A Numerical Study of Coalbed Dewatering
- J.P. Seidle (Amoco Production Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Rocky Mountain Regional Meeting, 18-21 May, Casper, Wyoming
- Publication Date
- Document Type
- Conference Paper
- 1992. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 5.5 Reservoir Simulation, 5.5.8 History Matching, 5.8.3 Coal Seam Gas, 4.1.4 Gas Processing
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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.
A conventional gas well is most prolific when it is first put on production. As reservoir pressure is depleted, productivity of a conventional gas well declines. Productive capacity of a coal well, however, is not readily apparent from initial production. Until the coal drainage area is essentially dewatered, a coal well References and illustrations at end of paper. will often have a low, nearly constant gas rate and a high, nearly constant water rate. Equipment problems, especially pump problems, can obscure this constant rate behavior, as shown in Figure 1. Although production rates of a new coal well are often erratic, the gas-water ratio (GWR) of a new coal well is often remarkably constant.
After a dewatering period ranging from months to years, gas production from a coal well begins to rise, such behavior is termed "negative decline," the water rate begins to fall, and the gas-water ratio shows a clear break. A numerical simulation study was undertaken to investigate saturation and pressure behavior during this initial dewatering period and to develop methods to predict its duration.
This study employed conventional black oil models modified for coalbed methane simulation as described by Seidle and Arri (Ref. 1). Use of black oil models for history matching coal well performance has been documented by Volz (Ref. 2). Only bounded coal wells will experience negative decline, unbounded wells will not. Consequently, in this study a single well with a no-flow boundary was employed in all simulations. The well was controlled by specifying a constant bottomhole flowing pressure, typical of a pumped-off new coal well. Most simulations had only a single coal seam. In all cases, a single model layer was used to simulate each coal seam. The one multiseam simulation, Case F, had no vertical permeability, reflecting geological constraints.
Case A - Warrior Basin Well No. 1
The first set of simulations used a dual porosity coning model with logarithmically spaced radial grid nodes.
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