper fracture (MZF) completion pattern. The growth of a hydraulic fracture in an XFEM region were verified by Khristianovic–Geertsma–de Klerk (KGD) zero toughness solution and good agreement with a negligible error of <2.5% was obtained. A series of simulation scenarios was carried out with various flow rates and three well spacings of 40, 60, and 80 m, over a long injection period to identify the optimum injection time in which a fracture grows straight with minimal deviation. The results show that to create smooth and deeper multiple hydraulic fractures in zipper fracturing, an increase in the flow rate is preferred, but the duration of the injection period should be optimized. The results of this study could be used to better address operational difficulties, such as multiple-fracture crossing and closure that is associated with unfavorable injection times, well spacing, and flow rates in MZF operations.

Numerical Analysis of Multiple Factors Affecting Hydraulic Fracturing in Heterogeneous Reservoirs Using a Coupled Hydraulic-Mechanical-Damage Model

Hydraulic fracturing performance, affected by multiple factors, was essential to the economic exploitation of oil and gas in heterogeneous unconventional reservoirs. Multifactor analysis can gain insight into the fracturing response of reservoirs and in turn optimize the treatment design. Based on characterizations of the geological setting of a heterogeneous glutenite reservoir, the hydraulic fracture (HF) initiation and propagation process, as well as the stimulated reservoir volume (SRV), were simulated and analyzed using a coupled hydraulic-mechanical-damage model. The Weibull distribution was employed to describe rock heterogeneity. The numerical model was verified with microseism (MS) interpretation results of HF geometry. A multifactor analysis and optimization workflow integrating response surface methodology, central composite design (CCD), and numerical simulations was proposed to investigate the coupling effects of multiple geomechanical and hydrofracturing factors on SRV and identify the optimum design of fracturing treatment. The results showed that the horizontal stress difference and injection rate were the most significant factors to control the SRV. Increasing the injection rate and reducing fluid viscosity may contribute to improving the SRV. It is more difficult to increase the SRV at higher horizontal stress difference than at lower horizontal stress difference. The multifactor analysis and optimization workflow introduced in this work was a practical and effective method to control the HF geometry and improve the SRV. This study provided a deep understanding of the hydraulic fracturing mechanism and possessed theoretical significance for treatment design.

Complexity and tortuosity hydraulic fracture morphology due to near-wellbore nonplanar propagation from perforated horizontal wells

Perforated fracturing enhances single well production in the tight sandstones formations with low porosity and permeability effectively, however, some issues are caused after such achievements, such as high breakdown pressure, short fracture length, and limited proppant migration. All are tightly related to hydraulic fracture complex propagation behavior near wellbore. To understand its mechanism, a 3D near-wellbore fracture propagation model is established based on the combination of finite element method and post-failure damage mechanism. The model is applied on a cased and perforated horizontal well to investigate fracture initiation characteristics, near-wellbore fracture non-planar propagation behavior, and torturous hydraulic fracture morphology. Moreover, the effects of stress shadow and perforation parameters are discussed. Based on induced hydraulic fracture initiation position and propagation speed, three main fracture propagation behaviors are observed under the stress shadow effect in simulation results: limited propagation, nonuniform circular propagation, and opposite direction propagation. These nonplanar propagation behaviors cause complexity and tortuosity hydraulic fracture morphologies near the wellbore, which could be mainly divided into four types: spiral-shaped fracture, multi-spiral shaped fracture, horizontal-vertical crossing shaped fracture, and multi-layered shaped fracture. In addition, the reasonable perforation parameter design could reduce torturous fracture surface, resulting in a near flat fracture. The increase in perforation density or perforation diameter could shorten perforation spacing to provide a small diversion angle for induced fractures to interact with each other and form a small spiral degree fracture geometry. However, increased perforation phase causes many perforations in the same direction to create a longitudinal fracture that has a short fracture length for the horizontal well. Therefore, based on above analyses, high perforation density, small perforation phase, and large perforation diameter are suggested to be an effective perforation parameter combination to conduct the perforated fracturing in tight sandstone formation, in order to create a near flat fracture surface with a minimum tortuosity as well as to increase hydraulic fracture propagation area.

