TRAPPED ANNULUS AND WELLHEAD GROWTH ANALYSIS-A CASE STUDY OF TWO SUBSEA WELL COMPLETIONS OFFSHORE EQUATORIAL GUINEA
Field Developments and Case Studies
Kim Boles2, Christopher Arehia1
1Eclipse Petroleum Technology, 2Hess Corporation
Trapped annulus and Wellhead growth studies were carried out for 2 subsea well completions in Ceiba field offshore Equatorial Guinea. The objective was to review, and set new Well Operating Envelopes (WOE), because the criteria for the old operating limits were not clearly defined.
In subsea-completed wells, fluids are usually trapped in casing annuli above the top of cement and below the wellhead. When these trapped fluids are heated by the passage of warm produced fluids, thermal expansion can create high pressures in excess of base loads, and consequently cause casing, and tubing collapse, as well as wellhead growth that may result in wellhead integrity issues.
The approach used was to derive the overall well system operating limits, by integrating well capacities which were separately set at drawdown limits, with well mechanical integrity at different production operations conditions over the well life-cycle.
Two- Case scenarios for Well Operating Envelopes (WOE) were derived as follows:
Surface facilities for crude evacuation restricted the minimum wellhead pressures to 600 psig, and management of trapped annulus pressures at a level of 1000 psig. A production gain of 25% over and above current level was predicted.
Post debottle-necking to remove the crude evacuation constraint, which would allow production at much lower wellhead pressures to 150 psig, while the trapped annulus pressures could be managed at higher level of 4000 psig. A production gain of 50% over and above current production were predicted.
The recommendations from the study were implemented in the field, and a gain of 30% production was realised.