Casing deformation in the Cooper Basin.
Engineering Honours Degree 2008
University of Adelaide
The project objectives were: to establish the cause of casing deformation, research casing deformation mitigation methods and identify deformation prediction methods. The consequence of casing deformation is the inability for full-bore work over tools e.g. high-pressure casing patches to gain access to the wellbore.
A review of historical data was performed and showed that since January 2007 approximately half of wells that were fracture stimulated are suspected or confirmed to have casing deformation. Case studies were performed on wells that calipers had confirmed deformation in. Analysis showed that there is a correlation between casing deformation and coals particularly the VC-50 or coal-sand interfaces with proximity of fracture stimulation stages having a significant effect. A cost analysis was performed showing the initial cost due to casing deformation in Fly Lake 21 equated to $719,000. Pressure differential was eliminated as the cause for the deformation.
A Finite Elements Analysis (FEA) was performed by Stress Engineering Services to determine the force required to cause the observed deformation. Casing deformation mitigation methods such as increasing casing size and weight were investigated. The results of the FEA were inconclusive in this area as the actual force supplied was undeterminable from FEA methods. Approximately twice the force is required to deform 5 inch 24.1 lb/ft blast joints compared to 4.5 inch 13.5 lb/ft casing. At high ovalities the deformation force required for 4.5 inch 15.1 lb/ft was equivalent to that of 13.5 lb/ft casing. Decision and Risk Analysis (DRA) should be utilised where possible to financially justify fracture stimulation.
Fracture stimulation is the likely cause of rock shear and the resulting deformation with proximity to high-risk lithology a significant factor. Pressure differential is not the cause. No predictive methods were defined due to multiple contributing factors and limited data points to establish regional correlations.
Recommendations include a trial of 5 inch 24.1 lb/ft blast joints. A DRA should be performed when fracturing within 50 ft of the VC-50 coal to justify the decision. Further research is necessary into the use of more-ductile cements, their ability to withstand deformation and the incremental cost. Further data collection should take place on future wells, with these wells added to the regional map. This information should be used to apply the DRA to a larger range of zones.