Hydraulic Fracturing Of Coal Seam Research Articles

Microseismic activity characteristics and range evaluation of hydraulic fracturing in coal seam

Accurately evaluating hydraulic fracturing (HF) activity characteristics and impact range is critical to optimizing the HF scheme design. This study proposes an adaptive threshold peak detection method and a wavelet transform-support vector machine (WT-SVM) polarity analysis method based on microseismic (MS) monitoring data from a coal mine in Guizhou, China. The results show that the adaptive threshold peak detection method effectively improves the accuracy and stability of the P-wave initial arrival pickup for low signal-to-noise ratio (SNR) signals. The main frequency band of the MS signal of HF is focused on 150–300 Hz. The improved particle swarm optimization algorithm was used by introducing a random-weight strategy to determine the high-density impact areas of the HF within approximately 20 m on both sides of the fracturing borehole. Density cloud map analysis revealed a small area of the blank zone at the junction of the fracturing section, which provided a basis for fracturing parameter design and scheme optimization. The directional characteristics of the MS waveform were quantitatively described by polarity analysis, where the dip angle distribution ranged from 39° to 49°, the strike angle ranged from 61° to 74°, the slip angle was concentrated near 44°, and the stretch angle ranged from 61° to 74°. The WT-SVM proposed in this study overcame the effects of low SNR signals and the limitations of a small-area station network; 62% of the shear ruptures and 38% of the tensile ruptures of the HF were obtained. Shear cracks were the primary expression of the HF rupture type.

Effect of size and anisotropy on mode I fracture toughness of coal

Obtaining the accurate fracture toughness of coal considering bedding angle and independent of size is of great significance to the evaluation of the initiation pressure in the hydraulic fracturing of coal seam. In this study, the newly proposed modified semi-circular bending test was used to conduct mode I fracture testing of coal samples with different sizes and bedding angles. A four-dimensional lattice spring model (4D-LSM) was established based on the cohesive zone model considering bedding planes to study the effects of specimen size and bedding angle the fracture toughness and fracture pattern of coal. The size effect and anisotropy characteristics of fracture toughness of coal were analyzed by using linear elastic fracture mechanics and Bazant size effect law (SEL) theory. The results show that the fracture toughness of coal has obvious size effect and anisotropy. In quasi-brittle materials, the fracture toughness increases with the increase of specimen size due to the fracture process zone (FPZ). However, the fracture toughness of coal affected by microcracks generally decreases with the increase of specimen size. The size effect of fracture toughness of coal is jointly determined by the FPZ with increasing effect and the microcrack with reducing effect. The fracture toughness of coal increases with the increase of bedding angle. The bedding angle has no influence on the fracture toughness size effect of coal, but the microcracks associated with the bedding plane have a greater influence on the fracture toughness size effect. The specimen size only affects the fracture toughness value, but has no obvious effect on the fracture toughness anisotropy. Based on Bazant SEL theory, a fracture toughness size effect model considering microcracks and bedding angles was established. The research results have reference significance for crack initiation and propagation in hydraulic fracturing of coal seam.

Evaluation of Hydraulic Fracturing in Coal Seam using Ground Microseismic Monitoring and Source Location

Evaluation of Hydraulic Fracturing in Coal Seam using Ground Microseismic Monitoring and Source Location

An Experimental Study on the Impact of the Particle Size and Proportion of Composite Proppant on the Conductivity of Propped Fractures in Coalbed Methane Reservoirs following Pulverized Coal Fines Infiltration

