Learning from Three Years Annular Injection of Drill Wastes in Alwyn and Dunbar Fields in the North Sea

摘要

TOTAL conducted DCRI (Drill Cuttings Re-Injection) operations for discharging oil contaminated cuttings coming from the phases 12 1/4" and 8 1/2” of Alwyn and Dunbar wells, two fields situated in the UK sector of the North Sea, to meet requirements of new discharge regulations in that area. Re-injection was preferred to skip and ship based on economical evaluation of both alternatives. Several options were considered for DCRI: annular injection into a production well, annular injection in the same well being drilled or injection through a dedicated well. The major benefit of annular injection, provided that this option is technically viable, is the saving of the cost of drilling and completing a dedicated well. Given the number of wells and the corresponding amount of cuttings that remained to be discharged in Alwyn and Dunbar, it was decided to proceed with annular injection in existing wells. The depth of injection, the lithology, and the well deviation at that depth were dictated by the position of 13 3/8” casing shoe and open hole interval between to cement behind the casing 9 5/8”. Injectivity tests were performed in the annulus of several wells1; however few of them were identified as potential injectors. The initial design was based on the possibility of injecting up to 55,000 barrels of slurry per well. Severe difficulties were encountered during the first injections with frequent rapid loss of injectivity and total injected volume per well far below the theoretical storage capacity. A complete review of the injection parameters, mainly injection pressure and rates, and of injection procedures that include the control of slurry rheology and the way of suspending the injection annulus during DCRI shut down led to dramatic improvements regarding the injection conditions and the total volume injected in each well. This paper presents the learning from more than three years experience gained with annular DCRI on Alwyn and Dunbar fields.