Initial Pressure Research Articles

HELLP Syndrome Developing at 14 Weeks of Gestation: An Extremely Rare Case Report and a Literature Review

Introduction: Hemolysis, elevated liver enzymes and low platelets (HELLP) syndrome is a disease of pregnancy that occurs very rarely before 20 weeks of gestation. We report a case of HELLP syndrome developing at 14 weeks and 2 days of gestation. Case Presentation: A 33-year-old Asian primipara at 14 weeks and 2 days of gestation visited the emergency room with a fever and headache. Initial blood pressure was 140/70 mm Hg, temperature 38.5°C, heart rate 130 beats/min with tachycardia. Her prenatal examination has been unremarkable, and fetal ultrasonography was within normal range. The laboratory results showed low platelet count with elevated liver enzymes, D-dimer, and fibrinogen but no sign of jaundice. Her WBC differential suggested a bacterial infection. Thus, we diagnosed early HELLP syndrome and immediately started conservative treatments. One day after admission, symptoms and laboratory results showed aggravation of the disease. We performed termination, followed by dilation and curettage for retained placenta. Her general condition improved rapidly after the operation. Placental biopsy showed both acute and chronic inflammation. She also had anticardiolipin antibody IgM, and after discharge, she was referred to a rheumatology specialist to address the antiphospholipid syndrome issue. Discussion: Although the triggers of HELLP syndrome are unclear, a recent inflammatory hypothesis suggests that placenta-derived inflammatory cytokines are involved. In our case, the anti-cardiolipin antibody may have triggered microangiopathy of the placenta. Our analysis of published HELLP cases revealed that, apart from the three diagnostic criteria, the most common abnormal laboratory finding was antiphospholipid antibodies. Therefore, despite its rarity, if a sign of inflammation is present in a patient, it is important to consider HELLP syndrome regardless of gestational age

Initiation Pressure and Fracture Path of Near-Field Symmetric Pre-cut Hydraulic Fracturing

Initiation Pressure and Fracture Path of Near-Field Symmetric Pre-cut Hydraulic Fracturing

Study on Near-Wellbore Fracture Initiation and Propagation with Fixed-Plane Perforation in Horizontal Well for Unconventional Reservoirs

At present, in the process of volume fracturing for a tight reservoir, employing the spiral perforation method to induce the fracture propagation direction would always obtain an unsatisfactory effect, which causes the deflection and tortuosity of hydraulic fractures. Therefore, researchers presented the fixed-plane perforation method for enhancing the effect on volume fracturing. In this paper, the three-dimensional discrete lattice method is used to study the initiation and propagation law of horizontal well fixed-plane perforation in unconventional reservoirs under two different stress states. The results show that it is more suitable to use fixed-plane perforation for reducing the initiation pressure. When employing the fixed-plane perforation method, fracture always initiates in the perforation plane, presents as an irregular fan-shaped failure surface, and then propagates along the wellbore. The initiation pressure is highly correlated to the phasing angle between adjacent perforations under different conditions, and the rate of increase in the initiation pressure decreases by around 1.59~6.38% when the phasing angle reaches 30°. The fracturing pressure is inversely correlated with the diameter and tunnel length of the perforations and the horizontal stress difference. When the diameter increases to 17 mm, the tunnel length increases to 25 cm or the horizontal stress difference reaches 8 MPa. These results reveal an insignificant effect of the above parameters on the initiation pressure.

Experimental Study on Improving the Recovery Rate of Low-Pressure Tight Oil Reservoirs Using Molecular Deposition Film Technology

