The Effects of Hole Wash-Out on Wellbore Centralization and Effective Zonal Isolation in Consolidated and Unconsolidated Gas Reservoirs

Abstract

Formation evaluation and geological characteristics of the reservoir have been key factors that help in determining the best practices in drilling and completion operations. Primary cementing in unconsolidated or consolidated gas formations presents even bigger challenges due to the volatility of gas and a high risk involved with respect to maintaining good well control across the various phases involved in delivering a well. To achieve good zonal isolation through cement placement, it is important to ensure that best operational cementing practices are properly incorporated with a robust slurry design based on specific formation data. Beyond the reliability of these factors, it is important to have accurate hole data which inform the optimum centralizer program and volume determinations. One key factor that hinders the accuracy of this information is the extent of wash-outs in an open hole. Hole wash-outs and the level of wash-outs in a wellbore depends on various factors some of which includes, the degree of consolidation of the formation environment, the practices deployed during the drilling phase of the wellbore and the operationally-influenced delays due to challenges experienced in the drilling phase of the hole section. Over time, the levels of wash-out experienced in a wellbore have been seen to greatly influence the centralization effectiveness, and depth accuracy with respect to annular slurry placement. This paper will be analyzing case studies from gas wells that were drilled in both consolidated and unconsolidated formations, the effect of wash outs on the centralization stand-off and how these wash-outs affect the quality of zonal isolation across these hole sections. The paper will include best practices to improve zonal isolation when dealing with wash-out intervals.