Preventation of shale formation damage by fracturing fluid-rock interactions plays an important role in well production performance and subsequent refracturing design. In this paper, the effect of converting typically hydrophilic fracture surface to hydrophobic ones on fluid-induced damage and softening is investigated. In particular, nanoindentation experiments were employed to characterize changes in the mechanical properties of shale samples with different surface treatments.
Shale samples were treated by octadecyltrimethoxysilane (OTMS) to convert the initially hydrophilic surfaces to hydrophobic ones, followed by inundation in water to allow for fluid-rock interactions. Nanoindentation testing was then conducted on the samples to characterize hardness, Young's modulus, and fracture toughness to examine the effect of fluid-rock interactions on shale mechanical properties and whether surface treatment can mitigate shale-softening. Its unique advantage is that the near-surface properties (but not the entire bulk sample) can be characterized, thus elucidating how water-rock interactions migrate from the surface to the interior of the rock.
Results obtained from nanoindentation testing were analyzed by statistical analysis and by a proposed clay matrix criterion. Different treatments lead to variable alterations to rock properties. The hardness and Young's modulus exhibit a reciprocal steady state trend at certain depths. Samples with hydrophobic surfaces exhibit a higher resistance to fluid-shale interactions, as reflected by a 40% higher hardness, a 25% higher Young's modulus, and a 35% higher fracture toughness, when compared with untreated shale samples. The untreated rock actually shows a 48% reduction in hardness, a 27% reduction in Young's modulus, and a 43% reduction in fracture toughness in the rock interior. This study also demonstrates that nanoindentation testing provides multiple advantages in characterizing the fracturing fluid-rock interactions at the micro/nano scale, since these interactions start from the rock surface and propagate to the interior without significantly involving the bulk rock. Nanoindentation testing is competent in characterizing micromechanically fluid-shale interactions and how surface treatment can mitigate the softening of shales.