Breakdown Pressure Research Articles

Precipitants transport during post-flush and its impacts

Wellbore stimulation is often needed for different situations in the field ranging from deepwater frac-packs to enhanced geothermal systems. Acidizing is commonly designed for cleaning perforation, recovering perforation functions, and improving injectivity during completion operations such as frac-pack, aiming to reduce the fracturing pressure and enhance productivity. However, some field observations have shown unexpected breakdown pressure increases after the post-flush period of acid treatment in sandstone reservoirs. To understand this problem, a 3D model has been established in this paper to simulate the acidizing process through a perforation hole, and then the subsequent post-flush resulting in precipitant transport. Finally, the breakdown process where fluid is injected into the rock is modeled to compare breakdown pressures in different cases. Our numerical results reveal that although minerals can be effectively dissolved by chemical reactions during the main flush period, the generated precipitants transported during the post-flush period may affect the rock breakdown process. The precipitant transport might develop high-porosity channels, potentially leading to the breakthrough along the precipitant deposits and establishing a string hydraulic communication between the perforation hole and the surrounding rock. The existence of these high-porosity channels enables fluids to flow swiftly into the surroundings, impeding enough pressure accumulation to crack the rock. The formation of high-porosity channels in the post-flush period and its potential impact on the breakdown pressure is influenced by several factors such as post-flush fluid injection rate, acid concentration and viscosity, and the initial rock permeability.

Numerical analysis of hydraulic fracture propagation in deep shale reservoir with different injection strategies

Deep shale reservoirs are characterized by elevated breakdown pressures, diminished fracture complexity, and reduced modified volumes compared to medium and shallow reservoirs. Therefore, it is urgent to investigate particular injection strategies that can optimize breakdown pressure and fracturing efficiency to address the increasing demands for deep shale reservoir stimulation. In this study, the efficiency of various stimulation strategies, including multi-cluster simultaneous fracturing, modified alternating fracturing, alternating shut-in fracturing, and cyclic alternating fracturing, was evaluated. Subsequently, the sensitivity of factors such as the cycle index, shut-in time, cluster spacing, and horizontal permeability was investigated. Additionally, the flow distribution effect within the wellbore was discussed. The results indicate that relative to multi-cluster simultaneous fracturing, modified alternating fracturing exhibits reduced susceptibility to the stress shadow effect, which results in earlier breakdown, extended hydraulic fracture lengths, and more consistent propagation despite an increase in breakdown pressure. The alternating shut-in fracturing benefits the increase of fracture length, which is closely related to the shut-in time. Furthermore, cyclic alternating fracturing markedly lowers breakdown pressure and contributes to uniform fracture propagation, in which the cycle count plays an important role. Modified alternating fracturing demonstrates insensitivity to variations in cluster spacing, whereas horizontal permeability is a critical factor affecting fracture length. The wellbore effect restrains the accumulation of pressure and flow near the perforation, delaying the initiation of hydraulic fractures. The simulation results can provide valuable numerical insights for optimizing injection strategies for deep shale hydraulic fracturing.

In-situ laboratory study on influencing factors of pre-SC-CO2 hybrid fracturing effect in shale oil reservoirs

Supercritical CO2 (SC-CO2) fracturing, being a waterless fracturing technology, has garnered increasing attention in the shale oil reservoir exploitation industry. Recently, a novel pre-SC-CO2 hybrid fracturing method has been proposed, which combines the advantages of SC-CO2 fracturing and hydraulic fracturing. However, the specific impacts of different pre–SC-CO2 injection conditions on the physical parameters, mechanical properties, and crack propagation behavior of shale reservoirs remain unclear. In this study, we utilize a newly developed “pre-SC-CO2 injection → water-based fracturing” integrated experimental device. Through experimentation under in-situ conditions, the impact of pre-SC-CO2 injection displacement and volume on the shale mineral composition, mechanical parameters, and fracture propagation behavior are investigated. The findings of the study demonstrate that the pre-injection SC-CO2 leads to a reduction in clay and carbonate mineral content, while increasing the quartz content. The correlation between quartz content and SC-CO2 injection volume is positive, while a negative correlation is observed with injection displacement. The elastic modulus and compressive strength exhibit a declining trend, while Poisson's ratio shows an increasing trend. The weakening of shale mechanics caused by pre-injection of SC-CO2 is positively correlated with the injection displacement and volume. Additionally, pre-injection of SC-CO2 enhances the plastic deformation behavior of shale, and its breakdown pressure is 16.6% lower than that of hydraulic fracturing. The breakdown pressure demonstrates a non-linear downward trend with the gradual increase of pre-SC-CO2 injection parameters. Unlike hydraulic fracturing, which typically generates primary fractures along the direction of the maximum principal stress, pre-SC-CO2 hybrid fracturing leads to a more complex fracture network. With increasing pre-SC-CO2 injection displacement, intersecting double Y-shaped complex fractures are formed along the vertical axis. On the other hand, increasing the injection rate generates secondary fractures along the direction of non-principal stress. The insights gained from this study are valuable for guiding the design of preSC-CO2 hybrid fracturing in shale oil reservoirs.

