Permeability variation associated with fines production from anthracite coal during water injection

Abstract

Produced water during coal seam gas (CSG) production carries fines. A few laboratory studies reported in open media suggest that fines migration can cause variation in coal permeability during water flow. However, a detailed laboratory study has not been reported so far to explain this variation in coal permeability. The characterization of coal and produced fines and water quality may shed light on the root causes of variation in coal permeability.This paper presents an experimental study on an anthracite coal sample from a CSG field in China to investigate the impact of fines migration on coal permeability. Proximate, petrographic and XRD tests are conducted for a robust characterization of the coal sample. The coal sample is first covered with Araldite epoxy to minimize any confining stress effect during the flow test. Then the coal sample is saturated with filtered distilled water which is also injected to the coal sample. The injection pressure is kept constant during the flow and the production rate is measured continuously to calculate permeability. Once the measured permeability stabilizes, the injection pressure is increased to see the effect of pressure gradient on fines migration and permeability variation. Effluent water is collected frequently and analyzed by a laser particle counter to determine the concentration and size of the produced fines. The fines are then separated from the water samples using membrane filters and analyzed under a scanning electron microscope and electron dispersive X-ray (SEM–EDX) to investigate their composition and morphology.The proximate test shows 9.6% ash (air-dried-basis) in the coal sample while the low-temperature ashing XRD shows kaolinite (38.5%), illite (26.2%) and chloride (2.8%) clay in the mineral matters. Production of fines and permeability increases and decreases are observed during water injection. The permeability decrease is attributed to the blockage of cleats by fines whereas the permeability increase indicates the mobilization of trapped fines. The characterization of produced fines shows that the majority of fines are clay particles with some coal fines also observed.