Method of drilling a subterranean borehole

1. A method of drilling a subterranean well bore using a tubular drill string, the method including the steps of injecting a drilling fluid into the well bore via the drill string and removing said drilling fluid from an annular space in the well bore around the drill string via a return line, wherein the method further includes oscillating the pressure of the fluid in the annular space in the well bore at a predetermined frequency, and monitoring the rate of flow of fluid along the return line.

2. A method according to claim 1 wherein the return line is provided with a choke which restricts the flow of fluid along the return line and which is operable to vary the degree to which the flow of fluid along the return line is restricted, and the oscillating of the pressure of the fluid in the annular space in the well bore is achieved by oscillating the choke to alternately increase and decrease the degree to which the flow of fluid along the return line is restricted.

3. A method according to claim 2 wherein the return line is provided with a main choke and an auxiliary choke, the auxiliary choke being located in a branch line which extends from the return line upstream of the main choke to the return line downstream of the main choke.

4. A method according to claim 3 wherein the rate of flow of the drilling fluid along the return line is monitored using a flow meter, the flow meter being located in the return line upstream of the main choke.

5. A method according to claim 3 wherein the rate of flow of the drilling fluid along the return line is monitored using a flow meter, the flow meter being located in the return line upstream of the auxiliary choke.

6. A method according to claim 3 wherein the oscillating of the pressure of the fluid in the well bore is preferably achieved by oscillating the auxiliary choke to alternately increase and decrease the degree to which the flow of fluid along the return line is restricted.

7. A method according to claim 1 wherein the rate of flow of the drilling fluid along the return line is monitored using a flow meter which is connected to a processor which records the rate of flow of fluid along the return line over time.

8. A method according to claim 1 wherein the method further includes the steps of comparing the rate of flow of fluid along the return line when oscillating the pressure of the fluid in the well bore prior to drilling into a formation with the rate of flow of fluid along the return line when oscillating the pressure of the fluid in the well bore whilst drilling through a formation including a reservoir of formation fluid.

9. A method according to claim 1 wherein the method further includes the steps of, whilst drilling through a formation including a reservoir of formation fluid, progressively increasing the mean pressure of fluid in the well bore whilst oscillating the pressure of fluid in the well bore.

10. A method according to claim 1 wherein the method further includes the steps of, whilst drilling through a formation including a reservoir of formation fluid, progressively decreasing the mean pressure of fluid in the well bore whilst oscillating the pressure of fluid in the well bore.

11. A method of drilling a subterranean well bore using a tubular drill string, the method including the steps of injecting a drilling fluid into the well bore via the drill string and removing said drilling fluid from an annular space in the well bore around the drill string via a return line, wherein the method further includes oscillating the pressure of the fluid in the annular space in the well bore, the amplitude of the pressure oscillations being maintained at a generally constant level, and monitoring the rate of flow of fluid along the return line.

12. A method of drilling a subterranean well bore using a tubular drill string, the method including the steps of injecting a drilling fluid into the well bore via the drill string and removing said drilling fluid from an annular space in the well bore around the drill string via a return line, wherein the method further includes oscillating the pressure of the fluid in the annular space in the well bore at a predetermined frequency so that a variation in the pressure takes a waveform, and monitoring the rate of flow of fluid along the return line.

13. The method according to claim 12 , wherein the waveform is selected from a group consisting of sinusoidal, square, and spike wave forms.