Fracture Initiation Pressure Research Articles

A New Experimental Method for Acid Pretreatment in Perforated Horizontal Wells: A Case Study of Mahu Conglomerate Reservoir

The Mahu Oilfield in Xinjiang, China, is the world’s largest conglomerate oilfield, with a daily output of more than 8000 tons. In the fracturing of perforated horizontal wells, the breakdown pressure is high, resulting in difficulties in their treatment. Acid pretreatment has been applied to reduce the breakdown pressure in the field, but with poor performance. Few studies have been conducted on how acid pretreatment affects the breakdown pressure under perforation conditions in the Mahu conglomerate reservoir. Also, existing fracturing or acid pretreatment experiments cannot simulate perforation well. Therefore, a new method was developed to make perforations by hydro jetting the fracturing specimens (300 mm × 300 mm × 300 mm) first. It can achieve specified perforation parameters, including the perforation angle, position, and length. Subsequently, true triaxial hydraulic fracturing experiments were conducted on the conglomerate specimens obtained from the Mahu area. The effects of the acid pretreatment on the fracture initiation and breakdown pressure were investigated by injecting the perforation section of the conglomerate using various acid systems. The study results showed that the perforation made by the new apparatus was extremely close to the perforation on-site. The acid pretreatment effectively dissolved minerals and could decrease the breakdown pressure down to 7.7 MPa. A combination of 6%HF + 10%HCl was recommended for the acid pretreatment in the Mahu conglomerate reservoir. A total of 60 min acid–rock contact time is necessary for sufficient rock dissolution. The mechanism of the acid pretreatment to decrease the breakdown pressure is that the rock dissolution by the acid reduces the rock tensile strength and increases the permeability. The raised permeability increases the fluid pressure of the reservoir near the wellbore so as to reduce the breakdown pressure of the formation. The research results provide technical support for reducing construction difficulty and optimizing parameters in the Mahu Oilfield.

Optimization of fracturing stages/clusters in horizontal well based on unsupervised clustering of bottomhole mechanical specific energy on the bit

Multi-cluster perforation and multi-staged fracturing of horizontal well is one of the main technologies in volumetric fracturing stimulation of unconventional oil and gas reservoirs, but unconventional reservoirs in China are generally of strong heterogeneity, which causes different fracture initiation pressures in different positions of lateral, making it difficult to ensure the balanced fracture initiation and propagation between clusters in multi-cluster perforating. It is in urgent need to precisely evaluate the difference in rock strength in lateral and determine the well section with similar rock strength to deploy fractures, so as to reach the goal of balanced stimulation. Based on the drilling and logging data, this paper establishes an unsupervised clustering model of mechanical specific energy of bit at the bottomhole the lateral. Then, the influence of drill string friction, composite drilling and jet-assisted rock breaking on the mechanical specific energy is analyzed, and the distribution and clustering categories of bottomhole mechanical specific energy with decimeter spatial resolution are obtained. Finally, a fracture deployment optimization method for horizontal well volumetric fracturing aiming balanced stimulation is developed by comprehensively considering inter-fracture interference, casing collar position, plug position, and clustering result of bottomhole mechanical specific energy. The following results are obtained. First, compared with brittleness index, Poisson's ratio and stress difference, perforation erosion area is in a stronger correlation with the mechanical specific energy, and the mechanical specific energy can effectively characterize the difference in the amount of proppant injected into the perforation clusters in the lateral, so it can be served as one of the important indicators for the selection of fracture deployment position. Second, the drilling and logging data cleaning and smoothing and the clustering number selection by the elbow method are the key steps to obtain the clustering results of bottomhole mechanical specific energy, which can tell the difference in the mechanical specific energy with decimeter-level resolution. Third, the interval with mechanical specific energy within 10% of the average value in the section is selected for deploying perforation clusters, and the compiled computer algorithm can automatically determine the optimal position of fracturing section and cluster, so as to realize the differential design of stage spacing and cluster spacing. In conclusion, the research results can further improve the fractures deployment efficiency and balanced stimulation of volumetric fracturing in unconventional oil and gas reservoirs, and this technology is expected to provide ideas and new methods for the fracture deployment optimization of horizontal well volumetric fracturing in unconventional oil and gas reservoirs.

