Optimisation of Dewatering Rates to Maximise Coal Seam Gas Production

This study investigates the evolution of coal seam gas (CSG) reservoir properties in relation to a gradual and immediate decline in Bottom-Hole Pressure (BHP) to determine an optimum dewatering strategy to maximize gas production rates. It has been reported in literature that drawing down the BHP too low and/or too quickly may lead to permanent closure of the cleat fracture system and lead to reduced peak gas production rates in the short term, and reduced gas recovery in the long term. This work conducted a series of simulation studies and investigated CSG well production behavior under different drawdown strategies and examine the effects of a range of relative permeability characteristics, geomechanical properties, and Langmuir isotherms. Results of the sensitivity analysis show that different coal relative permeability, geomechanical properties, and Langmuir isotherm properties have significant effects on dewatering efficiency and gas productivity. The results reveal that under all simulated scenarios, lower BHP leads to higher rates of gas production. Therefore, these findings suggest that BHP should be decreased as quickly as possible subject to other geomechanical or operations constraints such as wellbore stability and mobilization of fines. This conclusion contradicts some prior literature studies that there may be benefits to be gained in terms of productivity in staging the reduction in BHP. The contradiction could be due to other factors that were not considered in our simulations.