Shale formations are characterized by low permeability and require hydraulic fracturing to extract oil and gas. During the hydraulic stimulation process, proppants are used to keep the fractures open, allowing for the passage of hydrocarbons and water-based fluids such as hydraulic fracturing fluids and formation water. These fluids can chemically react with and weaken the fracture surfaces and the adjacent shale matrix, leading to proppant embedment. In this study, we utilized nanoindentation to measure the changes in hardness and modulus resulting from shale-fluid interactions. The most prominent chemical reaction observed was carbonate dissolution, which caused a significant decrease in both hardness and modulus for a shale sample containing 31% calcite. However, a sample with 5% calcite only showed a minor decrease in hardness and modulus. The sample with 1% calcite did not change in mechanical properties, although we observed clay mineral dissolution and ferric oxide dissolution/precipitation in the altered zone. These findings provide quantitative insights into the relationship between chemical alteration and mechanical properties in shale, offering valuable information for estimating proppant embedment after shale-fluid interactions.
Natural gas and oil recovery from shale formations has revolutionized the energy industry over the past decade. Hydraulic fracturing, or fracking, is widely used to enhance the permeability of these formations and increase production. This process involves injecting large volumes of water-based hydraulic fracturing fluids (or frac fluids) into the formation. The injection typically starts with a strong hydrochloric acid, which is injected for several minutes, followed by slickwater that fractures the formation (FrackOptima, 2014; Li et al., 2016; OilfieldBasics, 2019). Proppants are then delivered to support the fracture aperture throughout the production stage and potentially continuous utilization of the reservoir.
However, as observed since the 1980s, proppants have been found to embed into shale matrices (Lacy et al., 1998; Volk et al., 1981), leading to a loss of fracture connectivity over time. This phenomenon is mainly caused by proppant crushing, proppant movement, rock-fluid interactions, and/or secondary mineral precipitation on proppants (Katende et al., 2021).
