Control method and apparatus for well operations

1. A method of controlling the annular pressure in a well during a well construction operation, the operation comprising pumping a fluid down a tubing located within the well and extracting the fluid that flows back through an annulus within said well and surrounding the tubing, the method comprising:

defining a set pressure p ref ;

determining a fluid pressure p within said annulus and determining a pressure offset of the determined pressure p with respect to said set pressure p ref ;

calculating a desired extraction flow rate q c of fluid from said annulus based on an analytical model depending upon the set pressure p ref , said pressure offset scaled by a constant gain factor K p , and a pumped flow rate into the annulus; and

configuring an extraction path to achieve said desired extraction flow rate.

2. The method according to claim 1 , said step of determining a desired extraction flow rate being additionally made in dependence upon a determined or estimated influx or efflux q res through the well walls or a part of the well walls.

3. The method according to claim 1 , wherein said step of determining a fluid pressure within the annulus comprises measuring a fluid pressure at a downhole end of the annulus.

4. The method according to claim 1 , wherein said step of determining a fluid pressure within the annulus comprises measuring a fluid pressure at a topside end of the annulus.

5. The method according to claim 1 , wherein a step of using said pressure offset to determine a desired extraction flow rate of fluid from the annulus comprises scaling said pressure offset to compensate for compression of the fluid within the annulus.

6. The method according to claim 1 , wherein said pressure offset is scaled by a factor V a /β a , where V a is the volume of said annulus and β a is the effective bulk modulus of the fluid within said annulus.

7. The method according to claim 1 , wherein said step of determining a desired extraction flow rate q c comprises evaluating at least one of the following terms:

−{dot over (V)} a , where {dot over (V)} a is the rate of change of the volume of a wellbore annulus within the system;

q bit , wherein q bit is the flow of fluid into the annulus through a bottom hole apparatus;

q res , wherein q res is the flow of fluid into the annulus from a reservoir; and

{dot over (p)} ref , scaled with

V

a

β

a

,

 wherein {dot over (p)} ref is the rate of change of the said set pressure p ref .

8. The method according to claim 7 , wherein said step of determining a desired extraction flow rate q c comprises summing two or more of the evaluated terms.

9. The method according to claim 1 , said step of determining a desired extraction flow rate from the annulus being additionally made in dependence upon a determined or estimated rate of change of a volume {dot over (V)} a of the wellbore, excluding the displacement volume of the tubing and any attached bottom hole apparatus.

10. The method according to claim 1 , and further comprising determining a flow rate q bit through a bottom hole apparatus attached to an end of the tubing in order to provide said pumped flow rate into the annulus.

11. The method according to claim 1 , wherein said step of configuring an extraction path to achieve said desired extraction flow rate comprises setting the operating points of one or more valves and/or a pressure back pump in the extraction path.

12. The method according to claim 1 , wherein said well construction operation is one of: drilling; drilling during start and/or stop of a rig pump; drilling during power loss at the rig pump; tripping of a tubing into the well; cementing of the well; and fishing within the well.

13. The method according to claim 1 , said step of determining a desired extraction flow rate q c comprising evaluating the equation

q

c

=

-

α

⁢

V

.

a

+

δ

⁢

⁢

q

bit

+

λ

⁢

⁢

q

res

+

γ

⁡

[

V

a

β

a

⁢

K

p

⁢

g

⁡

(

p

,

p

ref

,

t

)

]

where V a is the volume of the wellbore annulus, {dot over (V)} a is the rate of change of V a , q bit is the flow of fluid into the annulus through a bottom hole apparatus, q res is the flow of fluid into the annulus from a reservoir, β a is the effective bulk modulus of the fluid in the annulus, K p is the controller gain, g is p−p ref , or a nonlinear, time-varying, monotonically increasing function of p−p ref , and wherein at least two of α, δ, λ and γ=1, and each of the remaining two of α, δ, λ and γ=0 or 1.

14. The method according to claim 1 , said step of determining a desired extraction flow rate q c comprising evaluating the equation

q

c

=

-

α

⁢

V

.

a

+

δ

⁢

⁢

q

bit

+

λ

⁢

⁢

q

res

+

ϕ

⁢

V

a

β

a

⁢

p

.

ref

+

γ

⁡

[

V

a

β

a

⁢

K

p

⁢

g

⁡

(

p

,

p

ref

,

t

)

]

where V a is the volume of the wellbore annulus, {dot over (V)} a is the rate of change of V a , q bit is the flow of fluid into the annulus through a bottom hole apparatus, q res is the flow of fluid into the annulus from a reservoir, β a is the effective bulk modulus of the fluid in the annulus, K p is the controller gain, g is p−p ref or a nonlinear, time-varying, monotonically increasing function of p−p ref , and wherein at least two of α, δ, λ, φ and γ=1, and each of the remaining two of α, δ, λ, φ and γ=0 or 1.

15. A controller for controlling the pressure within an annulus during a well construction operation, the operation comprising pumping a fluid down a tubing and extracting the fluid that flows back through an annulus within said wellbore and surrounding the tubing, the controller comprising:

a pressure setting unit for defining a set pressure p ref ,

a pressure monitor for determining a fluid pressure p within said wellbore and for determining a pressure offset of the determined pressure p with respect to the set pressure p ref ,

a flow rate determiner for using said pressure offset to determine a desired extraction flow rate q c of fluid from said annulus, wherein the desired extraction flow rate q c of fluid from said annulus is calculated based on an analytical model depending upon the set pressure p ref , said pressure offset scaled by a constant gain factor K p , and a pumped flow rate into the annulus; and

a flow rate setting unit for configuring an extraction path to achieve said desired extraction flow rate.

16. The controller according to claim 15 , said flow rate determiner being configured to determine a desired extraction flow rate q c by evaluating the equation

q

c

=

-

α

⁢

V

.

a

+

δ

⁢

⁢

q

bit

+

λ

⁢

⁢

q

res

+

γ

⁡

[

V

a

β

a

⁢

K

p

⁢

g

⁡

(

p

,

p

ref

,

t

)

]

where V a is the volume of the wellbore annulus, {dot over (V)} a is the rate of change of V a , q bit is the flow of fluid into the annulus through a bottom hole apparatus, q res is the flow of fluid into the annulus from a reservoir, β a is the effective bulk modulus of the fluid in the annulus, K p is the controller gain, g is p−p ref , or a nonlinear, time-varying, monotonically increasing function of p−p ref , and wherein at least two of α, δ, λ and γ=1, and each of the remaining two of α, δ, λ and γ=0 or 1.

17. The controller according to claim 15 , said flow rate determiner being configured to determine a desired extraction flow rate q c by evaluating the equation

q

c

=

-

α

⁢

V

.

a

+

δ

⁢

⁢

q

bit

+

λ

⁢

⁢

q

res

+

ϕ

⁢

V

a

β

a

⁢

p

.

ref

+

γ

⁡

[

V

a

β

a

⁢

K

p

⁢

g

⁡

(

p

,

p

ref

,

t

)

]

where V a is the volume of the wellbore annulus, {dot over (V)} a is the rate of change of V a , q bit is the flow of fluid into the annulus through a bottom hole apparatus, q res is the flow of fluid into the annulus from a reservoir, β a is the effective bulk modulus of the fluid in the annulus, K p is the controller gain, g is p−p ref , or a nonlinear, time-varying, monotonically increasing function of p−p ref , and wherein at least two of α, δ, λ, φ and γ=1, and each of the remaining two of α, δ, λ, φ and γ=0 or 1.