Drilling Salt – Effect of Drilling Fluid on Penetration Rate and Hole Size

Abstract Drilling a deep, high-pressure sub-salt well is both expensive and a mechanical challenge. There are a number of issues that create difficulty. Drilling fluid selection in the past has focused on drilling a gauge hole, with little concern on the issue of penetration rates.(rate-of-penetration.) Drilling a gauge hole is still an important issue for an exploration well and a critical issue for a development well is to prevent well-bore collapse. High operating day-cost for deep-water drilling is placing additional emphasis on drilling rate, as reflected by the rate of penetration (ROP) and rotating hours. This concern raises the issues of how to most effectively improve penetration rate and/or how to drill the well bore with a single pass. Should a water base mud (WBM) or a synthetic oil base mud (SBM) be used. Should you drill a pilot hole and then under-ream, under-ream while drilling, or drill without under-reaming by using a bi-center bit? If water base muds are selected, can you use a controlled wash-out approach to either replace under-reaming, or make it faster? This paper will address the issue of SBM versus WBM and the efficacy of using a controlled wash-out approach with WBM. Case histories for the Gulf of Mexico will be discussed.