Experimental investigation on mode I fracture characteristics of Longmaxi formation shale after cyclic thermal shock and high-temperature acid etching treatments

Fracture toughness is a key parameter for shale hydraulic fracturing design, which directly affects the fracture initiation pressure. In this paper, the influence of cyclic thermal shock and high-temperature acid etching on the Mode I fracture of Longmaxi Formation shale was investigated. The crack propagation was obtained by using acoustic emission (AE) and digital image correlation (DIC) techniques. Meanwhile, the fracture surface morphology and microstructure of the specimens were analyzed using white light interferometer and scanning electron microscope (SEM). The experimental results demonstrate that cyclic thermal shock severely degrades the nominal strength and fracture toughness of shale. The average fracture toughness is found to be only 0.67 MPa·m0.5, which accounts for a reduction of 25.9 %. AE analysis reveals that microfracture is the dominant mechanism, with the highest amplitude of AE source only 78 dB. The critical strain value of the fracture propagation zone (FPZ) is determined to be 1.35 ‰, and SEM images indicate that along-crystal fracture is the primary fracture mechanism. On the other hand, the shale specimen subjected to high-temperature acid-etching treatment exhibits an average fracture toughness of 0.86 MPa·m0.5, with the AE source amplitude reaching 86 dB. The critical strain value of the FPZ was measured to be 2.38 ‰. The fracture surface appears smooth and is mainly characterized by perforated-crystal rupture. The findings of this study not only enhance our understanding of shale mode I fracture toughness but also provide valuable insights for optimizing process parameters to reduce initiation pressure in deep shale formations.

Combined effects of bedding anisotropy and matrix heterogeneity on hydraulic fracturing of shales from Changning and Lushan, South China: An experimental investigation

Shales are intrinsically anisotropic and heterogeneous due to the existence of bedding planes and natural microfractures. Although the roles of shale bedding anisotropy and inherent heterogeneity have been well studied, their combined effects remain unclear. To fill this knowledge gap, we performed six hydraulic fracturing tests on shales with varying bedding inclinations (0°, 45° and 90°) collected from Changing (Sichuan) and Lushan (Jiangxi), South China. The mineral components and microstructure of shales collected from these two places were compared. The results indicate that the shale from Lushan is more heterogeneous due to its higher content of microfractures and pores than the shale from Changning. Through hydraulic fracturing analysis, we show that the fracture initiation pressure and breakdown pressure first increase and then decrease with increasing bedding inclination, and a strong linear correlation between the two pressures is found to be independent of shale heterogeneity. Second, we find inconsistency between the high breakdown pressure and large hoop deformation, which can be attributed to the anisotropy effect of shale’s bedding strength relative to the maximum principal stress. In addition, the analysis of the amplitudes and frequency of acoustic emission signals suggests that tensile cracks preferentially occur in a short-transverse mode, while deviating the bedding plane from the axial direction suppresses the initiation of tensile cracks. Shale heterogeneity is mainly reflected in reducing the magnitude of critical pressures and deflecting local fracture branch behaviors, which are less significant than the impacts of bedding anisotropy during the hydraulic fracturing process.

The influences of perforating phase and bedding planes on the fracture deflection in laminated shale

The perforating phase leads to complex and diverse hydraulic fracture propagation behaviors in laminated shale formations. In this paper, a 2D high-speed imaging scheme which can capture the interaction between perforating phase and natural shale bedding planes was proposed. The phase field method was used to simulate the same conditions as in the experiment for verification and hydraulic fracture propagation mechanism under the competition of perforating phase and bedding planes was discussed. The results indicate that the bedding planes appear to be no influence on fracture propagation while the perforating phase is perpendicular to the bedding planes, and the fracture propagates along the perforating phase without deflection. When the perforating phase algins with the bedding planes, the fracture initiation pressure reserves the lowest value, and no deflection occurs during fracture propagation. When the perforating phase is the angle 45°, 60° and 75° of bedding planes, the bedding planes begin to play a key role on the fracture deflection. The maximum deflection degree is reached at the perforating phase of 75°. Numerical simulation provides evidence that the existence of shale bedding planes is not exactly equivalent to anisotropy for fracture propagation and the difference of mechanical properties between different shale layers is the fundamental reason for fracture deflection. The findings help to understand the intrinsic characteristics of shale and provide a theoretical basis for the optimization design of field perforation parameters.