Numerical simulation of the laws of fracture propagation of multi-hole linear co-directional hydraulic fracturing

Directional rupture is one of the most important and most common problems related to rock breaking. The goal of directional rock breaking can be effectively achieved via multi-hole linear co-directional hydraulic fracturing. In this paper, the XSite software was utilized to verify the experimental results of multi-hole linear co-directional hydraulic fracturing., and its basic law is studied. The results indicate that the process of multi-hole linear co-directional hydraulic fracturing can be divided into four stages: water injection boost, hydraulic fracture initiation, and the unstable and stable propagation of hydraulic fracture. The stable expansion stage lasts longer and produces more microcracks than the unstable expansion stage. Due to the existence of the borehole-sealing device, the three-dimensional hydraulic fracture first initiates and expands along the axial direction in the bare borehole section, then extends along the axial direction in the non-bare hole section and finally expands along the axial direction in the rock mass without the borehole. The network formed by hydraulic fracture in rock is not a pure plane, but rather a curved spatial surface. The curved spatial surface passes through both the centre of the borehole and the axial direction relative to the borehole. Due to the boundary effect, the curved spatial surface goes toward the plane in which the maximum principal stress occurs. The local ground stress field is changed due to the initiation and propagation of hydraulic fractures. The propagation direction of the fractures between the fracturing boreholes will be deflected. A fracture propagation pressure that is greater than the minimum principle stress and a tension field that is induced in the leading edge of the fracture end, will aid to fracture intersection; as a result, the possibility of connecting the boreholes will increase.

A Semianalytical Modeling Approach for Hydraulic Fracture Initiation and Orientation from Perforated Wells

Summary Accurate prediction of fracture initiation pressure and orientation is paramount to the design of a hydraulic fracture stimulation treatment and is a major factor in the treatment's eventual success. In this study, closed-form analytical approximations of the fracturing stresses are used to develop orientation criteria for relative-to-the-wellbore (longitudinal or transverse) fracture initiation from perforated wells. These criteria were assessed numerically and found to overestimate the occurrence of transverse fracture initiation, which only takes place under a narrow range of conditions in which the tensile strength of the rock formation is lower than a critical value, and the breakdown pressure falls within a “window.” For a case study performed on the Barnett Shale, transverse fracture initiation is shown to take place for breakdown pressures below 4,762 psi, provided that the formation's tensile strength is below 2,482 psi. A robust 3D finite volume numerical model is used to evaluate solutions for the longitudinal and transverse fracturing stresses for a variable wellbore pressure, hence developing correction factors for the existing closed-form approximations. Geomechanical inputs from the Barnett Shale are considered for a horizontal well aligned parallel to the direction of the least compressive horizontal principal stress. The corrected numerically derived expressions can predict initiation pressures for a specific orientation of fracture initiation. Similarly, at known breakdown pressures, the corrected expressions are used to predict the orientation of fracture initiation. Besides wellbore trajectory, the results depend on the perforation direction. For the Barnett Shale case study, which is under a normal faulting stress regime, the perforations on the side of the borehole yield a wider breakdown pressure window by 71% and higher critical tensile strength by 32.5%, compared to perforations on top of the borehole, implying better promotion of transverse fracture initiation. Leakage of fracturing fluid around the wellbore, between the cemented casing and the surrounding rock, reduces the breakdown pressure window by 11% and the critical tensile strength by 65%. Dimensionless plots are employed to present the range of in-situ stress states in which longitudinal or transverse hydraulic fracture initiation is promoted. This is useful for completion engineers; when targeting low permeability formations such as shale reservoirs, multiple transverse fractures must be induced from the horizontal wells, as opposed to longitudinal fracture initiation, which is desired in higher permeability reservoirs or “frac-and-pack” operations.