Coalbed methane reservoirs exhibit a low strength and high heterogeneity, rendering them susceptible to coal fines generation during hydraulic fracturing operations. The detrimental impact of coal fines on the conductivity of the propped fracture has been overlooked, leading to a substantial negative effect on the later-stage recovery of coalbed methane reservoirs. Moreover, the particle size distribution of the composite proppant also affects the conductivity of the propped fracture. To mitigate the damage caused by coal fines to the conductivity of the proppant pack in CBM reservoirs, this study conducted conductivity tests on actual coal rock fractures. The aim was to assess the effect of various particle size ratios in composite proppant blends on the conductivity of complex fractures in CBM reservoirs. The ultimate goal was to identify an optimized proppant blending approach that is suitable for hydraulic fracturing in coal seams. The results indicated that, in terms of the short-term conductivity of coalbed methane reservoirs, the conductivity of composite proppants is primarily influenced by the proportion of large or small particles. A higher proportion of large particles corresponds to a stronger conductivity (e.g., the conductivity is highest at a particle ratio of 5:1:1 for large, medium, and small particles). On the other hand, a higher proportion of small particles leads to a poorer conductivity (the conductivity is lowest when the particle ratio is 1:1:5). In the long-term conductivity of coalbed methane reservoirs, the fluid flushing of the fracture surfaces generates coal fines, and small particles can fill the gaps between larger particles, hindering the infiltration of coal fines. Therefore, it is important to control the particle size ratio of composite proppants, with a predominant proportion of larger particles. This approach can maintain long-term conductivity and prevent the excessive infiltration of coal fines, thereby avoiding fracture blockage (e.g., the conductivity is highest at a particle ratio of 5:1:5, followed by a ratio of 3:1:3). Furthermore, considering the influence of proppant placement methods and the support effect on near-wellbore opening fractures and far-end sliding fractures, segmented placement is utilized to fully fill the fractures for short-term conductivity, whereas mixed placement is employed for long-term conductivity to achieve a balance in particle gaps and hinder the infiltration of coal fines. The findings of this study contribute to the understanding of proppant selection and placement strategies for efficient hydraulic fracturing in coalbed methane reservoirs.

Numerical Simulation Study on the Evolution Law of Stress and Crack in Coal Seam Hydraulic Fracturing

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.

Research on the evolution law of hot spots in the field of coal seam hydraulic fracturing based on bibliometric analysis: review from a new scientific perspective.

As an unconventional natural gas, the calorific value of coal seam gas (CSG) is equivalent to that of natural gas. It is a high-quality, clean, and efficient green low-carbon energy source. Coal seam hydraulic fracturing is an important permeability enhancement measure in the process of CSG drainage. In order to further understand the overall research progress in the field of coal seam hydraulic fracturing, the Web of Science (WOS) database is used as a sample source, and the bibliometric analysis of the literature is carried out by CiteSpace software. The visual knowledge maps of the number of publications, the research countries, institutions, and keyword clustering are drawn. The research shows that it has gone through two stages of slow development and rapid growth in terms of time distribution. In terms of cooperation networks, the main active countries include China, the USA, Australia, Russia, and Canada, composed of China University of Mining and Technology, Chongqing University, Henan Polytechnic University, and China University of Petroleum as the core research institutions. Taking keywords as the theme, the coal seam hydraulic fracturing research field mainly involves high-frequency keywords such as hydraulic fracturing, permeability, model, and numerical simulation. The hotspot evolution law and frontier development trend of keywords with time are analyzed and obtained. On this basis, from a new perspective, the "scientific research landscape map" in the field of coal seam hydraulic fracturing is outlined, in order to provide a scientific reference for the research in this field.

Multi-scale multivariate detection method for the effective impact range of hydraulic fracturing in coal seam

