The Relation of Diagenetic Clays and Sulfates to the Treatment of Coalbed Methane Reservoirs

Abstract

Abstract Field procedures and laboratory experiments to stimulate production from coalbed methane reservoirs have largely been designed from experience with fractured reservoirs and coal geochemistry. This paper descibes experiments to investigate the occurrence of cements in the permeability pathways in coal samples from mines and cores and how these diagenetic materials interact with completion and stimulation fluids based on core flow tests. Core flow testing of coals generally suggests damage by polymers and stimulation by several acids. Examination of cleat surfaces from the Mary Lee and Black Creek coal seams in the Black Warrior Basin and Fruitland coal from the San Juan Basin by light microscopy, SEM, and XRD analyses show the presence of carbonate, quartz and iron sulfide cements in impermeable cleats and quartz, illite, kaolinite, barite, gypsum, and iron sulfates in permeable fractures. Fracture morphology is also complex with many branching shapes and surface textures. Illite is typically the most abundant clay and is also interlaminated with or interspersed in the coal. Kaolinite is most likely to migrate and its occurrence in meniscus forms and packed against steps in the cleats document mobility. Coal fines are also present in the cleats, concentrated with clays at steps and bends in the cleats. Injection of fluorescin tagged guar through coal plugs demonstrated the polymer’s affinity for clay and identified permeability pathways in coal.