The intricate geological characteristics of tight oil reservoirs, characterized by extremely low porosity and permeability as well as pronounced heterogeneity, have led to a decline in reservoir pressure, substantial gas expulsion, an accelerated decrease in oil production rates, and the inadequacy of traditional water injection methods for enhancing oil recovery. As a result, operators encounter heightened operational costs and prolonged timelines necessary to achieve optimal production levels. This situation underscores the increasing demand for advanced techniques specifically designed for tight oil reservoirs. An internal evaluation is presented, focusing on the application of molecular deposition film techniques for enhanced oil recovery from tight oil reservoirs, with the aim of elucidating the underlying mechanisms of this approach. The research addresses fluid flow resistance by employing aqueous solutions as transmission media and leverages electrostatic interactions to generate nanometer-thin films that enhance the surface properties of the reservoir while modifying the interaction dynamics between oil and rock. This facilitates the more efficient displacement of injected fluids to replace oil during pore flushing processes, thereby achieving enhanced oil recovery objectives. The experimental results indicate that an improvement in oil displacement efficiency is attained by increasing the concentration of the molecular deposition film agent, with 400 mg/L identified as the optimal concentration from an economic perspective. It is advisable to commence with a concentration of 500 mg/L before transitioning to 400 mg/L, considering the adsorption effects near the well zone and dilution phenomena within the reservoir. Molecular deposition films can effectively reduce injection pressure, enhance injection capacity, and lower initiation pressure. These improvements significantly optimize flow conditions within the reservoir and increase core permeability, resulting in a 7.82% enhancement in oil recovery. This molecular deposition film oil recovery technology presents a promising innovative approach for enhanced oil recovery, serving as a viable alternative to conventional water flooding methods.

Sensitivity Analysis of Depth-Controlled Oriented Perforation in Horizontal Wells Based on the 3D Lattice Method

The main method used to exploit unconventional oil and gas reservoirs involves multi-cluster perforation combined with hydraulic fracturing in horizontal wells. However, as the use of this technology has expanded, challenges like reduced perforation efficiency and elevated fracture initiation pressure have surfaced. The depth-controlled oriented perforation technique helps achieve uniform fracture initiation, enhance efficiency, and lower initiation pressure. In this study, a hydraulic fracturing fluid–solid coupling model at the perforation scale was established using the 3D lattice method to compare the near-wellbore fracture morphologies of depth-controlled oriented perforation, spiral perforation, and oriented perforation. Additionally, this study analyzes the effects of injection rate, reservoir elastic modulus, and horizontal stress difference on the fracture morphology and initiation pressure of depth-controlled oriented perforation. This study clarifies the applicability of depth-controlled oriented perforation in different types of reservoirs for the first time. The results indicate that intermediate fractures between spiral and oriented perforations are hindered, while depth-controlled oriented perforation ensures uniform fracture initiation. In the injection rate range of 0.144 to 0.360 L/min, an increase in injection rate accelerates the rise of fluid pressure within the perforations, leading to an increase in fracture initiation pressure. Therefore, excessively high injection rates are unfavorable for fracture initiation. Through depth-controlled oriented perforation, long and singular fractures can be formed in reservoirs with significant horizontal stress differences and high elastic moduli.

Experiment and Numerical Prediction on Shock Sensitivity of HMX–Based Booster Explosive with Small Scale Gap Test at Low and Elevated Temperatures

In order to analyze the effect of temperature changes on the shock initiation performance of HMX–based booster explosive, which consists of 95% HMX and 5% FPM2602 by weight, a temperature calibration test of acceptor was designed. The temperature changes in the booster at low and elevated temperatures under the constraint of steel sleeve were obtained. Based on the temperature calibration results, polymethyl methacrylate (PMMA) was selected as gap material to conduct a small scale gap test (SSGT) of HMX–based booster under different temperature conditions. The corresponding critical gap thickness was tested. Based on SSGT results at different temperatures, the shock initiation processes were simulated by adjusting parameters of ignition and growth reactive rate model. The critical gap thickness and critical initiation pressure of HMX–based booster at different temperatures were numerically predicted. By combining SSGT experimental data and simulation results, the attenuation law and fitting prediction formula of the critical initiation pressure of HMX–based booster were proposed. The mechanism of temperature effect on the shock sensitivity of HMX–based booster explosive was analyzed. The research results indicate that the critical gap thickness of HMX–based booster gradually increases with the rise in temperature, and the critical initiation pressure gradually decreases during the shock initiation process under the heating temperature conditions. In addition, the simulation results show that the heated HMX–based booster under steel constraints becomes more sensitive at high temperatures (>120 °C), while the cooled booster is more insensitive, but its critical initial pressure does not change significantly between 88 °C and 120 °C. The experimental and numerical prediction results are important for the shock initiation safety and design of an insensitive booster explosive.

