A Prediction Model for Pressure Propagation and Production Boundary during Coalbed Methane Development

In coalbed methane field development, an accurate understanding of dynamic propagation of reservoir pressure is crucial for predicting production potential, enhancing recovery, and engineering planning. Based on actual reservoir conditions and production data, a semi-analytical mathematical model that considers the self-regulating effects of coal reservoirs was established for both nonstimulated and hydraulically fractured vertical wells in undersaturated coal reservoirs. In the model, undersaturated coal seam is divided into the drainage area and desorption area. Material balance equations (MBE) are deduced based on the unique gas–water production mechanism of coal seams, and pressure profiles are expressed as continuous succession of steady state flow. The pressure propagation can be finally predicted by combining MBE and pressure profiles in different areas. Three hydraulically fractured vertical wells with different production characteristics in the Shizhuangnan Block, southern of Qinshui Basin are taken as examples to verify the applicability of the model. Results show that dynamic pressure propagation of a single well is accurately characterized in the early production stage, and the shape of the pressure propagation area translates from an ellipse into a circle during production; actual interference boundaries or damage boundaries are precisely predicted when interwell pressure interference is formed or a poor production strategy is adopted, respectively. Additionally, mechanism analysis under a mathematic model and orthogonal experiment evaluation are adopted to quantify the sensitivity of various parameters on the prediction results. Results show that coal reservoir physical properties, especially critical desorption pressure and porosity, mainly affect drainage radius expansion, whereas human factors and adsorption performance are the key factors influencing the desorption radius. The proposed model develops an accurate and efficient mathematical method for evaluating dynamic characteristics of pressure propagation in undersaturated coal reservoir and predicting production boundary and finally provides valuable guidance for the formulation of reasonable development plans.