Successful Campaign Using Coiled Tubing to Perforate Five HPHT Wells in the UK Central North Sea

Abstract

This paper details the planning, design and execution of a successful campaign to perforate five HPHT wells, using Coiled Tubing (CT), in the UK Central North Sea. The project goal was to perforate extended intervals, in live well conditions, in one run and leaving no guns or restrictions downhole. The well conditions presented challenges to the design and operation of CT in this campaign. Challenging factors included: The use of high yield strength CT (130,000ksi grade material). Wellhead pressure up to 9,000 psi. Bottomhole pressure up to 12,500 psi. Estimated BHT 375° F. Estimated FWHT 320° F. Well depths of 21,000 ft MDRKB. Deployment of up to 1,645 ft of 2 7/8″ perforating guns. Perforating gun string retrieval to surface. No rathole to drop guns due to: Penetration of the reservoir water bearing zone, potentially leading to produced water and scaling issues in later well life. Access required across the formation for data acquisition and reperforation operations. Cost and time involved drilling hard HPHT formations. Job design was critical to the success of the operation and consideration given to learnings from similar previous operation. This included selection and analysis of CT material, size, wall thickness and managing potential CT collapse pressure following reservoir perforation. Further analysis, included calculation of CT stretch, circulation pressure, and wellbore solids removal studies. Reverse circulation through the CT was carried out. This would increase the effectiveness of fluid displacement when creating an underbalance situation prior to perforation. In order to maintain pressure barriers specific tooling was required to minimise the risk during this operation. The completion design is discussed with specific attention to the process of eliminating the requirement to deploy and recover perforating guns whilst maintaining control of the surface pressure. The ability to maintain a dual pressure barrier within the completion saved considerable time over the conventional method of recovering long gun strings with pressure at the wellhead. An important element contribution to the success of the operation was the completion of additional equipment tests completed prior to the commencement of operations. The testing demonstrated equipment was fit for purpose and established critical interfaces as well as creating synergies with the multi-vendor delivery team. The fine-tuned operational and procedural aspects of the job and led to several modifications to equipment and procedures. Further testing was completed to confirm functionality the data acquisition, allowing monitoring and recording of pressures, circulation rate and depth control. The job design, planning, qualification of equipment and the use of dedicated competent personnel lead to the successful perforation campaign of five HPHT wells without any accidents, environmental spills or lost time and within budget.