Mud lift drilling system using ejector assembly in mud return line

Abstract

A method is described for controlling pressure in a wellbore drilled below the bottom of a body of water wherein the wellbore has a fluid outlet in communication with a working fluid inlet of an ejector assembly to return drilling fluid to a drilling platform on the water surface. The method includes pumping drilling fluid into a drill string extending from the drilling platform into the wellbore and at least one of, (i) introducing fluid into a power fluid inlet of the ejector assembly at a rate selected to remove fluid from the wellbore fluid outlet at a selected rate and (ii) operating a controllable flow restriction in a flow path from the wellbore fluid outlet to the working fluid inlet of the ejector assembly, in order to maintain a selected wellbore pressure.

What is claimed is: 
     
         1 . A method for controlling pressure in a wellbore drilled below the bottom of a body of water, the wellbore having a fluid outlet in fluid communication with a working fluid inlet of an ejector assembly to return drilling fluid to a drilling platform on the water surface, the method comprising:
 pumping drilling fluid into a drill string extending from the drilling platform into the wellbore; and   at least one of, (i) introducing fluid into a power fluid inlet of the ejector assembly at a rate selected to remove fluid from the wellbore at a selected rate and (ii) operating a controllable flow restriction in a fluid path from the wellbore to the working fluid inlet of the ejector assembly, to maintain a selected wellbore pressure.   
     
     
         2 . The method of  claim 1  wherein the maintaining the selected wellbore pressure comprises maintaining a fluid level in a riser extending from a wellhead disposed on the bottom of the body of water to a drilling platform above the water surface. 
     
     
         3 . The method of  claim 1  further comprising removing the ejector assembly from a fluid return line extending from the drilling platform using at least one of a wireline, slickline, coiled tubing and a semi-stiff spoolable intervention rod extended into the fluid return line. 
     
     
         4 . The method of  claim 1  further comprising sealing an annular space between the drill string and an interior of a drilling riser extending from a wellhead proximate the bottom of the body of water to proximate the drilling platform. 
     
     
         5 . A wellbore drilling system comprising:
 a drill string extending into the wellbore from a drilling platform at a surface of a body of water;   a fluid pump coupled to an interior passage in the drill string;   a fluid outlet connected to the wellbore, the fluid outlet in fluid communication with a working fluid inlet of an ejector assembly, an outlet of the ejector assembly in fluid communication with a fluid return line extending to the drilling platform;   a pressure sensor in pressure communication with an interior of the wellbore; and   means for pumping a power fluid into an inlet therefor in the ejector assembly, the means for pumping controllable in response to signals from the pressure sensor to enable maintaining a selected wellbore pressure.   
     
     
         6 . The wellbore drilling system of  claim 5  further comprising a variable flow restriction between the fluid outlet and the ejector assembly. 
     
     
         7 . The wellbore drilling system of  claim 5  further comprising a riser extending from the bottom of the body of water to the drilling platform and wherein the pressure sensor is in fluid communication with an interior of the riser. 
     
     
         8 . The wellbore drilling system of  claim 5  wherein the fluid return line is substantially vertical and the ejector assembly is disposed in the fluid return line such that the ejector assembly is removable from the fluid return line by at least one of a wireline, slickline, coiled tubing and a semi-stiff spoolable intervention rod extended into the fluid return line. 
     
     
         9 . The wellbore drilling system of  claim 5  further comprising a seal disposed in an annular space between the drill string and an interior of a drilling riser extending from a wellhead proximate a bottom of a body of water to proximate the drilling platform.