The influence of water saturation on the stress sensitivity of medium rank coal reservoirs

In order to ascertain the effect of water saturation on the stress sensitivity of middle-rank coal reservoirs, a self-designed experimental device was used to develop the dryness, water-bearing and water-saturation conditions of seams No.1+3 and No.15. The liquid and gas permeability susceptibility tests were performed to analyze the characteristics of coal sample permeability, permeability damage rate, irreversible damage rate, and stress sensitivity coefficient, and the reservoir gas recovery and drainage of coalbed methane wells were investigated, which possesses a guiding significance for the reservoir reconstruction and drainage work of coalbed gas well. Results show that with the increase of water saturation, the permeability sensitivity of medium-rank coal liquid permeability increases, and the irreparable damage rate caused by loading and unloading increases. The increase of water saturation leads to the decrease of the initial gas permeability, the high degree of fracture closure of the dry coal samples, and the high saturation of the saturated coal-like irreducible water, then it has strong gas-permeability stress sensitivity. The dry coal and water samples are suitable to characterize the relationship between permeability and effective stress of non-dimensional gas by power function. The saturated coal sample is more suitable to characterize the relationship by negative exponent function. During the fracturing and drainage of medium-rank coal reservoirs, attention should be paid to reservoir protection. When the coal reservoirs contain relatively weak water, priority should be given to adopt CO₂ or N₂ foam fracturing methods for reservoir reconstruction.