Managed pressure drilling system having well control mode

Abstract

A method of drilling a subsea wellbore includes drilling the subsea wellbore and, while drilling the subsea wellbore: measuring a flow rate of the drilling fluid injected into a tubular string; measuring a flow rate of returns; comparing the returns flow rate to the drilling fluid flow rate to detect a kick by a formation being drilled; and exerting backpressure on the returns using a first variable choke valve. The method further includes, in response to detecting the kick: closing a blowout preventer of a subsea pressure control assembly (PCA) against the tubular string; and diverting the flow of returns from the PCA, through a choke line having a second variable choke valve, and through the first variable choke valve.

1 . A method of drilling a subsea wellbore, comprising:
 drilling the subsea wellbore by:
 injecting drilling fluid through a tubular string extending into the wellbore from an offshore drilling unit (ODU); and 
 rotating a drill bit disposed on a bottom of the tubular string, wherein: 
 the drilling fluid exits the drill bit and carries cuttings from the drill bit, 
 the drilling fluid and cuttings (returns) flow to a subsea wellhead via an annulus defined by an outer surface of the tubular string and an inner surface of the wellbore, and 
 the returns flow from the subsea wellhead to the ODU via a marine riser; 
   while drilling the subsea wellbore:
 measuring a flow rate of the drilling fluid injected into the tubular string; 
 measuring a flow rate of the returns; 
 comparing the returns flow rate to the drilling fluid flow rate to detect a kick by a formation being drilled; and 
 exerting backpressure on the returns using a first variable choke valve; and 
   in response to detecting the kick:
 closing a blowout preventer of a subsea pressure control assembly (PCA) against the tubular string; and 
 diverting the flow of returns from the PCA, through a choke line having a second variable choke valve, and through the first variable choke valve. 
   
     
     
         2 . The method of  claim 1 , further comprising, in response to detecting the kick, exerting backpressure on the returns using the first and second variable choke valves to alleviate pressure on the first variable choke valve. 
     
     
         3 . The method of  claim 2 , further comprising measuring the flow rate of the returns while exerting backpressure using the first and second variable choke valves. 
     
     
         4 . The method of  claim 1 ,
 further comprising increasing the backpressure exerted on the returns in response to detecting the kick,   wherein the backpressure is increased until the kick is controlled.   
     
     
         5 . The method of  claim 4 , further comprising:
 determining a pore pressure of the formation in response to controlling the kick;   determining a pore pressure gradient using the pore pressure; and   increasing a density of the drilling fluid to correspond to the pore pressure gradient.   
     
     
         6 . The method of  claim 5 , further comprising resuming drilling using the increased density drilling fluid. 
     
     
         7 . The method of  claim 1 , further comprising degassing the marine riser. 
     
     
         8 . The method of  claim 7 , further comprising operating a gas detector in fluid communication with the returns during drilling and in response to detecting the kick. 
     
     
         9 . The method of  claim 1 , wherein during drilling, the returns are diverted from the marine riser and through the first variable choke valve using a rotating control device located adjacent to an upper end of the marine riser. 
     
     
         10 . The method of  claim 1 , wherein the returns flow rate is measured using a mass flow meter. 
     
     
         11 . The method of  claim 10 , wherein:
 the mass flow meter is part of the PCA, and   the PCA is connected to the subsea wellhead.   
     
     
         12 . The method of  claim 11 , wherein the returns are diverted from the PCA and through the mass flow meter by a rotating control device of the PCA. 
     
     
         13 . A managed pressure drilling system, comprising:
 a first rotating control device (RCD) for connection to a marine riser;   a first variable choke valve for connection to an outlet of the first RCD;   a first mass flow meter for connection to an outlet of the first variable choke valve;   a splice for connecting an inlet of the first variable choke valve to an outlet of a second variable choke valve; and   a programmable logic controller (PLC) in communication with the first variable choke valve and the first mass flow meter, and configured to perform an operation, comprising:
 during drilling of a subsea wellbore:
 measuring a flow rate of returns using the first mass flow meter; 
 comparing the returns flow rate to a drilling fluid flow rate to detect a kick by a formation being drilled; and 
 exerting backpressure on the returns using the first variable choke valve; and 
 
 in response to detecting the kick, diverting the returns through the second variable choke valve, the splice, and the first variable choke valve to alleviate pressure on the first variable choke valve. 
   
     
     
         14 . The managed pressure drilling system of  claim 13 , wherein:
 the operation further comprises increasing the backpressure exerted on the returns in response to detecting the kick, and   the backpressure is increased until the kick is controlled.   
     
     
         15 . The managed pressure drilling system of  claim 14  wherein the operation further comprises:
 determining a pore pressure of the formation in response to controlling the kick; and 
 determining a pore pressure gradient using the pore pressure. 
 
     
     
         16 . The managed pressure drilling system of  claim 13 , further comprising:
 a second RCD for assembly as part of a subsea pressure control assembly; and   a subsea mass flow meter for connection to an outlet of the second RCD.   
     
     
         17 . The managed pressure drilling system of  claim 13 , further comprising a gas detector for connection to an outlet of the first mass flow meter. 
     
     
         18 . The managed pressure drilling system of  claim 17 , wherein the operation further comprises:
 monitoring the returns for gas during drilling; and   monitoring degassing of the marine riser using the the gas detector.   
     
     
         19 . A method of drilling a subsea wellbore, comprising:
 drilling the subsea wellbore;   while drilling the subsea wellbore:
 measuring a flow rate of drilling fluid injected into a tubular string having a drill bit; 
 measuring a flow rate of drilling returns using a subsea mass flow meter; and 
 comparing the returns flow rate to the drilling fluid flow rate to detect a kick by a formation being drilled; and 
   in response to detecting the kick:
 closing a blowout preventer of a subsea pressure control assembly (PCA) against the tubular string; and 
 diverting the flow of returns from the PCA, through a choke line having a second variable choke valve, and through a first variable choke valve. 
   
     
     
         20 . The method of  claim 19 , wherein:
 the subsea mass flow meter is part of the PCA, and   the PCA is connected to a subsea wellhead.   
     
     
         21 . The method of  claim 20 , wherein the returns are diverted from the PCA and through the mass flow meter by a rotating control device of the PCA. 
     
     
         22 . A managed pressure drilling system, comprising:
 a first rotating control device (RCD) for connection to a marine riser;   a first variable choke valve for connection to an outlet of the first RCD;   a first mass flow meter for connection to an outlet of the first variable choke valve;   a splice for connecting an inlet of the first variable choke valve to an outlet of a second variable choke valve;   a second RCD for assembly as part of a subsea pressure control assembly;   a subsea mass flow meter for connection to an outlet of the second RCD; and   a programmable logic controller (PLC) in communication with the first variable choke valve and the first and second mass flow meters.