Toolkit and drillstring valve for subsea mudlift drilling

Abstract

With the depleting reservoirs on land and increasing demand for hydrocarbons, the exploration for reservoirs has been moving to deeper and deeper water. The current technology has reached its limit in water depth, and new technology needs to be introduced if exploration is to be continued into even deeper water. One proposed method is subsea mudlift drilling (SMD), which is a joint industry project. The method uses a seafloor pump, which pumps the mud from the annulus at seafloor, through a separate small diameter pipe, such as the choke and kill line, to the rig floor. Because of the expected problems with u-tubing, a drillstring valve (DSV) has been developed to be installed in the drillstring. With new drilling technology comes new well control procedures. The toolkit is a well control tool that is used for almost any well control situation that may arise. The purpose of the research at hand was to develop and present a toolkit for subsea mudlift drilling on the basis of conventional toolkits. The DSV had not previously been modeled, and its response to changes in mudweight, flowrate, water depth etc. has been unknown to this point in time. The toolkit includes a model for the DSV, which can generate plots of pressure drop over the DSV for any changes. The results were examined for several cases, and the pressure response and characteristics of the DSV were mapped. The optimum location for the DSV in the drillpipe also was investigated. It was found that if the DSV were located just above the blow out preventer (BOP), the pressure differential over the DSV would be maximized. Maximizing the pressure difference over and under the flowcone on the DSV will result in a larger opening in the DSV, and thus less pressure will be lost through the DSV choke. If logging tools are to be run in the drillstring, the DSV must be placed in the bottom of the drillstring, near the bit. It is also recommended that all measurement while drilling (MWD) tools be installed between the bit and the DSV, as this will also increase the pressure differential over the DSV flowcone. During conventional drilling a straight-line approximation is often used to predict the drillpipe pressure-decline schedule. This research also established that for low flowrates, with no geometric well deviation, such a method might also be used for SMD. However, using a well control method as that is tailored for subsea mudlift drilling, the flowrate will be relatively high and the straight-line approximation will not be valid. In the case of high flowrates and a deviated well, the drillpipe pressure-decline schedule should always be calculated using the toolkit.