Casing collapse and fracture may be caused by the rise of casing annulus temperature and pressure in the initial stage of production and testing of deepwater wells. Based on the casing program and wellbore heat transfer process, a pressure calculation model for casing annulus was established, which provides basis for annulus pressure control.
Fluid trapped in the annulus of subsea wells can cause casing strings to fail. This condition occurs when casing annuli attain a closed-volume circumstance (when a well is cased, cemented, and head seals are set). During production, the heat transfer of the produced fluids to the casing strings causes the trapped fluid to increase in pressure. This condition is magnified in deepwater because annular fluids are cooler due to the cold deepwater environment.
With the annulus between testing string and production casing as an example, a casing annulus temperature calculation method was derived according to the principle of conservation of energy and wellbore heat transfer analysis. The casing annulus pressure prediction model was established for typical deepwater wells according to the volume change of casing annulus fluid with temperature and pressure, and in combination of the casing annulus PVT equation and annulus temperature equation.
The casing annulus temperature and pressure calculation methods were successfully applied in the casing annulus temperature and pressure management operation in deepwater wells in West Africa, and the predicted temperature and pressure were compared with the actual ones, which indicates that casing annulus temperature and pressure prediction methods have good accuracy.