Sea Water Drilling Fluid Interphase during Riserless Drilling
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The unique challenges of offshore drilling, unable to be overcome by conventional drilling approaches, have forced the industry to aggressively pursue alternative methods to the “pump and dump” top hole drilling, which simply spills the drilling fluid and cuttings onto the seabed. Drilling topholes using riser is banned on the Norwegian continental shelf due to the risky relation to shallow gas pockets. The most prevalent tophole drilling problem associated with the use of the “pump and dump” approach is the dispelling of drilling fluid and cuttings, which has obvious environmental implications. Heightened focus on the discharge to the marine environment has propelled the demand of a technology to mitigate the seabed disturbance, as authorities have begun to place restrictions on the amount of discharge emitted to sea. With new regulations, the demand for new technologies arises. The development of RMR was impelled by the vision of zero hazardous discharge, and offers environmental benefits by capturing drilling waste at wellhead and pumping it back to the rig for conditioning and reuse. By implementing RMR it is possible to bypass some destructive discharge, but new challenges related to the drilling process may arise. When drilling through potential hazardous zones, the mud cap on the SMO will be open to sea and allow for drilling fluid to breach out. The knowledge of the corals vulnerability of drilling fluid is insufficient. However, the corals in Norwegian waters are assumed to have some resistance to sedimentation due to high current intensity and high particle density in the water. Even though it assumed that the corals are able to re-colonize, inadequate documentation on coral settlement makes it important to limit destructive discharge. As mentioned, the Suction Module of the Riserless Mud Recovery System is open to sea and the different sub-activities of the drilling process will generate drilling fluid discharge. Pulling the drillstring out of the hole was deliberated to be the procedure causing the most mixing in the drilling fluid-sea water interphase and a model was developed for simulation purpose. Different rheology drilling fluids were mixed where the viscosity and density was altered in order to evolve rheology profiles. Baker Hughes provided drilling fluids commonly used during RMR projects. When experiments commenced it was evident that diffusion could be excluded as a mass transfer mechanism disturbing the interphase. Advection was superior and did cause, in some cases, complete contamination of the fluids. The acquired results indicate that density will have the least effect of impeding the advection. The only prerequisite was that there had to be a density difference between the two fluids. A density of 1,1 kg/l turned out to be sufficient as opposed to the first drilling muds tested with densities in the 1,02 kg/l-1,05 kg/l range. When the density requirement was fulfilled, viscosity was the property with most influence of the fluid contamination. The common denominator of the drilling fluid that managed to impede the advection and showed resistance towards mixing was a high plastic viscosity. If the drilling fluid in addition had a high yield point the mixing degree was lowered. The obtained results from the experiments indicate a marshalling sequence of: Plastic Viscosity Yield Point Density RMR is employed when drilling the top hole sections, hence a high yield point of the drilling fluid is not required to be able to lift the cuttings out of the annulus. However, when drilling in environmentally vulnerable areas the operator should consider raising the viscosity and yield point in order to strengthen the resistance towards mixing. The apparent results regarding duration of the tripping procedure, indicated that time have little effect on the fluid contamination. The mass transfer reached its peak and the mixing degree seemed to stay constant as time went on. The velocity of the tripping did have a greater influence on the mixing degree than duration. It was found that the high density mud with low viscosity and yield point, especially, will profit on the environmental account by slowing down the tripping velocity. However, reducing the speed increases the time consumption and increases the overall cost of the project. In most areas the drilling fluids will be allowed emitted to the sea, however RMR may be adapted in environmental vulnerable areas where the discharge regulations is more stringent.