Non-destructive testing principles and accurate evaluation of the hydraulic measure impact range using the DC method

Compared with blast mining only, blast mining after on-site hydraulic fracturing can make the mining easier and obtain better mining outcomes. To explore the effects of hydraulic fracturing on the blasting damages in coal seam, blasting experiments were carried out under biaxial confining pressure using the synthetic coal briquettes. The coal briquettes with the same mechanical properties as coal seam were prepared and the mica sheets with different radii and thicknesses were added to simulate the internal hydraulic fractures of different radii and openings. The internal damage distributions and stress attenuations of the coal briquette specimens with different hydraulic fracture radii and openings after the blasting were then measured using a rock ultrasonic tester and a static-dynamic strainmeter. Based on the rock blasting theory, the effects of hydraulic fractures with different radii and openings on the blast fracture propagation and coal seam damage were analyzed. The following conclusions are drawn: (1) The increases in hydraulic fracture radius mainly enhance the damages in the vertical direction to the hydraulic fracture, and can increase the vertical range of the severely damaged area by 20–25 cm. The increases in the hydraulic fracture opening mainly cause more severe damages along the direction of the hydraulic fracture and increase the horizontal range of the severely damaged area by 30 cm. (2) The area of the severely damaged area caused by blasting increases by 550 cm2 as the hydraulic fracture radius increased from 5 to 15 cm. As the hydraulic fracture opening increased from 2 to 10 mm, and the area of the severely damaged area caused by blasting increases by 650 cm2. Therefore, the hydraulic fracture opening has greater impacts on the severely damaged area. (3) The increase in the hydraulic fracture length reduces the compression phase attenuation of the blast stress in the radial direction. Both the increases of the hydraulic fracture length and opening increase the absolute value of the tensile phase in the radial direction. (4) Increasing the hydraulic fracture radius and opening can greatly promote the development of blast fractures and enhance the damages to coal seam. Therefore, the coal seam mining effect can be improved by increasing the radii or openings of hydraulic fractures to adjust the main action direction of blast fracture.

We have repeatedly measured the electrical resistivity of Oshima Volcano using bipole-bipole electrode arrangements across the central cone, Mihara-yama. Apparent resistivities obtained by three different pairs of electrodes varied according to the vicissitudes of the volcanic activity. Changes associated with the eruption on November 15, 1986, were particularly remarkable. The increase of the apparent resistivity obtained by the electrode pairs the nearest to the central crater vent became noticeable about one and half years before the eruption, and one day after the breakout of the eruption the apparent resistivity reached a value 17% higher than its original resistivity value. The increase of the apparent resistivity observed with these electrode pairs has been interpreted as a topographical effect caused by the ascent of magma through the volcanic vent. The apparent resistivity obtained by the farthest pairs of electrodes began to change about 10 months before the eruption. The decrease for the last 3 months before the eruption was extremely rapid and large, and the apparent resistivity became as small as about 50% of its original value. This is considered to reflect the change in the true resistivity at a few hundred meters' depth beneath the central crater.

The No.15 coal seam of Sijiazhuang Coal Mine belongs to high gas and low permeability coal seam. Hydraulic fracturing, as a technical measure for outburst prevention, is more and more widely used in high gas outburst mines. The influence of hydraulic fracturing high-pressure water on stress and displacement of roadway is analyzed, and the feasibility of hydraulic fracturing is verified. The industrial test of hydraulic fracturing through layers was carried out and the effect of gas extraction was investigated. The average concentration and pure volume of gas extraction unit in the fracturing area were 3.1 times and 5.7 times of those in the non fracturing area, respectively, which verified the permeability increasing effect of hydraulic fracturing measures. Through fracturing test, the extraction effect has been significantly improved. This paper provides important theoretical support and practical basis for hydraulic fracturing under similar conditions.

