Single-phase water flow quasi-steady-state drainage model and its application effect analysis in coal-bed methane wells

In order to realize the quantitative control of coal-bed methane wells,effectively improve the gas production per well,the high-rank coal in the Mabidong experimental area in the southern Qinshui Basin was taken as the re- search object. According to the single-phase seepage mechanism of saturated water in fracture reservoir,the influence of cracks on the drainage effect after fracture backflow is analyzed. Combined with fracturing and drainage data,the characteristics of pressure propagation within the range of fracturing transformation are discussed. The drainage area is divided into single-phase flow elastic drainage area and two-phase flow elastic drainage area. By establishing a vertical flow linear flow quasi-steady-state model,the drainage control principle consistent with the geological conditions of the test area is determined. The research shows that in the stage of saturated water single-phase flow,the fracture crack is the main seepage channel of the water phase,which is the strong flow region of the water phase,exogenous and micro- cracks of 100 μm (permeability 1×10-15 -1． 2×10-15 m2 ) are most sensitive to coal seam stress,pressure propagation and gas desorption,which is a weak flow zone of water phase. The nonlinear seepage effect caused by low pressure gra- dient and stress sensitive effect should be considered simultaneously in the drainage process. The drainage model shows that in the pseudo-steady-state seepage stage of vertical cracks,the reservoir pressure distribution is parabolic,and the water production in the near-well area is higher than that in the boundary area,which is more likely to desorb. The seepage resistance in the single-phase water flow period is mainly affected by various factors such as coal rock permea- bility,drainage time, coal rock compressibility coefficient, crack size, fracturing transformation radius, displacement pressure gradient,and pore pressure. In order to ensure the continuous and high-efficiency output of desorption gas,the Mabidong test area has a reasonable drainage limit (0. 05 -0. 10 MPa / d) according to the principle of " fast-slow- slow" drainage. For high rank fractured coal,the form of water production curve can be divided into stable,rising and falling types without the pressure drop of external water supply,and reflects the water supply characteristics of different coal reservoirs. Gas flooding spikes usually appear before desorption.