A laboratory set up for the measurement of wellbore instability due to erosion of unconsolidated formations

Abstract

Borehole instability is a recurring issue encountered during drilling operations in the oil and gas, and mining industries. In exploration drilling using coiled tubing technology, boreholes are slim and the annular space is usually tight. Similar situations are also encountered around the drill-collar section in standard drilling where the annular space can be tight; therefore, the drilling fluid has a high flow velocity and a turbulent flow regime is dominant. The flow conditions are the governing conditions for borehole erosion, which can lead to borehole instability. Erosion of the borehole introduces more cuttings into the annulus space, which is detrimental to the efficiency of cuttings transport and brings contamination to samples being retrieved at the surface for geochemistry and mineral analyses. In this peer-reviewed paper a new laboratory set-up aimed at estimating borehole erosion in pre-drilled rock samples is reported. A flow loop unit is used, which allows the injection of high flow-rates of any type of fluid composed of chemical additives and simulated cuttings. Early results of an exploratory study are reported in terms of changes in the internal borehole diameter (ID) of the sample before and after the erosion experiment. These were recorded using X-ray CT scanning. This change in ID acts as a measure of the formation’s erodibility, from which shear stress is estimated.