Pseudoisotherm: A Revised Concept of Desorption Phenomena of Coalbed Gas Reservoirs Integrating Isotherm, Material Balance, and Pressure for Optimized Field Development

Summary Gas in the coal is found mainly in the form of adsorbed methane on the surface of micropores. Methane desorbs and diffuses from micropores to cleat systems; then, the cleat network delivers the gas to the wellbore. The Langmuir isotherm empirically explains this phenomenon, a plot between pressure and adsorbed gas content (GC). The isotherm is generated in a laboratory, indicating how a particular reservoir will desorb the gas from the coal surface with a reduction in pressure throughout the production stage from startup to abandonment. This study aims to validate the laboratory-derived isotherm after a few years of field production, impacting the revised development plan for infill wells. Assuming that the initially established isotherm is correct, the difference between the initial and current adsorbed gas volume should match closely with the produced gas volume. However, production analysis after 5 years indicates that the calculated produced gas volume is significantly higher than the observed, thus invalidating the fundamental material balance equation. This triggers the question: What is wrong with using the initially established adsorption isotherm in recovery calculations of depleted coal reservoirs, and how can it be corrected? This necessitates revision/review of isotherms to match current pressure and GC/gas in place (GIP = area × current GC × thickness × density). As explained before, for coalbed methane (CBM) reservoirs, GC, cumulative gas production, and current reservoir pressure are direct measurements and require a solid logic to change the parameters. On the other hand, a Langmuir isotherm is a laboratory-derived isotherm from a representative sample of coal with inherent uncertainties from sampling to measurement. Hence, the isotherm shape is modified based on production data analysis and associated material balance results. Considering all these empirical calculations, the laboratory isotherm is revised to satisfy both initial and current reservoir conditions. This solution is validated with hard field data in the form of new core holes drilled after 3 years of production. The pressure and GC measured in those core holes were plotted, which matched the pseudoisotherm, validating the new concept. Instead of using the laboratory-derived core-scale isotherm throughout field life, it is modified with respect to production data analysis and material balance to make it a field-scale isotherm. In the coalbed gas or CBM industry, a new name is coined as “pseudoisotherm” to differentiate it from laboratory-derived isotherm. This method can be used in other reservoirs to optimize field development through infill wells during the production stage and unlock the full potential of coal-gas reservoirs.