Laboratory investigation on injection of supercritical CO2 into basalt: Implication for CO2 sequestration via mineralization

Hydraulic fracturing can create a fracture network that enables fluid penetration of a basalt formation with otherwise low porosity, offering a site for rapid CO2 mineralization sequestration. Supercritical carbon dioxide (SC-CO2) is a promising fracturing fluid with unique properties, making it well-suited for unconventional oil and gas reservoir stimulation. In this study, experiments of fracturing with water and SC-CO2 were conducted under different injection schemes and formation temperatures. The frackability of basalts was investigated from the perspective of breakdown pressure and fracture morphology. The findings revealed that SC-CO2 fracturing with a low pressurization rate could become an optimized option for enhancing the stimulated effect. The potential of carbon sequestration was roughly estimated based on the area of induced fractures, suggesting that fostering a developed fracture network would aid in the in-situ mineralization and storage of carbon. The variation of element composition obtained from basalt slices containing fractures induced by SC-CO2 under different reaction conditions verified the role of water participation and time scale in the mineralization effect.

Fracture propagation morphology and parameter optimization design of pre-CO2 hybrid fracturing in shale oil reservoirs, Ordos Basin

Shale reservoirs typically exhibit the characteristics of large burial depth, low permeability, and well-developed bedding. Despite the implementation of water-based fracturing techniques with high displacement, attaining the optimal reservoir stimulation effect remains a formidable challenge. Consequently, in recent years, a novel method of hybrid fracturing method using pre-CO2 + sand-carrying slick water has been proposed. Within this framework, the meticulous design of fracturing operation parameters stands out as a pivotal factor influencing the efficacy of reservoir stimulation. This paper presents a coupled 3-D hydraulic fracture propagation model, investigating the influence of injection displacement, injection volume, and three-stage pre-injection fluid volume ratio on the fracture propagation. Two quantitative evaluation metrics, namely fracture surface area S and fracture transverse plane fractal dimension D, are precisely defined. Additionally, real-time analysis of bottom-hole pressure is conducted to further comprehend the dynamics of the process. Numerical investigations have revealed that both pre-CO2 injection displacement and volume exert a substantial influence on fracture half-length and complexity. Furthermore, the proportion of hybrid fracturing fluid demonstrates a notable impact on fracture height and complexity. The design of injection parameters does not exhibit a significant effect on the formation breakdown pressure. This study quantitatively evaluates fracture propagation under different working conditions from the perspective of fracture morphology, and serve as practical guidance for optimizing the process parameters design for hybrid fracturing in shale oil reservoirs, Ordos Basin.

