Compressive and Tensile Strength Anisotropy of Friable Sandstone and Thin-bed Shale of the Pleistocene Erin Formation – Insights for safe operating mud weight windows for vertical and horizontal wells

Abstract

Abstract The compressive and tensile strengths are essential parameters used for determining the safe mud weight window (SMWW) during borehole drilling, especially in friable rocks where the SMWW tends to be narrow and therefore, the margin for wellbore instability issues is high. However, these parameters are seldomly reported for friable rocks due to the difficulties and challenges faced while recovering and preparing friable cores and outcrop samples for strength testing. This study evaluates the compressive and tensile strengths of friable rocks under dry and saturated conditions, and investigates how strength anisotropy affect the SMWW. Unconfined compressive strength, confined compressive strength (up to 130 MPa effective pressure), and Brazilian (tensile strength) measurements were made perpendicular and parallel to the outcrop bedding of the friable sandstone and thin-bed shale lithofacies of the Erin Formation, Southern Trinidad. In spite of this Formation being one of the main petroleum reservoirs, there are no published data on the rock strength. The results show that strength anisotropy exist, which is larger under saturated conditions. Under confinement, the friable rocks accumulated large strain (an average of 20% strain) and experienced significant strain hardening causing the strength to be high. The ratio of the yield strength to failure strength is less than 0.2, under dry conditions. Thin section analysis of tested specimens shows intragranular fracturing across quartz grains, which led to grain crushing and pore collapse. Catalastic flow within the specimens and no stress drop after the peak stress suggest the friable rocks failed in a ductile manner. The effect of the strength anisotropy on the SMWW for the sandstone and thin-bed shale was determined for wells that are orientated in the principal stress directions (Sv, Shnmi and SHmax), of a normal faulting stress regime, at depths of 3400 to 6900 ft. The upper mud weight limit of the SMWW is not influenced by the tensile strength anisotropy. However, the lower mud weight of the SMWW was influenced by the compressive strength anisotropy, predominantly for Sv and Shnmi aligned wells that are penetrating through the saturated friable sandstone. Since the rocks are friable, drilling outside of the SMWW by 5% to 8%, if forecasted using the strongest compressive strength, can result in wellbore instability.