The effect of rock mechanical heterogeneity on induced stress and initiation pressure of multi-cluster fracturing

The stress interference between fracturing clusters in horizontal wells is key in affecting the initiation and propagation of multi-cluster fractures. However, in reservoirs such as shale, there are often changes in the mechanical properties of the rock along the direction of the wellbore. Such changes are often not considered in models for multi-fracture propagation. As a result, the impact of rock mechanical heterogeneity on induced stress is not fully understood. Therefore, in this paper, a prediction model of the three-dimensional induced stress field of hydraulic fractures is established, taking into account mechanical heterogeneity. Combined with the prediction model of fracturing pressure, the changes in induced stress and initiation pressure under different conditions are analyzed. The results indicate a significant difference between the induced stress results considering mechanical heterogeneity and those considering only a single homogeneous region. The change in elastic modulus has a profound influence on induced stress and initiation pressure. Perforating at locations with low elastic modulus in situ can help reduce initiation pressure. Additionally, temporarily blocking the wellbore and reducing the flow rate to decrease the net pressure in the fractures can facilitate more fractures to initiate.

The fracture propagation law of axially symmetric intersecting precut hydraulic fracturing in mine rock-breaking

As a mine rock-breaking technique, hydraulic fracturing technology can reduce the amount of explosives used, which enhance safety and reduce environmental pollution in mines. After precutting along the borehole axis, hydraulic fractures will expand along the precutting direction within a certain range and reduce initiation pressure. These hydraulic fractures cut through the rock mass, reducing its integrity and weakening its mechanical properties. Hydraulic fracturing with axially symmetric intersecting precut fractures not only controls the multi-directional propagation of fractures but also increases the fractures within rock mass. The lattice method simulated the hydraulic fracturing process, focusing on the parameters like angles between precut fractures and the minimum horizontal principal stress, the maximum horizontal principal stress, and angles between intersecting precut fractures. Results indicate that the hydraulic fractures propagate along intersecting precut fractures, forming main and interconnected secondary fractures. The directional cutting effect is influenced by the number of secondary fractures. With the increase in the angle between precut fractures and the minimum horizontal principal stress, the maximum horizontal principal stress, the angle between precut fractures, the area of secondary fractures decreased, and the expansion extent of main fractures along the precut fractures increased, indicating better directional effects. The study identifies relationships between initiation time, initiation pressure, and parameters. These findings provide valuable technical guidance for designing on-site construction plans for hydraulic fracturing projects involving intersecting precut fractures.

Application of Constant Pressure Explosive Floating Casing Technology in Large Displacement Horizontal Wells

Abstract Large displacement horizontal wells are commonly employed to enhance unconventional resource recovery. However, during the field process, challenges often arise, particularly in cases where the casing string cannot reach the designated position. This issue is particularly prominent in the development of shallow wells with long horizontal segments. Currently, widely adopted traditional floating casing methods fall into two main categories: mechanical and rupture disc types. Mechanical methods faces the challenge that after casing running, the mechanical separation structure remains in the wellbore, adding difficulties to subsequent operations. Rupture disc methods face the challenge of pressure misalignment due to factors such as excitement pressure in the liquid column, potentially leading to premature opening during descent. This paper presents a constant pressure explosive floating casing method, characterized by large reliability and the elimination of the need for additional drilling to remove the barrier structure after casing lowered. This paper focuses on two main aspects: firstly, the design of two-stage initiation mechanisms, corresponding to the initiation pressure of the exciting device and the crushing pressure of the rupture disc. This design ensures precise initiation at low pressure levels while maintaining pressure integrity. Secondly, for different well conditions, varying initiation pressures can be selected, allowing for targeted optimization of initiation and crushing pressures while keeping the pressure difference consistently within a safe range, preventing inadvertent opening and failure during the descent process. In field application, the constant pressure explosive floating casing technology has been successfully applied in over 100 wells, with the longest floating segment reaching 3270m. In all cases, the casing descended smoothly to the designated position, offering convenient, safe operations without the need for repeated plugging. This technology guarantees the attainment of the design-required opening pressure and ensures full wellbore diameter and sealing integrity. The technology effectively avoids the complexities associated with casing installation in long horizontal, large-displacement wells, reducing the difficulties in subsequent operations such as cementing and completion.

