A method for maintaining pressure in a
wellbore during drilling operations is disclosed. The method includes the steps of providing fluid from a reservoir through a
drill string, circulating the fluid from the
drill string to an annulus between the
drill string and the
wellbore, isolating pressure in the annulus, measuring pressure in the annulus, calculating a
set point backpressure, applying
back pressure to the annulus based on the
set point back pressure, diverting fluid from the annulus to a controllable
choke, controllably bleeding off pressurized fluid from the annulus, separating solids from the fluid, and directing the fluid back to the reservoir. An apparatus for maintaining pressure in a
wellbore during drilling operations that includes an adjustable
choke for controllably bleeding off pressurized fluid from the wellbore annulus, a backpressure pump for applying a calculated
set point backpressure, and a processor for controlling the adjustable
choke and backpressure pump is also disclosed.