Coal and rock mass is a kind of porous media consist of many cracks and pores, these cracks and pores in the original state is mostly mutually connected. When making water injection borehole, crack will produce in coal and rock around borehole. These fissure circles are interlaced with coal native fissures. Hydraulic Fracturing use the initially fissures of water injection borehole, putting high-pressure water into the coal rock, to overcome coal rock fracture surface minimum principal stress and the tensile strength of coal and rock, making these fractures occurred after effects of high pressure water expansion and extension, thereby forming an internal split seam, forming a number of interwoven multi-fractured connected network. The formation of the network had created conditions for the flow of the gas in the coal seam,1 INTRODUCTIONIn recent years, most mines of our country have entered a phase of deep mining, the stress increasing that comes along makes the seam and rock permeability get worse, gas pressure and gas content has greatly increased[1]. According to statistics, more than 95% of the high gas and outburst mines coal seam in our country have a low coal seam permeability, gas extraction is difficult, drainage is not ideal[2]. To improve the coal seam permeability, domestic and foreign scholars have proposed a variety of methods, such as hydraulic cutting, hydraulic punching, hydraulic fracturing, loosening blasting, protective layer of pressure relief. Due to the simple process and broad applicability, the hydraulic fracturing has been more and more applied[3-6].

The fluid–solid coupling effect is more pronounced in the process of deep coal seam development compared to shallow coalbed methane, exerting a greater influence on production, and cannot be disregarded. Throughout the extraction process, the interaction between effective stress and gas desorption triggers deformation within the coal seam, leading to dynamic changes in both porosity and permeability. This paper has developed a fully coupled gas flow and deformation model that contains the coal matrix and discrete fractures to describe the dynamic gas seepage behavior and deformation of deep coal seams within a coupled wellbore–hydraulic fractures–matrix system. The model's validity is corroborated through the examination of fracture aperture, employing the finite element numerical simulation capabilities of COMSOL Multiphysics. Subsequent to the model's validation, an in-depth investigation into the permeability and production variations under diverse parametric conditions is conducted. This analysis also encompasses the assessment of hydraulic fracture geometry's impact. The simulation outcomes reveal that the permeability alterations during coal seam development are subject to the counteracting influences of gas desorption and effective stress. Moreover, it is observed that an increase in the Langmuir volume strain constant and initial porosity correlates with enhanced production, whereas a diminution in the hydraulic fracture compression coefficient leads to increased cumulative production. Notably, the optimal production is attained when hydraulic fractures are oriented vertically yet asymmetrically relative to the horizontal well.

Hydraulic fracturing as a conventional reservoir permeability enhancement technique can effectively increase the production of coalbed methane, and it is important to study the stress and crack evolution law to evaluate the effect of coalbed fracturing and optimize the construction process. To accurately derive the evolution characteristics of stress and the propagation form of cracks during hydraulic fracturing of coal seams, a numerical model of hydraulic fracturing was established based on a three-point bending test of coal samples using the finite-discrete element method (FDEM). Based on a coal seam in a mining area in southwest China, a hydraulic fracturing model was established, and the reliability of the numerical model was verified by comparing the numerical simulation with the analytical expression. The model was used to study the evolution of stress and cracks with time during hydraulic fracturing, and the influence of elastic modulus and permeability on the evolution of stress and cracks was investigated. The results show that stress and cracks in the process of hydraulic fracturing belong to a mutual feeding mechanism during evolution, and the effective permeability range of fracturing is an ellipse with the crack as the long axis enclosed by the effective stress field. The greater the elastic modulus of the coal seam, the greater the crack initiation pressure and the shorter the crack initiation time, and a coal seam with a high elastic modulus is more likely to form complex cracks. The change in coal seam permeability has little effect on the initiation pressure and initiation time, but the crack propagation path is obviously different, and a coal seam with low permeability is more favorable to hydraulic fracturing.

Treatment pressures observed during hydraulic fracturing of coal seams are often different than for conventional formations. In particular, there is usually an early, steep pressure rise and overall treatment pressures often exceed the lithostatic pressure, but produce surprisingly little vertical fracture extension. In addition, instantaneous shut-in pressures are high following a fracture treatment, while closure pressures after treatment are in good agreement with in-situ stresses determined prior to initiating hydraulic fracture stimulation. These responses cannot be explained by conventional modeling approaches. On the other hand, there are cases for coal treatments where pressures remain at levels comparable to other formations. An investigation into the physical causes of the unusual behavior of coal seam stimulations resulted in the identification of a new physical mechanism for further consideration. The accumulation of coal chips at the wellbore and partial plugging of an advancing fracture tip by coal fines carried ahead of or within the fluid treatment pad are mechanisms which consistently explain all of the observed behavior. Hydraulic fracture design codes, which include the description of the coal fines accumulation at the fracture periphery, were used to assist in the design of hydraulic fracture treatments at the Gas Research Institute, Multiple Coal Seam Field Project, at Rock Creek in Alabama, and were used to analyze the actual treatments.