Interaction behavior between hydraulic fractures and interface in coal-rock complex rock layer: Experimental study and application exploration

To achieve long-term and efficient gas extraction in soft, low-permeability coal seams, this study conducted hydraulic fracturing experiments on coal-rock complexes under true triaxial conditions. The pattern of hydraulic fractures (HFs) was reconstructed based on the fractal dimension concept. The results indicate that the tendency of the complex rock layers to initiate fractures toward the coal weakens the trend of increasing fracture initiation pressure with rising geostress. When HFs interact with the interface, the extension pressure significantly decreases. With the lateral pressure coefficient decreasing, HFs tend to extend toward the coal and be captured by the interface, transitioning from a single-wing to a double-wing shape and approaching a symmetrical conjugate state. Only when the vertical principal stress is sufficiently large can HFs separate from the interface. Based on the derived distribution function of induced stress in the coal-rock matrix around the HFs, the displacement conditions of the coal, rock, and interface were examined. The interaction process of rock layer HFs and the interface was divided into three stages: deflection, capture, and separation. The applicability of this study to high-gas soft coal seams was discussed, and a gas management plan involving roof fracturing and full-period extraction was proposed, with the aim of providing a theoretical foundation for the co-extraction and efficient utilization of coal and gas in mines.

Research on microscale displacement characteristics of supercritical CO2 fracturing in shale oil reservoirs

AbstractThe great success of CO2 fracturing in shale oil reservoirs has not only increased the production capacity of shale oil, but also effectively carried out CO2 geological storage. In this paper, focusing on the microscale displacement characteristics of CO2 fracturing in shale oil reservoirs, first, the impact of oil saturation and gas invasion pressure on gas invasion is discussed. Then, the coupling control mode of oil saturation/pressure for various mechanisms of gas invasion is revealed. Results show that: (a) at lower displacement, the rock core has a lower initiation pressure and is fractured in a shorter period of time; (b) at higher displacement, the required fracturing time is longer and the fracturing pressure increases, but the fracturing effect is good and it is easy to form complex fracture networks; and (c) under higher pressure conditions, more complex fractures can be formed, which is beneficial for reservoir transformation and production improvement.

Optimization of Supercritical CO2 Fracturing Based on Random Forest-Particle Swarm Optimization Model and Pre-existing Fracture Network

Summary On a global scale, shale oil/gas has become an important alternative energy source for conventional oil and gas. The potential advantages of supercritical CO2 (ScCO2) make it an ideal alternative to hydraulic fracturing, used for shale reservoir transformation and production increase while also promoting the geological storage of CO2, which is in-line with today’s carbon capture, utilization, and storage technology and helps to address the challenges of global climate change. To further study the fracture propagation and optimization of a complex fracture network (CFN) in ScCO2 fracturing under complex geological conditions using the cohesive module of ABAQUS to establish a fluid structure coupling model and completing indoor and field experimental verification, we introduce the global embedded cohesion zone model (CZM) combined with Python to generate two natural-fracture (NF) distribution models, conjugate and power law, to establish a dispersed mesh model. Based on this model, we studied the fracture propagation problem of ScCO2 fracturing under different engineering and geological conditions. The simulation results will be used as data-driven data to establish an optimization model of the random forest-particle swarm optimization algorithm (RF-PSO) and optimize the CFN. Research has shown that (1) ScCO2 is more inclined to pass through NFs and propagate in the rock matrix, and hydraulic fractures (HFs) combine better with NFs. Compared with hydraulic fracturing, ScCO2 fracturing has significant advantages (only the fracture width is lower than hydraulic fracturing, its initiation pressure and fracture length are much better than hydraulic fracturing, and there are more small fractures, making it easier to form a CFN). (2) During the process of fracture propagation, once dominant fractures form, the trend of the “Matthew effect” is inevitable. The process of fracture propagation is influenced by multiple factors, especially the distribution of NFs; the larger the reservoir filtration coefficient is, the more ScCO2 fracturing fluid that is lost, which is more unfavorable for fracturing construction. While maintaining the same amount of fracturing fluid injection, as the displacement increases, the fracture complexity increases, and the fracturing control range expands. Compared with other parameters, the effect of fracturing fluid temperature (FFT) on the expansion of ScCO2 fracturing fractures is not significant. (3) The established RF-PSO optimization model has an error of 2.89%, which can well adapt to CFN optimization problems under complex NF conditions and reduce uncertainty. We propose in this article a research method for fracture network optimization from fracture modeling, dynamic simulation, and optimization modeling. By combining numerical simulation and machine learning, the CFN optimization design of ScCO2 fracturing under CFN conditions is achieved, providing a research approach for the optimization of fracturing in fractured reservoirs.