Impact of Wellbore Cross-Sectional Elongation on the Hydraulic Fracturing Breakdown Pressure and Fracture Initiation Direction

Investigation of breakdown pressure in wellbores in complex conditions is of great importance, both in fracture design and in wellbore log interpretation for in situ stress estimation. In this research, using a two-dimensional numerical model, the breakdown pressure is determined in ellipsoidal and breakout wellbores. To find the breakdown pressure, the mixed criterion is used, in which the toughness and the tensile strength criteria must be satisfied concurrently. In breakout boreholes, the breakdown pressure is lower than the circular wellbores; indeed, the ratio of the breakdown pressure of the breakout wellbore to the breakdown pressure in the circular wellbore is between 1 and 0.04, depending on the deviatoric stress and the width and depth of the breakout zone. In breakout wellbores, the fracture initiation position depends on the deviatoric stress. In small deviatoric stresses, the fracture initiation position is aligned with the minimum in situ stress, unlike circular boreholes; and in large deviatoric stresses, the fracture initiates in the direction of the major principal stress. In large wellbores, the breakdown pressure is controlled by the tensile strength of the rock; and in small wellbores, the breakdown pressure is under the control of the energy spent to create new crack surfaces.

Propagation and complex morphology of hydraulic fractures in lamellar shales based on finite-discrete element modeling

We explore the controls of stress magnitude and orientation relative to bedding on the resulting morphology/topology of hydraulic fractures using a combined finite-discrete element method (FDEM). Behavior is shown conditioned by the ratio of principal stresses λ=σ3/σ1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda ={\\sigma }_{3}/{\\sigma }_{1}$$\\end{document} and relative inclination of the bedding. When the lateral pressure coefficient (λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda$$\\end{document}) is less than 0.67, hydraulic fractures predominantly initiate as tensile fractures along the wellbore, aligning with the maximum principal stress direction. Conversely, for λ≥0.67\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda \\ge 0.67$$\\end{document}, shear cracks are favored to initiate for the minor stress difference, leading to a less predictable initiation and extension direction. Simultaneously, diminished stress differences correspond to elevated reservoir breakdown pressures, displaying a linear correlation with lateral pressure coefficients and little influenced by equivalent bedding orientation. Bedding plane orientation significantly impacts the mode and morphology of hydraulic fracture propagation. Bedding parallel to the direction of the minimum principal stress (σ3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\sigma }_{3}$$\\end{document}) favors layer-penetrating and bifurcated fractures, whereas inclined bedding facilitates the emergence of numerous steering-type and capture-type fractures. Especially at steeper inclinations (β=60∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta =60^\\circ$$\\end{document}), hydraulic fractures readily extend along the bedding surface, inducing macroscopic shear slip failure. Under high-stress disparities, the breakdown pressure exhibits greater sensitivity to bedding inclination, and its influence pattern aligns with the variations in tensile strength, typically reaching maximum and minimum values at bedding inclination angles of 0° and 60°, respectively.

Modeling the Impact of Thermal Stresses Induced by Wellbore Cooldown on the Breakdown Pressure and Geometry of a Hydraulic Fracture

Modeling the Impact of Thermal Stresses Induced by Wellbore Cooldown on the Breakdown Pressure and Geometry of a Hydraulic Fracture

Experimental study of hydraulic fracture propagation with multi-cluster in-plane perforations in a horizontal well

Tri-axial fracturing studies were carried out to understand the impact of lateral mechanical parameters on fracture propagation from multiple in-plane perforations in horizontal wells. Additionally, the discussion covered the effects of geology, treatment, and perforation characteristics on the non-planar propagation behavior. According to experimental findings, two parallel transverse fractures can be successfully initiated from in-plane perforation clusters in the horizontal well because of the in-plane perforation, the guide nonuniform fishbone structure fracture propagation still can be exhibited. The emergence of transverse fractures and axial fractures combined as complex fractures under low horizontal principal stress difference and large pump rate conditions. The injection pressure was also investigated, and the largest breakdown pressure can be also found for samples under these conditions. The increase in perforation number or decrease in the cluster spacing could provide more chances to increase the complexity of the target stimulated zone, thus affecting the pressure fluctuation. In a contrast, the increase in fracturing fluid viscosity can reduce the multiple fracture complexity. The fracture propagation is significantly affected by the change in the rock mechanical properties. The fracture geometry in the high brittle zone seems to be complicated and tends to induce fracture reorientation from the weak-brittle zone. The stress shadow effect can be used to explain the fracture attraction, branch, connection, and repulsion in the multiple perforation clusters for the horizontal well. The increase in the rock heterogeneity can enhance the stress shadow effect, resulting in more complex fracture geometry. In addition, the variable density perforation and temporary plugging fracturing were also conducted, demonstrating higher likelihood for non-uniform multiple fracture propagation. Thus, to increase the perforation efficiency along the horizontal well, it is necessary to consider the lateral fracability of the horizontal well on target formation.

