Method of drilling a subterranean borehole

1. A method of drilling a subterranean wellbore using a drill string comprising the steps of:

a. injecting a drilling fluid into the well bore via the drill string and removing the drilling fluid from an annular space around the drill string (the annulus) via an annulus return line in which a main control choke is provided,

b. oscillating the pressure of the fluid in the annulus, the oscillation of the pressure in the annulus being achieved by oscillating the main control choke so that a degree to which the main control choke restricts a fluid flow along the annulus return line is alternately decreased and increased,

c. determining a wellbore storage volume and a wellbore storage coefficient for each fluid pressure oscillation, the wellbore storage volume being a change in a measured flow rate over a time period, and the wellbore storage coefficient being the wellbore storage volume divided by a pressure change over the time period,

d. using the wellbore storage volume and wellbore storage coefficient to determine the proportion by volume of gas and the proportion by volume of liquid in the annulus during that pressure oscillation, and

e. controlling a gain setting of the main control choke in accordance with the proportion by volume of gas in the annulus,

wherein,

the main control choke has a higher gain setting when the proportion by volume of gas in the annulus is higher,

the main control choke has a lower gain setting when the proportion by volume of gas in the annulus is lower,

the higher gain setting allows the main control choke to adjust a flow rate by a predetermined amount in a first time,

the lower gain setting allows the main control choke to adjust the flow rate by the predetermined amount in a second time, and

the first time is shorter than the second time.

2. The method according to claim 1 , wherein the wellbore storage volume and wellbore storage coefficient are determined by monitoring the rate of flow of fluid along the annulus return line.

3. The method according to claim 1 , wherein the wellbore storage volume and wellbore storage coefficient are determined by monitoring the fluid pressure at the top of the annulus.

4. The method according to claim 1 , wherein the volume percentage of gas in the annulus and the fluid pressure in the annulus are used to obtain an estimate of the maximum pressure of the gas when the gas enters the annulus return line.

5. The method according to claim 4 , wherein drilling is stopped and a blowout preventer closed around the drill string if it is determined that the estimated maximum pressure of the gas when the gas enters the annulus return line exceeds a predetermined value.

6. The method according to claim 1 , wherein

a main control choke is provided in the annulus return line,

an auxiliary choke is provided in a branch line which extends from the annulus return line upstream of the main control choke to the annulus return line downstream of the main control choke, and

the oscillation of the pressure in the annulus is achieved by oscillating the auxiliary choke so that the degree to which the auxiliary choke restricts a fluid flow along the branch line is alternately decreased and increased.

7. The method according to claim 1 , further including the steps of:

monitoring the fluid pressure at the bottom of the wellbore; and

controlling the main control choke to maintain the fluid pressure at the bottom of the wellbore at a predetermined level.

8. The method according to claim 7 , wherein the main control choke is operated to increase its restriction of a fluid flow along the annulus return line immediately after the presence of gas in the annulus is detected.

9. The method according to claim 1 , wherein an estimate of the position of any gas in the annulus is determined by analysing the shift in the frequency of the returning pressure pulse compared with the frequency of the applied pressure pulse.

10. The method according to claim 1 , further including the step of:

controlling a gain setting of the main control choke,

wherein,

a control unit calculates the gain setting based on a correlation of the proportion of volume of gas to the proportion of volume of liquid in the annulus during the pressure oscillation and, if an increase or decrease of the gain setting is required, transmits the increase or decrease of the gain setting to a controller of the main control choke which then updates the gain setting of the main control choke,

the control unit calculates and transmits an increase of the gain setting to the controller of the main control choke if the proportion by volume of gas to the proportion by volume of liquid in the annulus increases though the main control choke during the pressure oscillation, and calculates and transmits a decrease of the gain setting to the controller of the main control choke if the proportion by volume of gas to the proportion by volume of liquid in the annulus decreases though the main control choke during the pressure oscillation,

a higher gain setting allows the main control choke to adjust a flow rate by a predetermined amount in a first time,

a lower gain setting allows the main control choke to adjust the flow rate by the predetermined amount in a second time, and

the first time is shorter than the second time.

11. A method of drilling a subterranean wellbore using a drill string comprising the steps of:

injecting a drilling fluid into the well bore via the drill string and removing the drilling fluid from an annular space around the drill string (the annulus) via an annulus return line;

oscillating the pressure of the drilling fluid in the annulus by oscillating a main control choke provided in the annulus return line so that a degree to which the main control choke restricts a fluid flow along the annulus return line is alternately decreased and increased;

determining a wellbore storage volume and a wellbore storage coefficient for each fluid pressure oscillation, the wellbore storage volume being a change in a measured flow rate over a time period, and the wellbore storage coefficient being the wellbore storage volume divided by a pressure change over the time period;

using the wellbore storage volume and wellbore storage coefficient to determine the proportion by volume of gas and the proportion by volume of liquid in the annulus during that pressure oscillation; and

controlling a gain setting of the main control choke in accordance with the proportion by volume of gas in the annulus,

wherein,

the main control choke has a higher gain setting when the proportion by volume of gas in the annulus is higher,

the main control choke has a lower gain setting when the proportion by volume of gas in the annulus is lower,

the higher gain setting allows the main control choke to adjust a flow rate by a predetermined amount in a first time,

the lower gain setting allows the main control choke to adjust the flow rate by the predetermined amount in a second time, and

the first time is shorter than the second time.