Assessment of Salt Loading on Well Casings
- S.M. Willson (BP America Inc.) | A.F. Fossum (Sandia Natl. Laboratories) | J.T. Fredrich (Sandia Natl. Laboratories)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2003
- Document Type
- Journal Paper
- 13 - 21
- 2003. Society of Petroleum Engineers
- 1.14.1 Casing Design, 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 5.3.4 Integration of geomechanics in models, 5.1.2 Faults and Fracture Characterisation, 2.4.3 Sand/Solids Control, 1.2.3 Rock properties, 1.5 Drill Bits, 4.3.4 Scale, 1.6.10 Running and Setting Casing, 1.2.2 Geomechanics, 5.3.1 Flow in Porous Media, 4.1.5 Processing Equipment, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 1.14 Casing and Cementing
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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.
The Gulf of Mexico is the most active deepwater region in the world, currently providing some of the greatest challenges in scope and opportunity for the industry. Undiscovered, recoverable resources are estimated to be at least ~13 billion barrels of oil equivalent (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. A well lifetime of 10 to 25 years is integral to successful economic development (where the cost of a single well can be from U.S. $20 to 60 million). A significant majority of the wells will potentially penetrate considerable salt thicknesses, with 1,000 to 6,000 ft of salt not uncommon. Therefore, assuring the longevity of well casings drilled through salt is a major casing-design requirement for these subsalt developments.
Though the behavior of salt is well described from a geologic standpoint, our knowledge of the influence of salt deformation on both a well and reservoir scale (both temporally and spatially) is poor. However, the nature of the deformation occurring during field life is considered more likely to be detrimental than beneficial. In subsalt reservoirs in which the salt is laterally extensive and in close vertical proximity to the reservoir formations, there may be a tendency for the salt to flow laterally to fill "subsidence bowls" formed by compaction of the reservoir interval. This lateral movement could jeopardize the integrity of well casings drilled through the deforming salt because of anisotropic loading and induced shears at the bounding formation interfaces.
It is important, therefore, that loading by salt is properly defined and incorporated in any casing design. Because salt may typically be encountered at relatively shallow depths below mudline in deepwater wells, well failure caused by salt loading (e.g., collapsed or ruptured casings) may require redrilling the entire well. Unforeseen failures can, in deep water, make entire field development projects uneconomical.
Casing Design Adjacent to Salt - Historical Perspective
Within the petrochemical industry and the geomechanics community, previous assessments of salt behavior pertaining to well design have principally fallen into one of four categories, addressed in the following sections.
These investigations are principally concerned with the constitutive behavior of salt deformation without necessarily relating the mechanics to a specific engineering problem; good examples of these approaches include Refs. 1 through 6. The work undertaken in support of the SPR and WIPP projects1-3 is particularly thorough. Of particular relevance to casing design, the work by Cheatham and McEver, 6 though published in 1964, is still a pioneering work. Their experimental work, subjecting copper tubes to various loading conditions and transmitting by blocks of rock salt (from Hockley, Texas), set the ground rules for three subsequent decades of assessing casing design adjacent to salt.
It is not economically feasible to design casing for the most severe situations of nonuniform loading.
When the annulus is completely filled, by cement or salt, casing is subjected to a nearly uniform loading approximately equal to the overburden pressure.
Incorporating Nonuniform Loading Effects Into the Casing Design.
Recognizing that salt has the potential to apply nonuniform loading, several studies have proposed methodologies for including this in the casing design.7-11 Incorporation of nonuniform loading has application beyond salt loading, in tectonically stressed areas, for example.12 The principal thrust of these studies has been to include the effects of a unidirectional load, an opposed line load, or loading by means of a limited contact arc (see Fig. 1). With some modification in parameter values to achieve field-specific calibration, these methodologies have proven to be largely successful. As discussed later, several of the models assume a simplistic (and in some circumstances, a potentially unrealistic) mode of salt deformation that bears little correspondence to present-day knowledge of the mechanics of salt flow around underground openings.
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