It has been illustrated that the production efficiency of coalbed methane (CBM) is significantly sensitive to the producing pressure difference and flow rate of fluids. To study the controlling mechanisms of producing pressure difference sensitivity and flow rate sensitivity of CBM, several medium- and high-rank coal samples were collected from three coal mines in the Qinshui Basin for the aim of carrying out relevant sensitivity experimental analysis. The results show that the permeability of coal samples shows varying laws with the changes of the producing pressure difference and flow rate, and both display a critical value for the permeability change. Specifically, the permeability of coal samples shows a rapid decreasing trend with the increase of the producing pressure difference after the critical pressure difference. However, at the beginning stages with the injection of a slow flow rate of fluids, the permeability in coals increases, but with the increasing injection speed of fluids, the permeability rapidly arrived to the peak value at the critical flow rate and then significantly decreased. The critical pressure difference in the study area ranges from 0.051 to 0.062 MPa, and the damage rate of permeability varies from weak to moderately strong. The average critical flow velocities in Sihe, Xishan, and Zhaozhuang coal mines are 6.92, 7.77, and 1.21 m/day, respectively. The flow damage rate of fluids in coals increases with the increase of porosity, permeability, and flow rate. It is also found that the larger the porosity and permeability of the coal reservoir, the smaller the critical pressure difference. Additionally, the damage rate of flow velocity for coal reservoir increases with the increased clay mineral contents in coals.

No abstract

During the drainage and production of coalbed methane (CBM) wells, the Constant changes in stress and water saturation of reservoir restricts the dynamic change of the reservoir permeability. By carrying out stress sensitivity experiments with different water saturations in coal, the correlation between permeability and the coupling of effective stress and water saturation was analyzed. The water saturation sensitivity and stress sensitivity of reservoir were evaluated by the stress sensitivity index (S), permeability damage rate (PDR), and stress sensitivity coefficient (αk), and the change law of coal permeability under different stresses with different water saturations was revealed. The results showed that the coal reservoir permeability decreased with increasing stress following a negative exponential function and decreased nearly linearly with increasing water saturation. In addition, the coal water saturation sensitivity was positively correlated with effective stress, and the coal stress sensitivity was positively correlated with water saturation. Finally, a mathematical model for predicting coal permeability that considered the impacts of water saturation and effective stress was established, revealing the controlled mechanism affecting the permeability change.

No abstract

During the drainage and production of coalbed methane (CBM) wells, the Constant changes in stress and water saturation of reservoir restricts the dynamic change of the reservoir permeability. By carrying out stress sensitivity experiments with different water saturations in coal, the correlation between permeability and the coupling of effective stress and water saturation was analyzed. The water saturation sensitivity and stress sensitivity of reservoir were evaluated by the stress sensitivity index (S), permeability damage rate (PDR), and stress sensitivity coefficient (αk), and the change law of coal permeability under different stresses with different water saturations was revealed. The results showed that the coal reservoir permeability decreased with increasing stress following a negative exponential function and decreased nearly linearly with increasing water saturation. In addition, the coal water saturation sensitivity was positively correlated with effective stress, and the coal stress sensitivity was positively correlated with water saturation. Finally, a mathematical model for predicting coal permeability that considered the impacts of water saturation and effective stress was established, revealing the controlled mechanism affecting the permeability change.