Fracture propagation laws of staged hydraulic fracture in fractured geothermal reservoir based on phase field model

Hydraulic fracturing is widely used in geothermal resource exploitation, and many natural fractures exist in hot dry rock reservoirs due to in-situ stress and faults. However, the influence of natural fractures on hydraulic fracture propagation is not considered in the current study. In this paper, based on the phase field model, a thermo-hydro-mechanical coupled hydraulic fracture propagation model was established to reveal the influence of injection time, fracturing method, injection flow rate, and natural fracture distribution on the fracture propagation mechanism. The results show that fracture complexity increases with an increase in injection time. The stress disturbance causes the fracture initiation pressure of the second cluster significantly higher than that of the first and third clusters. The zipper-type fracturing method can reduce the degree of stress disturbance and increase fracture complexity by 7.2% compared to simultaneous hydraulic fracturing. Both low and high injection flow rate lead to a decrease in fracture propagation time, which is not conducive to an increase in fracture complexity. An increase in the natural fracture angle leads to hydraulic fracture crossing natural fracture, but has a lesser effect on fracture complexity. In this paper, we analyzed the influence of different factors on initiation pressure and fracture complexity, providing valuable guidance for the exploitation of geothermal resources.

Correction to: Impact of Toughness Anisotropy on Fracture Initiation Pressure and Morphology in Shale Rocks

Correction to: Impact of Toughness Anisotropy on Fracture Initiation Pressure and Morphology in Shale Rocks

Impact of Toughness Anisotropy on Fracture Initiation Pressure and Morphology in Shale Rocks

Impact of Toughness Anisotropy on Fracture Initiation Pressure and Morphology in Shale Rocks

Thermo-hydro-mechanical coupling simulation for fracture propagation in CO2 fracturing based on phase-field model

CO2 fracturing is extensive used in unconventional oil and gas reservoir, geothermal resources, metal uranium mining, and CO2 geological storage. At present, the fluid-solid coupling model for fracture propagation with CO2 does not consider the influence of heat exchange between CO2 fracturing fluid and the formation, which leads to the deviation between the simulation results and field. Based on the phase-field models, considering the thermal stress and CO2 physical properties change induce by thermal effect, a Thermo-hydro-mechanical coupling model is established for CO2 fracturing. This paper studies the influence of formation horizontal stress difference, approaching angle and fracturing fluid injection temperature. The results indicate that, compared with hydraulic fracturing, CO2 fracturing can significantly reduce the fracture initiation pressure and time, and obtain a more complex fracture morphology with the same stress difference. For natural fractures, the approaching angle is linearly related to the initiation pressure and initiation time of opening new fractures. With the growth of approaching angle, the initiation pressure and initiation time increase respectively. The fracture shows increase in fracture morphology complexity induced by a stronger thermal stress with the growth of temperature differences. CO2 fracturing simulation thermal stress should be considered for a precision of fracture morphology.

Investigation of Mechanical Properties Evolution and Crack Initiation Mechanisms of Deep Carbonate Rocks Affected by Acid Erosion

Deep tight-gas carbonate reservoirs have huge reserves, with the advantages of having clean and low-carbon characteristics in addition to being a sustainable and stable supply which leads to very high-quality green energy, despite its difficult extraction. The reservoirs are usually modified using acid fracturing before exploitation, but due to acid erosion, the continuous alteration of the mechanical properties of the reservoir rocks complicates the process of predicting the crack initiation pressure. This paper aims to address the difficulties in predicting the crack initiation pressure by conducting a series of acid-etching experiments on carbonate rock samples subjected to splitting and uniaxial compression tests. By examining the variations in the elastic modulus, Poisson’s ratio, tensile strength under distinct acid systems, and acid-etching durations and temperatures, a quantified mathematical model was developed. This model was integrated into a fracture-initiation pressure prediction framework, resulting in a practical and user-friendly tool for the acid fracture-initiation pressure prediction model, which was further demonstrated through field engineering validation. The findings reveal that the elastic modulus, Poisson’s ratio, and tensile strength of carbonate rocks exhibit an inverse relationship with acid-etching time and temperature. Extended acid fracturing durations and high reservoir temperatures are conducive to acid-fracturing transformations. The fracture-initiation pressure-prediction-model analysis disclosed that, compared to the gelled acid, the diverting acid demonstrates a more pronounced reduction in the reservoir fracture pressure under high-temperature and short-duration conditions. An acid system preference diagram was constructed to provide a theoretical foundation for practical engineering applications, delivering valuable insights for optimizing acid fracturing treatments in carbonate reservoirs to provide a boost for the green energy extraction of tight gas.

