One of the well construction challenges facing today's deepwater drilling environment is the need for multiple casing strings and drilling liners reaching the reservoir zones at greater depths. This challenges cement placement and fluid displacement for achieving zonal isolation confidence across prospective intervals having narrow pore and fracture pressures margins and higher equivalent circulating densities (ECD).

Dual gradient in this paper refers to one hydrostatic gradient of seawater density that begins at rig floor and a second hydrostatic gradient of mud density that begins at seafloor. Cement placement with dual gradient mode technically follows similar steps and principles as single gradient cementing. However, dual gradient will add a certain complexity to the operations as the result of the following factors:

  • The entire cement placement is on vacuum, i.e. the pumped fluids are continously U-tubing (free falling) due to hydraulic pressure imbalance that exists between the tubing and annulus continuously as such no surface pressure can be observed until the very end of displacement.

  • Darts and plugs shearing indications may not be noticeable through surface pressure signature.

  • Pronounced U-tubing can result in excessive fluid velocity, hence excessive friction and potentially breaking the formation.

  • Downhole anomalies are sometimes only detected from the subsea pump rate and active pit volume changes.

The above considerations must be taken into account under different job configurations and scenarios. Real-time monitoring of job parameters is crucial to understand downhole cement placement and fluid displacement, in order to make quick and informed decisions towards achieving successful cement placement.

A cementing hydraulics simulator is critical to understanding pressures, fluid flow rates and temperature during the cement job. A cementing hydraulics simulator has been developed specifically for dual gradient cementing application. The cementing hydraulics simulator plays a critical role to predict, in addition to cementing hydraulics and temperature, the vacuum length and the subsea pump inlet pressure and rate response throughout cement placement. The simulator can also account for the effect of compressible fluids and internal constrictions on surface and downhole pressures responses.

This paper will briefly explain the dual gradient drilling concept with associated cementing challenges and potential advantages. The cementing hydraulics simulator is capable of computing and displaying dual gradient pressure profile in deepwater environment. This requires understanding U-tubing during pre-job mud circulation and cementing stages, subsea pump inlet pressures and rates responses, Equivalent Circulating Density (ECD) during circulation with different subsea pump inputs setup, optimum circulation rate, bottom-hole pressure and differential pressure at any depth of interest and finally wellbore integrity.

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