Proppant damage mechanisms in coal seam reservoirs during the hydraulic fracturing process: A review

Coal seam gas has attracted global interest due to its economic feasibility, high gas storage capacity, and its capacity to cater to the world energy crisis in the long run. One of the most effective ways of extracting coal seam gas is through the hydraulic fracturing process. For the success of the hydraulic fracturing process, the proppant performance plays a key role. Poor proppant performance is a common observation in the field, and studies on shale and siltstone reservoirs show that the primary damage mechanisms associated with hydraulic fracturing is proppant crushing, proppant diagenesis, and proppant embedment. However, coal material due to its softer rock surface is more prone to proppant embedment than the rest of the degradation mechanisms. Primary factors affecting the proppant behaviour in coal seam reservoirs are: 1) the implementation of the hydraulic fracturing process - this includes the proppant concentration, size, type of fracturing fluid used, and the proppant type selected. The selection of proppants is critical in the coal seam gas extraction process, where the softer coal rock surface restricts the use of high strength proppants due to its vulnerability of embedding into the surface. 2) coal properties - elastic modulus of coal is the most important factor deciding the proppant behaviour in the extraction process. The elastic modulus is affected by mainly geological factors (i.e. confining pressure, temperature, tectonic deformation etc.) and the maturity of coal. Moreover, we highlight the changes in elastic modulus of coal that are occurred due to the coal-water and coal-methane interaction, in which the effect of moisture on coal properties is significantly higher than methane interaction, for the reason that the coal matrix tends to chemically bind well with the water molecules. However, due to the heterogeneous nature of coal and the changing of reservoir conditions, we infer that influential factors must be re-evaluated for targeted seams, and thereby develop a suitable proppant implementation process.