CONTROL OF MANAGED PRESSURE DRILLING

Method for controlling bottomhole pressure in a borehole during managed pressure drilling. In the drilling procedure fluid is circulated down a drillstring and to surface through an annulus between the drillstring and an inner-wall of the borehole. A compressor/pump is used to inject gas into the returning drilling fluid at a gas injection point. The method comprises receiving a target bottomhole pressure; determining actual bottomhole pressure during the procedure; and adjusting a variable as directed by a feedback controller, which operates to reduce the difference between the target pressure and the measured pressure. The manipulated variable may be one or any combination of: (1) pressure of drilling fluid at the entry of the fluid into the drillstring, (2) the flow rate of drilling fluid out of the drillstring into the annulus, (3) pressure of the gas exiting from the compressor, and (4) the injection rate of the gas.