Coal seam gas (CSG) production involves water flow through coal cleats. Creep effect and fines migration can cause a severe damage to coal permeability during water flow. This study presents a systematic methodology to analyze coupled effect of creep and fines migration in a water injection test in a coal sample. Previous studies investigating permeability changes in coal during water flow do not incorporate both of these effects.
We perform two flow tests on coal samples from a mine in China. Proximate, petrographic and XRD tests were conducted for a robust characterization of the coal sample. The coal samples are saturated with ultra-pure water. Afterwards, the same water is injected through the saturated coal sample. The test temperature and injection pressure was kept constant during the flow and production rate was measured continuously to calculate permeability. Fines concentration of the produced water samples was tested by a laser particle counter. Afterwards, the produced fines were separated from the water samples using micron membrane filters. The separated fines were then analyzed using SEM-EDX method. The only difference in two tests procedure is that in first test, the sample is mounted in a Hasseler coreholder under 300psi confining stress while the second test is done under zero confining stress. This allows us to remove any creep effects in the 2nd test while the first test results are affected by both creep effect and fines migration.
The samples in this study are characterized to be bituminous coal containing mainly kaolinite and dolomite in mineral matters. The permeability is reduced to 10% and 73% of the original value in the first and second test, respectively. Although, creep effect is more pronounced as compared to fines migration effect, the 2nd flow test results demonstrate significant permeability damage during the water injection through the coal samples, along with appreciable fines production. The produced fines appear mainly in form of a mixture of coal and clay minerals. The strain associated with the creep effect in the first test contributes to a relatively high fines production as compared to the second test.