Fracturing Technology Research Articles

Study on rock fracture mechanism and hydraulic fracturing propagation law of heterogeneous tight sandstone reservoir.

Hydraulic fracturing technology is an effective way to develop tight sandstone reservoirs with low porosity and permeability. The tight sandstone reservoir is heterogeneous and the heterogeneity characteristics has an important influence on fracture propagation. To investigate hydraulic fracture performance in heterogeneous tight reservoir, the X-ray diffraction experiments are carried out, the Weibull distribution method and finite element method are applied to establish the uniaxial compression model and the hydraulic fracture propagation model of heterogeneous tight sandstone. Meanwhile, the sensitivity of different heterogeneity characterization factors and the multi-fracture propagation mechanism during hydraulic fracture propagation is analyzed. The results indicate that the pressure transfer in the heterogeneous reservoir is non-uniform, showing a multi-point initiation fracture mode. For different heterogeneity characterization factors, the heterogeneity characteristics based on elastic modulus are the most sensitive. The multi-fracture propagation of heterogeneous tight sandstone reservoir is different from that of homogeneous reservoir, the fracture propagation morphology is more complex. With the increase of stress difference, the fracture propagation length increases. With the increase of injection rate, the fracture propagation length increases. With the increase of cluster spacing, the propagation length of multiple fractures tends to propagate evenly. This study clarifies the influence of heterogeneity on fracture propagation and provides some guidance for fracturing optimization of tight sandstone reservoirs.

Research on the Optimization of Coalbed Methane Utilization Based on Hydraulic Fracturing Enhanced Permeability Technology

This study presents the solution of the extraction and coal seam. Through the combination of numerical simulation study and field investigation, the fracturing radius is 12 meters after 30 days of hydraulic fracturing in the 6th coal seam, and the implementation measures of fracturing hole spacing of 20 meters and 5 meters were formulated. The results showed that the hydraulic fracturing technology significantly increased the gas concentration of drilling extraction to 93%, and the attenuation rate of gas concentration in 120 days was 14%. This technology has significantly increased the extraction flow, and effectively controlled the occurrence of coal seam fire. The methods and results of this study are instructive for coalbed methane utilization in other mines.

The practice and knowledge of Maokou-Qixia Formation depth: Acid fracturing in the Hechuan Tongnan regione

The Maokou Qixia Formation in the Hechuan Tongnan region is characterized by tight reservoirs, high pressure, high temperature, high sulfur content, multiple layers, and strong heterogeneity, Mainly using highly deviated wells in conjunction with general acidification to achieve production, although certain stimulation effects have been achieved, there are also problems such as uneven acid distribution, poor targeted acid fracturing technology, large production differences between wells, and rapid production decline. In order to improve the development effectiveness of the Hechuan Tongnan area, based on the physical characteristics of the Maokou Qixia Formation reservoir in the Hechuan Tongnan area, Combining the influencing factors and process research of deep acid fracturing transformation, through experimental evaluation of acid corrosion effect and crack simulation, a set of deep acid fracturing transformation process with alternating injection of “high and low viscosity gelling acid+turning acid” as the core has been explored and formed. This technological achievement has increased the volume of vertical and horizontal acid fracturing transformation, and has been applied in HS6 on-site for 5 levels, with a tested daily gas production of 118.2×104 m3, The production increase effect is significant, with 100% effective rate for deep acid fracturing transformation and 100% effective rate for production increase. This provides a theoretical basis and technical guidance for the deep acid fracturing transformation of the Maokou Qixia Formation in the Hechuan Tongnan region.

