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

Abstract

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.

Research on the Movement of Overlying Strata in Shallow Coal Seams with High Mining Heights and Ultralong Working Faces

“…(3) Periodic loading step distance After experiencing the initial pressure, as the working face continued to advance, the structure formed by the basic roof strata always underwent a change from stable to unstable and then back to stable, which was a cyclic process called periodic pressure [30,31]. During the excavation process of the model, the working face underwent four cycles of pressure, with pressure steps of 19.2 m, 10.4 m, 20 m, and 15.2 m.…”

CO2 Gas Fracturing in High Dip Angled Coal Seams for Improved Gas Drainage Efficiency at Hashatu Coal Mine

“…The surface borehole drainage, termed as mine predrainage, through either vertical or horizontal wells, has been widely implemented for gassy coal mines in USA, Canada, Australia, China, and others. The predrainage surface boreholes were found to be effective for moderate and high-permeability seams with permeability exceeding 1 mD. − For the multiseam coal mines, the protective seam mining technology is developed for effective gas drainage through stress releasing for the underlying/overlying seams, and this technology is apparently not applicable for single coal seam mine. , The underlying rock tunnel associated with upward borehole drainage has been developed for a gassy mine in China, but it is limited for uniform application due to its high capital cost due to a large drivage volume. , In-seam borehole hydraulic fracturing technology has been widely used for low permeability coal gas drainage and the results are encouraging; however, the underground hydraulic fracturing requires a complex equipment set and long period operation, which hinder its application due to the confined space in the mine. − Shock wave, , ultrasound, − and N 2 gas displacement , have been tested in a few mines and are showing mixed results. A large number of field tests have proven that explosive loosening blasting can effectively improve the permeability of coal seams, but the technology has not been well received and adopted by coal operators because the misfire of explosive would pose a huge threat to mine safety with the undetonated shots. − …”