Study on the Microscale Tensile Properties of Lower Cambrian Niutitang Formation Shale Based on Digital Images

Tensile strength is an important parameter that affects the initiation and propagation of shale reservoir fractures during hydraulic fracturing. Shale is often filled with minerals such as calcite. To explore the effect of calcite minerals on the tensile strength and failure mode of shale, in this paper, lower Cambrian shale cores were observed by microslice observations and core X-ray whole-rock mineral diffraction analysis, and 7 groups of numerical direct tensile tests were performed on simulated shale samples with different azimuth angles. The test results show that as the azimuth angle α increases, the tensile strength of the samples gradually decreases, and the fracture rate also shows a decreasing trend. The failure modes can be summarized as root-shaped (0° and 15°), step-shaped (30 and 45°), fishbone-shaped (60°), and river-shaped (75° and 90°) fracturing. The smaller the azimuth angle α, the easier it is for hydraulic fractures to propagate along the direction of the calcite veins and inhibit the formation of fracture networks in the shale matrix. Considering the correlation between the acoustic emission characteristics and failure mode, the fractal dimension is used to reflect the microscopic failure mode of shale. The larger the fractal dimension, the higher the fracture rate is, the more microcracks exist at the edge of the main crack, the more severe the internal damage is, and the more complex the failure mode of the sample is. As the azimuth angle α increases, the fractal dimension shows a decreasing trend, and the crack becomes smoother. This research has important reference value for the study of hydraulic fracture initiation mechanisms and natural fracture propagation.

Experimental and Numerical Evaluation of Hydraulic Fracturing under High Temperature and Embedded Fractures in Large Concrete Samples

In order to study the mechanism of hydraulic fracturing in enhanced geothermal systems, we analyzed the influence of high temperatures and embedded fractures on the initiation and propagation of hydraulic fractures using a laboratory test and numerical simulation. The analysis was conducted via large-scale true triaxial hydraulic fracturing tests with acoustic emission monitoring. Moreover, we discussed and established the elastic-plastic criterion of hydraulic fracturing initiation. The corresponding fracturing procedure was designed and embedded into the FLAC3D software. Then, a numerical simulation was conducted and compared with the laboratory test to verify the accuracy of the fracturing procedure. The influence of high temperatures on hydraulic fracturing presented the following features. First, multi-fractures were created, especially in the near-well region. Second, fracturing pressure, extension pressure, and fracture flow resistance became larger than those at room temperature. 3D acoustic fracturing emission results indicated that the influence of the spatial distribution pattern of embedded fractures on hydraulic fracturing direction was larger than that of triaxial stress. Furthermore, the fracturing and extension pressures decreased with the increase of embedded fracture density. For hydraulic fracturing in a high temperature reservoir, a plastic zone was generated near the borehole, and this zone increased as the injection pressure increased until the well wall failed.

Study on non-plane propagation characteristics of fracturing fractures in tight sandstone of Qiulin block under high geo-stress

In order to study the propagation characteristics of hydraulic fracturing fractures in tight sandstone reservoirs in Qiulin block, Sichuan Basin, hydraulic fracturing of tight sandstone under high triaxial in-situ stress were carried out by using a large true triaxial fracturing physical experimental system. The fracture morphology and pump pressure curves under different in-situ stress, fracturing fluid types and displacement were obtained. The experimental results show that: (1) under high geo-stress conditions, the fracture morphology of tight sandstone after hydraulic fracturing is relatively simple, and complex fracture characteristics are less formed. The sedimentary characteristics and the development of internal natural fractures are the main control factors affecting the initiation and propagation of hydraulic fracturing fractures. (2) The characteristics of in-situ stress affect the fracture initiation and propagation direction. When the vertical in-situ stress is equal to the horizontal minimum principal stress, the horizontal fracturing fractures perpendicular to the vertical in-situ stress direction can be formed, and most of the fracturing fractures are non-plane propagation fractures. (3) When the low displacement slick water is used for fracturing, the fracturing fluid is easy to enter into the micro fractures inside the tight sandstone, which is helpful to open the original micro fractures, and the fracture morphology after fracturing is relatively complex; when the high displacement slick water or high viscosity linear gel is used for fracturing, the fractures with non-planar expansion are formed after fracturing. The research results can provide some technical support for the reservoir reconstruction of the second member of ES_2 formation in Qiulin block, Sichuan.

