Rate-transient analysis of an undersaturated CBM reservoir in Australia: Accounting for effective permeability changes above and below desorption pressure

Abstract

Rate-transient analysis (RTA) of coalbed methane (CBM) wells is an on-going reservoir engineering challenge because of the dynamic nature of coal reservoirs. During their lifetime, producing CBM wells completed in initially undersaturated reservoirs (flowing above desorption pressure) experience a change from single-phase water to two-phase (gas + water) production as desorption pressure is reached, and the production characteristics of these wells are affected by a multitude of dynamic CBM reservoir properties. Amongst the most important dynamic properties influencing production is absolute permeability of the natural fracture (cleat) network.Prolific coalbed methane wells in the Fairview Field, central Queensland, eastern Australia, exhibit stress-dependent permeability changes during early dewatering, followed by strong desorption-dependent permeability effects below desorption pressure. These effects, combined with the switch from single-phase to two-phase flow at desorption pressure, make quantitative production data analysis for reservoir properties (e.g. permeability and original gas-in-place) particularly challenging.In this study, a combination of flowing material balance (FMB) equations for gas and water, modified for dynamic changes in effective permeability, and analytical simulation is used to analyze a Fairview Field CBM well completed in coals of the Bandanna Formation using the cavitation technique. The subject well exhibits strong permeability changes for the first several years after desorption pressure is reached, after which permeability changes appear to occur more gradually. Single-phase water production data above desorption pressure is analyzed using an FMB method modified for stress-dependent permeability; gas production data below desorption pressure is analyzed using an FMB method modified to account for relative permeability and desorption-dependent permeability. Because of uncertainty in below-desorption pressure FMB model inputs, the FMB equations and analytical simulation are linked. This approach allows for a consistent, if not unique, analysis.The RTA approach outlined herein provides a reasonable starting point for more rigorous numerical simulation, which in turn can be used for reserves forecasting and development planning. Because of the multitude of reservoir properties (several of the key properties being dynamic) affecting the production characteristics of CBM wells such as those in the Fairview Field, it is important to advance RTA methods to constrain numerical model inputs.