Prediction of Fracture Initiation Pressure for Slotting-Directional Hydraulic Fracturing Based on the Anisotropy of Coal

Abstract The precise estimation of fracture initiation pressure is crucial for the effective implementation of slotting-directional hydraulic fracturing methods in coal seams. Nonetheless, current models fail to account for the impact of the morphology of the slotted borehole and the anisotropy of coal. To address this issue, a three-dimensional model was created in this study, which simplified the slotted borehole as an elliptical medium and the coal as an orthotropic medium. Laboratory experiments were conducted to validate the model, and the findings regarding the changes in fracture initiation pressure and deflection angle due to various factors were presented. The calculated outcomes of the proposed model align with the observed pattern of the experimental results, and the numerical discrepancy falls within the acceptable range of 7%, showcasing the precision of the proposed model. A rise in the horizontal stress difference and a decrease in the depth of the slots will result in an elevation of the fracture initiation pressure and deflection angle. In addition, the slotting angle will impact the distribution pattern of the fracture initiation pressure and deflection angle, underscoring the significance of these factors in the hydraulic fracturing of slotted boreholes.

Impacts of wellbore orientation with respect to bedding inclination and injection rate on laboratory hydraulic fracturing characteristics of Lushan shale

A series of hydraulic fracturing experiments were conducted using Lushan shale cores (50 mm in diameter and 100 mm in length) to unravel the formation mechanism of fracture networks under the combined effects of wellbore type (vertical or horizontal well), wellbore orientations relative to the bedding plane (0°, 30°,45°, 60°, and 90°), bedding inclinations (0°, 30°,45°, 60°, and 90°), and injection rates (0.03, 0.3, 3, and 30 mL/min). We monitored acoustic emission and surface displacements during the test and conducted post failure inspection via a fluorescent tracer and a 3D laser scanner. We show that adjusting the maximum principal stress perpendicular to the wellbore is beneficial for reducing the breakdown pressure. Inclined bedding tends to initiate along the bedding plane with relatively low fracture initiation pressure and breakdown pressure. Deploying the wellbore inclined to bedding inclination and/or adopting a low injection rate is beneficial to reduce the shale breakdown strength and create a complex fracture network. However, this will correspondingly increase the fracture surface roughness, which may impede the effective transportation of proppant. These findings can provide an important reference for field hydraulic fracturing.

Numerical simulations of supercritical carbon dioxide fracturing: A review

As an emerging waterless fracturing technology, supercritical carbon dioxide (SC-CO 2 ) fracturing can reduce reservoir damage and dependence on water resources, and can also promote the reservoir stimulation and geological storage of carbon dioxide (CO 2 ). It is vital to figure out the laws in SC-CO 2 fracturing for the large-scale field implementation of this technology. This paper reviews the numerical simulations of wellbore flow and heat transfer, fracture initiation and propagation, and proppant transport in SC-CO 2 fracturing, including the numerical approaches and the obtained findings. It shows that the variations of wellbore temperature and pressure are complex and strongly transient. The wellhead pressure can be reduced by tubing and annulus co-injection or adding drag reducers into the fracturing fluid. Increasing the temperature of CO 2 with wellhead heating can promote CO 2 to reach the well bottom in the supercritical state. Compared with hydraulic fracturing, SC-CO 2 fracturing has a lower fracture initiation pressure and can form a more complex fracture network, but the fracture width is narrower. The technology of SC-CO 2 fracturing followed by thickened SC-CO 2 fracturing, which combines with high injection rates and ultra-light proppants, can improve the placement effect of proppants while improving the complexity and width of fractures. The follow-up research is required to get a deeper insight into the SC-CO 2 fracturing mechanisms and develop cost-effective drag reducers, thickeners, and ultra-light proppants. This paper can guide further research and promote the field application of SC-CO 2 fracturing technology.

