Study on vertical cross-interface expansion and permeability characteristics of N2 foam fracturing cracks in coal measures reservoirs
CHAI Wangyang1, LI Wenda1, LIANG Weiguo1, 2, WANG Zaiyong1, REN Sentao1, LUO Hongye1
(1. Key Laboratory of In-situ Modified Mining Ministry of Education, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China; 2. School of Mining Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China)
Abstract： Indirect fracturing technology is frequently employed to enhance the exploitation of coalbed methane (CBM) in fractured, low-permeability coal seams. Investigating the vertical cross-interface propagation behavior of fracturing cracks in different fracturing media is crucial for improving vertical communication and permeability enhancement in reservoirs. This study conducted experimental research on the cross-interface propagation of true triaxial hydraulic fractures in coal-rock combinations, utilizing water, N2 gas, and N2 foam under varying vertical stress difference coefficients (Kv). The evolution law of injection pressure, dynamic response characteristics of acoustic emission (AE), fracture propagation morphology and fracture surface conductivity difference were compared and analysed, and the vertical communication and permeability enhancement effects of different fracturing media were comprehensively evaluated. The results indicate that: (1) Regarding the evolution of liquid injection and AE dynamic response, under identical Kv conditions, N2 foam fracturing exhibits characteristics of extended pressurization time, higher fracture pressure, greater AE energy, fewer cumulative ringing counts, and concentrated frequencies of fracture initiation and propagation events. (2) Concerning fracture network morphology, N2 foam fracturing tends to create a penetrating two-wing fracture network with extensive vertical extension and expansion. Compared to water and N2 gas, it facilitates better vertical communication and permeability enhancement in multi-layered reservoirs during indirect fracturing. (3) For vertical cross-interface expansion of fracturing cracks, the critical cross-layer Kv values for N2 foam, water, and N2 gas are 0.28, 0.39, and 0.50, respectively. N2 foam can achieve cross-interface expansion of coal and rock masses under lower Kv conditions, broadening the application range of indirect fracturing technology. (4) In terms of fracture network conductivity, at Kv = 0.50, the in-situ reinjection pressure for N2 foam fracturing is 9.14 and 10.19 MPa lower than that for water and N2 gas, respectively. Additionally, the fracture opening post-fracturing is 0.108 5 and 0.169 6 mm larger than those for water and N2 gas, respectively. The lower in-situ reinjection pressure and larger fracture opening suggest that the conductivity of the N2 foam fracture network is superior. (5) Regarding permeability characteristics, seepage simulation results reveal that the permeability of the fracture network post-N2 foam pressure is 2.95 and 11.86 times that of N2 gas and water, respectively. In summary, compared to water and N2 gas fracturing, N2 foam, with its high viscosity and low filtration properties, more effectively promotes fracture vertical cross-interface expansion and permeability enhancement, making it more suitable for the indirect fracturing of coal measure reservoirs.
柴望阳1，李文达1，梁卫国1，2，王在勇1，任森涛1，落弘业1. 煤系储层N2泡沫压裂裂缝垂向跨界面扩展与渗透性特征研究[J]. 岩石力学与工程学报, 2025, 44(6): 1596-1611.
CHAI Wangyang1, LI Wenda1, LIANG Weiguo1, 2, WANG Zaiyong1, REN Sentao1, LUO Hongye1. Study on vertical cross-interface expansion and permeability characteristics of N2 foam fracturing cracks in coal measures reservoirs. , 2025, 44(6): 1596-1611.
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2025, Vol. 44 
