Lessons Learned and Reduction in Rig Time with the Use of Water-Based Mud to Bottom Hole Drilling in the Pre-Salt

Abstract

Abstract The drilling of the evaporitic section using water-based mud (WBM) was always a challenge: even though there are already some wells drilled with saturated aqueous fluid, their use still divides opinions within the technical-academic community of the oil industry. Basically, there are two currents: one that believes that salt drilling with WBM causes an enlargement so great that it becomes impracticable to cement the casing and, consequently, to obtain a leak off test (LOT) or a formation integrity test (FIT) sufficient for drilling of the next phase. The other line of thinking is based on that the break-in created by the WBM is similar to that caused by the synthetic-based mud (SBM) and this increase in diameter would not have a negative influence on the quality of the cementing job. In 2015-2016, Petrobras drilled 13 wells in Santos Basin Pre-Salt using WBM for bottom hole drilling (BHD). In this way, this work aims to report the lessons learned in each well, comparing fluid costs with correlation wells drilled with SBM's. The use of WBM, besides not negatively impacting the drilling time of the phases, allows a reduction of the costs with chemicals products and a decrease in equipment costs, since the drying system of cuttings is not used. In addition, rig and borehole cleaning are simpler and require less time and WBM is less aggressive for people working in direct contact with the drilling fluid. To compare the costs of fabrication and treatment of fluids, an average calculation was used for the correlation wells. Thus, an economy achieved is the difference between average cost for correlation wells and the well in question. The drilling of the evaporitic section with WBM's makes: (i) no significant salt washout than that obtained when using SBM and (ii) as WBM does not require a cuttings drying system, rate of penetration (ROP) obtained during a salt drilling are higher than those obtained when working with SBM. For the drilling of the carbonate reservoir, the performance of WBM is similar to that of SBM and the use of WBM in BHD allows faster fluid exchange and generates less residue for wells with open hole single completions. By drilling 13 wells using WBM, it was possible to save 576.9 hours of rig time.