Long-Term Gas Deliverability of a Dewatered Coalbed

Abstract

Summary Gas and water production from a coal deposit can be divided into three distinct stages. In the first, gas and water production rates are almost constant. The second stage, beginning production rates are almost constant. The second stage, beginning once pseudosteady state is reached, is characterized by a "negative decline" in gas production rate and a declining water rate. The third stage begins when the peak gas rate is reached. This final stage spans most of the economic life of a coal well and is characterized by a gentle decline in gas rate and negligible water production. Gas production in this stage is described by two production. Gas production in this stage is described by two equations. The first is a gas-deliverability equation using the real-gas pseudo pressure developed by Al-Hussainy et al. This equation is applied to published data from a well in the Deerlick Creek field in the Warrior basin of Alabama. The second is a mass-balance equation combining elements from conventional and shale gas reservoirs. The two equations can be coupled to predict gas rates and recoveries of dewatered coal wells. Published simulation results from the Warrior basin Brookwood field and a San Juan basin coal well are compared. The deliverability equation also is used to construct gas-deliverability curves for a typical Brookwood coal well. The method presented here is applicable to coal wells completed in dry coalbeds. Introduction An important characteristic of a gas well is its deliverability over the course of reservoir depletion. A considerable body of literature describes conventional gas-well behavior mathematically. Theoretical aspects of wells completed in conventional dry gas reservoirs have been discussed thoroughly. Gas flow in porous media can be described by three methods: pressure, pressure-squared, and the real-gas pseudo pressure of Al-Hussainy et al. and Al-Hussainy and Ramey. While the last method is more cumbersome to use than the first two, it is more rigorous and is the only reliable method when large drawdowns exist. This paper shows that the traditional real-gas pseudo pressure can be extended to describe gas deliverability of a dewatered coal well. Coal deposits are naturally fractured gas reservoirs. Initially, the natural fractures, or cleats, of the coal typically are water saturated and the coal matrix adsorbs most, if not all, of the gas. Coal gas is mostly methane with minor amounts of CO2, nitrogen, and heavier hydrocarbons, similar to dry natural gas from conventional reservoirs. Wells completed in coal deposits progress through three distinct stages, as Fig. 1 illustrates. In the first stage, gas and water production rates are almost constant. The second stage is characterized by a negative decline in gas production rate and a declining water rate. Once the peak gas rate is attained, the third stage of production begins where the gas production rate shows a gentle decline and water production is often negligible. This final stage spans most of the economic life of a coal well.