Deepwater Workover Experience With Extreme Hydrostatic Overbalance

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 134378, ’Deepwater Workover Experience With Extreme Hydrostatic Overbalance, 2002-2009’ by G. Myers, SPE, D. Cahill, SPE, P. Clay, SPE, B. Esquinance, B. Smith, SPE, and R. Zeringue, SPE, Shell E&P Co.; C. Davis, SPE, Shell International E&P; and T. Kenney, Crown Consulting, originally prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The full-length paper presents planning and execution details for fluid-loss control during workovers with “extreme-hydrostatic-overbalance” (EHO) conditions (i.e., between 1,500 and 4,000 psi). Actual results for several wells from three deepwater fields in the Gulf of Mexico (GOM) are included. The information is applicable to operators that face similar requirements to maintain well control throughout the workover, mitigate any damage to the installed sand control or reservoir, and complete removal of the fluid-loss barrier to re-establish production. Introduction For many years, the industry has dealt with large hydrostatic overbalances in a variety of ways during workovers. Onshore, operators of tight gas fields initially may pump fluid to kill the well for rig up, then add fluid intermittently during the job as required to work safely. Offshore, operators of shallow-water, higher-permeability fields may initially kill the well with seawater for rig up, then continuously feed or “let the well drink” seawater to maintain hydrostatic overbalance during the job. The full-length paper focuses on direct-vertical-access (surface tree) wells in deepwater GOM fields. These wells are characterized by high-permeability, thick sand intervals; downhole sand control; and very high fluid-loss rates after perforating or stimulation. Profitability is tied to maintaining high production rates without significant solids production. It is the combination of extreme hydrostatic overbalance, high permeability, downhole sand control, and high production potential that makes these wells so challenging. Subsea wells in deepwater GOM fields must be handled differently because of the potential for riser collapse, use of riser fill valves, and the daily cost of floating rigs. EHO is a term used to describe conditions significantly outside the normal. It is common to execute workovers in the GOM with 200- to 300-psi hydrostatic overbalance—that is, the difference between the hydrostatic-column pressure at the perforations and the near-wellbore reservoir pressure. Depending on the reservoir-drive mechanism, workovers in later life may be executed with 300- to 800-psi hydrostatic over-balance (moderate) or 800- to 1,500-psi overbalance (high). The term EHO is used to describe wells with 1,500-psi or greater overbalance and is not a precise definition. Depending on the job, 1,200-psi hydrostatic overbalance may require similar planning. To date, jobs have been planned with overbalance approaching 4,000-psi, typically using seawater as the workover fluid.