A Generalized Material Balance Equation for Coalbed Methane Reservoirs

Abstract

Abstract The objective of this paper is to present three contributions regarding the mathematical development of material-balance-based methods for analyzing unconventional gas reservoirs, particularly coal bed methane (" CBM") reservoirs. These three contributions are: Contribution 1: A generalized material balance equation (" MBE") that accounts for and incorporates the Langmuir isotherm, initial free gas, water expansion, and formation compaction. This particular form of material balancing can be used to estimate the original gas-in-place and, unlike other methods, does not require an iterative process to solve the equations involved. Contribution 2: Prudent CBM reservoir management and optimization requires knowledge of reservoir pressure. An iterative method has been developed to predict reservoir pressure throughout the producing life of the reservoir using only initial reservoir pressure, Langmuir isotherm, and cumulative gas production. Use of the proposed method has the potential to significantly reduce operating costs by minimizing the number of pressure surveys. Contribution 3: A contribution designed to provide an efficient iterative scheme that incorporates the gas-water relative permeability data to predict future performance of the CBM reservoir. The paper documents the practical applications of the proposed MBE and verifies its accuracy through comparison with results of numerical simulation on several field examples. The proposed MBE can provide an independent source of validation for numerical simulators and is applicable to any coal which behaves according to the Langmuir isotherm equation. The Method can accurately predict the future production and pressure performance and, hence, it has the potential to significantly reduce operating costs by minimizing the number of pressure surveys required. Introduction The term "coal" refers to sedimentary rocks that contain more than 50% by weight and more than 70% by volume of organic materials consisting mainly of carbon, hydrogen, and oxygen in addition to inherent moisture. Coals generate an extensive suite of hydrocarbons and non-hydrocarbon components. Although the term "methane" is used frequently in the industry, in reality the produced gas is typically a mixture of C1, C2, traces of C3 and heavier, N2 and CO2. Methane, as one such hydrocarbon constituent of coal, is of special interest for two reasons: Methane is usually present in high concentration, in coal, depending on composition, temperature, pressure, and other factors