Hydraulic fracturing enhances coal seam permeability and reduces original gas content. Determining the effective range of hydraulic fracturing is essential in evaluating its effectiveness. This paper analyzes physical field changes during hydraulic fracturing and proposes a multi-scale multivariate detection method for determining the effective range of hydraulic fracturing in coal seams based on changes in wave velocity and resistivity due to fracture expansion.The method was validated through field experiments. The result shows that vibration wave computed tomography (VWCT) is a passive, large-scale area detection method. In contrast, the direct current (DC) method is a small-scale, local active detection method. In the hydraulic fracturing process, the microseismic (MS) events increase. After fracturing, the wave velocity of coal seam decreases significantly, indicating the formation of fractures around the fractured holes and a fracture zone. The fracturing expands the pre-existing fractures adjacent to fractured holes, forming a fracture network between them and natural fractures, leading to a decrease in wave velocity around the coal seam. The DC detection results indicate a noticeable development of pores around the fractured holes following hydraulic fracturing. The increased resistance value area is mainly concentrated on both sides of the 9–4 drilling, with an influence range of approximately 30 m, and an increased range of 100–400 Ω·m, indicating a hydraulic fracturing pore expansion range of 30 m in coal seams. Comprehensively, the maximum impact area of fracturing is the area surrounding the fractured holes, and the impact area is not uniform. The DC method test results reflect the extension of fracturing around fractured holes, while the VWCT test results represent the extension of fracturing around fractured holes and the connectivity of primary fractures, both of which include each other.

Research status and development trend of optimization Angle of hydraulic fracturing in coal seams

Drilling along the coal seam is an effective method for large-scale gas control. The distribution law of the stress field is determined by the parameters affecting the drilling along the coal seam, thereby affecting the drainage effect of the drilling along the coal seam. In order to study the distribution law and drilling parameters of the borehole stress field along the coal seam, the residual plastic zone, the radius of the plastic softening zone, the radius of The radial displacement of the coal body on the hole wall and the axial plastic range are used to construct the calculation model of the sealing length; the calculation model of the sealing strength is established through the micro-element analysis of the packer rubber cylinder. The research finally determined that the most suitable drilling position is the original rock stress area; the reasonable sealing length and sealing strength were determined.

An Improved Coupled Modeling Method for Coalbed Methane Extraction after Hydraulic Fracturing

This paper proposes an improved coupled modeling method for coalbed methane extraction after hydraulic fracturing. Based on the damage mechanics theory, we introduce a state damage variable to record the damage changes at different times and construct the coupling governing equations of the seepage field and solid mechanics field in the hydraulic fracturing process. Then, the results of hydraulic fracturing simulation are used as initial values to perform coupling modeling for coalbed methane extraction. The method is implemented by the combination of different modules in COMSOL to solve the state damage variable, displacement, and pressure. Additionally, two 2D numerical examples are used to verify the precision of this method, and a 3D coalbed methane extraction simulation is performed to analyze the extraction effects and the evolution characteristics of the permeability before and after hydraulic fracturing. Numerical examples show that the proposed coupling modeling method can not only simplify the calculation steps of the CwM method and reduce the numerical errors caused, but also characterize the damage, porosity, permeability, and extraction effect caused by hydraulic fracturing in coal seams with permeability anisotropy.

A numerical investigation of hydraulic fracturing on coal seam permeability based on PFC-COMSOL coupling method

Hydraulic fracturing and permeability enhancement are effective methods to improve low-permeability coal seams. However, few studies focused on methods to increase permeability, and there are no suitable prediction methods for engineering applications. In this work, PFC2D software was used to simulate coal seam hydraulic fracturing. The results were used in a coupled mathematical model of the interaction between coal seam deformation and gas flow. The results show that the displacement and velocity of particles increase in the direction of minimum principal stress, and the cracks propagate in the direction of maximum principal stress. The gas pressure drop rate and permeability increase rate of the fracture model are higher than that of the non-fracture model. Both parameters decrease rapidly with an increase in the drainage time and approach 0. The longer the hydraulic fracturing time, the more complex the fracture network is, and the faster the gas pressure drops. However, the impact of fracturing on the gas drainage effect declines over time. As the fracturing time increases, the difference between the horizontal and vertical permeability increases. However, this difference decreases as the gas drainage time increases. The higher the initial void pressure, the faster the gas pressure drops, and the greater the permeability increase is. However, the influence of the initial void pressure on the permeability declines over time. The research results provide guidance for predicting the anti-reflection effect of hydraulic fracturing in underground coal mines.

