Apparatus for and Method of Drilling a Subterranean Borehole

Abstract

A control apparatus for use in controlling the fluid pressure in a borehole during drilling of the borehole, the apparatus including an inlet suitable for connection to a pump, a main outlet suitable for connection to a drill pipe, and an overflow outlet suitable for connection to a fluid reservoir, the inlet being connected to the main outlet by a main flow conduit, and the overflow outlet being connected to the main flow conduit by means of an overflow conduit, wherein the overflow conduit is provided with a pressure regulator by means of which the pressure of fluid in the main flow conduit may be maintained at or around a predetermined pressure value.

1 . A method of drilling a borehole using a drilling system including a control apparatus, the control apparatus having an inlet, a main outlet and an overflow outlet, the inlet being connected to the main outlet by a main flow conduit and the overflow outlet being connected to the main conduit by an overflow conduit, the overflow conduit being provided with a pressure regulator, the drilling system further including a drill pipe connected to the main outlet of the control apparatus, a pump connected the inlet of the control apparatus, and a fluid reservoir connected to the overflow outlet, wherein the method includes the steps of operating the pump to pump fluid into the drill pipe via the main flow conduit of the control apparatus, characterized that the method further includes operating the pressure regulator to maintain the pressure of fluid in the main flow conduit at or around a predetermined level while drilling the borehole. 
     
     
         2 . A method of drilling a borehole according to  claim 1  wherein the pressure regulator comprises an adjustable overflow choke. 
     
     
         3 . A method of drilling a borehole according to  claim 1  wherein the control apparatus also includes a further choke which is located in the main conduit between the overflow conduit and the main outlet, and the method also includes the step of operating the main choke to alter the extent to which flow of fluid along the main conduit is restricted. 
     
     
         4 . A method of drilling a borehole according to  claim 1  wherein the pump is connected to the mud reservoir such that operation of the pump causes fluid in the reservoir to be pumped into the control apparatus. 
     
     
         5 . A method of drilling a borehole according to  claim 1  wherein the drilling system includes an annulus return conduit which extends between the annular region around the drill pipe and the fluid reservoir, and the annulus return conduit is provided with a choke which is adjustable to restrict flow of fluid along the annulus return conduit to a greater or lesser extent, and the method further includes the steps of operating the choke to bring the pressure in the annular region around the drill pipe to or maintain the pressure at a desired level. 
     
     
         6 . A method of drilling a borehole according to  claim 5  wherein the drilling system further includes a main choke which is located in the main flow conduit between the overflow conduit and the main outlet and which is adjustable to vary the degree of restriction of flow of fluid along the main conduit, and an annulus conduit which extends from the main conduit to the annular region around the drill pipe, the annulus conduit being provided with an annulus choke which is adjustable to restrict flow of fluid along the annulus conduit to a greater or lesser extent, the method further including the steps of, during operation of the pump to pump fluid into the main flow conduit, adjusting the annulus choke to decrease the restriction on flow of fluid along the annulus conduit until the rate of flow of fluid along the annulus conduit reaches a predetermined amount, then adjusting the main choke to increase the restriction of flow of fluid along the main conduit, whilst at the same time adjusting the annulus return choke to increase the restriction of flow of fluid along the annulus return conduit. 
     
     
         7 . A method of drilling a borehole according to  claim 6  wherein the method further includes the step of adjusting the main choke until flow of fluid through the main choke is substantially prevented, and then disconnecting the drill pipe from the main outlet. 
     
     
         8 . A method of drilling a borehole according to  claim 5  wherein the drilling system further includes a main choke which is located in the main flow conduit between the overflow conduit and the main outlet and which is adjustable to vary the degree of restriction of flow of fluid along the main conduit, and an annulus conduit which extends from the main conduit to the annular region around the drill pipe, the annulus conduit being provided with an annulus choke which is adjustable to restrict flow of fluid along the annulus conduit to a greater or lesser extent, the method further including the steps of, during operation of the pump to pump fluid into the main flow conduit, adjusting the main choke to decrease the restriction of flow of fluid along the main conduit, whilst at the same time adjusting the annulus return choke to decrease the restriction of flow of fluid along the annulus return conduit, and then adjusting the annulus choke to increase the restriction on flow of fluid along the annulus conduit until the rate of flow of fluid along the annulus conduit is substantially prevented. 
     
     
         9 . A method of drilling a borehole according to  claim 1  wherein the method includes the steps of adjusting the choke to increase restriction of fluid flow along the overflow conduit if the pressure of fluid in the overflow conduit is below the predetermined level or adjusting the choke to decrease restriction of fluid flow along the overflow conduit if the pressure of fluid in the main flow conduit is above the predetermined level.