Method for Controlling Fluid Pressure in a Borehole Using a Dynamic Annular Pressure Control System

Abstract

A method for controlling formation pressure during the drilling of a borehole through a subterranean formation includes selectively pumping a drilling fluid through a drill string extended into a borehole, out a drill bit at the bottom end of the drill string, and into an annular space between drill string and the borehole. The drilling fluid is discharged from the borehole to a reservoir to clean the drilling fluid for reuse. An annular space fluid pressure is selectively increased to maintain a selected fluid pressure proximate the bottom of the borehole by applying fluid pressure to the annular space. The selective increasing includes controlling an aperture of an orifice operatively coupled between the annular space and the reservoir. A formation fluid pressure is predicted at a depth below a lowermost depth to which the borehole is drilled and the aperture is adjusted to maintain a selected pressure in the borehole in response to the predicted formation fluid pressure.

1 . A method for controlling formation pressure during the drilling of a borehole through a subterranean formation, comprising:
 selectively pumping a drilling fluid through a drill string extended into a borehole, out a drill bit at the bottom end of the drill string, and into an annular space between drill string and the borehole;   discharging the drilling fluid to a reservoir to clean the drilling fluid for reuse;   selectively increasing annular space fluid pressure to maintain a selected fluid pressure proximate the bottom of the borehole by applying fluid pressure to the annular space, the selective increasing including controlling an aperture of an orifice operatively coupled between the annular space and the reservoir;   predicting a formation fluid pressure at a depth below a lowermost depth to which the borehole is drilled; and   adjusting the aperture to maintain a selected pressure in the borehole in response to the predicted formation fluid pressure.   
   
   
       2 . The method of  claim 1  wherein the predicting comprises measuring d-exponent. 
   
   
       3 . The method of  claim 1  wherein the predicting comprises measuring formation acoustic velocity. 
   
   
       4 . The method of  claim 1  wherein the predicting comprises measuring formation electrical resistivity. 
   
   
       5 . The method of  claim 1  wherein the predicting comprises performing seismic velocity analysis. 
   
   
       6 . A method for controlling formation pressure during the drilling of a borehole through a subterranean formation, comprising:
 selectively pumping a drilling fluid through a drill string extended into a borehole, out a drill bit at the bottom end of the drill string, and into an annular space between drill string and the borehole;   discharging the drilling fluid to a reservoir to clean the drilling fluid for reuse;   selectively increasing annular space fluid pressure to maintain a selected fluid pressure proximate the bottom of the borehole by applying fluid pressure to the annular space, the selective increasing including controlling an aperture of an orifice operatively coupled between the annular space and the reservoir, the selected fluid pressure calculated by an hydraulics model;   measuring fluid pressure in the annular space at a plurality of depths; and   calibrating the hydraulics model based on the measured fluid pressures.   
   
   
       7 . The method of  claim 6  wherein the calibrating comprises correcting the model for changes in shape of the borehole. 
   
   
       8 . The method of  claim 6  wherein the calibrating comprises correcting the model for variations in temperature along the borehole. 
   
   
       9 . The method of  claim 6  wherein the calibrating comprises detecting circulation in the annular space of fluid entered into the borehole from an Earth formation adjacent the borehole.