A numerical method of hydraulic fracturing in coal seam for enhancing gas drainage

Hydraulic fracturing technology, as one of the measures to enhance the coal seam permeability artificially and increase the coalbed-methane gas extraction rate, has its remarkable advantages. Although the interaction between fluid and coal has been studied comprehensively in the process of fracturing and gas drainage, fewer researchers focus on the combination of solid-gas-water three-phase flow, which brings great deviations to the design of hydraulic fracturing enhanced underground gas drainage. In this paper, a fracture-pore equivalent model is proposed, which establishes the equivalent relationship between the fracture aperture under hydraulic fracturing and the permeability coefficient of coal in the process of gas extraction, thus realizing the weak coupling between gas-solid-water. The hydraulic fracturing takes into account the deformation and fracture of solids driven by fluids. And the gas extraction considering simultaneously adsorption and desorption effect and the permeability coefficient caused by hydraulic fracturing is realized by the coupling of solid stress field and pore seepage field. The accuracy of this method is verified by comparing with the theoretical solution of the KGD model. Taking the fracturing boreholes in a mine’s field test as the background, the change of the cracks distribution with the injection pressure is analysed quantitatively. And the relationship between gas extraction rate and time is verified by the field monitoring data. The results show that the model has positive significance for the optimal design of gas extraction scheme under hydraulic fracturing.

Variation law of roof stress and permeability enhancement effect of repeated hydraulic fracturing in low‐permeability coal seam

AbstractHydraulic fracturing experiments were conducted to study the change of initial fracture pressure and roof stress under repeated hydraulic fracturing in coal mines and to investigate the actual penetration enhancement effect on coal seams. The results show that as the fracturing is repeated, a lower initiation pressure is required. The roof stress changes consistently with water injection pressure. First, the stress in the area around the water injection hole gradually increases, and then stress transfers to the surroundings. With the re‐implementation of fracturing, the area of increased stress expands, and the scope of influence of hydraulic fracturing also becomes larger. When fracturing in a coal mine, for the same total fracturing time, the total water injection volume of repeated fracturing is 35% higher than that of single fracturing, and the affected area is increased by 31%. The concentration of repeated hydraulic fracturing is about 36% higher than that of a single hydraulic fracturing, and the pure flow of gas drainage is increased by about 1.3 times. The results of this study can guide the efficient implementation of hydraulic fracturing underground in coal mines, so as to achieve a larger impact range of coal seam hydraulic fracturing and better gas drainage effects.

Measuring system to investigate geo- and gas-dynamic processes in hydraulic fracturing of coal seams

A measuring system has been developed for a test bench allowing the study of gas-dynamic processes in large-sized rock samples when performing hydraulic fracturing of a coal seam in a borehole model. This system provides collection of the data on pressure in a hydraulic fracturing device and loading of pneumatic jacks which create compressive stresses in the borehole zone, it also records acoustic emission during crack formation. An additional capability is implemented to measure the flow rate of gas supplied to sample through the holes on its side wall and pumped from the borehole.

Experimental Study on Stress Transfer and Fracture Law During the Hydraulic Fracturing of Coal Seams

Hydraulic fracturing is a key technology for increasing the permeability of coal seams and improving the extraction effect of coalbed methane. Exploring the parameters that represent the infiltration effect is an essential part of the optimization process of hydraulic fracturing. In this work, based on a similarity principle design, a hydraulic fracturing test was carried out on large raw coal samples using an indoor hydraulic fracturing physical simulation test system. Based on the distribution of primary cracks in the coal samples, the law of crack extension during hydraulic fracturing was studied. The stress variations before and after hydraulic fracturing of the coal seams were monitored, and the stress transfer laws during confining pressure loading and hydraulic fracturing were investigated. The experiment showed that the new fissures in the fracturing process are more likely to extend to the primary fissure development area. When the applied coal body stress exceeds the strength of the coal sample body, the internal cracks in the coal sample are completely penetrated. During this complete penetration, the energy accumulated by pressure water injection is continuously released, and the count detected by the acoustic emission increases sharply. The coal is mainly subjected to compressive stress during the confining pressure loading process, whereas the coal body is mainly subjected to a shear stress during the hydraulic fracturing process. Both the stresses can be transferred through the coal samples. When the stress increase during hydraulic fracturing is greater than the confining load, the overall effect is due to different stresses. As the cracks continue to expand and extend during hydraulic fracturing, the stress transfer effect becomes weaker. Through the exploration of the law of crack extension and stress transmission, this experiment showed that large-scale physical simulation experiments in a laboratory setting can help effectively simulate on-site hydraulic fracturing conditions. The test method and results reported herein provide a reference and basis for the design and optimization of on-site hydraulic fracturing parameters.