Study on the Fracture Extension Pattern of Shale Reservoir Fracturing under the Influence of Mineral Content.

This study aimed to investigate the effect of the microstructure of shale on fracture initiation and extension during hydraulic fracturing. The Longmaxi Formation shale reservoir in the Sichuan Basin was considered as the research object; its structure was modeled from a microscopic perspective, and a zero-thickness cohesive unit was embedded within the solid unit. Numerical simulations were performed to study the effect of mineral content on the microextension of the hydraulic fracture, extension behavior, and evolution law of shale. The results showed that changes in the mineral content resulted in changes in the forces between molecules within the minerals, which, in turn, affected the shale's brittleness. The percentages of brittle mineral content in the Long I, II, and III reservoir sections are 60.37, 47.60, and 53.56%, respectively. The fracture initiation pressures of the three reservoirs were 29.22, 31.42, and 30.22 MPa, respectively, and a linear correlation was found between the fracture initiation pressures and the brittle mineral contents of the reservoir sections. An increase in the reservoirs' percentage of brittle mineral content facilitated the fracture initiation, with a corresponding gradual decrease in the resistance to fracture initiation. The pore pressures of the fractures in the three reservoirs after fracture initiation were 0.90, 1.18, and 1.00 MPa, respectively. The larger the percentage of brittle minerals was, the lower was the fracture pore pressure. The greater the length, number, area, and width of the cracks were, the more likely they were to form longer and wider cracks. Hence, reservoirs with a high percentage of brittle minerals should be prioritized as the target formations for hydraulic fracturing operations. The results of this study reveal how the mineral content affects the extension of microscopic hydraulic fractures in shale reservoirs. As such, this work can provide a theoretical basis for rationally selecting a hydraulic fracturing operation layer in shale gas reservoirs.

Research on the Influencing Factors of Pulsed Hydraulic Fracturing Effects in Hot Dry Rock Based on Numerical Simulation

Geothermal energy, representing one of the five principal non-carbon-based renewable resources, is crucial in the global energy transition and in mitigating greenhouse gas emissions. Hot Dry Rock (HDR) reservoirs, rich in geothermal resources, predominantly utilize hydraulic fracturing, the most extensively researched and applied technology. However, fracturing these robust HDR formations poses significant challenges compared to traditional hydraulic fracturing in sedimentary rocks in the oil and gas industry. Cyclic pulse fracturing, an innovative mode of fracturing, has received limited academic attention, particularly regarding its permeability enhancement mechanism under varying thermal and pressure conditions, which remains largely unexplored. Building on prior laboratory experiments, this study employs the pore pressure cohesive zone model within ABAQUS software to elucidate the initiation and expansion of hydraulic fractures. Numerical simulations investigated the influence of critical parameters such as pulse pattern, temperature, geostress differential, and confining pressure on fracture propagation. The findings indicate that cyclic pulse hydraulic fracturing can significantly reduce the breakdown pressure in HDR, with reductions up to 37.68% under certain conditions. Notably, at constant amplitude, increased frequency exacerbates damage to hot dry rocks; conversely, at constant frequency, higher amplitudes result in greater damage. Additionally, pulse hydraulic fracturing improves the fracturing process, yielding a more intricate fracture network. In HDR pulse hydraulic fracturing, the temperature disparity between the fracturing fluid and the rock markedly impacts the breakdown pressure. Cold water interacting with a high-temperature geo-thermal reservoir triggers a thermal shock effect, diminishing initiation pressure and expediting fracture development and expansion. The impact of geostress differential and confining pressure on pulse hydraulic fracturing is complex, influencing not only the breakdown pressure but also the direction and extent of fracture expansion.

