Modeling fishbones in coalbed methane reservoirs using a hybrid model formulation: Gas/water production performance in various lateral-cleat- network geometries

摘要

Fishbone well is an efficient approach to exploit coalbed methane (CBM) reservoirs due to large reservoir-wellbore contact. Estimating fishbone-well production performance is challenging, because of the complex lateral-cleat networks and gas/water two-phase flow.To study flow characteristics of this complex system, a hybrid analyticalumerical model is proposed. Each fishbone lateral is treated as a fracture with flowing properties equivalent to the laterals. We explicitly model the laterals and connected cleats as discretized segments and connected nodes. Various physical and dimensional properties can be assigned for each of the segment. Hence, the model has sufficient flexibility to consider any complex lateral-cleat-network configuration. An analytical transient-reservoir-flow model is coupled with a numerical solution for fracture flow on discretized segments and connected nodes. The gas/water two-phase flow is incorporated by dynamically updating the multiphase-flow factors including gas/water saturation, relative permeability and capillary pressure at each segment and node. The accuracy of the model is confirmed by a numerical reservoir simulator. Then, a series of case studies based on the reservoir data from Qinshui Basin, China, are comprehensively performed to identify the influences of lateral-cleat-network configurations on gas-and water-production performance. In addition, the results show its computational efficiency as the complexity of lateral-cleat-network increases.This work presents a computationally less demanding approach to model gas/water two-phase flow in fishbone well in CBM reservoirs. The results of the simulation highlight the importance of the optimum design for the fishbones patterns and the connections between the laterals and the pre-existing cleats. The key findings are expected to provide valuable insights into the production enhancement in the CBM reservoirs.