Cyclical hydraulic pressure pulses reduce breakdown pressure and initiate staged fracture growth in PMMA

Using unique experimental equipment on large bench-scale samples of Polymethylmethacrylate, used in the literature as an analogue for shale, we investigate the potential benefits of applying cyclical hydraulic pressure pulses to enhance the near-well connectivity through hydraulic fracturing treatment. Under unconfined and confined stresses, equivalent to a depth of up to 530 m, we use dynamic high-resolution strain measurements from fibre optic cables, complemented by optical recordings of fracture development, and investigate the impact of cyclical hydraulic pressure pulses on the number of cycles to failure in Polymethylmethacrylate at different temperatures. Our results indicate that a significant reduction in breakdown pressure can be achieved. This suggests that cyclic pressure pulses could require lower power consumption, as well as reduced fluid injection volumes and injection rates during stimulation, which could minimise the occurrence of the largest induced seismic events. Our results show that fractures develop in stages under repeated pressure cycles. This suggests that Cyclic Fluid Pressurization Systems could be effective in managing damage build-up and increasing permeability. This is achieved by forming numerous small fractures and reducing the size and occurrence of large fracturing events that produce large seismic events. Our results offer new insight into cyclical hydraulic fracturing treatments and provide a unique data set for benchmarking numerical models of fracture initiation and propagation.

Pushed and pulled onto the streets: Perspectives of street children in Accra, Ghana

Many Non-Governmental Organizations (NGOs) that work with street children in Ghana have been relevant as they complement the limited services provided by the government. Notwithstanding the efforts made by NGOs to address this social phenomenon, some of their beneficiaries maintain their presence on the streets. This study therefore explored the reasons why some street children keep going back to the streets despite the resources and time spent on their behalf by NGOs. Utilizing the qualitative research design, forty participants, comprising of thirty street children and ten staff of two NGOs in Accra were purposively selected for this study. In-depth interviews and focus groups discussions were used to gather data that were analyzed thematically. The study found that the children decided to resort to street life for survival due to various factors, including inadequate family support, abuse, family breakdown, and peer pressure. The findings also indicated that NGOs provided services, such as vocational training, shelter, as well as other necessities, such as food to street children. Nonetheless, some of their beneficiaries remained on the streets due to issues, such as inadequate organizational support, peer influence, and limited freedom at the premises of the NGOs. Based on the findings of the study, recommendations were made for policy and practice.

Optimizing extreme limited entry design for lateral heterogeneous reservoirs

Extreme limited entry (XLE) method can enhance hydraulic fracture uniformity by elevating wellbore pressure to overcome reservoir heterogeneity. However, it increases surface pressure and decreases hydraulic fracturing efficiency. In this paper, the effects of stress shadow and heterogeneity of the lateral breakdown pressure on fracture propagation are numerically studied, from which an optimization method is proposed for the XLE design with the minimal requirement of fracturing pressure. Initially, the unconventional fracture model is applied to analyze fracture propagation in formations with homogeneous geomechanical parameters and various perforation designs. Fracture length distribution is correlated with perforation friction, revealing insights into the influence of geomechanical parameters and fracture spacing. Then, the uniformity of fracture propagation is assessed for different XLE designs by varying lateral breakdown pressures. Simulation results are utilized to develop a cross-plot of fracture length distributions vs normalized perforation frictions for optimizing an XLE design. In homogeneous reservoirs, decreasing perforation spacing and increasing perforation hole erosion rate slow down the reduction in the fracture length distribution span with perforation friction, necessitating a higher perforation friction for uniform fractures. In all simulated cases, a similar trend is observed when the distribution of fracture lengths is plotted vs the normalized perforation friction to the difference of breakdown pressure within one fracturing stage. It is recommended that this normalized value be maintained above 2 in an XLE design. This new plot makes the conventional horn-shaped plot more applicable to reservoirs with heterogeneous lateral breakdown pressures.