Although numerous studies have tried to explain the mechanism of directional hydraulic fracturing in a coal seam, few of them have been conducted on gas migration stimulated by directional hydraulic fracturing during coal mine methane extraction. In this study, a fully coupled multi-scale model to stimulate gas extraction from a coal seam stimulated by directional hydraulic fracturing was developed and calculated by a finite element approach. The model considers gas flow and heat transfer within the hydraulic fractures, the coal matrix, and cleat system, and it accounts for coal deformation. The model was verified using gas amount data from the NO.8 coal seam at Fengchun mine, Chongqing, Southwest China. Model simulation results show that slots and hydraulic fracture can expand the area of gas pressure drop and decrease the time needed to complete the extraction. The evolution of hydraulic fracture apertures and permeability in coal seams is greatly influenced by the effective stress and coal matrix deformation. A series of sensitivity analyses were performed to investigate the impacts of key factors on gas extraction time of completion. The study shows that hydraulic fracture aperture and the cleat permeability of coal seams play crucial roles in gas extraction from a coal seam stimulated by directional hydraulic fracturing. In addition, the reasonable arrangement of directional boreholes could improve the gas extraction efficiency. A large coal seam dip angle and high temperature help to enhance coal mine methane extraction from the coal seam.

Experimental and numerical study on hydraulic fracture propagation in coalbed methane reservoir

The impact of cleats on hydraulic fracture initiation and propagation in coal seams

GE's technology deployed on two Chinese units

Enhancing coal seam permeability can significantly improve the efficiency of coalbed methane (CBM) extraction. In view of the research gap in the practical application of gas–liquid combined fracturing coal seam permeability enhancement technology (first liquid CO2 phase transition blasting fracturing of coal seam, and then hydraulic fracturing), this study conducted field trials at the 2215 coal mining face of the Liu'jia'liang Coal Mine, Xuan'gang Coal, and Electricity Co., Ltd. (a subsidiary of Jinneng Holding Coal Group). Based on customized fracturing devices, the practical implementation and effectiveness evaluation of gas–liquid combined fracturing technology were systematically investigated. The results demonstrated that, compared to hydraulic fracturing alone, the gas–liquid combined approach effectively reduced the initial fracture initiation pressure and propagation time of hydraulic fractures. Among them, the fracture initial initiation pressure of the hydraulic fracture is reduced by 22%–35%, and its propagation time is shortened by 26%–27%. Under dual-borehole cooperative fracturing conditions, blast-induced fractures achieved dual induction at both the starting and terminal ends of hydraulic fractures. Gas emission volumes from gas–liquid combined fracturing boreholes were 2.49 times higher than those from hydraulic fracturing boreholes, confirming its superior capability in promoting gas desorption and drainage. Comparative analysis of drainage effects across different permeability enhancement technologies further validated the efficacy of the gas–liquid combined method. Additionally, this study proposed a multi-borehole cooperative gas–liquid fracturing technique to eliminate permeability “blank zones” in coal seams. This technique effectively controlled the propagation direction of primary hydraulic fractures within coal seams, thereby improving fracture uniformity. The findings not only provide a high-efficiency technical solution for CBM development but also establish a practical foundation for the broader application of gas–liquid combined fracturing, contributing to the sustainable advancement of the CBM industry.

