Method for tuning choke operation in a managed pressure drilling system

Abstract

A method for tuning a managed pressure drilling system comprising a variable orifice choke to control fluid flow from a drilling well includes (a) characterizing change in flow rate through the choke with respect to choke opening at a substantially constant pressure drop; (b) characterizing change in fluid pressure in the well with respect to change in choke opening at a substantially constant flow rate into the well; (c) characterizing a response time of pressure in the well to changes in the choke opening, a delay time of pressure response after a change in choke opening and a back pressure applied to the well with respect to choke opening. The characterizations in at least one of (a) and (b), and the characterization in (c) are used as control parameters for a proportional integral differential controller having as output the choke opening and as input the fluid pressure in the well before the choke.

What is claimed is: 
     
         1 . A method for tuning a managed pressure drilling system comprising a variable orifice choke operating to controllably restrict fluid flow from a drilling well, the method comprising:
 (a) characterizing change in flow rate of fluid through the choke with respect to fractional choke opening at a substantially constant pressure drop across the choke;   (b) characterizing change in fluid pressure in the well with respect to change in fractional choke opening at a substantially constant fluid flow rate into the well;   (c) characterizing a response time of pressure in the well to changes in the fractional choke opening, a delay time of pressure response after a change in fractional choke opening and a surface back pressure applied to the well with respect to fractional choke opening; and   using the characterizations in at least one of (a), and (b) and the characterization in (c) to calculate control parameters for a proportional integral differential controller having as a controlled output the fractional choke opening and as a control input the fluid pressure in the well before the choke.   
     
     
         2 . The method of  claim 1  wherein the characterizing change in flow rate of fluid through the choke with respect to fractional choke opening at a substantially constant pressure drop across the choke comprises pumping fluid through a conduit extended into the well, measuring flow rate of fluid leaving the well through an annular space between the conduit and a wall of the well, measuring pressure of fluid in the well upstream of the choke, adjusting the fractional choke opening and changing a rate of the pumping fluid so that the measured pressure remains substantially constant. 
     
     
         3 . The method of  claim 1  wherein the characterizing a response time of pressure in the well to changes in the fractional choke opening, a delay time of pressure response after a change in fractional choke opening and a surface back pressure applied to the well with respect to fractional choke opening comprises measuring pressure in the well upstream of the choke, measuring pressure upstream of the choke, and change in the measured pressure with respect to change in the fractional choke opening while pumping fluid into the well at a substantially constant rate. 
     
     
         4 . The method of  claim 1  wherein the characterizing a response time of pressure in the well to changes in the fractional choke opening comprises measuring pressure in the well upstream of the choke with respect to time, and determining an elapsed time between change in the fractional choke opening and a change in the measured pressure while pumping fluid into the well at a substantially constant rate.