Fines migration during coal bed methane production: mathematical and laboratory modelling, field cases

Abstract

Fines migration in coalbed methane (CBM) fields comprises a serious environmental and gas-production challenge. The literature widely reports two kinds of fines: potential coal fines, which are a part of the coal body and can be detached by breakage under a significant drag force exerted from the inflowing water, and detrital coal fines, which are attached to the coal body by electrostatic forces. The theory for detrital coal fines migration is well developed. A theory for potential coal fines, where the drag deforms the coal asperities and detaches fines by rock failure, is not available. The objectives of this study are (1) to derive the governing equations for fines generation by breakage using failure criteria, and (2) to predict well productivity during dewatering and gas production using laboratory-based modelling. The micro-model developed is based on beam theory and comprises static rock deformation by the flow-through water and calculating failure criteria by tensile and shear stresses. The failure condition determines the number of fines that detach after the application of each flow rate, allowing determining the maximum retention function of potential coal fines. The breakage micro-model is incorporated into filtration equations that account for fines mobilisation, migration, straining and consequent permeability decline. Eight series of lab flooding data with coal cores have been treated. The close match between the lab and model validates the model developed. The model allows predicting productivity decline due to permeability reduction by fines breakage and straining.