Research on Production Prediction Method of Multi-stage Fractured Shale Gas Horizontal Well

Abstract Shale gas is a crucial component of unconventional energy. Productivity evaluation of gas wells is essential to ensure efficient and stable production. However, predicting productivity has been challenging due to the complex characteristics of shale gas reservoirs and the use of multi-stage fractured horizontal wells (MFHW) technology in the development process. This study compares traditional production prediction methods with an optimized least squares support vector machine model (LSSVM). The traditional productivity prediction method involves establishing a mathematical model of the MFHW in a fully enclosed rectangular shale formation. The model considers the effects of shale gas adsorption, diffusion, and pressure sensitivity. The model’s analytical solution is obtained using Duhamel’s principle, Laplace transform, and inverse transform. An independent production data analysis software is developed based on the analytical solution of bottom-hole pressure to predict production. To implement the LSSVM model, the model’s input parameters must be determined first. The LSSVM model’s regularization parameters and kernel parameters are obtained through the particle swarm optimization (PSO) algorithm, and the prediction model is established. The model’s matching is evaluated by calculating the coefficient of determination (R2) and the normalized root mean square error (NRMSE). The results indicate that the traditional production capacity prediction method is suitable for stable production in terms of applicability. However, the LSSVM model does not have this limitation and generally provides more accurate predictions throughout the entire production process. For complex shale gas reservoirs that frequently switch wells and use multi-stage fracturing technology, the LSSVM model is more suitable for predicting shale gas well productivity.

Research on Flow Patterns of Two-Phase Fracturing Fluid in Hydraulic Fracture Considering the Fluid Leak-off.

In in situ-generated proppant fracturing technology without proppant injection, the distribution of the flow pattern of two-phase fracturing fluid in the fracture determines the concentration of proppant particles formed by phase change in different positions. Therefore, the study of a two-phase fracturing fluid flow pattern is of great significance to reveal the formation mechanisms of different flow patterns and guide the on-site implementation of the technology. This paper establishes a mathematical model for the two-phase fracturing fluids in fractures based on their physical properties and presents numerical experiments on the flow pattern of two-phase fracturing fluids under different conditions of injection displacement, interfacial tension, and phase change liquid (PCL) ratio. The results show that at lower injection displacements, such as 3 or 4 m3/min, it is easier to form striped shape distributions, and at higher injection displacements, such as 5 or 6 m3/min, it is easier to form droplet shape distributions. When the interfacial tension is low (15 mN/m), PCL shrinks less and is distributed in strips; when the interfacial tension is high (25, 35, and 45 mN/m), PCL shrinks more and mainly forms droplet-shaped distributions. PCL tends to form discrete droplet shape distributions at PCL volume fractions of 10, and 20%. At 30% volume fraction, PCL is distributed in strips, and at 40% volume fraction, PCL forms strips of a larger size. These findings reveal the changing pattern of two-phase fracturing fluid flow and enrich the theoretical system of in situ-generated proppant fracturing technology, which can provide theoretical support for the on-site implementation of this technology.

Parameter optimization study of three-dimensional well network-fracture network coupled fracturing in jimsar shale oil

The exploration of sweet spots in the Jimsar shale oil reservoir in Xinjiang involves creating a complex fracture network through three-dimensional well networks and advanced fracturing technology, crucial for successful shale oil reservoir development. However, the extremely low permeability of shale oil and limited natural flow capacity of crude oil pose significant challenges. The interconnection between three-dimensional well networks and artificial fracture networks, and the relationship between fracturing parameters and fracture morphology, remain unclear. This study focuses on the P2l12−2 and P2l12−3 layers of the Lucaogou Formation. Utilizing the Petrel geological engineering integrated platform and the Kinetix fracturing module, we conducted numerical simulations to explore coupled fracturing in different sweet spots, with a specific emphasis on well network and fracture network coupling. This study identified relevant optimized engineering parameters. Research findings indicate that, during single-well single-factor optimization, the viscosity optimization range for Class II reservoirs is smaller compared to Class I reservoirs. However, for other factors such as injection rate, liquid volume, proppant concentration, cluster count, etc., the optimization ranges are greater for Class II reservoirs than for Class I reservoirs. In the case of single-factor optimization for well networks, increasing well spacing leads to larger optimization ranges for proppant concentration and perforation numbers. Under the same well spacing, an alternating wellbore arrangement results in a smaller optimization range for proppant concentration but a larger range for perforation numbers compared to a directly opposite wellbore arrangement. Additionally, this paper summarizes the optimization ranges and provides relevant tables and figures, aiming to offer guidance for on-site construction.

