Investigating the proppant damage mechanisms expected in a propped coal fracture and its effect on fracture flow

Abstract

Although mixing of proppants with the fracturing fluid can significantly improve the productivity of many unconventional gas reservoirs, application of this technique to coal seam gas reservoirs has been challenging. This is particularly due to the unique characteristics of coal, including its highly soft nature that promotes proppants damage mechanisms in coal. Therefore, correctly understanding the fracture conductivity of proppant supported fractures under coal seam gas reservoirs' conditions is essential in enhancing the coal seam gas (CSG) extraction. A series of advanced laboratory experiments combined with an X-ray CT analysis was thus conducted under in-situ reservoir stress conditions to understand the behaviour of two widely using proppants types in coal (sand, and ceramic proppants). According to the results, although sand proppants are more effective compared to stronger ceramic proppants at shallow depth-conditions, in-efficient at deeper depths, at which ceramic proppants are more effective. Importantly, the fracture width variation is strongly influenced by the shape of the proppants, where sand propped fractures can be subjected to a significant fracture width variation, even under a low confining stress condition available at a shallow depth due to the high angularity and associated particles edges’ disintegration happens in the sand. At deeper depths (>6 MPa confinement), sand proppants are more vulnerable to being crushed under higher reservoir pressures, and ceramic proppants are more susceptible to embed into the coal surface. Overall, the study delivers a fundamental understanding of the proppant damage mechanisms in propped coal fractures under various types of proppants and their effect on the fracture flow, during the CSG production process. These findings would be essential benchmarks for the related future field applications.