Assessment of Salt Loading on Well Casings

Abstract

Abstract In the next decade significant new oil field developments will be brought on-stream in the deep water Gulf of Mexico. A large number of these wells will be drilled through salt. Assuring the integrity of these wells over the life of the field is a major drilling engineering challenge, as the consequences of well failures may result in billions of dollars remedial costs and lost production. To address these challenges for BP's deepwater developments, advanced numerical finite element analyses of salt / casing interaction were undertaken by Sandia National Laboratories using state-of-the-art computational modeling developed as part of extensive research supporting the Strategic Petroleum Reserve (SPR) and the Waste Isolation Pilot Plant (WIPP) projects. The paper summarizes, briefly, the salient features of salt mechanical behavior of relevance to well integrity. The results of numerical modeling of casing loading are presented which focus on the importance of assessing the possibility of both uniform and non-uniform loading by the salt; and the impact that production-induced heating of the salt has on the rate and magnitude of casing loading. Based on the findings of these analyses, strategies for minimizing the magnitude of loading on well casings are discussed. The relative merits of cementing the annulus through salt are compared with the option of leaving the annulus open in order to defer the time at which salt loading occurs. Introduction The deepwater Gulf of Mexico (GoM) is the most active deepwater region in the world and currently provides some of the greatest challenges in scope and opportunity for the industry. The deepwater GoM is estimated to contain undiscovered recoverable resources of at least ~13 billion boe, and is known to harbor some exceptional reservoirs such as the recent Crazy Horse discovery at over 6,000 feet water depth with estimated recoverable reserves of at least 1 billion boe. However, the complex salt tectonics and extreme water and reservoir depths necessitate very high development costs, in addition to requiring innovative technology to bring these fields on stream. Integral to the successful economic development of these fields (where the cost of a single well can be $20-$60 MM) is a well lifetime of 10 to 20 years. A significant majority of these wells will penetrate potentially considerable thicknesses of salt, 1000 ft to 6000 ft of salt not being uncommon. Therefore, assuring the longevity of well casings drilled through salt is a major requirement in the casing design for these sub-salt developments. Though the behavior of salt from a geologic standpoint is quite well described, our knowledge of the influence of salt deformation on both a well scale, and reservoir scale (both temporal and spatial), is poor. However, the nature of the deformation occurring over field life is considered more likely to be detrimental than beneficial. In sub-salt reservoirs, where the salt is laterally extensive and in close vertical proximity to the reservoir formations there will be a tendency for the salt to flow laterally to fill 'compaction bowl's formed by production from the reservoir interval. This lateral movement of the salt could jeopardize the integrity of well casings drilled through the deforming salt as a consequence of anisotropic loading and induced shears at the bounding formation interfaces. It is important, therefore, that the loading by salt is properly defined and included in the casing design. The consequences of well failures can be very severe. As salt may typically be encountered at relatively shallow depths below mudline in deepwater wells, failure of the well due to salt loading (e.g. collapsed or ruptured casings) may require the re-drilling of the entire well. Unforeseen failures could, in deepwater, make the field development project uneconomic were they to occur.