UNSTEADY-STATE FLOW OF METHANE AND WATER IN COALBEDS

A hypothesis on the transition of a fully water-saturated coalbed methane reservoir from single-phase flow through unsaturated flow to the two-phase flow regime was developed and substantiated using field data. The concept of "parameter measurement windows" was introduced to obtain the required reservoir parameters from flow testing. While some history matching is required, the computational effort is reduced by approximately two and a half orders of magnitude compared to the conventional history-matching approach.; A new empirical relationship for capillary pressure vs. saturation, referred to as the Fermi distribution, was developed and shown to fit the capillary pressure-saturation data for coal, sandstone, and other consolidated and unconsolidated materials. New approximate analytic solutions for flow to a sink from an infinite coalbed reservoir with desorbing methane have been developed and verified using numerical models for the following cases: (1) single-phase gas flow with desorption; (2) single-phase water flow in fully water-saturated and unsaturated flow regimes. A numerical model based on finite-difference method for simultaneous flow of water and gas was also developed.; Analysis of Glover Well No. 1, drawdown test no. 2 data using unsaturated flow theory resulted in defining both relative permeability and capillary pressure as functions of saturation. The remaining parameters were then obtained using a numerical model of two-phase flow.; Since the slope of the semi-log plot of pseudo-pressure drop vs. pseudo-time is unaffected by desorption, permeability can be obtained by analyzing the single-phase gas flow data as if desorption effects were not present. The effect of desorption shows up as an increased effective compressibility and could be easily misinterpreted as a negative skin factor.; A new method to determine matrix compressibility without a pump test was developed. The new method uses only baseline well pressure and surface pressure data as a function of time. The values of matrix compressibility obtained by this method are not very sensitive to reasonable choices of porosity and initial gas saturation.