Analysis of hydraulic fracture behavior and well pattern optimization in anisotropic coal reservoirs

Macrolithotypes control the pore-fracture distribution heterogeneity in coal, which impacts stimulation via hydrofracturing and coalbed methane (CBM) production in the reservoir. Here, the hydraulic fracture was evaluated using the microseismic signal behavior for each macrolithotype with microfracture imaging technology, and the impact of the macrolithotype on hydraulic fracture initiation and propagation was investigated systematically. The result showed that the propagation types of hydraulic fractures are controlled by the macrolithotype. Due to the well-developed natural fracture network, the fracture in the bright coal is more likely to form the “complex fracture network”, and the “simple” case often happens in the dull coal. The hydraulic fracture differences are likely to impact the permeability pathways and the well productivity appears to vary when developing different coal macrolithtypes. Thus, considering the difference of hydraulic fracture and permeability, the CBM productivity characteristics controlled by coal petrology were simulated by numerical simulation software, and the rationality of well pattern optimization factors for each coal macrolithotype was demonstrated. The results showed the square well pattern is more suitable for dull coal and semi-dull coal with undeveloped natural fractures, while diamond and rectangular well pattern is more suitable for semi-bright coal and bright coal with more developed natural fractures and more complex fracturing fracture network; the optimum wells spacing of bright coal and semi-bright coal is 300 m and 250 m, while that of semi-dull coal and dull coal is just 200 m.

A Simple Method to Predict Tensile Strength of Gravelly Soil Using Shear Strength Indexes

The tensile strength of gravelly soil plays a decisive role in the formation or initiation of longitudinal cracks or hydraulic fracturing in the core of earth and rockfill dams. For this study, a series of uniaxial tensile tests and consolidated drained triaxial shear tests was carried out to study the relationships between the tensile strength and shear strength indexes of gravelly soils with gravel contents ranging from 0% to 50%. The test results indicate that the tensile strength decreases linearly with increasing gravel content when the soil is at its maximum dry density and optimum water content. An equation for estimating tensile strength of gravelly soil without tensile strength testing is proposed. The relevant test results can provide guidance for anti-crack designs for earthcore rockfill dams.

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.

Advances in Laboratory-Scale Hydraulic Fracturing Experiments

Hydraulic fracturing has been widely applied to stimulate the natural gas and oil production from unconventional reservoirs. To optimize the design of hydraulic fracturing in this application, an accurate estimation of the initiation and propagation of hydraulic fractures is indispensable. However, it still remains challenging as a result of the complex stress state and geological conditions. On account of their ability to complete control some significant factors and efficient observation of fracture geometry, laboratory-scale hydraulic fracturing experiments have received abundant research attention in recent years. This paper presents a review of the state of the art of laboratory-scale hydraulic fracturing experiments, focusing on the scaling analysis, experimental setup, fracturing fluids, and sample preparation. A discussion of the directions for future research is also provided with the intention of stimulating the development of the experimental technique for investigating hydraulic fracturing.