Phase-field cohesive zone modeling of hydro-thermally induced fracture in hot dry rock during liquid nitrogen fracturing

Liquid nitrogen (LN2) fracturing is regarded as a viable alternative for the efficient development of hot dry rock (HDR) resources due to its main advantage in producing complex fracture networks and lowering breakdown pressure. However, the fracturing mechanisms and major factors controlling the LN2 fracturing efficiency are still poorly understood. A thermo-hydro-mechanical-damage (THMD) coupling model is proposed to study the fracture initiation/propagation behavior of LN2 fracturing under various fracturing parameters and reservoir conditions based on a phase-field cohesive zone method (PF-CZM). The characteristic differences between LN2 and water fracturing are compared. Results indicate that the tensile stress of LN2 fracturing is concentrated at the tip of fractures. When the HDR reservoirs are undertaken lower stress level with higher initial rock temperature, the damage ratio can be enhanced. Young's modulus and thermal expansion coefficient are the key petrophysical and mechanical properties affecting the LN2 fracturing performance. The optimal pressurization rate is 0.2 MPa/s. It indicated that a balanced contribution of thermal stress and fluid pressure is crucial to the bifurcation of major fracture and forming complex fracture networks. The major findings of the study are expected to provide theoretical guidance and computational simulation basis for the LN2 fracturing on HDR reservoirs.

Experimental study on the effects of chemical or microwave treatment on the tensile strength of hot dry rock

The Brazilian disc (BD) specimens were heated to high temperatures up to 100–500 °C followed by immersion in chemical solutions or irradiation in a microwave oven. The Brazilian splitting tests were then performed at high temperatures, and acoustic emission (AE) events were recorded. Meanwhile, the longitudinal wave velocity and open porosity of the rocks were measured, and the scanning electron microscope (SEM) was applied to identify the influence of chemical or microwave treatment on the microstructure in specimens. The experimental results show that: (1) the chemical or microwave treatment exacerbates the damage to the heat-treated rocks and that either treatment has significant effect on heat-treated rocks with diverse lithologies. (2) The specimens with a high proportion of soluble minerals are chemically dissolved and the specimens with a high percentage of powerful microwave absorption minerals were irradiated by the microwaves, resulting in lower longitudinal wave velocity, lower tensile strength, higher open porosity, and more severe microstructural damages. (3) Rock tensile strength is closely related to the breakdown pressure of hydraulic fracturing, implying that chemical or microwave-assisted fracturing techniques have significant impact on various hot dry rocks, and these auxiliary fracturing techniques considering rock lithology are novel and important for EGS projects.

Multiscale characterization of carbonate-rich shale: From micro structure to macro mechanical properties, a case study in Hongxing Block, China

As a new growth point of shale gas production, carbonate-rich shale has recently attracted great interest. However, gas production from this reservoir was poor since the properties of the carbonate-rich shale have not been systematically studied. This is an urgent need to be addressed. Downhole cores collected from an exploration well were analyzed by micro-structure experiments (X-ray diffraction, mineral quantitative evaluation, low-pressure nitrogen adsorption, and mercury intrusion piezometry test) and macro-mechanical tests (triaxial compression, Brazilian splitting, and fracture toughness test) to obtain multiscale characterization of carbonate-rich shale. The intrinsic relationship between the microstructural and mechanical characteristics of this reservoir has been revealed. The results showed that (1) The content of carbonate minerals exceeds 30%, just slightly lower than that of quartz. Calcite has large particle sizes and is closely cemented with quartz to form a dense rock skeleton. (2) The maximum adsorption volume, specific surface area, total pore volume and average pore diameter values are poor, which is unfavorable for shale gas accumulation. Limited porosity and permeability and great tortuosity can be obstacles to the release of accumulated gas. (3) High peak deviatoric stress, Young's modulus, and Poisson's ratio were contributed by dense rock skeleton. (4) Narrow maximum main fracture width is not favorable to place proppants and further to create effective gas transport channels. (5) High tensile strength and fracture toughness enhance high breakdown and extension pressure. These comprehensive insights are of great significance for both the exploration and development of carbonate-rich shale reservoirs.

