Sustained casing pressure (SCP), exhibited by many wells, is defined as any measurable casing pressure that rebuilds after being bled down, attributable to cause(s) other than artificially applied pressures or temperature fluctuations in the well. Gas leakage, leading to SCP, may occur through the poor cement bond between the casing and the formation, packer, and/or the casing itself. All safety regulators require SCP elimination. However, test-data analysis is mostly qualitative and limited to arbitrary criteria, thereby lacking consensus. This paper attempts to provide a theoretical frame and a model for quantitative analysis of SCP-test data.
Specifically, we adapted a model for SCP that is rooted in the transport processes of the system. The model accounts for mud compressibility and assumes gas is leaking into the annulus and migrating up the annular liquid column. Some robust assumptions about the transport processes allow for expressing the governing equation as a first-order, linear differential equation that is solved with appropriate boundary conditions. The resulting algebraic expression for the casing-pressure rise (Annulus A) with time is easy to apply. A comparison of the performance of the model with field data suggests that gas influx causes a casing-pressure (Annulus A) increase in wells.