Experimental Study of the Simultaneous Initiation of Multiple Hydraulic Fractures Driven by Static Fatigue and Pressure Shock

One of the most important challenges in horizontal well fracturing is to create multiple tightly-spaced hydraulic fractures (HFs) within each fracturing stage in tight reservoirs, especially with the strong heterogeneity. This study explores the influences of static fatigue and pressure shock on the simultaneous initiation of multi-HF from an open-hole horizontal wellbore in a formation with the variability in the rock strength. Laboratory-scale fracturing experiments were conducted on cuboid-shaped specimens (8 cm × 8 cm × 10 cm) of varying brittleness under the tri-axial confining stress. The rock strength variability in a specimen was illustrated as the variation in the width or length of the preset natural fractures (NFs) within the open-hole section (OHS). Inside the wellbore, three pressurization regimes, corresponding to the normal, static fatigue and pressure shock conditions, were obtained by the constant-rate injection, constant-pressure injection, and high-pressure build-up and rapid release, respectively. The results demonstrate that under the pressure shock conditions, bottom hole pressure (BHP) can be forced to exceed the normal breakdown pressure in a conventional fracturing, and generate a relatively high breakdown pressure. It has the potential to improve the simultaneous initiation of multiple transverse HFs from NFs of different strengths and that of longitudinal HF within a single fracturing stage. A lower limit BHP is required to overcome the variations in strength across different potential initiation points. Meanwhile, the low-BHP, which is lower than the normal breakdown pressure, is maintained to induce the static fatigue failure of the surrounding rock. Under static fatigue conditions, a single HF is generally initiated from the weak NF. The time to breakdown decreases evidently with the increase in the BHP and rock brittleness. In low-brittle specimens, the time-dependent initiation of HF may be difficult in the OHS without connecting with the NFs. In practice, simultaneous initiation of multi-HF may be enhanced by the combined effects of static fatigue and pressure shock.

Study on dynamic propagation of hydraulic fractures in enhanced thermal reservoir

Micro-seismic data and post-fracturing production data have confirmed that the hydraulic fracture is an effective method to improve the production performance in enhanced thermal reservoir. While the mechanism of complex fracture network in fractured reservoirs is still not fully understood. This paper proposes a comprehensive model to simulate the fracture propagation by accounting for interactions with numerous natural fractures. We present a robust finite element model with adaptive insertion of two-dimensional cohesive elements for fracture propagation through the intact rock as well as the network of intersecting natural fractures. Cohesive elements are coupled with general Darcy's flow to incorporate fluid flow as well as elastic and plastic deformations of rock during initiation, propagation and closure of hydraulic fractures. From the simulation, it can be concluded that the composite stress factor was more suitable to indicate the ability of generating complex fracture network in fractured formation. In the simulation, there is a good correlation between composite stress factor and fracture complex index. The lower the composite stress factor is, the higher the fracture complex index is. When the composite stress factor is less than 4.6, the fracture complex index is greater than 3.5, which meant that it is much easier to generate complex fracture network when the composite stress factor is less than 4.6 under the same formation and operating technology.

Fully-coupled hydro-mechanical cracking using XFEM in 3D for application to complex flow in discontinuities including drainage system

The presence of uplift pressures in cracked plain concrete hydraulic structures is a major concern for their durability, serviceability and stability. To assess the performance of cracked structures several mechanical and hydraulic response parameters must be computed. This paper presents the development, implementation and application of a new nonlinear combined segregated fully-coupled hydromechanical model for application to non-planar 3D hydraulic fractures where complex flow is occurring. The eXtended Finite Element Method (XFEM) formulation is used, as it facilitates the computation of the crack aperture as well as the application of water pressure on crack surfaces for the simulation of hydraulic fracture initiation and propagation. To take into consideration the effects of drainage in numerical modeling and simulation, two coupled (multi-physics) finite element models are used; (1) one for the uplift pressures where a hydraulic mesh with refinement around drains is used, and (2) one for the mechanical response coupled with the computation of the unsaturated interstitial seepage problem. The two subproblems are solved using a partitioned procedure, as they have different resolution requirements. Finally, the mechanical mesh and the hydraulic mesh have non-matching discrete interfaces that must be coupled while respecting the equilibrium of the applied loads. Applications of the proposed hydromechanical constitutive model and numerical solution strategy to different problems are presented on a wedge splitting test and a full scale gravity dam including multiple drainage configurations. A case study adapted from a real dam is also given with a complex 3D non-planar discontinuity surface.