Optimization method of oriented perforation parameters improving uneven fractures initiation for horizontal well fracturing

Horizontal well oriented perforation fracturing is a common technique for unconventional oil and gas production. With the wide application of this technology, issues such as uneven fracture initiation and high initiation pressure appeared, which limited the fracturing effect for unconventional oil and gas. To improve the current situation for oriented perforation fracturing, we establish a numerical model of oriented perforation fracturing for horizontal wells using three-dimensional lattice method. The effects of perforation depth and diameter on fracture initiation pressure, fracture morphology and stimulating area are obtained through numerical simulation. Meanwhile, we propose a depth-controlled and a diameter-controlled oriented perforation design method for uniform fracture initiation. The research results show that, the growth of perforation depth and diameter would lower fracture initiation pressure. Increasing the perforation depth can improve the issue of uneven fracture initiation for oriented perforation fracturing in horizontal wells. Compared with conventional oriented perforation, the depth-controlled and diameter-controlled oriented perforation can significantly improve the uneven fracture initiation issue. The depth-controlled oriented perforation can effectively increase the stimulating area, and induce narrow and long fractures in high elastic modulus formations. The diameter-controlled oriented perforation is suitable for high in-site stress and can reduce fracture initiation pressure. The depth-controlled oriented perforation optimal design is applied to an objective well, Well H-21 of the Chang 73 target layer in Changqing Oilfield, China. Numerical results show that the proposed method can reduce the fracture initiation pressure by 12.6% and increase the fracture area by 11.80%, which can effectively solve the issue of uneven fracture initiation.

Fracture initiation and propagation of supercritical carbon dioxide fracturing in calcite-rich shale: A coupled thermal-hydraulic-mechanical-chemical simulation

The initiation and propagation mechanisms of supercritical carbon dioxide (scCO2) fracturing are more complex than those of hydraulic fracturing in calcite-rich shale due to the temperature‒pressure sensitivity of scCO2 and scCO2-brine-rock interactions. In this study, we present an enhanced thermal-hydraulic-mechanical-chemical (THMC) simulator called CMPSF, which incorporates a rock damage model and a CO2 physical property model into the COMSOL-MATLAB-PHREEQC (CMP) coupling framework via MATLAB. Furthermore, the coupling procedure of CMPSF is optimized from a previous sequential non-iterative approach to a sequential partly-iterative approach. We then utilize CMPSF validated with numerical and experimental examples to analyze the evolution of THMC fields and examine the impact of calcite content and injection temperature on fracture initiation and propagation of scCO2 fracturing in calcite-rich shale. The simulation results reveal that the decreased mass fraction of calcite is 1.5‰ and the porosity increment from calcite dissolution is 0.33‰ at most within 12 min. A higher calcite content and injection temperature lead to a decrease in fracture initiation pressure and an increase in rock damage ratio. Specifically, as the calcite content increases from 50 wt% to 80 wt%, the fracture initiation pressure decreases by 0.1 MPa, and the final damage ratio increases by 0.25%. As the injection temperature increases from 30 °C to 60 °C, the fracture initiation pressure decreases by 4.3 MPa, and the final damage ratio increases by 1.55%. Calcite dissolution has a minor impact on fracture initiation and propagation at the early stage of scCO2 fracturing only from the perspective of affecting seepage, and its long-term effects require further investigation. Our findings provide insight into the complex THMC coupling processes of scCO2 fracturing in calcite-rich shale.

Study on dynamic fracture growth mechanism of continental shale under compression failure

Microscale and nanoscale fractures induced by stress transfer can effectively improve pore connectivity, which can provide significant benefits by improving well productivity in shale reservoirs. In this research, representative samples of four shale lithofacies are investigated. The rock deformation and fracture propagation of shale samples under stress are numerically simulated by integrating various technical approaches, such as characterization of the mineral composition and distribution, digital core modeling, micromechanical tests, and finite element method-based simulations. The results show that the numerical model developed in this research can accurately replicate the microscale characteristics of the heterogeneity of shale and thus effectively clarify the effects of control of the composition, morphology, and distribution of minerals and structural defects on induced fractures in rock masses. Under uniaxial compression, the failure mechanism of rock is dominated by tensile fracturing, which generates tensile fractures parallel to the loading direction. However, compression‒shear fractures perpendicular to the loading direction also form due to the presence of lamination seams. The content of brittle minerals in different lithofacies is relatively stable, but the characteristics of the morphology and distribution of such minerals are highly differentiated, which leads to significant differences in mechanical properties and failure mechanisms. Both laminated felsic and diamictic shale (LFS and LDS) can present high fracture complexity in cases of low fracture energy. In argillaceous shale (AS), fractures initiate at low strengths, yet fracture propagation requires high energy. In contrast, the fracture initiation pressure and fracture propagation energy consumption of dolomitic shale (DS) are both high and unfavorable for the propagation of induced fractures. This research provides a technical approach for studying the formation mechanism and dynamic evolution of induced fractures, and the corresponding findings are of great theoretical significance for the identification of sweet spots and design of shale reservoirs for fracturing applications.

