Subsea Mudlift Drilling: evaluation of the pressure differential problem with subsea pump

Abstract

The petroleum industry is trying to develop new and improved technology to safely, successfully and profitably extract hydrocarbons in deep water. One such technology under development is subsea mudlift drilling (SMD), a joint industry project with nine companies participating. In subsea mudlift drilling the mud returns do not go through the drilling riser as in conventional drilling, but instead they go through one or more smaller diameter return lines back to the surface. This is made possible by the use of a set of subsea pumps located on the ocean floor that will take suction from the annulus side of the wellbore. The subsea pump acts as a choke that can be "opened" or "closed" by adjusting the pump rate in order to keep the bottom hole pressure constant. The pump is designed to do this as long as the outlet pressure does not fall more than 500 psi below the inlet pressure. If the outlet pressure falls more than 500 psi below the inlet pressure, the pump will not be able to handle this differential and the fluid will flow through the pump due to a pressure reversal. This would result in a loss of bottom hole pressure. The purpose of this thesis is to investigate this pressure differential problem to determine the operating conditions under which a pressure reversal will occur, and to see for what kick scenarios this becomes a problem. One way to counteract the pressure reversal is to apply backpressure at the surface choke. The effect of applying various levels of backpressure is investigated in this thesis. Possible complications resulting from this remedy, such as increased flow rates of gas from the well during well control operations, are also evaluated. A major conclusion from this study is that the solution of applying backpressure through a surface choke is a valid and functional solution. This thesis also explains the effect of various variables on u-tubing rate and mud level drop in subsea mudlift drilling. The major conclusion here is that the driving forces behind the u-tube are water depth and mud weight.