Dual-gradient drilling using nitrogen injection

Abstract

Methods and apparatus for drilling subsea wells in deep water using dual-gradient drilling techniques. One preferred embodiment includes a high pressure riser running from a drilling platform at the surface to the seafloor. The base of the riser is connected to a wellhead that is anchored to the seafloor. Pressure control equipment is disposed on the upper end of the riser at the drilling platform. A coiled tubing drill string is run through the riser and into the subsea formation. The drill string preferably includes a pressure sensing device that can be used in transmitting real-time downhole pressure data to the surface. A riser injection system is provided to inject a lower density fluid into the riser annulus in order to reduce the density of the fluids in the riser annulus and therefore reduce the hydrostatic pressure within the wellbore.

What is claimed is:  
     
         1 . A system for drilling a well from a platform at the sea surface comprising: 
 a riser providing a conduit between the platform and the well;    a coiled tubing drill string extending from the platform, through said riser, and into the well, thereby forming an annulus between the drill string and the well or riser;    a fluid circulation system adapted to circulate a first fluid from the surface, down the well through the drill string and back to the surface through the annulus;    a fluid injection system adapted to inject a second fluid into the annulus so as to lower the density of the fluid in the annulus; and    a pressure sensing device adapted to provide pressure data from the bottom of the well to a control station at the surface, wherein fluid injected into said riser through said fluid injection system is used to control the pressure at the bottom of the well.    
     
     
         2 . The system of  claim 1  further comprising pressure control equipment disposed on the platform wherein said riser is a high-pressure riser adapted to contain the full pressure of the well.  
     
     
         3 . The system of  claim 2  wherein said pressure control equipment comprises a choke to regulate the pressure within the riser.  
     
     
         4 . The system of  claim 1  wherein the injected fluid is a gas.  
     
     
         5 . The system of  claim 1  wherein the injected fluid is nitrogen.  
     
     
         6 . The system of  claim 1  further comprising a bottom hole assembly disposed on the end of said coiled tubing drill string.  
     
     
         7 . The system of  claim 6  wherein said pressure sensing device is disposed on said bottom hole assembly.  
     
     
         8 . The system of  claim 1  wherein the pressure data sent from said pressure sensing device to the surface is transmitted by electrical signals along said coiled tubing string.  
     
     
         9 . The system of  claim 8  wherein said coiled tubing string is constructed from a composite material.  
     
     
         10 . The system of  claim 1  wherein the pressure data is provided in real-time from said pressure sensing device.  
     
     
         11 . A method for controlling the pressure developed in a subsea well comprising: 
 extending a drill string from a surface platform, through a riser, and into a subsea well;    circulating a drilling fluid from the surface platform into the well through the drill string and back to the surface platform through the annulus between the riser or well and the drill string;    determining the pressure in the well using a pressure sensing device disposed on the drill string; and    injecting a fluid into the annulus to reduce the pressure in the annulus.    
     
     
         12 . The method of  claim 11  further comprising maintaining a back pressure on the riser annulus with pressure control equipment located at the surface platform, wherein the riser is a high-pressure riser adapted to contain the full pressure of the well.  
     
     
         13 . The method of  claim 12  wherein said pressure control equipment comprises a choke used to regulate the pressure within said riser.  
     
     
         14 . The method of  claim 11  wherein the injected fluid is a gas.  
     
     
         15 . The method of  claim 11  wherein the injected fluid is nitrogen.  
     
     
         16 . The method of  claim 11  wherein the pressure sensing device is disposed on a bottom hole assembly supported by the drill string.  
     
     
         17 . The method of  claim 11  wherein the pressure sensing device transmits pressure data to the surface by sending electrical signals along said coiled tubing string.  
     
     
         18 . The method of  claim 17  wherein said coiled tubing string is constructed from a composite material.  
     
     
         19 . The method of  claim 11  wherein the pressure is provided in real-time from said pressure sensing device.  
     
     
         20 . A system for controlling pressure within a subsea wellbore connected to the surface by a riser, the system comprising: 
 a drill string disposed within the wellbore;    an annulus formed between said drillstring and the wellbore and the riser;    a first fluid circulated through said drill string and said annulus;    a second fluid injected into said annulus;    a means for determining the mass flow rate of the first and second fluids into the wellbore;    a means for determining the mass flow rate of the first and second fluids out of the wellbore; and    a means for comparing the mass flow rates of the first and second fluid into and out of the wellbore.    
     
     
         21 . The system of  claim 20  further comprising a pumping system adapted to pump said first fluid through said drill string into the wellbore.  
     
     
         22 . The system of  claim 21  further comprising an injection system adapted to inject said second fluid into said annulus.  
     
     
         23 . The system of  claim 22  further comprising a control system adapted to adjust said pumping system in response to a difference in the mass flow rates into and out of the wellbore.  
     
     
         24 . The system of  claim 22  further comprising a control system adapted to adjust said injection system in response to a difference in the mass flow rates into and out of the wellbore.  
     
     
         25 . A method for controlling wellbore pressure in a well comprising: 
 pumping a first fluid through a drill string through a riser into a wellbore;    injecting a second fluid into an annulus between the drill string and the riser and the wellbore;    determining the mass flow rate of the first and second fluids flowing into the wellbore;    determining the mass flow rate of the first and second fluids flowing out of the wellbore; and    comparing the mass flow rate of the first and second fluids flowing into the wellbore to the mass flow rates of the first and second fluids flowing out of the wellbore.    
     
     
         26 . The method of  claim 25  further comprising adjusting the pumping of the first fluid in response to a difference between the mass flow rates of fluid into and out of the wellbore.  
     
     
         27 . The method of  claim 25  further comprising adjusting the injection of the second fluid in response to a difference between the mass flow rates of fluid into and out of the wellbore.