Evolution of rock mass stress, movement, and deformation during injection-production processes in coal mines that have been converted into natural gas storage facilities

Abstract

The rapid growth in natural gas consumption globally has been accompanied by a significant demand increase for storage capacity. Abandoned underground coal mines can be converted into natural gas storage facilities with relatively low construction costs and short construction periods. However, the underground coal pillars and surrounding rock masses are at risk of movement and deformation under the dual effects of injection-production pressures and creep effect. By using mined-out areas of a selected coal mine as the setting to examine performance and effectiveness, this study preliminarily proposes a simplified technical approach for converting abandoned room-and-pillar coal mines into natural gas storage facilities. Also, this study presents and interprets the rock movements, deformations, and fracture characteristics during the injection-production processes. The research shows that: (1) The initial deflection of the roof was 142.91 mm, it increased at a decreasing rate to 226.06 mm after 50 cycles. (2) After injection-production processes the stress and resulting displacement on pillars showed "arched" distributions. (3) As injection-production stress increased from 1.8 MPa to 8.2 MPa, the difference in vertical displacement between the roof and pillars decreased from 108.22 mm to 91.28 mm. The instability risk is found when the injection-production pressure approaches in-situ conditions. (4) More injection-production cycles increased the proportional number of fractures in the overlying rock mass, as well as the length and aperture of the fractures. The research findings provide theoretical underpinnings for the conversion of abandoned coal mines to gas storage facilities from engineering practices.