Assessment on the sustained casing pressure (SCP) with consideration of the infiltration of formation fluid into the annulus of deepwater wells

The sustained casing pressure (SCP) is a common problem in deepwater wells, which is caused by the coupling effects of temperature and pressure and brings about significant challenges to the wellbore integrity. In this paper, the analysis model of temperature distribution along the wellbore has been presented firstly based on the casing program and the basic principle of heat transfer, which is adopted as the boundary condition and initial condition of the SCP model. In this model, the annular spaces of the oil tubing-production casing, production casing-intermediate casing, intermediate casing-surface casing are named as annulus A, annulus B and annulus C, respectively. Secondly, the mechanical model and governing equation of the SCP have been established based on the thermal elastic mechanics with consideration of the formation fluid infiltration into the annulus. Then, the SCP and its maximum allowable value have been figured out by solving the governing equation through the iteration calculation approach. At last, a SCP relief tool has been proposed to improve the stress state of casing string and its performance has been evaluated. On this basis, the parameter sensitivity analysis has been carried out, where the influence of yield, production time, the comprehensive permeability of cement sheath, physical parameters of fluid in the annuli and the formation fluid infiltration on the SCP are discussed in detail. Analysis results show that the ranking order of temperature variation is annulus A, annulus B, annulus C, while the ranking order of the SCP is annulus B, annulus C, and annulus A. The SCP at wellhead increases approximately linearly with the yield and the thermal expansion coefficient of the fluid in annulus. At the initial stage of production, the well temperature and SCP gradually increase with the time, and then remain constant with the production going on. However, the time of the SCP reaching stable decreases with the comprehensive permeability of the cement sheath. In addition, the maximum allowable value of the SCP is mainly restricted by the collapse resistance of the production casing, which occurs under the circumstance of annulus A is tunneled. The relief tool can significantly reduce the SCP in annulus B and C with about 30% of the reduction rate, which plays a positive role in improving the mechanical state of the casing. This paper has reference significance for prediction of the SCP and evaluation of the well integrity of deepwater wells.