Reverse circulation cement placement is the technique when cement slurries are pumped down the annulus and up the casing, as opposed to conventional primary cementing where fluids are pumped down the casing. Reverse circulation can reduce bottom hole pressures compared to conventional cementing, making it particularly attractive for cementing zones where margins to the fracture pressure are small. Since the fluids are not mechanically separated in the annulus, density and viscosity hierarchies need to be carefully designed to minimize mixing and slurry contamination. We investigate the effect of variations in density and viscosity on the displacement efficiency by means of computational fluid dynamics to improve the design of a successful reverse circulation cementing operation.
The simulations are performed using an open-source computational fluid dynamics software, enabling a parameter study of the effect of flow rate, inclination, standoff and fluid parameters such as density and viscosity on the displacement process. We compare the reversed circulation displacement efficiency and the hydraulic pressure in the annulus to corresponding conventional primary cementing operations.
The displacement flows involve complex non-Newtonian viscosities in eccentric annuli, and the flow is typically fully three-dimensional. The efficiency and quality of the fluid-fluid displacement is governed by the hierarchy of fluid properties between the displaced and displacing fluids for both conventional and reverse circulation cementing. Furthermore, it is shown how flow rate and geometric constraints such as inclination and standoff affect the efficiency.
Previous work has focused primarily on hydraulic pressure and downhole temperature calculation. We investigate the effect of fluid hierarchies on cement contamination during reverse circulation cementing. The combination of fluid hierarchies and flow rate need to be carefully designed to avoid cement contamination while maintaining low bottom hole pressures during reverse circulation.
A transient flow model capable of modeling gas solubility will be used to perform a sensitivity analysis of kick behavior in a subsea backpressure MPD system when using oil based mud. The parameters of interest are choke pressures, pit volume, and return rates. At HPHT conditions, gas kicks can be entirely dissolved in oil based mud. However, when being circulated upwards, free gas will emerge at a certain depth. The required choke pressure to maintain a constant bottomhole pressure depends on the amount of gas released from the mud and where this occurs. Another parameter that impacts both choke pressure and return flow is the geometry, whether we have a wide riser for subsea MPD or are considering an MPD operation from a fixed installation with a narrower geometry. In this paper, the riser geometry will be varied. The paper will contribute in showing how transient models can assist in the planning of MPD operations. It will also provide insight into influx behavior and its impact on surface parameters with focus on oil based mud.