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Hydraulic fracturing or hydro-frac fluids can impede well production due to the damage caused to the reservoir formation and fracture face, generated from adverse interactions with reservoir rock. Understanding the mechanisms of hydraulic fracturing, optimum treatment designs, and pumping/pressure profiles is critical for hydro-frac success. However, to realize the full potential of fracturing and the mitigation strategies for reservoir and fracture conductivity damage during and after its occurrence, fracturing must be considered during the design phase itself. This article provides a brief overview of hydro-frac techniques, including design, optimization, modeling, commonly used proppants, and fracturing fluid benefits and consequences based on critically reviewed case studies. However, the primary focus of this article is on the potential of fracture conductivity damage and the intrinsic mechanisms in hydraulic fracturing. The article presents updated information on various damage mitigation processes established through laboratory investigation and field implementation. The authors expect that the provided workflow in this article will be helpful to researchers and stimulate engineers to a great extent.

Metal flow tends to be complex and difficult to predict in the combined forward-backward extrusion (CFBE) process. Piercing and surface-crack defects are phenomenal in forming fasteners featuring a forward extruded pin and a backward extruded cup. In this work, a series of the CFBE tests with various combinations of the forward extrusion ratio (FER) and the backward extrusion ratio (BER) were conducted. A forming limit diagram, detailed with the piercing and surface-crack defects on the forward extruded pin or the backward extruded cup, was developed to provide a conception in choosing appropriate extrusion ratios in forming fasteners with such pin-and-cup features. With the aid of the forming load-stroke curves and the finite element analysis of fracture damage, the fracturing mechanism for the CFBE process was provided.

This study comprehensively investigates the damage and fracturing behaviors of sandstone specimens containing a single flaw under stepwise cyclic loading using digital image correlation (DIC) and acoustic emission (AE) techniques. The degradation of rocks is characterized by the evolution of residual strain, energy density, and cracking behaviors of flawed specimens while considering the effect of flaw inclination angle on the mechanical properties and fracturing behaviors of rocks. Experimental results reveal that residual strain gradually increases with an increasing number of cycles, and the increase in stress level induces a sudden rise in both elastic and dissipated energy density. The dissipation factor decreases initially and then reaches a constant value as the upper-stress limit increases. Moreover, the energy dissipation behavior becomes more consistent among the five cycles as the stress levels increase. Tensile wing cracks propagate stably during the stepwise cyclic loading process, accompanied by scattered low-amplitude AE events and a linear increase in cumulative AE counts. The analysis of normal and shear displacements indicates that wing cracks are primarily tensile, with significant normal opening displacements and negligible shear displacements. Horsetail cracks and anti-wing cracks initiate within fan-shaped strain zones of great size, driven by high compressive-shear stress, and rapidly propagate in the last one or two stress levels, leading to the detection of abundant high-amplitude AE events. Horsetail cracks and anti-wing cracks exhibit comparable displacement jumps in both normal and tangential directions, suggesting a mixed tensile-shear mode of crack propagation.

To circumvent the numerous deficiencies inherent to water-based fracturing fluids and the associated greenhouse effect, CO2 fracturing fluids are employed as a novel reservoir working fluid for reservoir reconstruction in unconventional oil fields. Herein, a mathematical model of CO2 fracturing crack propagation based on seepage–stress–damage coupling was constructed for analysing the effects of different drilling fluid components and reservoir parameters on the crack propagation behaviour of low permeability reservoirs. Additionally, the fracture expansion mechanism of CO2 fracturing fluid on low permeability reservoirs was elucidated through mechanical and chemical analysis. The findings demonstrated that CO2 fracturing fluid can effectively facilitate the expansion of cracks in low-permeability reservoirs, and thickener content, reservoir pressure, and reservoir parameters were identified as influencing factors in the expansion of reservoir cracks and the evolution of rock damage. The 5% CO2 thickener can increase the apparent viscosity and fracture length of CO2 fracturing fluid to 5.12 mPa·s and 58 m, respectively, which are significantly higher than the fluid viscosity (0.04 mPa·s) and expansion capacity (13 m) of pure CO2 fracturing fluid. Furthermore, various other factors significantly influence the fracture expansion capacity of CO2 fracturing fluid, thereby offering technical support for fracture propagation in low-permeability reservoirs and enhancing oil recovery.