A Combined Denoising Method for Microseismic Signals from Coal Seam Hydraulic Fracturing: Multithreshold Wavelet Packet Transform and Improved Hilbert-Huang Transform

Coal seam hydraulic fracturing (CSHF) has recently been applied to mitigate frequent regional rockburst risk in deep mines before mining practice, as an effective substitute for conventional labor-intensive and time-consuming rockburst prevention measures. Due to the complex nature of CSHF microseismic signals—e.g., nonstationary, transient, and low signal-to-noise ratio—conventional denoising methods tend to yield undesirable results that may preclude reliable evaluation of hydraulic fracturing performance using microseismic data. We propose an advanced denoising method MWPT-IHHT to achieve twice denoising in a fine and adaptive manner. This method combines a multithreshold wavelet packet transform (MWPT) and an improved Hilbert-Huang transform (IHHT), with each being improved compared to their conventional counterparts. A quantitative comparison using synthetic signals suggests the outperformance of the proposed method over the commonly used denoising methods in suppressing noises in terms of signal-to-noise ratio, signal similarity, and energy percentage. The desirable denoising results of two typical real CSHF signals in a CSHF test at Huafeng Coal Mine further demonstrate the applicability and effectiveness of the proposed MWPT-IHHT method.

Effect of packer design on hydraulic fracturing of coal seam

The article addresses the issue of in-situ methane drainage from coal seams using multiple hydraulic fracturing. The authors undertake the stress–strain analysis of rock mass in the vicinity of a drainage borehole subjected to loading by sealing packers using the finite element method. The pressures at the packer and borehole wall contacts are calculated. It is shown how the length and spacing of the packers influence the values of the maximum axial tensile stresses on the borehole wall.

Influence of water injection parameters on fracturing effect in coal seam hydraulic fracturing

According to the basic parameters of Yangquan mining area, based on the determined meso physical and mechanical parameters and geological parameters, several groups of numerical calculation models considering different fracturing parameters (water injection flow and water injection time) are established. Then, the numerical simulation test of water injection fracturing is carried out by using the particle discrete element program, and the influence effect of related parameters of coal seam hydraulic fracturing is analyzed numerically, the influence of flow rate, pressure, time and other parameters on water injection flow is obtained.

Prediction of fracture initiation pressure in multiple failure hydraulic fracturing modes: Three-dimensional stress model considering borehole deformation

Hydraulic fracturing technology has been widely used as the main method of exploiting coalbed methane (CBM). The core of this technology is the accurate prediction of the fracture initiation pressure. As the exploitation depth increases, the influence of the in-situ stress obviously increases. Because the existing model of calculating the fracture initiation pressure ignores the influence of in-situ stress on the extraction borehole (CBM well) deformation, and the fracture initiation failure mode is single, the fracture initiation pressure of hydraulic fracturing cannot be accurately predicted in the field. In this study, a new model for predicting the fracture initiation pressure in hydraulic fracturing under a three-dimensional stress state was established. For the first time, the deformation of a circular borehole is related to the in-situ stress, and the size of the elliptical borehole can be accurately obtained after deformation. Eleven fracture initiation failure modes for the hydraulic fracturing of coal seams are proposed. The fracture initiation pressure of each failure mode was calculated in MATLAB, and the fracture initiation pressure of the shear failure of the ρz plane (a) was the smallest. The effects of the horizontal maximum principal stress, horizontal minimum principal stress, vertical intermediate principal stress, coal seam dip angle, horizontal azimuth of borehole, and gas pressure on the fracturing pressure were investigated using the proposed model. The coal seam dip angle exerted the greatest influence, and a new hydraulic fracturing construction procedure is proposed: the pump pressure should be gradually increased from the minimum initiation pressure according to the fracture initiation pressure gradient required by each failure mode. A field hydraulic fracturing test was conducted, and the experimental results are in good agreement with the theoretical calculation results. The maximum error was 5.5%, which validates the proposed theoretical model. The accuracy of the proposed theoretical model in the calculation of the fracture initiation pressure increased by 364.4% compared with the classical hydraulic fracturing theory. The results obtained by this study can provide more precise guidance to hydraulic fracturing projects.

