Drilling through overburden shale often presents operational challenges, particularly when drilling high-angle wells. Chemical imbalance between the shale and the drilling fluid can lead to time-dependent wellbore instability resulting in stuck pipe; cavings; and, under extreme circumstances, loss of the well. This study provides a unique workflow combining a suite of laboratory measurements to evaluate rock-fluid interaction between shale and drilling fluids. The testing suite includes X-ray diffraction (XRD) for mineralogical composition; tight rock analysis (TRA) for fluid saturations and petrophysical properties; mercury injection capillary pressure (MICP) for pore throat radius analysis, pore water composition and water activity (aw) that is measured at ambient and elevated pressure using a new test system (patent pending); pressure penetration and membrane efficiency tests to evaluate pressure transmission through shale when exposed to drilling fluids; and modified thick-walled cylinder (mTWC) tests for screening potential adverse effects of drilling fluid on rock strength. Test results show how water activity measurements using synthetic brines can vary significantly from measurements made directly from the rock and under in-situ stress conditions. Water activity measurements made under stress correlate well with clay content and pore throat radius. From mTWC tests, the imbalance of aw can generate shale swelling and strength reduction. The pressure transmission tests show how the drilling fluids can plug the pores and reduce the pressure transmission rate to the formation. These data are instrumental in providing mitigating measures to avoid instability problems during drilling through overburden shales.


Drilling high-angle development wells in shale can pose many operational risks requiring optimization of casing points, knowledge of safe mud weight profiles, and selection of the drilling fluid to counter rock-fluid interaction. Often the mud weight needs to be high enough to prevent shear collapse but low enough to not induce weak-bedding-plane failure (mechanical tensile failure) in the overburden. To determine the drilling window and optimum drilling mud weight for the well design, basic well logs and drilling reports are not enough to solve the problem alone; core measurements and advanced sonic are key ingredients to the wellbore stability model and subsequent predictions. Laboratory tests address both mechanical behavior through plane of weakness testing and modeling coupled with anisotropic characterization (Xi et al., 2020; Shaver et al., 2020) and evaluation of chemical shale interaction with drilling fluids to determine the effects of drilling time-dependent instability. This paper provides a novel laboratory testing approach to characterize shale time-dependent behavior to optimize drilling fluids used for shale formations.

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