Assessment of Salt Loading on Well Casings

Abstract

Summary Assuring the integrity of subsalt wells in the deepwater of the Gulf of Mexico throughout the field's life is a major drilling engineering challenge. The consequences of well failures may result in billions of dollars in remedial costs and lost production. On the other hand, the costs associated with overly conservative well design are significant, which motivates systematic analysis of casing loading for scenarios of interest. Simplified hole-closure and casing-design guidelines for salt, many developed for the western U.S. Overthrust Belt, are not appropriate for the relatively pure, slow-moving halite found along the Gulf Coast. Instead, our work applies knowledge gained by Sandia Natl. Laboratories from technical research and development (R&D) investigations of the Waste Isolation Pilot Plant (WIPP) and the Strategic Petroleum Reserve (SPR) to determine the magnitude and timing of salt loading on well casings in the Gulf of Mexico. If hole quality can be assured, the analyses presented show that it is not always necessary to cement the casing/borehole annulus through the salt because the subsequent uniform loading is insufficient to substantially deform the casing. This poses no threat to drilling operations or impingement on the inner casing string in the long term and results in considerable cost savings. However, if hole quality is poor, a cemented annulus is necessary, as the cement effectively transforms the potentially nonuniform loading situation into one of uniform loading. Significant benefits can accrue from quantifying the magnitude and timing of salt loading. Difficult cementing jobs and liner tiebacks can be omitted and a more aggressive well design adopted. The simplified well design, and the elimination of potentially troublesome operations, leads to millions of dollars in cost savings in individual wells.