Environmental risks in the application of hydraulic fracturing technology in the oil industry

Environmental risks in the application of hydraulic fracturing technology in the oil industry

Анализ патентной и научной литературы в области гидроразрыва пласта

The article searches and analyzes patent and scientific literature in the field of application of hydraulic fracturing technologies. A review and analysis of publications from open sources is presented, in which articles and abstracts on technologies used during hydraulic fracturing were published. As a result, leading journals were identified that presented publications on applied hydraulic fracturing technologies. The leaders in the publication of articles related to hydraulic fracturing technologies are: Bulletin of Science, Exposition “Oil, Gas”. A search and analysis of patent documentation for hydraulic fracturing technologies that are directly used in the Russian Federation was also carried out. Patent analysis showed that the main patent holder for technologies used during hydraulic fracturing in the Russian Federation is the Public Joint Stock Company Tatneft named after V.D. Shashin. This analysis will allow us to assess the potential of solutions proposed by scientific and industrial teams in the field of hydraulic fracturing.

Investigation of Rock Mechanical Properties under Liquid Nitrogen Environment

In order to promote sustainable energy development and reduce the impact of fossil fuels on the environment, it is crucial to strengthen the development and utilization of clean and renewable geothermal energy. Liquid nitrogen fracturing, as an emerging waterless fracturing technology, has outstanding advantages in rock fracturing effect and thermal exchange ability with hot dry rock and is more environmentally friendly. In order to evaluate the influence of liquid nitrogen on the mechanical properties, acoustic emission characteristics, and cross-sectional crack propagation characteristics of granite at different initial temperatures, this paper carried out three-point bending tests and acoustic emission detection on granite treated by high-temperature heating and liquid nitrogen cooling. Finally, based on the cross-sectional scanning test, the expansion characteristics of microcracks in granite were analyzed. The results show that the higher the initial temperature of granite, the stronger the cold impact of liquid nitrogen on granite, and the faster the rock’s mechanical performance declines. The acoustic emission ringing count is closely related to the development of microcracks in granite, and as the initial temperature of granite increases, the more ringing counts there are, indicating that the huge temperature difference induces more microcracks inside the rock. In addition, the cold impact of liquid nitrogen can effectively promote the fracturing of granite. After liquid nitrogen treatment, the fractal dimension of the granite cross-section increases, the shape of the cross-section becomes rough, and many micropores appear. This study can provide a scientific basis for the engineering application of liquid nitrogen fracturing technology.

HORIZONTAL WELL COMPLETION SYSTEMS WITH MULTI-STAGE HYDRAULIC FRACTURING FOR LOW-PERMEABILITY, POORLY DRAINED, HETEROGENEOUS AND DISMEMBERED RESERVOIRS

Currently, the most effective method of intensifying the inflow of hydrocarbons and increasing oil recovery in productive formations in wells, in particular with a horizontal well, remains the technology of hydraulic fracturing (HF). In many regions, according to some domestic and foreign researchers, this is the only technology for developing fields with hard-to-recover hydrocarbon reserves confined to lowpermeability, poorly drained, heterogeneous and dissected reservoirs, which can significantly increase hydrocarbon production and make wells economically viable.Multi-stage hydraulic fracturing (MSHF) has established itself as one of the main methods of reservoir stimulation in the Russian Federation. The main goal of this method is to create highly conductive fractures in an oil and gas saturated reservoir throughout the horizontal section of the wellbore. To do this, an alternate series of hydraulic fracturing is performed, the result of which is the maximum production rate of hydrocarbons.Over time, oil and gas companies in the Russian Federation are becoming more demanding on the technical and technological parameters of MSHF in terms of the number of stages, the length of the horizontal section of the well, the possibility of re-fracturing and management of intervals/ports after its commissioning, and much more.The authors of the article propose to consider and analyze modern horizontal well completion systems with multi-stage hydraulic fracturing for low-permeability, low-drainage, heterogeneous and dissected reservoirs in order to form their rating.