Productivity Model Study of Water-Bearing Tight Gas Reservoirs Considering Micro- to Nano-Scale Effects

Tight sandstone is rich in micron- and nano-scale pores, making the two-phase flow of gas and water complex. Establishing reliable relative permeability and productivity models is an urgent issue. In this study, we first used a slip model to correct the gas phase’s no-slip Hagen–Poiseuille equation for nano- and micropores. Then, combined with the fractal theory of porous media and the tortuous capillary bundle model, we established two-phase relative permeability models for nanopores and micropores. These relative permeability models comprehensively consider the gas slippage effect, the initiation pressure gradient, the pores’ fractal characteristics, and water film mechanisms. Based on these models, we developed a three-region coupling productivity model for water-bearing tight gas reservoirs with multi-stage fractured horizontal wells. This productivity model considered the micro- and nano-scale effects and the heterogeneity of fracture networks. Then, the model was solved and validated with a field case. The results indicated that the three-region composite unsteady productivity model for water-bearing tight gas reservoirs, which incorporated micro- and nano-scale effects (with consideration of micro-scale and nano-scale phenomena in the fluid flow), could accurately predict a gas well’s productivity. An analysis of the factors influencing productivity showed that ignoring the micro- and nano-scale effects in water-bearing tight gas reservoirs will underestimate the reservoir’s productivity. The initial water saturation, the two-phase flow’s initiation pressure gradient, and capillary force are all negatively correlated with the productivity of gas wells, while the conductivity of the fractures is positively correlated with gas well productivity.

Effect of aspect ratio and axial tensile load on the inflation of cylindrical tubes

We explore the snap-through instability in hyper-elastic cylindrical tubes during inflation, specifically investigating the influences of geometry and imposed axial tensile loads on both the bulging shape profiles and the initiation pressure of the bulge. We perform bulging experiments on latex rubber tubes with different parameters such as the length-to-diameter aspect ratio and axial tension. To complement these experiments, finite element simulations across various geometries and a theoretical analysis of an infinite-length tube are conducted. Our simulations reveal a critical aspect ratio that divides the bulging into two possibilities: short tubes exhibit whole bulging, while longer tubes show localized bulging. Both experimental and simulation findings indicate that as the aspect ratio and axial tensile load increase, the initiation pressure diminishes and then converges. Notably, when the axial tensile load surpasses the shear modulus, it obstructs snap-through in shorter tubes and neutralizes the influence of the aspect ratio on the initiation pressure. The outcomes of this research offer valuable perspectives on modulating the bulging mode and initiation pressure in tubular structures within soft devices, including soft pneumatic actuators and energy harvesters.

Modeling the Hydraulic Fracturing Processes in Shale Formations Using a Meshless Method

Complex bedding properties and in situ stress conditions of shale formation lead to complex hydraulic fracturing morphologies. However, due to the limitations of traditional numerical methods, the simulation of hydraulic fracturing in shale formation still needs further development. Based on this, the liquid–solid interaction modes and the SPH governing equations considering liquid–solid interaction force have been introduced. The smoothing kernel function in the traditional SPH method is improved by introducing the fracture mark ξ, which can realize the simulation of rock hydraulic fracturing processes. The stress boundary of the SPH method is applied by stress mapping of “stress particles”, and the feasibility and correctness of the method are verified by two numerical examples. Then, the simulation of hydraulic fracturing processes of bedding shale formations are carried out. With the increase of horizontal stress ratio, the total number of damaged particles decreases, but the initiation and extension pressure increase gradually. The initiation stress of small bedding dip angles (θ < 45°) is larger than that of big bedding dip angles (θ > 45°). The hydraulic fracture propagation range at low horizontal stress ratio is wider and the fracture is along the direction of maximum principal stress, while the hydraulic fracture propagation range at high horizontal stress ratio is limited to the perforation. The hydraulic fracture will propagate through the bedding with small dip angles. However, when the bedding dip angle is larger, the hydraulic fracture will propagate along the bedding direction.

