Response Characteristics of Coal Measure Strata Subjected to Hydraulic Fracturing: Insights from a Field Test

Hydraulic fracturing (HF) is an effective method to improve the permeability of coal seams and enhance coalbed methane recovery in underground coal reservoirs. However, the induced stress and strain evolution behaviors triggered by HF engineering practice remain unclear, which hinders our perception and field applications on HF technology. In this work, an HF field experiment was conducted in a deep buried coal mine, and a hollow inclusion cell (HI cell) was proposed to investigate the induced strain and stress evolution behaviors of HF. In addition, the HF complete evolution process was sectioned and comprehensively studied. The results showed that the influence range of this HF field test exceeds 44 m, and the induced strain primarily experiences three stages when subjected to HF: initial constancy, strain fluctuation, and later, stability. The induced stress experiences intense variations when subjected to HF, with the minimum principal stress increment being the greatest. Principal stress tends to recover to the initial stress state after HF, but the recovery speed declines with time. Further, azimuth and dip angle experience intense changes under HF. The change law is irregular, but they all tend to return to the primary state. Moreover, combining water flow and pressure, the HF evolution process can be divided into five stages: prehydraulic fracturing, unstable crack propagation, stable crack propagation, posthydraulic fracturing, and pump off stage, which can reflect the real evolution behaviors accurately. The research results have profound implications for coalbed methane recovery and coal mine gas governance.