CALCULATION OF AN ALTERNATIVE DEVELOPMENT SYSTEM FOR THE MAIN DEPOSIT OF THE DANILOVSKOYE OIL AND GAS CONDENSATE FIELD

The article provides a calculation of an alternative development system for deposit 3 (Main deposit ) of the Danilovskoye oil and gas condensate field. At the time of the analysis, this deposit is in the pilot development stage, with six development variants proposed in the trial operation project, which assume a principled arrangement of wells along an areal homogeneous grid. At the same time, due to the need to maintain the energy of the formation (that is, certain values of thermodynamic conditions), three provide for a transition to pressure maintenance, with the resulting four-point scheme of effect on the deposit (intensity 1:2; one well for injection, two for production). In the alternative model under consideration, the development method also involves a subsequent transition to pressure maintenance, but with a more active, five-point waterflooding system (intensity 1:1, equal ratio of production and injection wells), if necessary, as will be shown in the work, easily convertible to a nine-point one. The latter is an objective advantage, since the formation has low permeability and high vertical and lateral heterogeneity, and during waterflooding, zones not affected by the impact may appear, and, consequently, zones with high residual oil saturation. When calculating the characteristics of the proposed alternative method, modeling using the Peaceman method is used. Peaceman’s method is based on determining the well-reservoir connectivity of each individually opened cell, used to calculate flow rates (taking into account different values of average water cut for wells). The method is classical and, although it includes a large number of limiting assumptions, it is applicable everywhere and can provide sufficient justification for the effectiveness of the chosen development variant. General recommendations for the development of the entire field are also given (taking into account the low viscosity and lightness of oil and the properties of reservoir rocks). Such as the use of acid fracturing technology, the use of surfactants, hydrochloric acid treatments (HAT) and a complexing phosphorus-containing reagent — AFC.

Establishment and application of propped hydraulic fracture conductivity theoretical model based on fracturing efficiency index

Hydraulic fracturing technology is a key technology for the development of unconventional oil and gas reservoirs, and the conductivity of hydraulic fractures has a great impact on their production. The paper introduces the "fracturing efficiency index" as means of characterising the dynamic relationship between hydraulic fracture expansion and proppant filling. It also suggests a way to figure out the proppant fracture inflow capacity. This approach addresses issues such as uneven fracturing fluid dosage, a wide range of injection discharges, and inconsistent sand spreading concentration encountered during fracturing operations. Based on this model, the influencing factors of the flow-conducting capacity such as proppant particle size, sand spreading concentration, liquid injection displacement, and proppant crushing rate, were analyzed, and the calculated results of this model are closer to the experimental results of the API flow-conducting capacity test compared with the calculated results of the reference model. Finally, this model was used to optimize the fluid volume, sand volume, and displacement of hydraulic fracturing in the Jiping 1H shale gas field. The research results show that: (1) the fracturing efficiency index is negatively correlated with the conductivity of hydraulic fractures, whereas is positively correlated with the enhancement of conductivity; (2) our model is applicable to the calculation of supported fracture inflow capacity for high sand concentration, large particle size, intermediate fracture flow and low crushing rate, and the results are close to more than 98.58%, 98.29%, 95.68% and 95.08% of the experimental results, respectively; (3) On-site hydraulic fracturing does not result in higher production capacity with more fracturing fluid dosage, higher displacement and more sand addition; there are optimal fluid, optimal displacement and optimal sand intervals, which are optimized to increase production by 26.92%. It can be seen that the hydraulic fracture conductivity model can optimize various parameters of on-site fracturing construction.

