When a hydraulic fracture is poorly supported by sub-monolayer proppant, acid treatment of a brittle, carbonate-rich shale fracture may be able to reduce proppant crushing and fracture closure by making the rock matrix more ductile. To test this hypothesis, a series of laboratory fracture closure experiments were conducted on clay-rich and carbonate-rich Wolfcamp shale samples, under elevated stress (effective stress ~27 MPa) and temperature (~122°C). A specially designed oedometric compaction cell was used, which allowed optical in-situ visualization of the interaction between the sample surface and a sub-monolayer (~50% surface coverage) of quartz proppant grains. Two-week-long loading tests showed some reductions in the fracture compaction with an acid-treated carbonate-rich samples compared to an untreated counterpart and clay-rich samples, but the impact of dissolution was subtle in spite of strong indications that the local proppant-shale matrix interaction was altered by the mineral-type-dependent changes of the shale texture.
In an unconventional, low-permeability oil and gas reservoir in shale, the use of proppant for preserving the aperture of induced hydraulic fractures is critical for efficient and economical resource extraction. However, particularly in a poorly supported fracture containing a sub-monolayer (less than 100% surface coverage) of proppant in brittle shale, crushing of proppant grains can lead to excessive reductions in the permeability, due to fracture aperture reductions and debris-induced clogging. Such behavior may potentially be manipulated by chemically increasing the ductility of the shale matrix by dissolving hard carbonate grains near the fracture surface. Conversely, increasing the ductility of the shale matrix may also lead to enhanced proppant embedment which can contribute to fracture closure and permeability reduction.
In order to investigate the relative importance of proppant crushing and embedment in shale fracture closure, we conducted a series of laboratory experiments investigating time-dependent proppant-shale interaction under effective stress up to 27 MPa and temperature at ~122°C. The key objectives of the experiments are (1) to determine the time-dependent compaction behavior of propped fractures for shale samples with different mineral compositions, and (2) to examine the effect of acid treatment on the ductility of the shale matrix and proppant crushing. These experiments were conducted using a custom-built oedometric cell with a high-pressure viewing window which allowed real-time visualization of fracture closure and proppant deformation and embedment. The visualization was facilitated by the UV fluorescence of injected pore fluid (Nakagawa and Borglin, 2019). The samples used in the experiments were both clay-rich and carbonate-rich Wolfcamp shale samples which were obtained via HFTS (Hydraulic Fracture Test Sites) project (e.g., Birkholzer et al., 2021).
