The Effect of Borehole Pressure on the Drilling Process in Salt

Abstract

Summary Wells penetrate long intervals of salt to access hydrocarbon reservoirs in many locations. High mud weights are frequently used to stabilize the wellbore while drilling through salt formations. High borehole pressures reduce rates of penetration (ROPs) in many sedimentary rocks. This effect is significant and contributes greatly to the low ROPs and high costs associated with deep drilling. This paper presents the results of an integrated laboratory study of the effect of borehole pressure on the drilling process in salt. The study included full-scale laboratory-drilling tests, visual single-point-cutter tests, and triaxial-compression tests. The drilling tests were performed by use of a polycrystalline-diamond-compact (PDC) bit and a synthetic-based drilling fluid typical of field practice. The visual single-point-cutter tests used a clear drilling fluid, enabling observation and video recording of the salt-cutting process at elevated borehole pressures along with simultaneous measurement of the cutting forces. The effect of borehole pressure on drilling salt is seen to be small; increasing borehole pressure does not reduce ROPs in salt as much as it does in many other sedimentary rocks. In contrast to other sedimentary rocks, salt deforms plastically to relatively large strains in compression without loss of mechanical integrity, even at low confining pressures. In addition, the shear stress to achieve a given strain does not increase greatly with confining pressure. Rock-cutting forces in the single-cutter tests increased only moderately as borehole pressure was increased from 300 to 6,000 psi. Full-scale bit-penetration rates also decreased by relatively small amounts over the borehole-pressure range investigated (1,000–6,000 psi). Mechanical-specific-energy (MSE) levels showed corresponding small increases. These levels, however, were consistently much higher than the compressive stresses required to make even large strains at a confining pressure equivalent to the borehole pressure. Observations of the deformation and cutting processes in salt combined with results of finite-element analyses of rock and metal cutting explain the low sensitivity to borehole pressure of halite-penetration rates and its relatively high specific energy. This study fills a gap in industry knowledge of pressure effects in drilling and sheds light on the processes involved in drilling rock salt. The key practical conclusions are that ROPs and MSE in salt are not greatly influenced by borehole pressure. Consequently, if a high borehole pressure is required to stabilize the wellbore through a salt formation, there is little penetration-rate penalty to be paid, except perhaps if increasing the mud weight involves significant changes to the drilling-fluid properties other than its density. The comparison of results from visual single-point-cutter and full-scale bit tests suggests, unsurprisingly, that reducing bit-cutter density and modifying drilling-fluid properties may both offer routes to increase ROPs in salt. The current study does not, however, investigate directly the effect of fluid properties on the drilling process in salt.