Study on catastrophe mechanism of hydraulic fracturing fracks initiation in radial boreholes

Radial drilling hydraulic fracturing has become a new stimulation method. The hydraulic fracturing effect of radial drilling on site is not ideal, and the stimulation effect of some wells is not obvious. The hydraulic fracturing effect of radial drilling is closely related to the crack initiation position after radial drilling fracturing. In the past researches on fracture initiation in radial drilling hydraulic fracturing, it is believed that the fracture initiation position is the radial drilling position and extends along the drilling direction. In this paper, a new model is established to solve the problem of hydraulic fracturing initiation mechanism in radial drilling. By studying the crustal stress analysis around radial drilling, it is found that when the parameters such as angle or crustal stress reach a certain degree, the initiation position appears near the wellhead. The occurrence of this mutation is the main reason for the unsatisfactory effect of hydraulic fracturing in radial drilling. The research results in this paper provide a reliable basis for radial drilling fracturing reconstruction of unconventional oil and gas reservoirs, and reference for the design of construction parameters of radial drilling.

Numerical and experimental investigation of hydraulic fracture using the synthesized PMMA

Hydraulic fracturing is a technique used for stimulation of unconventional reservoirs including shale plays. Several parameters, including rock properties, state of stresses, and fracturing fluid characteristics, influence successful design and implementation of a hydraulic fracturing operation. Monitoring of real-time hydraulic fracture initiation and propagation in the field to improve the design parameters is costly. Several hydraulic fracturing laboratory studies have been reported; however, similar to the field operations, real-time observation of the fracturing fluid penetration to the rock sample is not practical throughout the test but the sample can only be inspected post-experiment. In this work, we present the specifications of a through see synthesized polymethyl methacrylate (PMMA) material which was made for the purpose of laboratory experimental hydraulic fracturing testing. The mechanical testing of the PMMA allowed us to obtain the mechanical properties, including tensile and compressive strengths as well as toughness are close to the typical tight samples used in the laboratory for hydraulic fracturing experiments. Hydraulic fracturing tests were performed on a number of cylindrical PMMA samples to investigate the effect of injecting fluid viscosity and flow rate as well as the geometry of the initial crack on fracture breakdown pressure and the characteristics of the propagating fracture. The ability to directly observe the fracture geometry throughout the experiment allowed us to make a good correspondence between the pressure–time curve and the evolution of the fracture geometry. Numerical modeling using ANSYS was performed to run multiple simulations and do sensitivity analysis of different parameters similar to the laboratory experiments. The results also were compared with some of the analytical solutions.

Variational Phase‐Field Modeling of Hydraulic Fracture Interaction With Natural Fractures and Application to Enhanced Geothermal Systems

AbstractIn every tight formation reservoir, natural fractures play an important role for mass and energy transport and stress distribution. Enhanced Geothermal Systems (EGS) make no exception, and stimulation aims at increasing the reservoir permeability to enhance fluid circulation and heat transport. EGS development relies upon the complex task of predicting accurate hydraulic fracture propagation pathway by taking into account reservoir heterogeneities and natural or preexisting fractures. In this contribution, we employ the variational phase‐field method, which handles hydraulic fracture initiation, propagation, and interaction with natural fractures and is tested under varying conditions of rock mechanical properties and natural fractures distributions. We run bidimensional finite element simulations employing the open‐source software OpenGeoSys and apply the model to simulate realistic stimulation scenarios, each one built from field data and considering complex natural fracture geometries in the order of a thousand of fractures. Key mechanical properties are derived from laboratory measurements on samples obtained in the field. Simulations results confirm the fundamental role played by natural fractures in stimulation's predictions, which is essential for developing successful EGS projects.