Prediction of the maximum allowable bottom hole pressure in CO2 injection wells

Abstract

Mechanical failure of well cement sheath can result in loss of zonal isolation, which is the cause of many problems, such as sustainable casing pressure, crossflows between formation zones, and leakage of fluid to surface. Pressure- and temperature-induced stresses in the cement sheath have been numerically analyzed by a number of previous investigators with sophisticated computer models. However, these investigations do not provide a practical method for predicting the maximum permissible pressure limited by the mechanical failure of cement sheath.An analytical model was derived in this work to predict the Maximum Permissible Pressure (MaxPP) in fluid injection and well stimulation. Results given by the model were verified with data from a CO2 sequestration operation. Case studies with the model suggest that if the cement placement efficiency in the annulus is 100% as detected by CBL, formation in-situ stress should be utilized to make a conservative prediction of the MaxPP. If the cement placement efficiency in the annulus is less than 100% detected by CBL, the outer radius of the cement sheath should be considered and formation pore pressure in the cap rock should be employed to make a conservative prediction of the MaxPP. Improving cement strength is not an effective means of increasing MaxPP. The MaxPP should be enhanced by maximizing the outer radius of the cement sheath through improving the cement placement efficiency. The Maximum Permissible Net Pressure is lower than the burst-pressure rating of casing, suggesting that cement sheath failure will occur before casing failure occurs in the burst condition. Therefore burst design should be carried out for cement sheath, not production casing. This paper provides engineers a practical guideline to design and apply fluid injection pressure in well stimulation and operations.