Liquid nitrogen (LN2) fracturing is a technology that can dramatically enhance the stimulation performances of high-temperature reservoirs, such as hot dry rock geothermal and deep/ultra-deep hydrocarbon reservoirs. The aim of the present study was to investigate the damage characteristics of high-temperature rocks subjected to LN2 thermal shock, which is a critical concern in the engineering application of LN2 fracturing. In our work, the rocks (granite, shale and sandstone) were slowly heated to different temperatures (25 °C, 150 °C and 260 °C) and maintained at the target temperatures for 10 h, followed by LN2 quenching. After thermal treatments, we tested the physical and mechanical properties of the rocks to evaluate their damages. Additionally, sensitivities of the three rocks to thermal shock were also compared and analyzed. According to our experiments, LN2 thermal shock can enhance the permeability of the rocks and deteriorate their mechanical properties significantly. Increasing rock temperature helps strengthen the effect of LN2 thermal shock, leading to more severe damage. Inter-granular cracking is the primary contribution to the rock damage in the LN2 cooling process. Compared with granite and shale, sandstone is less sensitive to LN2 thermal shock. The lower sensitivity of sandstone to thermal shock is mainly attributed to its larger pore spaces and weaker heterogeneity of mineral thermal expansion. The present paper can provide some guidance for the engineering application of LN2 fracturing technology.

This study aims to explore dynamic behaviours of fracturing and damage evolution of rock materials at the grain scale. A grain-based discrete element method (GB-DEM) is proposed to reveal microscale characterisation and mineral grain compositions of rock materials realistically. Micro-parameters of GB-DEM are obtained by calibrating quasi-static strengths, elastic modulus, stress–strain curves, and fracture characteristics of igneous rocks. Comprehensive numerical simulations are conducted to compare with dynamic experimental results obtained by the split Hopkinson pressure bar (SHPB). The reasonability of using the GB-DEM is presented to validate fundamental pre-requisites of the SHPB technique. Combined with crack strain and acoustic emissions, the rate dependency of crack initiation stress threshold and crack damage stress threshold is investigated. The dynamic damage evolution in the form of Weibull distribution is distinctively different from that in static tests and the shape/scale parameters are presented as functions of strain rate. Moreover, microcharacteristics of crack fracturing transition and fracturing patterns formation are discussed in detail. It is found that there exist two classes of mechanical behaviour (i.e., Class I and Class II) observed from stress–strain responses of dynamic tests. Main fracturing surfaces induced by intergranular fractures split the specimen along the direction of stress wave propagation in the type of Class I behaviour. Branching cracks derive the cracks’ nucleation and in turn increases the fragment degree. A shearing band formed near the fracture surface is caused by grain pulverisations, which eventually enhances the sustainability of rocks under dynamic loading. At last, we propose a generalised equation of dynamic increase factor in the range from 10− 5 to 500/s, and also discuss the characteristic strain rate.

Deep and ultra-deep reservoirs have gradually become the primary focus of hydrocarbon exploration as a result of a series of significant discoveries in deep hydrocarbon exploration worldwide. These reservoirs present unique challenges due to their deep burial depth (4500–8882 m), low matrix permeability, complex crustal stress conditions, high temperature and pressure (HTHP, 150–200 °C, 105–155 MPa), coupled with high salinity of formation water. Consequently, the costs associated with their exploitation and development are exceptionally high. In deep and ultra-deep reservoirs, hydraulic fracturing is commonly used to achieve high and stable production. During hydraulic fracturing, a substantial volume of fluid is injected into the reservoir. However, statistical analysis reveals that the flowback rate is typically less than 30%, leaving the majority of the fluid trapped within the reservoir. Therefore, hydraulic fracturing in deep reservoirs not only enhances the reservoir permeability by creating artificial fractures but also damages reservoirs due to the fracturing fluids involved. The challenging “three-high” environment of a deep reservoir, characterized by high temperature, high pressure, and high salinity, exacerbates conventional forms of damage, including water sensitivity, retention of fracturing fluids, rock creep, and proppant breakage. In addition, specific damage mechanisms come into play, such as fracturing fluid decomposition at elevated temperatures and proppant diagenetic reactions at HTHP conditions. Presently, the foremost concern in deep oil and gas development lies in effectively assessing the damage inflicted on these reservoirs by hydraulic fracturing, comprehending the underlying mechanisms, and selecting appropriate solutions. It's noteworthy that the majority of existing studies on reservoir damage primarily focus on conventional reservoirs, with limited attention given to deep reservoirs and a lack of systematic summaries. In light of this, our approach entails initially summarizing the current knowledge pertaining to the types of fracturing fluids employed in deep and ultra-deep reservoirs. Subsequently, we delve into a systematic examination of the damage processes and mechanisms caused by fracturing fluids within the context of hydraulic fracturing in deep reservoirs, taking into account the unique reservoir characteristics of high temperature, high pressure, and high in-situ stress. In addition, we provide an overview of research progress related to high-temperature deep reservoir fracturing fluid and the damage of aqueous fracturing fluids to rock matrix, both artificial and natural fractures, and sand-packed fractures. We conclude by offering a summary of current research advancements and future directions, which hold significant potential for facilitating the efficient development of deep oil and gas reservoirs while effectively mitigating reservoir damage.

