Analytical critical drawdown (CDD) failure model for real time sanding potential prediction based on Hoek and Brown failure criterion

Abstract

The risks of failure in sand reservoirs and consequent sand production is now a stark reality in the upstream oil and gas industry. As a result, failure analysis of reservoir rocks for sanding potential prediction purposes has become a routine activity more than ever before. Owing to the huge economic, operational and safety implications of risks of sand failure, the efficient management of these risks for field operation optimization requires a reliable failure model, which can capture the failure processes adequately in real time. Mohr coulomb failure criterion has been more widely applied for rock mechanics problems relating to sand failure analysis and production in the oil industry and elsewhere, and has been used as the basis or platform for more than 80% of the failure models being used in the industry today for rock failure analysis and sanding potential prediction. The major reasons for this could be attributed to: (a) Simplicity in understanding and use and (b) Description by a simple mathematical expression. The mathematical expression of Mohr Coulomb criterion defines shear stress as a linear function of the normal stress, which is depictive of a linear failure envelope. In addition Mohr Coulomb is only applicable to intact rocks and cannot be applied to already failed rock. Failure envelope in petroleum formation rock has however been proved to be non-linear and as such Mohr Coulomb failure criterion and the models based on it cannot be trusted to capture the failure processes adequately and reliably. In this study, Hoek and Brown failure criterion has been used as a platform to develop a new time-coupled analytical failure model for the analysis of sanding potential prediction in real time. The basis for using the Hoek and Brown failure criterion lies in its ability to capture rock failure as a non-linear process and applicability to both intact and failed rocks. This model has been tested and validated on some field data; in addition, it has been compared with another Mohr Coulomb-based drawdown failure model. The results obtained from the testing and validation scheme are very encouraging and show that Hoek and Brown criterion can indeed help overcome the inherent problems in Mohr Coulomb criterion Key words: Rock failure, sand prediction, failure criteria, critical drawdown, failure envelope, uniaxial compressive strength (UCS).