Analysis of Alternative Well-Control Methods for Dual-Density Deepwater Drilling

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98957, "Analysis of Alternative Well-Control Methods for Dual-Density Deepwater Drilling," by M. Stanislawek, SPE, Ensco Offshore Co., and J.R. Smith, SPE, Louisiana State U., prepared for the 2006 IADC/SPE Drilling Conference, Miami, Florida, 21-23 February. Dual-gradient drilling methods have been proposed to provide simpler, safer, and more economic well designs for deepwater gas resources. Riser gas lift was investigated as a means to implement a dual-gradient system. A primary concern was whether effective well control was possible in a system containing so many flow paths and fluids with different densities. A study with a transient, multiphase simulator found that well control was feasible by use of methods analogous to those for conventional operations. Introduction Deepwater Gulf of Mexico (GOM) resources are especially important for reversing the decline in U.S. production. However, a narrow margin between formation pore and fracture pressure exists in many overpressured basins including the GOM. This limited margin between pore and fracture pressure often becomes narrower with increasing water depth as a result of the reduced overburden pressure and shallow onset of abnormal pressure. As a result, reaching the target depth while retaining a useable borehole size often is difficult, which can limit deepwater resource development. Dual-gradient drilling methods have been proposed as a means to overcome this. A dual-density drilling concept using riser gas lift was studied as a way to implement a dual-gradient system. The new system would provide a simpler, more economic design consisting of nitrogen and mud with an average density equivalent to seawater in the riser annulus and mud in the wellbore. In concept, more standard equipment would be used than in the industry projects focused on use of seafloor pumps. The expected advantages of such a system would be fewer casing strings, larger mud-weight margins, and a larger production-casing size. Problem Description The major question addressed in this study is if effective well control is possible for a system with so many different fluid densities and relatively complex flow paths. Three critical phases of a well-control operation were addressed: kick detection, stoppage of formation inflow, and circulation to remove kick fluids. Each of these phases was simulated in sequence by use of a transient, multiphase numerical simulator to determine the best well-control method.