Reservoir compaction, shear impact on casing under study
One of the more dramatic instances of reservoir compaction has been the extensive casing damage resulting from chalk shrinkage and overburden settlement in the Ekofisk Field in the North Sea. The wellbore impact there has been primarily axial, resulting in buckling and collapse of the casing. However, there are other fields in the North Sea, the US Gulf of Mexico, and in California where formation shear and reservoir compaction have produced other types of casing problems. Among the dominant casing failure mechanisms are:
- Transverse shear loading: Transverse loading applied to vertical and horizontal wells will displace a section or the entire wellbore, stretching or shearing the casing in the area of the displacement.
- Radial compression loading: Wells aligned to the shear plane or approaching the zone at a low angle can experience casing ovaling when the loading is tangential or radial.
- Formation compaction loading: Axial compression can cause buckling in vertical wells (Ekofisk) and ovaling or collapse in horizontal wells. As more horizontal wells are drilled, formation and overburden collapse will dramatize this type of failure.
Where reservoir compaction conditions are thought to be potentially troublesome, engineers can take steps to angle the well or better protect the casing. The problem is that other than company-specific programs to prevent damage, no widespread effort to catalog troublesome formations and develop solutions has been undertaken. The Drilling Engineering Association hopes to remedy this with a study (DEA-99), to be undertaken by Terralog Technologies (Arcadia, California) and Stress Engineering (Houston), and sponsored by Chevron. Dr. Mike Bruno of Terralog has estimated that companies have already spent several hundred million dollars over the past 10 years fixing the problems created by collapsing formations. Terralog is looking for additional sponsors for the study.
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