Analysis of an Annular Pressure Buildup Failure During Drill Ahead

Abstract

Summary 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 drillpipe simultaneously became stuck. Subsequent calipers indicated the 16-in. casing to be deformed onto the drillpipe at approximately 250 ft. To the authors' knowledge, this situation 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 is solely from circulating drilling fluid. This paper provides a detailed post-analysis of the 16-in. casing failure as follows: 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 the possible resulting thermally induced pressure load. 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 drillpipe/16-in. casing/20-in. casing trio indicating that, in addition to the collapse of the 16-in. casing and subsequent sticking of the drillstring, the outer 20-in. casing was also damaged by the event. The latter item is particularly important because damage to outer strings can be easily overlooked if concentration is on the collapsed casing and drillpipe fish. A mechanical caliper confirms damage to the outer 20-in. string. Conclusions in the paper center on confirmation of the postulated failure mode by field measurement and recommendations for avoiding such an event.