Evaluation of Borehole Hydraulic Fracturing in Coal Seam Using the Microseismic Monitoring Method

Accurate evaluation of the influence range of borehole hydraulic fracturing (HF) in coal seam is crucial for optimizing the design scheme of HF. In this study, we adopted the microseismic (MS) monitoring technology to monitor and characterize the spatial shape of cracks caused by borehole HF in coal seam in an underground coal mine. And we also tested and analyzed the stress and moisture content changes of coal mass at different distances from the borehole after HF program. The number of MS waveforms and the energy of MS events show a good positive correlation with the water pressure curve. The response of the energy curve to the extension of the hydraulic cracks is ahead of the water pressure curve. Based on the short-time average/long-time average (STA/LTA), the interference signal recognition method (ISR) and the improved Akaike information criterion (AIC) method, we developed a comprehensive MS event detection and arrival time picking (CMDP) program that are suitable for the weak MS signals with low signal to noise ratio (SNR) induced by HF in coal seam. And then we were able to more accurately locate the MS events of the hydraulic cracks using the simplex source location method. We have conducted a comprehensive analysis of the relationship between the temporal and spatial distribution of MS events and hydraulic cracks propagation. The results show that there is an apparent correlation between the MS activities and HF operation. The expansion of hydraulic cracks generates MS events, and the larger the size of the cracks, the greater the energy of MS events. Based on the MS monitoring results, the HF produces a crack network of flat ellipsoid in the No. 6 coal seam, which indicates that there is obvious stimulated reservoir volume (SRV) fracturing effect during the borehole HF process. The influence radius of HF based on the moisture content is the smallest (about 20 m), followed by the stress monitoring (about 30 m), and the MS monitoring is the largest (about 40 m). The high-precision MS source location (location errors < 2.5 m) results combined with roadway roof watering phenomenon indicate that the influence radius of HF based on the moisture content and stress release may be underestimated. And the effective influence radius of the borehole HF is about 40 m for this HF program.

Study on Eliminating the Water Blocking Effect in Coal Seams Using Gas-Wetting Reversal Technology.

To eliminate or alleviate the water blocking effect (WBE) encountered while hydraulic fracturing of coal seams, the wettability of the coal body was changed, using gas-wetting reversal technology, thus improving gas drainage. After initial measurements of contact angles, surface tension, surface free energies, erosion resistance, self-absorption capacities, and self-absorption rates of coal samples, a hydrophobic and low-surface-tension gas surfactant was selected. After comparing the results of four candidate surfactants, two surfactants were selected, FC117 and FC134, which were both hydrophobic and exhibited low surface tension, to achieve the desired effect. Testing showed that with increased surfactant concentration contact angles increased and surface tension gradually decreased. After surfactant treatment, the coal surface energy decreased, self-absorption capacities and rates of coal samples decreased significantly, and their erosion resistance was good. After determination and verification of several evaluation parameters, the results showed that the two surfactants performed well and achieved the expected effect. By optimizing and evaluating gas-wetting reversal agents, our work has provided a theoretical basis and scientific guidelines for eliminating the WBE from coal seams, thereby facilitating gas drainage.