Cementation of wells, especially in deepwater has attracted a great deal of attention after recent loss of well control events and subsequent changes in regulatory requirements with respect to achieving effective zonal isolation, preventing the release of wellbore fluids to the environment and avoiding the need for cement job remediation[1, 2].
Deepwater well construction presents unique challenges for proper cementation. These challenges are driven by the greater complexity of deepwater (DW) wells. There is; narrower pore pressure (PP) and fracture gradient (FG) margin, use of oil based mud (OBM) or synthetic based mud (SBM) drilling fluid that is incompatible with water based cement slurries, additional length for the cementing fluids to travel due to the sea water depth increasing the risk of intermixing between fluids, technology pushed to the limit, and the cost of drilling in deepwater that is much higher than conventional wells.
Currently there are two main flow regimes used when planning for effective mud removal and cement placement: effective turbulent flow and effective laminar flow. Turbulent flow is generally considered the preferred method of efficiently removing fluids; however there are a number of requirements to be met in order for turbulent flow to be both safe and effective. Laminar flow is the most widely used due to the ease of placement design and the lower annular velocities required to achieve effective cement placement. Drilling fluids, weighted spacers and cement slurries can all be designed in effective laminar flow, since they exhibit a yield stress. Brines, water, and chemical washes are typically designed in effective turbulent flow.
In this paper the authors will discuss the factors to be taken into account when designing cement jobs using fluids in turbulent flow and some of the overlooked benefits of using turbulent flow design for mud removal. Case histories from DW wells cemented in the Gulf of Mexico region and risk factors will be discussed. It will be demonstrated that when designed and executed properly, the use of turbulent flow mud removal with Newtonian fluids can be beneficial in DW cementing operations.