Mechanical behavior and energy evolution of rock around the tunnel are critical for evaluating the instability of geotechnical engineering. To reveal the influence of tunnel section shape on deformation, stress distribution, and fracturing mechanism of rock around the tunnel, a series of physical model shear tests for rock around prefabricated rectangular and circular tunnels were carried out, and corresponding fracturing and energy evolution analysis were also presented. In the shear test, the cracking evolution of rock around tunnel specimens was monitored and recorded by a high-speed camera and acoustic emission monitor to reveal the macro- and meso-fracture features. In addition, to examine the continuous-discontinuous shear process, four typical numerical models of rock around the tunnel were exploited to explore meso-mechanical behavior and fracturing mechanism. In light of the first law of thermodynamics, energy conversion process, damage characteristics and rockburst tendency of rock around tunnel specimens were investigated. The test results manifested that fracturing evolution, energy characteristic conversion, and micro-cracks evolution of rock around tunnel specimens generally were classified as four unified stages. In terms of fracturing evolution, rock around tunnel specimens experienced shearing compression stage (stage I), elastic stage (stage II) dominated by crack initiation, shearing fracture stage (stage III) dominated by crack propagation, coalescence and shear-induced rockburst, and shearing friction stage (stage IV). In the aspect of energy characteristic conversion, rock around tunnel specimens were mainly elastic deformation before peak shearing load, and the plastic deformation was relatively small. Partial dissipated strain energy acted on closing hole and crack initiation, and the rest was stored as elastic strain energy. After peak shearing load, the shear strength dropped rapidly, and a large amount of strain energy was converted into dissipated strain energy for crack propagation, coalescence and shear-induced rockburst. In the evolution of micro-cracks, the specimens underwent crack quiet period (stage I), crack initial increase stage (stage II), crack rapid increase stage (stage III), and crack stable stage (stage IV). Interestingly, the damage stress and rockburst tendency of rock around prefabricated rectangular tunnels were superior to those of rock around prefabricated circular tunnels, indicating that the bearing capacity of rock around prefabricated rectangular tunnels was superior to that of rock around prefabricated circular tunnels, related to the deviatoric stress distribution and confining pressure. In addition, a novel impact tendency index (Sp et) was presented for evaluating shear-induced rockburst tendency, which carved the proportional relationship between elastic strain energy and dissipative strain energy at peak shearing load. The research results were conducive to recognize the fracturing mechanism of rock around a tunnel subjected to shear condition and provided a theoretical basis for the prevention and control of geotechnical engineering.HighlightsShear characteristic, energy characteristic conversion and micro-cracks number evolution of rock around tunnel specimens generally were classified as four unified stages.Bearing capacity of rock around a prefabricated rectangular tunnel was superior to that of rock around a prefabricated circular tunnel, related to the deviatoric stress distribution and confining pressure.A novel damage variable was proposed to quantify the damage degree of rockA novel impact tendency index (Sp et) was presented for evaluating the shear-induced rockburst tendency

To determine the effect of liquid nitrogen (LN2) cooling on the damage of heated rock, we conducted a series of physical and mechanical tests on Shandong granite samples. These granites were first slowly heated to the target temperatures (25~600 °C) and held for 10 h, followed by rapid cooling with a coolant. Three coolants were used and compared in our experiment: air, water and LN2. Physical properties and mechanical properties were tested after thermal treatments. Microstructural changes were also observed using scanning electron microscope and optical microscope. According to experimental results, permeability of the heated granites increases significantly after LN2-cooling, while density, P-wave velocity, strength and elastic modulus reduce. As heating temperature rises, changes in these properties become more pronounced. Compared to air-cooling and water-cooling, LN2-cooling induces greater changes in the physical and mechanical properties at any target temperature. This indicates that LN2-cooling can damage the heated rocks more remarkably than the other two cooling treatments. According to microscopic analysis, inter-granular cracking is the primary failure mode during thermal treatment, and most of the inter-granular cracks distribute at the boundaries of quartz. Our results in this paper are of great value for understanding the characteristics of thermal damage induced by rapid cooling.

This study focuses on investigating the damage characteristics and mechanisms of Slickwo clean fracturing fluid to the reservoir by using the deep coal seam in the Yan’an gas field as the research subject. During the experiment, fracturing fluids with varying A content were employed to displace coal and rock cores. The impact of these fluids on the permeability and pore structure of coal and rock was analyzed using a combination of nuclear magnetic resonance and high-pressure mercury injection technology. The findings indicate that the permeability damage rates of cores Y-1 and Y-2 post-displacement are 48.4% and 53.6% correspondingly, with the damage worsening as the agent A content increases. NMR data reveals that the fracturing fluid exhibits the highest retention in small pores, followed by medium-sized pores, and the least in large pores. The rise in agent A content enhanced the retention degree in individual pore throats and overall, increasing from 62.24% to 68.74%. The escalation in agent A content results in higher macromolecular residues, causing seepage channel blockages and enhancing the adsorption properties between fracturing fluid and coal rock. This phenomenon leads to inadequate backflow, primarily in smaller apertures. Simultaneously, the interaction between the gel breaker and clay minerals triggers particle migration, blockage, and expansion, consequently diminishing the permeability of coal and rock and inducing specific damages.