Recent Innovations in Offshore Completion and Workover Systems

Abstract

A major oil company has developed an offshore completion and workover system, for use with multiwell fixed platforms, that uses the tubingless completion and concentric tubing concept to reduce completion and workover costs. Introduction Tubingless completions have been used in the oil industry for many years. During the late 1950's this type of miniaturization became the key development system for some of the most active drilling areas. This type of completion is the cumulative result of developments in permanent-well-completion techniques, in modified cement, in through-tubing jet perforating guns, and in optimum perforating conditions, as well as of a large number of additional developments in equipment and techniques by the operating and service companies. The tubingless completion was more slowly adapted to offshore wells because of the prevalence of directional holes and different operating prevalence of directional holes and different operating conditions. It is more difficult to apply tubingless completion techniques to offshore wells, but a look at offshore economics will disclose the multiplied incentive for such major cost-cutting. The higher investments warrant careful consideration of every type of offshore completion, but particularly of the tubingless completion. It is as safe as a conventional completion, has both operating and economic advantages, and retains the integrity of the well. In addition to savings on the initial completion, there are great reductions in costs of workover operations; artificial lift maintenance is lower; and lost production from the other zones of multiple wells during workover or servicing of a given zone is materially reduced. Humble developed further the tubingless completion and concentric tubing concepts to obtain a new offshore completion and workover system that would be especially adapted to the multiwell fixed platform. The objective was to reduce the total cost of reserve exploitation, including costs of drilling and completion, production, artificial lift, workover, and well production, artificial lift, workover, and well servicing. An example of the comparative economics in one major offshore field is given in Fig. 1. For the development and exploitation program illustrated, the savings of almost $9 million over 20 years is enough to redrill all of the wells, if necessary, and still yield a profit. Evolution of the Basic Completion For Fixed Platform Development Advantages of the Concentric Tubing Rig Elimination or reduction of high cost operations during the drilling, completion or operational phases of offshore wells is best accomplished through the items that carry the highest per-day cost: the drilling rig and the workover rig. Transportation and personnel costs, along with those of minimum basic requirements, make the cost of workover rigs comparable with that of drilling rigs, when the workover rig is equipped for work in conventional-sized 7-in., 7 5/8-in., or 9 5/8-in. casing and with 2, 3, or 4 strings of 2 3/8-in. or 2 7/8-in. OD tubing installed. Through the adaptation of many well established techniques, the innovation of a number of ideas, and the development of many equipment items by the service industry, a concentric workover rig, such as the one shown in Fig. 2, can replace the higher-cost drilling rig for completion work and the higher-cost workover rig for later work, if the well is completed initially to take maximum advantage of concentric operations. JPT P. 17