Use of downhole isolation valve to sense annulus pressure

1. A method of drilling a wellbore, comprising:

deploying a drill string into the wellbore through a casing string disposed in the wellbore, the casing string having a pressure responsive element responsive to an annulus pressure inside the casing string and a hydraulic line in communication with the element and extending along the casing string;

drilling the wellbore into a formation by injecting drilling fluid through the drill string and rotating a drill bit of the drill sting; and

while drilling the formation, monitoring a pressure of the hydraulic line to identify a change in the annulus pressure.

2. The method of claim 1 , wherein:

the pressure responsive element is part of an isolation valve, and

the hydraulic line is a control line for operating the isolation valve.

3. The method of claim 2 , wherein:

the hydraulic control line is configured to open the isolation valve, and

the isolation valve is further operated by a second hydraulic control line extending along the casing string configured to close the isolation valve.

4. The method of claim 2 , wherein the pressure responsive element is a piston of the isolation valve.

5. The method of claim 2 , further comprising:

applying an annulus pressure to the pressure responsive element, wherein the pressure responsive element is a sleeve of the isolation valve; and

deflecting the pressure responsive element relative to the wellbore.

6. The method of claim 1 , wherein the pressure responsive element is part of a pressure sub.

7. The method of claim 6 , wherein the pressure responsive element is a piston of the pressure sub.

8. The method of claim 6 , wherein the pressure responsive element is a diaphragm of the pressure sub.

9. The method of claim 6 , further comprising:

applying an annulus pressure to the pressure responsive element; and

deflecting the pressure responsive element relative to the wellbore.

10. The method of claim 1 , further comprising, before drilling the formation, calibrating the pressure responsive element using a hydrostatic pressure of the drilling fluid.

11. The method of claim 10 , further comprising, while drilling the formation, converting the monitored pressure to a dynamic annulus pressure.

12. The method of claim 11 , further comprising, while drilling the formation, calculating an annulus pressure using the dynamic annulus pressure and the hydrostatic pressure.

13. The method of claim 12 , further comprising:

detecting a kick using the calculated annulus pressure; and

increasing a density of the drilling fluid to control the kick using the calculated annulus pressure.

14. The method of claim 12 , further comprising, while drilling the formation:

measuring a flow rate of the drilling fluid injected into the drill string;

measuring a flow rate of returns;

comparing the returns flow rate to the drilling fluid flow rate; and

using the calculated annulus pressure to predict bottom hole pressure.

15. The method of claim 12 , further comprising, while drilling the formation, exerting back pressure on the formation using a rotating control device, a variable choke valve, and the calculated annulus pressure.

16. The method of claim 1 , wherein the pressure is monitored for a sudden increase to detect a kick.

17. The method of claim 1 , wherein the pressure is monitored for a sudden decrease to detect lost circulation.

18. The method of claim 1 , further comprising, while deploying the drill string, monitoring the pressure of the hydraulic line.

19. The method of claim 18 , further comprising, while deploying the drill string, converting the monitored pressure to a dynamic annulus pressure.

20. The method of claim 19 , further comprising, while deploying the drill string, calculating an annulus pressure using the dynamic annulus pressure and a hydrostatic pressure.