Scratch Testing Applied To Drilling Fluid Filter Cakes: Effect Of Cutter Velocity And Comparison Of Different Mud Formulations

Abstract

Scratch testing to determine rock shear strength has been adapted to soft materials such as filter cakes. A soft metal blade with a strain gauge attached to it and calibrated to obtain the deformation to force correspondence replaces the conventional cutter in the scratch apparatus. The shear strength is obtained from the tangential force reading when the blade is moved at a constant applied velocity across a filter cake, for a given cutting depth. In this study, four commercially available drilling mud types (2 water-based and 2 oil-based drilling fluids) were filtered on ceramic discs. The resulting filter cakes were then scratch tested at various imposed cutter blade velocities, to determine the cake shear strength. The goal of this study was to investigate whether the scratch apparatus gives different strength values as a function of the cutter velocity. The results from this measurement campaign, where also a sandstone was tested, show that for the range of available cutter velocities, no effect can be detected on the computed strength values. The results thus suggest that no rheological effects such as shear-thinning occur at these velocities (for the filter cakes), as well as no undrained effects on the pore pressure. This is good news as it validates all previous scratch campaigns, removing the need for reviewing the relevance and interpretation of the scratch test, especially as applied to filter cakes.