Influence of Gas Migration on the Sustained Casing Pressure (SCP) of Subsea Wellhead in Deepwater Wells

Abstract

SCP is one of the significant factors affecting the integrity of subsea wellhead in deepwater wells. In this paper, a new model has been established for predicting the SCP in multilayer casings based on the Darcy percolation flow model and the pressure–volume–temperature equation of state, in which the effect of inter-coupling compression of multilayer casing and volume change of annulus and casing has been considered. An iterative calculation computer program is coded for solving the model. On this basis, the influence of the gas migration parameters, the casing program, the physical properties of fluid in the annulus, and the cement permeability on the SCP is discussed, respectively. Analysis results show that the gas migration has a significant influence on the SCP, which indicates that the importance of the cement sealing performance to the integrity of subsea wellhead. In particular, the gas migration would cause the SCP increase at the beginning and then tend to be stable finally. The increase in the cement permeability, fluid density, or cement top can decrease the time when SCP reaches to stability. On the contrary, the high fluid bulk modulus can increase the time. Casing deformation provided additional annular space for gas migration, the calculation results agree well with field data, and the study provides guidance for the casing program design, the cement job, and the prediction and control of the wellbore integrity in deepwater wells.