Abstract Unlike conventional gas reservoirs where gas saturation exists at its maximum value initially, coalbed methane (CBM) reservoirs are unique in a way that gas saturation in reservoir starts from zero and builds up to reach a peak value as the adsorbed gas in the coal matrix desorbs into the coal cleat system due to depressurization of coal reservoirs through CBM wells. In comparison to conventional gas wells, the increase in gas saturation with pressure depletion is multifold in CBM reservoirs, thus leading to the gas relative permeability dominate the CBM well performance. Moreover, cleat porosity and permeability of coal also dynamically change with pressure depletion. Given these subsurface phenomena, the Inflow Performance Relationship (IPR) of CBM wells involves additional complexities, thus making conventional IPR methods unsuitable for CBM wells. Similarly, the vertical lift performance (VLP) of CBM wells also differs from that of conventional oil and gas wells, given the typical CBM well configuration where gas is produced through the annulus and water is pumped through tubing. The flowing bottom hole pressure (FBHP) calculations for CBM wells involve additional parameters, such as depth of the water level in the annulus, the water-gas-ratio, the casing ID, and the tubing OD. In this context, the VLP of CBM wells entails further intricacies that are not captured in conventional methods of VLP generation. In this paper, for the first time, a mathematical model is developed, and a computational approach is propounded to construct the VLP of CBM wells. A mathematical model and process workflow have been defined for generating IPR of CBM wells considering the dynamic changes in cleat permeability and phase relative permeabilities with pressure depletion. The process workflow has been defined to integrate the IPR and VLP of CBM wells to determine the optimal operating point. The study shows that the VLP of CBM wells, unlike that of conventional gas wells, displays a decreasing trend in the FBHP with increasing surface gas rate. The study also reveals that the gas-phase IPR of CBM wells is driven mainly by the variation of \: kₑ₆{\: ₆B₆} with pressure depletion. The results of the mathematical model have been validated with in-field production data from a CBM well.
Upadhyay et al. (Mon,) studied this question.