Multiphase drilling systems and methods

Abstract

In one embodiment, a method for drilling a wellbore includes injecting drilling fluid through a drill string disposed in the wellbore and rotating a drill bit disposed on a bottom of the drill string. The drilling fluid includes a liquid and a gas. The drilling fluid is injected at the surface. The drilling fluid exits the drill bit and carries cuttings from the drill bit. The drilling fluid and cuttings (returns) flow to the surface via an annulus formed between the drill string and the wellbore. The liquid is injected at a rate so that a liquid velocity of the returns in the annulus is sufficient to transport the cuttings. The method further includes drilling through at least a portion of a non-productive formation.

1 . A method for drilling a wellbore, comprising:
 injecting drilling fluid through a drill string disposed in the wellbore;   rotating a drill bit disposed at a bottom of the drill string, wherein:
 the drilling fluid comprises a liquid and a gas; 
 the drilling fluid is injected at the surface; 
 the drilling fluid exits the drill bit and carries cuttings from the drill bit; 
 the drilling fluid and cuttings (returns) flow to the surface via an annulus formed between the drill string and the wellbore; and 
 the liquid is injected at a rate so that a liquid velocity of the returns in the annulus is sufficient to transport the cuttings; and 
   drilling through at least a portion of a non-productive formation with the drill bit.   
     
     
         2 . The method of  claim 1 , wherein an injection rate of the gas is controlled so that an equivalent circulating density (ECD) of the drilling fluid is less than a pore equivalent mud density (EMD) of the formation and greater than a stability EMD of the formation. 
     
     
         3 . The method of  claim 1 , wherein an equivalent circulating density (ECD) of the drilling fluid is substantially equal to a stability equivalent mud density (EMD) of the formation. 
     
     
         4 . The method of  claim 1 , wherein an injection rate of the gas is controlled so that an equivalent circulating density (ECD) of the drilling fluid is substantially less than a pore equivalent mud density (EMD) of the formation. 
     
     
         5 . The method of  claim 4 , wherein the ECD is less than or equal to two-thirds of the pore EMD. 
     
     
         6 . The method of  claim 4 , wherein the ECD is less than or equal to one-half of the pore EMD. 
     
     
         7 . The method of  claim 1 , wherein an injection rate of the gas is controlled to maximize rate of penetration. 
     
     
         8 . The method of  claim 1 , wherein a liquid volume fraction of the drilling fluid at standard temperature and pressure is less than or equal to 0.07 and greater than or equal to 0.01. 
     
     
         9 . The method of  claim 1 , wherein the drilling fluid is hydrostatically dominated. 
     
     
         10 . The method of  claim 1 , wherein the liquid is a base oil. 
     
     
         11 . The method of  claim 10 , wherein the drilling fluid further comprises water emulsified in the base oil. 
     
     
         12 . The method of  claim 11 , wherein the drilling fluid further comprises organophillic clay. 
     
     
         13 . The method of  claim 11 , wherein the drilling fluid further comprises a metal halide. 
     
     
         14 . The method of  claim 11 , wherein the drilling fluid further comprises a polymer prill. 
     
     
         15 . The method of  claim 1 , wherein the gas has an oxygen concentration less than the oxygen concentration sufficient for combustion. 
     
     
         16 . The method of  claim 15 , wherein the gas is substantially pure nitrogen. 
     
     
         17 . The method of  claim 16 , further comprising generating the nitrogen at the surface using air. 
     
     
         18 . The method of  claim 1 , wherein the drill string comprises a mud motor and wherein the drill bit is rotated by the mud motor. 
     
     
         19 . The method of  claim 1 , wherein a bottom of the wellbore is at a depth distal from a productive formation. 
     
     
         20 . The method of  claim 1 , wherein a rotating control device (RCD) engages an outer surface of the drill string at the surface and wherein the RCD diverts the returns from the annulus to an outlet line. 
     
     
         21 . The method of  claim 20 , wherein a separator is in fluid communication with the outlet line and wherein the separator separates gas from the returns. 
     
     
         22 . The method of  claim 21 , further comprising:
 flaring the separated gas; and   processing remaining returns using a solids shaker.   
     
     
         23 . The method of  claim 22 , wherein a variable choke is in fluid communication with the outlet line. 
     
     
         24 . The method of  claim 21 , further comprising recycling a portion of the separated gas. 
     
     
         25 . A method for drilling a wellbore, comprising:
 injecting drilling fluid through a drill string disposed in the wellbore;   rotating a drill bit disposed on a bottom of the drill string, wherein:
 the drilling fluid comprises a liquid and a gas; 
 the drilling fluid is injected at the surface; 
 the drilling fluid exits the drill bit and carries cuttings from the drill bit; 
 the drilling fluid and cuttings (returns) flow to the surface via an annulus formed between the drill string and the wellbore; and 
 a liquid volume fraction of the drilling fluid at standard temperature and pressure is greater than or equal to 0.01; and 
   drilling through at least a portion of a non-productive formation with the drill bit.   
     
     
         26 . The method of  claim 25 , wherein an injection rate of the gas is controlled so that a velocity of the drilling fluid is equal to or greater than a slip velocity of the cuttings.