Effective application of proppants during the hydraulic fracturing of coal seam gas reservoirs: Implications from laboratory testings of propped and unpropped coal fractures

Abstract

Commercial production of coal seam gas is a significant proportion of the total Worldwide natural gas supply due to its high gas storage capacity at shallow extraction depths. Coal seam gas recovery may be enhanced by pure fluid-based or proppant based hydraulic fracturing – the decision whether or not to use proppants depends on coal seam characteristics – in particular stress-permeability characteristics. Although proppant application has been common in deep reservoir fracturing, its application on soft rocks such as coal should be carefully investigated. Higher concentrations of proppant are known to minimise proppant damage in deep reservoirs, with stiffness and flow tortuosity also impacting the response. We explored the impact of proppant loading, coal type and fracture roughness on fluid flow characteristics in propped fractures by conducting a series of triaxial permeability experiments combined with micro-CT imaging. The results reveal that proppant application in coal is only effective for deep coal seam gas reservoirs with effective stress greater than 6 to 8 MPa (~700 to ~900 m). Further, proppant application may cause up to 1–2 folds of fracture conductivity improvement in coal seam gas reservoirs located at over 1.2–1.4 km depths (effective stresses > 12 MPa). The impact caused by proppant embedment on fracture conductivity degradation is much significant compared to other proppant damage mechanisms. The effect, however, depends on the stiffness of the coal rock matrix and the proppant type implemented.