Potential use of hollow spheres in dual gradient drilling

Abstract

The increasing number of significant deepwater discoveries has pushed the operator and service oil companies to focus their efforts on developing new technologies to drill in deeper water. Dual gradient drilling (DGD) will allow reaching deeper target depths with greater final hole size, which in turn will permit setting larger tubing strings, and hence allowing for higher production rates. DGD can be accomplished by either lifting the mud returns mechanically or diluting the mud returns at the seafloor level by injecting lightweight components. Recently, a novel concept involving the use of hollow spheres in DGD applications has been introduced. In this research, we have evaluated the technical feasibility of using hollow spheres in DGD. We found that hollow spheres have high potential for such an application. They are stable to the drilling fluid additives and components and decrease the density of the drilling mud. The effect on pressure reduction at the seafloor can be significant even when the concentration of spheres injected is smaller than that required to reduce the mud density to seawater density. If the base mud is the carrier fluid, the hollow spheres DGD systems do not require equipment at the seafloor. Additionally, the injection of spheres does not affect the wellbore hydraulics under dynamic conditions. We have identified the constraints for using hollow spheres in DGD. These include particle size of the spheres, collapse of first spheres to be injected at deeper water depths, achieving high concentrations for systems using the mud base as the carrier fluid, and lack of technology to separate the spheres from the mud. In this research, we have developed a friendly, in-house computer program to model features specific to hollow-spheres DGD systems, such as wellbore hydraulics under static and dynamic conditions and the u-tube phenomenon. The results generated by our model match those produced by a field-tested computer program that performs the same task for a similar application. Our findings can be used for further studies of the constraints on the spheres identified in this research, to field test the advantages we predict that hollow spheres will have, and to develop software to fully model hollow-spheres DGD systems.