Impact of capillary trapping on CSG recovery: an overlooked phenomenon

Abstract

The impact of water on gas production has been commonly reflected using relative permeability curves, which are obtained by measuring the flow behaviour of each phase through a core sample. This approach reflects the overall response of a core to flow but is unable to capture the capillary trapping phenomenon at the microscale, which is expected to vary significantly for coals with different microstructures. The overlook of trapping effect could potentially overestimate gas production, a topic that does not appear to be well explored. In this work, the impact of capillary trapping on gas recovery was investigated numerically through a fully coupled water-gas two-phase flow model. The characterisation of a coal microstructure, surface chemistry (e.g. coal wettability) and an isotherm curve was summarised for Bowen Basin coal. Various sensitivity studies were then conducted at coal matrix scale to quantify the amount of gas trapped by capillary forces under different reservoir conditions and production controls. Our results show that for the studied coal parameters from the Bowen Basin, the capillary trapping effect hinders gas breakthrough noticeably, causing unwanted high abandonment pressure and reduction in gas recovery rate. Among all investigated parameters, pore size has the most important effect on trapped gas percentage. If taking 3 MPa as initial reservoir pressure, 300 kPa as the abandonment pressure baseline, 63.58 kPa as the gas breakthrough pressure, then the trapped gas accounts up to 4.02% of the total predicted gas; the trapped gas percentage will increase considerably if the saturation of gas is very low, although this variability is largely dependent on reservoir condition.