Experimental research of novel true triaxial hydrothermal phase change impact fracturing

When extracting coal-bed methane (CBM) from deep ground, conventional hydraulic fracturing struggled to expand fractures due to both inadequate pressure and the water-lock phenomenon, while burgeoning fracturing techniques were unable to provide both high pressure and impact stress. The hydrothermal phase change impact fracturing technology can instantaneously discharge high-pressure thermal fluid and generate impact stress to volumetric fracturing. In order to research the fracturing mechanism of the new fracturing strategy, a separately designed experimental system was utilised for high-pressure thermohydraulic fracturing at different release pressures and different peripheral pressures. The experimental findings: (1) The pressurisation profile in the reactor was divided into five stages: accelerated reaction process, reaction homogeneity process, deceleration process, pressure relief shock stage and residual pressure seam formation process; (2) High-pressure hydrothermal fracture produces both radial and tearing cracks, with radial cracks being the main cracks; (3) The dimension of the fractal can be used as a damage variable to describe both the fracture morphology and the degree of damage.

Predicting gas flow rate in fractured shale reservoirs using discrete fracture model and GA-BP neural network method

In the process of shale gas exploitation, the hydraulic fracturing technology is generally used to generate the stimulate reservoir volume (SRV). The network connectivity between natural fractures (NFs) and hydraulic fractures (HFs) significantly affects gas production rate in the hydraulic fracturing stage. To study the influence of natural fracture characteristics on gas flow behavior, a discrete fracture model considering matrix and fracture behaviors is developed. It is found that the increase of natural fracture permeability and distribution density leads to the increase of gas production rate, whereas large orientation angle may cause its decrease. To determine the nonlinear relationship between the natural fracture parameters and gas production rate, the method of back propagation (BP) neural network with genetic algorithm (GA) was applied to make intelligent prediction for the shale gas production. The gas flow rate values for different natural fracture parameters (e.g., permeability, density, orientation) were calculated by discrete fracture model, and used as input data for training and prediction in the GA-BP neural network. Results demonstrate the applicability of forecasting model, which is suitable for shale gas prediction during the operation of horizontal wells if more data profiles could be collected.

Geological Characteristics of Low-Yield and Low-Efficiency CBM Wells and Practical Measures for Production Increase in the Qinshui Basin.

Many low-production and low-efficiency wells in the Zhengzhuang, Fanzhuang, Lu'an, and Yangquan blocks of the Qinshui Basin seriously hinder the development of coalbed methane in China. Through in-depth research on the geological conditions and development technology of coalbed methane, it was found that the main reasons for the existence of a large number of low-production and low-efficiency wells are the fragmentation of coal structure, poor adaptability of vertical well types, sizable well spacing, and mismatched stimulation measures. On this basis, it is proposed to adopt an L-shaped horizontal well and staged Fracking technology in the block with a complete coal structure but low permeability. For stress-concentration areas, the first fracturing of a vertical well has a single crack and a small coverage area; After a period of drainage and production, the use of repeated fracturing technology can increase the complexity of fractures and increase gas production; For the fractured area of coal structure, the use of roof fracturing technology effectively solves the problem of coal fragmentation. In natural fracture development areas, new fractures are formed using fracture turning technology to achieve the effect of increasing production. The above technologies have achieved good results in the Qinshui Basin engineering experiment. Therefore, in-depth research on the geological conditions of coalbed methane and matching related development technologies are the main ways to solve low-efficiency and low-production wells in coalbed methane development blocks.

