Case History of the Removal of a Hydrate Plug Formed During Deep Water Well Testing

Abstract

Abstract The potential for gas hydrate production is always present in deep water operations. The negative impact of gas hydrate formation on both drilling and completions activity in deep water has been documented previously1,2. Another speciality operation that holds equal hazardous potential is drill stem testing in deep water. This paper describes the cause and solution to a hydrate plug that formed during a deepwater well test operation off the UK Continental Shelf, West of Shetlands. During well test operations in a water depth of 2750', an 800’ hydrate plug formed in the string inside the riser above the mud line, prohibiting communication with the well below the SSTT. Diagnostic tests were carried out to determine the source of the blockage, and two potential culprits were initially identified - paraffin or hydrates. Coiled Tubing was mobilised to attempt removal using a heated glycol / brine solution. After several diagnostic operations, a coiled tubing jetting assembly and finally a mill and mud motor was successful in safely removing the hydrate blockage. This paper will discuss:How hydrate formation was masked by other aspects of the well testThe technical and operational issues considered while developing the procedure for removal of the hydrate plugThe outline of the final procedureThe result of the executionThe lessons learned Introduction Conoco UK Ltd. became involved in a deepwater well testing operation in Quad 204 in 2,750’ of water in September 1998. The well was located in deep water on the UK Atlantic Margin, West of the Shetland Islands. Strong currents and environmental sensitivity in the area added to the complexity of rig operations. While bringing the well in, a hydrate plug developed in the landing tubing string between the sea floor and the surface. This paper will provide details of why we believe the hydrate plug formed and what steps were considered to safely remove it. The execution phase of the operation is shared fully. Background Quad 204 lies west of Shetlands on the edge of the Faroes Trough in an environmentally sensitive area. A well test was designed for the first well drilled on the block in the event that hydrocarbons were found.In order to minimise the potential for an unplanned hydrocarbon release; the test was based on a specific cumulative volume to be extracted from the reservoir at a minimal flow rate. All hydrocarbons were to be contained at surface and shipped to shore for disposal. No oil was to be burned to the atmosphere. After logging the well, the decision was made to proceed with the well test. During the initial clean up period of the well test, a hydrate plug began to form in the landing string above the Sub-sea Test Tree (SSTT), and solidified during the initial pressure build up period of the test. The proprietary nature of the well does not allow full discussion of the details of the test, but this paper is at liberty to discuss the trends that took place to enable hydrates to be formed. Hydrate Formation during the Well Test Though hydrates is a risk in any deepwater operation, paraffin production was identified to be more of a risk during this test and remedial plans were put in place, including injection of chemical inhibitors at the SSTT. The well was perforated using Tubing Conveyed Perforating (TCP) guns and an attempt was made to clean up the well.