Successfully Managing Drilling-Fluid Losses in Multiple, Highly Depleted Sands

Abstract

Summary Successfully drilling through depleted sands to reach deeper targets often demands a delicate balancing act between maintaining mechanical stability in normal and overpressured shales and controlling fluid losses caused by hydraulic fracturing. A host of considerations go into the planning process, including whether further production from the depleted zones is planned. Because of the prohibitive cost of a new North Sea well, a stability analysis was performed to determine the feasibility of sidetracking from an existing well and then directionally drilling through two highly depleted sand formations with a single hole section. The case study presented compares the original mechanicalstability and hydraulic-fracturing estimates with the actual mud weights used to successfully complete this difficult drilling operation. Descriptions of hole instability and fluid losses are presented along with the drilling fluid parameters used to combat them. Methods used for estimating formation pore pressure, in-situ stresses, and formation rock properties are also described. In addition, possible explanations for the differences between the planned and actual stability estimates are explored, with future recommendations for similar operations.