Artificial fractures conductivity impairment in gas shale when fracturing with surface water

The shale gas fields are mainly in mountainous areas in China. Surface water is the major source of hydraulic fracturing fluid to develop shale gas. The clay mineral is generally rich in the shale gas reservoir. When encountered with low salinity surface water, the clay minerals are easy to hydrate in the process of fracturing and well shut-in. The hydration of clay minerals would result in the expansion, dispersion, and migration of mineral particles from the fracture surface, changing the structure and space of fractures, thereby affecting fracture conductivity. In this paper, shale artificial fracture core models were developed with rough fracture surfaces. With proppant-supported and self-supported fracture core models, the impact of hydration time, net pressure, and dislocation distance of fracture surfaces on the artificial conductivity were evaluated systematically. The results show that the conductivity of proppant-supported and self-supported fractures decreases by 85.3–95.1% under hydration. The turning point of conductivity is 8.0 hrs. and 4.3 hrs., respectively. At the initial stage of production, the artificial fracture could be protected better if the net pressure on the fracture is maintained from small to large. At the same time, the initial net pressure on fracture should be less than the proppant crushing pressure. Certain dislocation distances of self-supported fractures would augment the conductivity by 6.4–12.8 times. After hydration, illite and montmorillonite in the rock surface expanded and dispersed when encountered with water. The fracture surface strength decreases by 29.8%, while the surface roughness decreases by 46.1–51.6%, and some proppants were crushed. Then the fracture conductivity space was reduced. By adding KCl into the fracturing fluid, hydration can be effectively inhibited, which can increase the proppant-supported and self-supported fracture conductivity by 2.5–3.4 times, compared with fracturing fluid prepared with surface water. This study provides a theoretical basis for the optimization of shale gas reservoir fracturing technology and production strategy.

Damage characteristics of pore and fracture structures of coal with liquid nitrogen freeze thaw

Liquid nitrogen (LN2) fracturing technology is a novel waterless fracturing technology that has significant potential for application in the development of coalbed methane. However, the changes in the microstructure after coal samples are treated with LN2 freeze thaw are poorly understood. Therefore, a combination of mercury intrusion porosimetry and micro-computed tomography (micro CT) was employed to investigate the evolution of pore and fracture structure of coal samples treated with LN2. The experimental results showed that the pore volume and average pore size of coal samples increase after LN2 freeze thaw. After 12 freeze thaw cycles, the change in pore volume of micropores and minipores of coal samples was not significant, while the pore volume of mesopores and macropores increased significantly before LN2 freeze thaw. The specific surface area of the pores in different size ranges of coal samples increases with the increase in the number of LN2 freeze thaw cycles; the structure of micropores and miniopores were damaged by thermal stress and frost heave force during LN2 freeze thaw; and the pore size gradually increases to form mesopores and macropores. Micro-CT images of coal samples after LN2 freeze thaw indicated the primary fractures of coal sample expanded and generated a large number of secondary fractures. The primary and secondary fractures are interconnected and ultimately form penetrated fracture enhancing the connectivity of fractures, enhancing the connectivity of the fracture structure. The key finding study is expected to provide a theoretical basis for LN2 fracturing.

Fracture and control technology of hard and thick rock in mining engineering

The advent of non-pillar mining technology of self-formed roadway based on roof cutting theory (NPMTSFRRCT) has revolutionized the method of tackling the difficulties posed by hard suspended roofs in mining engineering. The design of roof cutting parameters plays a crucial role in determining the roof fracture characteristics. The principles of roof cutting parameter design have been analysed, with a focus on the key and challenging aspect of the collapse of single-layer thick hard rock under the condition of a thin immediate roof. A mechanical model of roof fracture has been established, and the impact of immediate roof thickness, roof cutting height, roof cutting angle, and main roof thickness on roof fracture has been analysed. A numerical model based on the UDEC software has been created, and the roof fracture, stress, and displacement variation characteristics have been studied using the failure criterion of polygonal blocks. The results of the theoretical analysis have been verified, and it was found that the roof fracture in partial roof cutting occurs in the form of hinge bite, while complete roof cutting results in step sinking. Engineering practice has shown that the deformation of the roadway surrounding rock has been effectively controlled.