Approach Investigates Need for Extreme-Limited-Entry Applications

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 204185, “Practical Design Considerations for Stage Length, Perforation Clusters, and Limited-Entry Pressure Intensities,” by Paul Huckabee, SPE, Chris Ledet, SPE, and Gustavo Ugueto, SPE, Shell, et al. The paper has not been peer reviewed. _ This paper presents design considerations for determining practical dimensions and limits for interdependencies associated with stage length, perforation clusters, and limited-entry pressures. Aggressive limited entry has been an enabler for successful extended-stage-length applications. The authors challenge the need for extreme limited entry in resource development across multiple North American basins. This synopsis does not focus on field trial applications, which are provided in the complete paper. Managing Threats to Stimulation Distribution Effectiveness (SDE) in New Completions Before any discussion of recommendations and trial results for stage length, perforation clusters, and limited-entry pressure intensities, it is important to consider the threats and risks to effective stimulation distribution. The authors detail the following risks in the complete paper: - Effective design and execution of limited-entry practices - Proppant transport in horizontal wellbores - Plug or ball leakage - Cement channeling - Casing erosion - Equipment reliability (pumps and injection lines or manifold leaks) How Much Limited-Entry Pressure Is Needed for Improved SDE? Effective limited-entry design and application must consider the differentials of the following interdependent geomechanical variables with the risks mentioned previously: - Fracture initiation pressure - Minimum in-situ stress - Fracture extension net pressure (process-zone stress) - Near-wellbore complexity pressure drop and changes with slurry erosion - Stress shadow effects (interstage, intrastage, and interwell) A diagnostic that the authors have used to enable quantification of some of these critical variabilities is coiled-tubing-actuated cemented-sleeve-stimulation systems. In these systems, sleeves are actuated by setting the bridge plug and shifting the sleeve to the treatment, or open, position. The bridge plug isolates treatment pressures in the wellbore of the active treatment sleeve from other downhole sleeves. The treatment proppant slurry is pumped down the coiled tubing by casing annulus. Pressure communication exists to the coiled tubing, which enables pseudomeasurements of bottomhole-treatment pressures and trends through a dead-leg pressure measurement at surface. Operational practices include pumping a very low rate down the coiled tubing to ensure that solids are not allowed to enter the coiled tubing side of the bottomhole assembly, so the authors recognize that these are pseudo-dead-leg measurements. The frictional pressure drop in the coiled tubing is assumed to be negligible for their calculations. If the downhole sleeves are left in the open position, an assessment of treatment isolation can be made. For sequential sleeve treatments, where good isolation exists between successive sleeves, some of the critical pressure differentials for different length dimensions in the lateral can be assessed. These length dimensions are limited to increments of the sleeve-spacing dimensions.

A Hybrid Data-Driven/Physics-Based Modeling Approach for Hydraulic Fracture Initiation and Early-Phase Propagation in Shale

