Understanding the Origin and Removal of Downhole Debris: Case Studies From the GOM

Abstract

Potential damage to the formation and generation of destructive wellbore debris can occur during perforating operations with shaped charges. One type of damage is to the formation, which is related in part to the powdered metals and metal alloys incorporated into the metal casings that contain the explosive charges in controlled debris perforating systems. Upon discharge, gun debris, including casing-metal residue and reaction products from the perforating system, can also contribute significantly to the overall amount of destructive wellbore debris generated. In addition, the debris often includes metal fragments from the wellbore casing, formation sand and other minerals, cement, and drilling fluid remnants that eventually are ejected into the wellbore. All of these materials can promote costly mechanical failures, contribute to well control problems, and hamper onsite operations. Various factors influence the ultimate generation of destructive wellbore debris. These factors include gun type, fluid type, fluid density, bottom hole pressure and temperature, and the specific wellbore configuration and completion scenario employed. While previous authors have addressed formation damage issues or the impact resulting from the use of various casing metals, this work focused on the actual downhole debris generated and collected from the wellbore just after perforating the well, and relates these findings to the influencing factors. With the use of advanced downhole filtration and collecting techniques, downhole debris was collected from a series of deepwater projects in the GOM. Debris analysis coupled with well completion information has been studied. Results from the examination of downhole wellbore debris collected after perforating the well is presented and discussed. Improved wellbore integrity and reduced risk are obtained through proper management of perforation and wellbore cleaning practices. Introduction Universally accepted, downhole debris is responsible for numerous problems associated with well completions, evidenced by the disclosures presented in each paper on this topic cited herein.1–10 Downhole debris has been associated with formation damage issues, failure in mechanical shifting of downhole flow control systems, retrieving temporary packer plugs, interfering with the setting of landing tools, and other problems. Much of the destructive wellbore debris is directly associated with tools used to perforate and the perforation process itself. Other debris can occur from material accidentally introduced from the surface. Several papers address the relative merits of zinc-cased perforating charges, steel-cased perforating charges, and a newly developed low-debris steel-cased perforating charges.1–4 Case histories demonstrating the usefulness of both zinc-cased2 and the newly developed steel-cased3,4 perforating charges are available in their respective publications. The perceived problems and benefits associated with zinc-cased charges has been described in some detail.5 Part of the potential damage from zinc-cased charges was found to be associated with the potential formation of complex zinc-hydroxy-chloride cements similar to the magnesia cements known as Sorel cements. In addition to acid, some products were reported effective in certain brine systems, but limited solubility of the product, incomplete control of precipitation, and potential formation damage prevented further application without modifying the product chemically. A method of preventing perforation damage from zinc-cased charges in high temperature applications by use of a long-chain organic acid was reported; 6 corrosion data was provided only for 22-chrome material.