Analysis of an Annular Pressure Buildup Failure during Drill Ahead

Abstract

Abstract While drilling ahead in salt on the Pompano A-31 (Gulf of Mexico) wellbore below the 16 in. shoe, the rotary stalled abruptly and the drill pipe simultaneously became stuck. Subsequent calipers indicated the 16 in. casing to be deformed onto the drill pipe at approximately 250 ft. To the authors' knowledge, this represents the first instance of an annular pressure buildup (APB) failure during drilling. APB is typically associated with extremes of temperature change resulting from production operations. In the current instance, the temperature change was solely from circulating drilling fluid. The manuscript provides a detailed post-analysis of the 16 in. casing failure:An overview of the conventional casing design for this well, indicating that normal operating conditions should not have resulted in a failure;A review of the failure event, substantiated by field measurement and photographs;An investigation of APB associated with inadvertently closing the 16 in. casing annulus at the surface, demonstrating the magnitude of thermally induced pressure load that can result;Consideration of an alternate failure mode, column buckling, to demonstrate that this failure mode was not active in this wellbore;A finite element model of the drill pipe/16 in. casing/20 in. casing trio indicating that, in addition to collapse of the 16 in. casing and subsequent sticking of the drill string, the outer 20 in. casing was also damaged by the event. The latter item is particularly important in that damage to outer strings can be easily overlooked as one concentrates on the collapsed casing and drill pipe fish. Damage to the outer 20 in. string is confirmed by a mechanical caliper. Conclusions in the paper center on confirmation of the postulated failure mode by field measurement and recommendations for avoiding such an event. Introduction While drilling ahead at 9132 ft. on the Pompano A-31 wellbore (Vioska Knoll 989 A-31, OCS-G-6898) below the 16 in. shoe (Fig. 1), the rotary stalled abruptly and the drill pipe simultaneously became stuck. An increase of mud flow out of the flow line surged over the gumbo buster. Approximately 500 psi pressure was bled off the 16 in. × 20 in. annulus. The fluid initially recovered from the 16 in. × 20 in. annulus was a clear 10 ppg brine, but later changed to a 10.5 ppg synthetic based mud composition tantamount to the fluid placed above the cement top in the 16 in. annulus.