Optimal hydraulic fracture (HF) initiation and early-phase propagation results in minimal near-wellbore tortuosity, decreasing the likelihood of screenouts and maximizing the resultant well productivity. While most predictive models for the HF geometry produced in a treatment consider the far-field region, the near-well vicinity should be an integral part of a properly-engineered reservoir stimulation strategy. In this work, a novel hybrid data-driven/physics-based approach is elaborated for modeling HF initiation and early-phase propagation from perforated horizontal wells.A treatment-optimization scheme via oriented perforating is presented using the developed hybrid model, considering the orientation of the induced HF initiation (longitudinal or transverse with respect to a well drilled along the minimum horizontal in-situ principal stress) and the resultant formation breakdown pressure (FBP). Transverse HF initiation and early-phase propagation is ideal for wells drilled in low-permeability “tight” formations, while FBP minimization suppresses the overall on-site hydraulic horsepower (HHP) requirements. The demonstrated optimization scheme is applied separately to the in-situ stress states of seven prolific shale plays from the U.S. and Argentina, suggesting oriented-perforating strategies that target the promotion of transverse HF initiation in two of these (Barnett and Marcellus), while suggesting oriented-perforating strategies that target FBP minimization in the remaining five (Bakken, Fayetteville, Haynesville, Niobrara, and Vaca Muerta). The effectiveness of oriented-perforating can potentially be compromised by fracturing fluid leakages around the casing’s circumference, which is shown to hinder transverse HF initiation.The hybrid-modeling approach is also used to estimate fracture initiation pressure (FIP) values for the seven shale plays studied, indicating significant discrepancies with analytical expressions traditionally used to approximate these FIPs in modern HF computational simulations. This sets the basis for expanding this hybrid-modeling approach over a range of in-situ stress states, incorporating aggregate data-driven (numerically-derived) correction factors in order to compensate for inaccuracies in the analytical approximations, which comprise the physics-based core of the proposed hybrid model.

Investigations of fracture behavior and pore structure change in pulse fracturing for cement block

Pulse fracturing can reduce fracture initiation pressure and increase stimulation effectiveness. To investigate the effect of frequency and amplitude on fracture propagation behavior in pulse fracturing, tri-axial experiments with acoustic emission (AE) monitoring are applied. The results demonstrate that pulse fracturing can generate mixed mode fractures, and the distinct deviation of the fracture propagation near-wellbore can be seen for pulse fracturing. Both pulse frequency and amplitude are critical parameters for the propagation path, measured AE accounts, accumulative energy and pore connectivity. The increase in amplitude is beneficial for large fracturing energy, but the increase in frequency is not necessarily helpful for fracture effectiveness. The harmony between the rock and pump frequency can yield the largest acoustic events and pore connectivity. The pore structure and microfracture change of rock before and after pulse fracturing at different distances are measured with nuclear magnetic resonance (NMR) and CT scans, indicating that pulse fracturing has a strong capability to create a larger stimulated area than the monotonic fracturing model. In addition, obvious microfracture generation and pore space expansion can be seen near the injection port. However, the pore structure comparison indicates that the pulse fracturing mode mainly improves the scale of macropores, which is primarily because of the relatively small pulse fracturing parameters used in these experiments. Thus, this study provides an understanding of pulse frequency and amplitude effects for stimulation effectiveness related to pulse fracturing of tight reservoirs.

Analytical Model of Hydraulic Fracturing for Low Permeability Hot Dry Rock Reservoirs and DEM Simulation Base on Fluid-Solid Coupling

The formation of a rich underground-seam network is the key problem in the development of low-permeability hot dry rock (HDR) resources. Considering the lack of macroscopic continuum theory to study hydraulic fracturing having preset fracture-interface element, the particle-flow method of micro-mechanical discrete-element theory is introduced to simulate the mechanical behavior of hydraulic fracturing for HDR low permeability reservoirs. The reservoir is simulated as a round particle; the fracturing fluid movement is described by the seepage field equation, and rock movement is described by the displacement field equation. Finally, the particle-flow numerical model of hydraulic fracturing for HDR low permeability reservoirs is established under the condition of fluid-solid coupling: the model contains two parts (rock and fracture). Based on the parallel-bond model, a definition of micro-fractures of hydraulic fracturing is given. The relation between the fracturing effect and influence parameters is discussed. The results show that the fracture-initiation pressure is proportional to the magnitude of minimum horizontal stress, particle normal-contact stiffness, and particle normal- and tangential-connection strengths; the pressure is also independent of maximal horizontal stress and tangential contact stiffness. At the same time, the formation temperature of dry hot rock will reduce the strength of the rock, so particle-flow numerical models of hydraulic fracturing in different temperatures are discussed. Results show that fracture length and width show a trend of increase before decrease with the increase of injection pressure, an inverse relationship with minimums horizontal principal stress, and a positive relationship with HDR reservoir permeability.