Influence of hydraulic fluctuations on the microstructure and mechanical properties of coal

IF 5.5 0 ENERGY & FUELS

Gas Science and Engineering Pub Date : 2025-07-18 DOI:10.1016/j.jgsce.2025.205735

Yiyu Lu , Shipu Miao , Zhaohui Lu , Junping Zhou , Baixue Wang , Jiankun Zhou , Shirong Meng

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引用次数: 0

Abstract

Utilizing periodic hydraulic fluctuations, Pulse Hydraulic Fracturing (PHF) effectively enhances fracture network development in low-permeability coal seams and helps coalbed methane (CBM) extraction. However, the influence of hydraulic fluctuations on structural evolution in coal remains unclear. This study systematically investigates microstructural and mechanical property alterations through X-ray diffraction (XRD), nuclear magnetic resonance (NMR), uniaxial compressive strength (UCS), and Brazilian tensile strength (BTS) tests. The results demonstrate that after hydraulic fluctuation action, coal's microstructure significantly improved. The clay mineral content in coal decreased by 16.30 %, mainly through physical processes including dissolution, transfer, and detachment caused by hydraulic fluctuations. Pore structure evolution showed a 6.45 % porosity increase accompanied by a 50.95 % expansion in macropore proportion and corresponding micropore reduction. Fractal dimensions of pores decreased, indicating smoother pores conducive to CBM transport. Conversely, hydraulic fluctuations weakened the mechanical properties of coal, inducing reductions of 39.76 %, 35.23 %, and 37.50 % in UCS, elastic modulus (E), and tensile strength (TS), respectively. We found that changes in coal porosity and macropore proportion constitute primary factors governing mechanical strength degradation. The theoretical relationship between pore structure and mechanical strength of coal under hydraulic fluctuations was established and a hydraulic fluctuations damage factor

D_{f}

was obtained. Furthermore, the mechanism of hydraulic fluctuations on coal structure was analyzed according to dynamic impact and fatigue damage effects, and a conceptual model was proposed to explain the structural evolution mechanism of coal under hydraulic fluctuations. This research can provide theoretical support for the exploration of PHF-enhanced CBM extraction technology in low-porosity coal seams.

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水力波动对煤微观结构和力学性能的影响

脉冲水力压裂利用水力的周期性波动，有效地促进了低渗透煤层裂缝网络的发育，有利于煤层气的开采。然而，水力波动对煤结构演化的影响尚不清楚。本研究通过x射线衍射（XRD）、核磁共振（NMR）、单轴抗压强度（UCS）和巴西抗拉强度（BTS）测试系统地研究了微观结构和力学性能的变化。结果表明，水力波动作用后，煤的微观结构有明显改善。煤中粘土矿物含量降低了16.30%，主要是由于水力波动引起的溶解、转移和剥离等物理过程。孔隙结构演化表现为孔隙度增加6.45%，大孔比例扩大50.95%，微孔减少。孔隙分形维数减小，表明孔隙越光滑，有利于煤层气运移。相反，水力波动削弱了煤的力学性能，使煤的UCS、弹性模量(E)和抗拉强度（TS）分别降低了39.76%、35.23%和37.50%。研究发现，孔隙率和大孔隙比例的变化是影响煤体力学强度退化的主要因素。建立了水力波动作用下煤的孔隙结构与力学强度的理论关系，得到了水力波动损伤因子Df。从动力冲击效应和疲劳损伤效应两方面分析了水力波动对煤结构的作用机理，提出了水力波动作用下煤结构演化的概念模型。该研究可为低孔隙度煤层phf强化煤层气开采技术的探索提供理论支持。

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来源期刊

Gas Science and Engineering

CiteScore

11.20

自引率

0.00%

发文量