Sustained Casing Pressure: One of the Most Important Factors Determining the Integrity of Deepwater Wells

Abstract

Abstract While drilling and completion of deepwater wells, the cement top is usually several hundred meters below the mudline, which results in a portion of completion fluid entrapped in the technical casing annuli between wellhead and the top of cement. The temperature of the wellbore would be redistributed due to the thermal fluid during well testing or production. Thus, high thermal stress and additional pressure would be induced in the annuli of technical casing, which is proved to be one of the major threats to well integrity. In this paper, a new model predicting the sustained casing pressure (SCP) is established, which is consisted of two sub-models: heat transfer model and sustained casing pressure model. The former model is thoughly considered to provide boundary and initial conditions for the latter model. The influence of wellbore heat transmission in the casing strings and properties of completion fluid in the annulus on the temperature distribution is considered in the heat transfer model. The mechanical model is presented with consideration of the coupling thermal effect of the casing and the fluid. On this basis, a numerical simulation is given to verify the validity of the theoretical model. Good consistency has been shown between the theoretical result and the numerical simulation. If the temperature increment is high enough, the SCP is enough destructive to destroy the casing and subsea wellbore. Analysis results also show that the physical parameters of the fluid in the annuli, the cement top and the annuli volume have significant influence on the SCP. Both the increase rate and the maximum limit of the SCP increase with the cement top. Appropriate increase of the annuli volume is an acceptable way to delay the pressure increase. The temperature and pressure coupling effect need to take special consideration in casing design of deepwater wells. The model in this paper provides more accurate boundary and conditions for SCP analysis. The calculation results have important theoretical and engineering significance, especially for the casing program design and the subsea wellbore integrity evaluation of deepwater wells.