Investigation of influence of confinement in proppant embedment in coal

During methane production, effective loading stress normal to the fracture surface is elevated, while effective confining stress applied on the surrounding coal body is also increased. For a soft rock material like coal, the influence of confinement in proppant embedment cannot be not neglected. We measure overall sample compaction (external LVDT) and local strain (strain gauge) in the matrix to deconvolve proppant embedment in a propped fracture for different conditions of confining stress. Five specimens of coal and one additional sample of shale are tested as a control group at incremented confinements of 0, 3, 6, and 9 MPa. The results suggest that coal has distinctively different embedment behavior from shale, though embedment increases with loading stress in all the cases. Embedment in shale shows a logarithmic relationship with increasing loading stress, suggesting indentation grows slower at high loading stresses. Conversely, the evolution of embedment in coal presents a convex upwards profile with stress where indented depth increases more rapidly as loading stress increases under a constant confinement. These behaviors are symptomatic of elastic (shale) and elastoplastic (coal) responses. In addition, the curvature of the embedment evolution profile for coal gradually disappears as confinement increases. Cyclic loading under confinement (9 MPa) shows that the majority of indentation is elastically recovered, but without confinement the maximum embedment increases rapidly with each successive loading cycle, eventually destroying the stability of the coal matrix and causing complete crushing of the sample.