Abstract
SummaryMitigation and prevention of shale-formation damage caused by hydraulic-fracturing fluid/rock interactions play an important role in well-production stability and subsequent refracturing design. In this paper, the effect of converting typically hydrophilic fractured surfaces to hydrophobic ones on fluid-induced softening of shales was investigated. Specifically, nanoindentation was used to characterize changes in the mechanical properties of shale samples after different surface treatments. A thin layer of octadecyltrimethoxysilane (OTMS) coating was deposited on the initially hydrophilic surface of shale, followed by inundation in water for certain periods of time to allow for fluid/rock interactions. Nanoindentation testing was then conducted on the treated shales to characterize their hardness, Young's modulus, and fracture toughness to examine the alteration of shale's mechanical properties caused by fluid/rock interactions and to check whether hydrophobic coating can mitigate shale-softening. Results from nanoindentation testing were analyzed by a newly proposed clay-matrix criterion for data screening. Different rock-surface treatments lead to changes in rock properties. Both the hardness and Young's modulus of the treated samples converge and stabilize at relatively large depths. Samples with hydrophobic surfaces exhibit a much lower degree of softening, as reflected by their better mechanical properties (e.g., 40% increase in hardness, 25% increase in Young's modulus, and 35% increase in fracture toughness), compared with untreated shale samples. The results of this study also demonstrate that the continuous-stiffness-measurement (CSM) method and the repeated loading method for nanoindentation loading yield similar ranges of micromechanical properties of the bulk shale. However, the CSM method, if combined with the newly proposed clay-matrix-based criterion for data screening can better define and characterize fluid/shale interactions or softening of shales.
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