| Popis: |
Drilling a surface hole with total losses through shallow aquifers is normal practice in Oman and most of the Gulf region. If losses cannot be cured while drilling, and the casing cement does not return to surface, required zonal isolation is most likely being compromised. During the life of the well, flow behind the casing from the shallow aquifers results in accelerated casing corrosion that compromises the integrity of the well and results in relatively costly workover operations to restore the well and allow it to be operated. Squeeze cement that repairs each corroded section of casing is a relatively high-risk and expensive operation with a low chance of success. During a workover on one of the water-injection wells in North Oman, severely corroded 9 5/8-in. production casing was confirmed. The initial investigation confirmed multiple leaks in the 9 5/8-in. casing, of which the exact location was difficult to establish; however, it was expected to have resulted from a failed 13 3/8-in. surface casing caused by corrosion from the shallow aquifers. Communication between the 9 5/8-in. production casing and the 13 3/8-in. surface casing was confirmed in multiple zones. Different options were evaluated to restore the integrity of this well. The options evaluated included the use of mechanical casing clads (expandables) to fix each corroded section in the 9 5/8-in. production casing before running a 7-in. cemented tieback string and the option to squeeze cement across each corroded section in the 9 5/8-in. production casing before running a 7-in. cemented tieback string. Both of these options were abandoned because of economic concerns and low chances of success. It was decided to approach this integrity repair differently by running a 7-in. tieback casing to surface and using foamed cement to cement the 7-in. tieback casing and existing corroded 9 5/8-in. casing to surface. To maximize the chance of success, the 7-in. tieback casing incorporated a multistage cementing collar to help ensure foamed cement returned to surface. The two-stage, foamed-cement job with controlled returns from two annuli might be considered to be the first of its kind, globally. The foamed-cement job was executed successfully as per program, with good foamed cement returns from both the 7-in. × 9 5/8-in. annulus and the 9 5/8-in. × 13 3/8-in. annulus. The overall job met all the objectives and this technique has been repeated successfully on subsequent wells with similar integrity issues. |
