Experimental Study on Effect of Three-dimensional Stresses on Permeability of Coalbed Gas-water Double State Flow

The absolute permeability of coal, relative permeability of gas-water are necessary parameters to forecasting the birth rate of coalbed methane and water in mining. Taking Tiandiwangpo Mine in Jincheng for example, using steady-state method, the experiment of absolute permeability of coal, relative permeability of gas-water double state flow in the coalbed methane(CBM) reservoir were simulated under the condition of changing load and pore pressure. The relative permeability curves of coalbed gas-water were obtained. The relationship between effective stress and effective permeability of gas and water were studied. Some new laws are presented as follows: (1)with the increasing of confining pressure, the relative permeability of gas-water are increased slightly, it is proved that confining pressure has a great effect on the absolute permeability of coal, (2)a lower critical value of effective stress exists, the effective permeability of gas and water will decrease rapidly with the increase of effective stress when it is smaller than it. For the coal sample No.3, the lower critical effective stress is about 2.7 MPa, (3) On the other hand, a upper critical value of effective stress is found, the effective permeability of gas and water will decrease slightly with the increase of effective stress when it is bigger than it. For the coal sample No.3, the upper critical effective stress is about 8.5 MPa, (4) along with the increasing of the pore and the cranny, the effective permeability of gas and water increased as the pore pressure increased. The relational expressions for effective stress and the effective permeability of water and gas were studied. The experimental results can provide a theoretical basis for reasonable drainage of coalbed methane(CBM).