Mineralogy of deepwater Gulf of Mexico salt formations and implications for constitutive behavior

Abstract

A majority of the wells for the most significant announced developments in the deepwater Gulf of Mexico will penetrate salt formations thousands of feet thick. Assuring the integrity of these wells over the field lifetime is a major drilling engineering challenge. The effect of salt on long-term well integrity is tied to the constitutive behavior of the salt, and more specifically, to its creep rate. The constitutive behavior of salt has been well studied in two previous geotechnical engineering applications. A sophisticated constitutive model for salt that considers transient and steady state creep was developed under the Waste Isolation Pilot Plant, a licensed repository for transuranic waste. Further studies of salt creep were performed for the Strategic Petroleum Reserve (SPR), a series of leached storage caverns located in salt domes along the U.S. Gulf Coast. However, only a single set of laboratory creep data exists for salt from a deepwater Gulf of Mexico diapir. To constrain the constitutive response of deepwater Gulf of Mexico salt diapirs, the mineralogic composition of salt diapirs from several regions, including Ship Shoal, Ewing Bank, Mississippi Canyon, Garden Banks, Green Canyon, and Walker Ridge, was determined by quantitative X-ray diffraction analyses of drill cuttings. Water depths for the wells ranged from 718 ft to 7590 ft, and salt sections up to nearly 11,000 ft thick were penetrated. The database provides insight into the large-scale geographic variations in salt mineralogy, as well as insight into variations along vertical (or near-vertical) transects through massive salt bodies in the deepwater Gulf of Mexico. To enable comparison with on-shore domal salts with known constitutive response, mineralogic analyses were also conducted on salt cores recovered from various on-shore SPR sites. We show that that the range in composition of the salt diapirs in the deepwater Gulf of Mexico is essentially identical to the observed range in composition of the salt domes located along the U.S. Gulf Coast (to which the deepwater salt diapirs are geologically related). In light of the observed constitutive response of the sole deepwater salt diapir tested to date, we conclude that the expected constitutive response of deepwater Gulf of Mexico salt diapirs is likely to be bracketed by the observed range in constitutive response of the well-characterized Gulf Coast domal salts. We demonstrate with an example how this knowledge can be directly applied in well casing design analyses.