Progress in and research direction of key technologies for normal-pressure shale gas exploration and development

Normal-pressure shale gas is an important object of shale gas reserves and production increase with broad resource prospects in China, but its large-scale benefit development is still confronted with technical bottlenecks. To promote the large-scale benefit development of normal-pressure shale gas, this paper systematically sorts out and summarizes the research achievements and technological progresses related to normal-pressure shale gas from the aspects of accumulation mechanism, enrichment theory, percolation mechanism, development technology, and low-cost engineering technology, and points out the difficulties and challenges to the benefit development of normal-pressure shale gas in the complex structure zones of southern China, by taking the shale gas in the Southeast Chongqing Area of the Sichuan Basin as the research object. In addition, the research direction of normal-pressure shale gas exploration and development is discussed in terms of sweet spot selection, development technology policy, low-cost drilling technology and high-efficiency fracturing technology. And the following research results are obtained. First, the accumulation mechanism of normal-pressure shale gas is clarified from the perspective of geological exploration theory; the hydrocarbon accumulation model of generation, expulsion, retention and accumulation is established; the enrichment theory of “three-factor controlling reservoir” is put forward; and the comprehensive sweetspot target evaluation system is formed. Second, as for development technology, the development technology policies of “multiple series of strata, variable well spacing, long horizontal section, small included angle, low elevation difference, strong stimulation and pressure difference controlling” are formulated. Third, as for drilling engineering, the optimal fast drilling and completion technology with “secondary structure + radical parameter + integrated guidance + unpressured leak-proof cementing” as the core is formed. Fourth, as for fracturing engineering, the low-cost and high-efficiency fracturing technology with “multi-cluster small-stage + limited-entry perforating + double temporary blocking + high-intensity sand injection + fully electric” as the core is formed. Fifth, normal-pressure shale gas is characterized by complex geological conditions, low pressure coefficient and gas content, poor resource endowment and so on, but its resource utilization still faces a series of challenges, such as uncertain productivity construction positions, low single-well productivity and ultimate recoverable reserve, high investment cost and poor economic benefit. In conclusion, the key research directions to realize the large-scale benefit development of low-grade normal-pressure shale gas are to deepen the research on the enrichment and high yield mechanism and sweet spot selection of normal-pressure shale gas, strengthen the research on the benefit development technology policy based on percolation mechanism and the key technologies for low-cost drilling, accelerate the research and development of the key technologies for low-cost and high-efficiency fracturing, and implement cost reduction and efficiency improvement continuously.

Evolution characteristics of pore structure of coal under freeze-thaw cycles combined with scanning electron microscope instrument and nitrogen adsorption experiment

ABSTRACT In this paper, anthracite in Shanxi region is taken as a research object, and the pore structure of coal under different freeze-thaw times is characterized and analyzed by scanning electron microscope and nitrogen adsorption experiment. The SEM experimental results show that the micron-scale cracks on the surface of anthracite gradually connect after freeze-thaw, and the secondary crack channels are formed around the pores and air holes, so that the pores also begin to connect. Further, the quantitative information of the picture was extracted, and it was found that the surface porosity and fractal dimension of the coal sample changed significantly after three freeze-thaw cycles, which increased by 163.3% and 15.9% respectively, and increased with the increase of freeze-thaw cycles. The results of low-temperature nitrogen adsorption experiments show that the coal samples after freeze-thaw are mainly crack, and the pore structure changes from simple to complex. In addition, the maximum nitrogen adsorption capacity, specific surface area and pore volume increased by 125.7%, 69.3% and 72.6%, respectively. With the increase of freeze-thaw times, the growth rates of maximum nitrogen adsorption capacity, specific surface area and pore volume are significantly different. The results of joint characterization show that the number of pores and cracks increases to different degrees after freezing and thawing. Comparing the increase in the degree of fracturing (163.3%) with the increase in the total pore volume (30.68%), we find that the degree of change of the crack after thawing is higher than that of the pore. In this paper, the damage effect of coal samples after freezing and thawing with liquid nitrogen is investigated from an experimental point of view, which accelerates the great leap of cyclic cryogenic fracturing technology from basic theory to technical practice.