Stability of highly inclined non-circular wellbores in isotropic formations

The shear and tensile stabilities of highly inclined non-circular wellbores are investigated in this study. Using the equivalent-ellipse hypothesis, the non-circular geometry was approximated as an ellipse, and the corresponding stress concentration equations are presented. With the new set of stress concentration equations, a comprehensive study of the tensile and shear stabilities of an elliptical borehole was conducted, including the impact of well inclination and azimuthal angles, horizontal stress difference, degree of ellipticity, and orientation of the maximum horizontal stress to the major axis of the ellipse. Using five commonly used shear failure criteria, we observed that both Mohr–Coulomb and Drucker Prager (inscribed) failure criteria predicted higher collapse pressures, relative to the others including Drucker Prager (inscribed), Mogi-Coulomb, and Modified Lade. While Drucker Prager's (circumscribed) failure criterion underestimates the collapse pressure. Both the linear elastic and poroelastic models were used in investigating the fracture initiation orientation and pressure of highly inclined elliptical boreholes. The prediction from the poroelastic model is always less than the linear elastic model. In some instances, they predict different fracture initiation orientations. From this study, we observed that generally, a near-circular wellbore is more stable than elliptical borehole in both shear and tension. Nevertheless, there are some well inclination and azimuthal angles than can make an elliptical borehole have more shear and tensile stabilities than a near-circular wellbore.

Experimental study on the design method of lost circulation materials for induced fractures

Owing to the influence of fluctuating wellbore pressure, the fracture width in formations with induced fractures changes dynamically. However, the traditional design method for fracture plugging materials is based on a static fracture width, which leads to the easy destruction of the plugging layer established underground and a high risk of secondary loss. Until now, the treatment of lost circulation in formations with induced fractures still faces significant challenges. Therefore, a dynamic fracture plugging experimental device was developed in this study to investigate the bridging–plugging process of lost circulation materials (LCMs) within induced fractures, as well as the design criteria for LCMs for induced fractures. Experimental results show that through adjusting the mass ratio and total concentration of the three-level bridging material, the plugging position of the bridging material can be changed, and the fracture reopen pressure can be increased. When the mass ratio of the three-level bridging material satisfies primary bridge particle: secondary bridge particle: tertiary bridge particle = 1:1:2 and the total concentration is greater than 5%, the plugging material compounded with the three-level bridging material is prone to retention in the induced fractures, exhibiting short bridging time, low permeability of the formed plugging layer, and high fracture initiation pressure. It can effectively prevent the opening of fractures or the generation of new induced fractures, thereby reducing the risk of secondary loss. The research results can provide a scientific theoretical basis for the loss control of induced fractures.

Study on the directional extension law of hydraulic fractures induced by pre-cast slot under tru-triaxial

Directional hydraulic fracturing technology serves as an effective means to overcome the stress constraints in deep coal mines and achieve targeted depressurization and permeability enhancement in specific areas. This study conducted directional hydraulic fracturing tests on samples with pre-cast slots under true triaxial conditions. Considering the azimuth angle (i.e., the angle between the long axis of the pre-cast slot and the maximum principal stress) and long axis length, the stress distribution along the fracture edge was determined using the method of complex variable function, and the evolution mechanism of the stress distribution along the fracture edge was investigated. The main conclusions are as follows: Experimental results indicate that the initiation pressure and directional extension effectiveness show a relative growth pattern of “slow-fast-slow” in the azimuth angle range of 30° to 90°, with the most significant decrease in initiation pressure occurring around 45°, and the enhancement trend of directional extension is most pronounced in the range of 45° to 60°. When the ratio of the long axis to the short axis of the pre-cast slot changes from 2:1 to 1:1 and 3:1, the initiation pressure increases by −30.44% and 28.08% respectively, while the amplification of directional extension effect is −74.6% and 166.8%. Considering the azimuth angle, an expression for the circumferential stress at the edge of the pre-cast slot was derived, and a criterion for determining the initiation of the pre-cast slot edge was proposed. As the azimuth angle increases from 0° to 90°, the circumferential stress at the edge of the pre-cast slot fluctuates, which is consistent with the trend of initiation pressure observed in the experimental results. As the long axis length increases, the initiation pressure decreases, and the initiation position moves closer to the tip.