Simulation research of coal reservoir pressure variation law

Accurately quantifying the real-time variation of reservoir pressure is the key factor to predict the variation characteristics of reservoir porosity and permeability and control the gas recovery effect. In order to further explore the characteristics of coal reservoir pressure dynamic variation in Baode area and its impact on the productivity of coalbed methane (CBM) wells, based on production dynamic data, a real-time simulation and detection technology of coal reservoir pressure was established while quantitatively described the pressure dynamics of saturated and unsaturated CBM reservoirs in Baode area, by considering the influence of positive and negative effects. By comparing the productivity under different pressure drop conditions, the prediction basis and theoretical guidance were provided for production. During the exploitation of CBM wells, the average reservoir pressure of saturated CBM reservoirs decreases linearly. Taking the average daily gas production of a single well as the selected standard, the pressure drop rate is divided into three types: fast falling type (more than 0.096 MPa/m), steady falling type (0.063～0.096 MPa/m) and slow falling type (less than 0.063 MPa/m). The productivity of CBM wells with steady-descent type (4648 m³/d) pressure drop system is significantly better than that of fast-descent type (2531 m³/d) and slow-descent type (2968 m³/d). Fast-descent type CBM wells have the lowest productivity. In the single-phase water flow stage of the undersaturated coalbed methane reservoir in the early stage of development, the average reservoir pressure drops rapidly, and the pressure drop rate is divided into three types: fast falling type (more than 0.38 MPa/m), steady falling type (0.228—0.38 MPa/m) and slow falling type (less than 0.228 MPa/m). The productivity of CBM wells with fast-descent type (526 m³/d) pressure drop system is significantly lower than that of steady-descent type (1021 m³/d) and slow-descent type (1054 m³/d). As the reservoir pressure drops to the critical desorption pressure, the reservoir pressure decreases linearly and slowly in the gas-water two-phase flow stage. It is suggested that the initial pressure drop rate of saturated CBM reservoirs should be maintained at 0.063～0.096MPa/m, and the initial pressure drop rate of unsaturated CBM reservoirs should not exceed 0.38 MPa/m in Baode area.