A novel mitigation on deepwater annular pressure buildup: Unidirectional control strategy

Abstract

When completing deepwater wells, a significant portion of the completion fluid is often trapped in the casing annuli because of widespread use of multi-layer casing technology. When transferred into testing or production, the temperature field in the immediate vicinity of the wellbore is redistributed by any present thermally conductive fluids. Raising trapped completion fluid to a high temperature can trigger high thermal stress, and this stress has been proven to present a grave threat to the safety of deepwater wells. In this paper, a thermal conductivity model of fluid-filled annuli based on a typical well structure used in the South China Sea is established to determine the effect of different production parameters during the well production or testing process on pressure behavior. A numerical simulation of the effects of fluid trapped in the casing annuli on temperature distribution was conducted. Based on this simulation, a novel method for mitigating the pressure of the trapped fluid by a way of a unidirectional control strategy was studied, and a unidirectional pressure control casing nipple tool was developed. Laboratory simulations were then conducted using a prototype tool to determine the technical feasibility of the unidirectional control strategy. The change in trapped fluid pressure with time was obtained for different release pressures and tool valve orifice diameters, showing the effects of different valve orifice diameters and volumetric injection rates on the pressure release performance. Experimental results demonstrate that the pressure envelope decreases quadratically with the increase in the pressure grade, while the valve working duration decreases linearly with the increase in valve orifice diameter. The increase in the volumetric injection rate was found to have little effect on the cumulative volume displacement. The proposed unidirectional pressure control strategy for controlling the accumulation of pressure in the annuli of deepwater wellbore casings can effectively eliminate the fatal dangers presented by thermal stress. It can also reduce the risk of casing deformation due to thermal stress within trapped annuli and reduce occurrences of wellhead sealing damage.