Minifrac tests are used to obtain key parameters of reservoirs for geomechanical modeling and hydraulic fracture treatment design. An ideal minifrac test can clearly indicate the leak-off pressure, fracture propagation pressure, shut-in pressure and fracture closure pressure. Repeated minifrac tests in the same interval provide additional value by verifying the fracture parameters with similar results. However, minifrac tests of two wells conducted in a coal seam in a coalbed methane (CBM) reservoir in China showed unconventional results - the fracture profile is less distinguishable for the fracture behaviors, and the subsequent test cycles in each well exhibited higher pressures than the precursors. A close examination of the test results with laboratory experiments and field microseismic monitoring results indicates that the discrepancies in pressure responses are correlated to the constitution of the coal seam. The coal seam contains many cleats, and the minifrac tests in most cases had opened these cleats instead of initiating new fractures. Opening a cleat whose plane is in line with the maximum horizontal stress orientation requires the least pressure but could be inhibited by connecting cleats with any other orientation that needs higher pressure to open or slide, which could lead to opening more cleats from different sites towards different directions. Eventually, multiple zigzag hydraulic fracture paths instead of a single planar fracture would be induced. The zigzag paths and simultaneously opening of multiple fracture paths will cause a stress increase in their vicinity, resulting in higher treating pressures for further advances. This implies that the magnitude of minimum horizontal stress Shmin shall be the least magnitude seen amongst the leak-off and fracture closure pressures of all tests, 0.0143 MPa/m for the case study. A conceptual model has been put forward to reveal the hydraulic fracturing in coalbeds. The mechanisms possibily contributing to the high treating pressure in hydraulically fracturing coalbeds have been explained from the geomechanics point of view, including poroelastic effects, fracture tortuosity effects and multiple fracture opening effects. This paper describes how the physical coal seam model reacts to fluid injection, which assists understanding the nature of minifrac tests and avoids the misinterpretation of the results. The approach can further help to quantify accurately the minimum horizontal stress magnitude from the complex minifrac tests, and hence contributes considerably to accurate geomechanical modelling and fracture treatment design for any CBM reservoirs. Introduction With horizontal drilling and hydraulic fracturing increasingly becoming economical and practical in developing shale gas, tight sand gas and coalbed methane reservoirs, knowledge of the minimum horizontal stress (Shmin) is critical in optimizing well trajectories and fracture treatment design (Ramakrishnan et al., 2009). Although there are a few methods available to assess the Shmin magnitude, the micro-fracture (microfrac) and mini-fracture (minifrac) as well as the extended leak-off test (XLOT) remain the most reliable methods (De Bree and Walters, 1989; White et al., 2002; Howkes et al., 2005; Lin et al., 2008). These methods are similar in principle: to obtain a fracture closure pressure by creating a pair of small fractures through injecting fluid in an isolated wellbore section (Fig. 1), followed by a shut-in process to allow the fracture to close. The fluid pressure varies corresponding to the fracture behavior during a complete test and the variation can indicate the moments of fracture creation, propagation and closure associated with fluid injection and shut-in process. The pressure at the time the fracture closes is termed the fracture closure pressure and is equivalent to the minimum horizontal stress. Repeated minifrac tests provide additional value by verifying the pressure parameters with similar results.

Hydraulic fracture initiation theory for a horizontal well in a coal seam

Hydraulic fracturing plays a vital role in the development of coalbed methane (CBM). Coal seam structures can, however, affect the fracturing operations, further affecting gas and water production from the coal seam. Within the research area—the Shizhuangnan Block, Southern Qinshui Basin, China—three types of coal body textures are relevant, including original, cataclastic, and granulated structure. This research describes the impacts of different coal seam types on fracturing operations and gas and water production. The results show that different types of coal seams have different impacts on hydraulic fracturing and the resultant production of gas and water. The coal seams are classified according to the vertical variation characteristics of the coal seam structures. The first type of coal seam (Type I) only develops the original and cataclastic coal structure. The effects of fracturing reconstruction for Type I are good. A high degree of coal breakage and fracture formation is observed. Gas production from CBM wells in Type I coalbeds is usually high because the water in the coal reservoir can be discharged smoothly at an early stage. The second type of coal seam (Type II) develops an additional layer of granulated coal compared with Type I. During the fracturing process, breakage of the coal seam is obvious, and the granulated coal can easily produce fine-grained coal that can block the pores and fractures, which causes an increase and fluctuation of oil pressure and can affect the effectiveness of fracturing. The gas production curve from CBM wells in Type II coal seams is mostly bimodal and water production depends on whether the pulverized coal blocks the pores and fractures. The third type of coal seam (Type III) develops two additional layers of granulated coal compared with Type I, with a greater proportion of granulated coal present. A low degree of coal breakage and fracture formation is observed, and fractures are easily blocked by pulverized coal. The effects of fracturing reconstruction for Type III are bad. Forming an effective seepage channel in this type of coal seam and extending the fracture to the far end is difficult. The gas production from CBM wells in Type III coal seams is usually low, and water production is generally low during the whole drainage period.