Laboratory study on acid fracturing performance in high temperature carbonate reservoirs

In this article, we investigate acid fracturing, a promising method to increase the effectiveness of fractures in high temperature carbonate reservoirs. In order to determine the performance of acid fracturing in carbonate reservoirs, we conducted a lab scaled true triaxial acid fracturing experiment on high temperature carbonate rock. The fracture initiation, propagation behavior, and conductivity under the action of acid solution were analyzed. The 3-D micro computed tomography was applied to illuminate the fracture network patterns; coefficient of stress difference, acid etching time, and acid concentration were also analyzed. Results show acidification exhibits the lowest breakdown pressure compared with no-acidification, reduced by 58%.

Experimental study on fracture propagation in anisotropy rock under cyclic hydraulic fracturing

Hydraulic fracturing is commonly used to enhance the hydraulic conductivity of geothermal, oil, and gas reservoirs, particularly those situated in enclosed and unconventional formations. Shale oil and gas reservoirs exhibit distinct characteristics in rock mechanics parameters, morphology, natural fracture geometry, and geological features. Structural anisotropy in materials generally arises from changes in grain type and size, weak layering, and variations in layering angles. This study aims to investigate the impact of two key factors, namely cyclic injection and structural anisotropy, on the hydraulic fracturing process. To achieve this goal, the phenomenon of hydraulic fatigue in rock was examined through various cyclic injection methods, and true-triaxial hydraulic fracturing tests were conducted on cubic samples composed of different materials and layers. To evaluate and analyze the fracture morphology of these samples, CT scan X-ray imaging technology was employed. The results revealed that samples subjected to cyclic injection exhibited an average breakdown pressure approximately 30% lower than those subjected to monotonic injection. Fracture geometry in the cyclic injection samples displayed a network pattern with multiple crack branches, while monotonic injection primarily resulted in a single-surface pattern. In samples with differences in horizontal stress, the anisotropy of the sample did not affect the fracture, and the single fracture surface extended in the direction of maximum stress. The energy generated by cyclic injection using the damage-controlled method (cycle duration: 16 s) was concentrated around the borehole, creating a complex fracture network with shorter crack lengths. Conversely, cyclic injection using the ramp signals method (cycle duration: eight seconds) produced more straightforward cracks with greater expansion and penetration. Examination of CT scan images of the fractured samples revealed average tortuosity and fracture volume fraction parameters of 1.117 and 0.027 for cyclic injection, respectively, in contrast to 1.026 and 0.008 for monotonic injection. Furthermore, the S-N damage model based on the results of true-triaxial hydraulic fracturing was introduced to describe rock damage behavior and the concept of fatigue under cyclic loads.

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.

An Experimental Investigation into Effects of Shale Hydration on Breakdown Pressure and the Fracture Characteristics During Hydraulic Fracture Initiation

An Experimental Investigation into Effects of Shale Hydration on Breakdown Pressure and the Fracture Characteristics During Hydraulic Fracture Initiation

Effect of Non-Uniform Minerals Distribution on Hydraulic Fracture Evolution during Unconventional Geoenergy Exploration

To study the effect of the non-uniform distribution of minerals on the development of microcracks within the rock during hydraulic fracturing, a novel numerical model considering multiple random mineral distributions was designed. The model investigated the impacts of mineral grain size, composition, and spatial arrangement on fracture initiation and propagation. The results indicate that the presence of the hard-phase mineral quartz can alter the propagation path of fractures, and increase the width of hydraulic fractures. In coarse-grained granite, the range of crack deflection is maximized, while in medium-grained granite, it is more prone to forming convoluted elongated cracks. A higher quartz content in granite further contributes to the formation of complex crack networks. Simultaneously, the evolution of granite fractures and variations in breakdown pressure in heterogeneous granite were investigated, considering the influence of core parameters such as fluid injection rate, fracturing fluid viscosity, and horizontal stress difference. The research reveals that a high injection rate promotes straight-line fracture expansion. Moreover, modest fluctuations in fracturing fluid viscosity have minimal effects on fracture morphology. When the fracture development avoids quartz, under the influence of high horizontal stress differential, it clearly turns toward the direction of the maximum principal stress. This study can offer insights into innovative and optimized deep reservoir fracturing techniques.