Unlike conventional reservoirs, the productive capacity of a coalbed is not immediately apparent. New coal wells typically produce at low rates and often exhibit low gas-water ratios. Only after a coalbed has been dewatered does gas rate begin to rise and water rate begin to fall. A numerical study was undertaken to investigate saturation and pressure behavior during the initial dewatering period and to develop methods to predict its duration. These simulations used typical coalbed and well properties encountered in Warrior and San Juan Basin coal wells. Behavior of gas saturation and pressure profiles during dewatering is discussed. It was found that, for engineering purposes, coalbed dewatering can be considered a constant pressure process. Two methods were developed to determine time and cumulative water production required to dewater a new coal well. The first method is based on the classical definition of pseudo-steady state flow, while the second method is based upon the gas-water ratio. Accuracy of both methods is discussed and a recommendations made for application to actual coal wells. Effective coalbed porosity is difficult to measure, but is necessary for reservoir engineering calculations and numerical simulations of coal well performance. Once a coal well has been dewatered, the gas-water ratio method developed here can be used to determine effective porosity. An example calculation is presented.

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