Patent ID: 8490719

Claim:
An apparatus for maintaining pressure in a wellbore during drilling operations, wherein the wellbore has casing set and cemented into place, the apparatus comprising: a reservoir containing fluid from the wellbore; a drill string in fluid communication with the reservoir, wherein an annulus is defined between the wellbore and the drill string; a pressure transducer in the drill string to measure pressure in the annulus; a rotating control device isolating pressure in the annulus and communicating fluid from the reservoir to the drill string and diverting fluid and solids from the annulus; a first flow meter for measuring a flow rate of fluid and solids diverted from the annulus; an adjustable choke in fluid communication with the rotating control device controllablly bleeding off pressurized fluid from the annulus, wherein the adjustable choke has a valve element with a position controlled by a first control pressure signal and an opposing second control pressure signal; solids control equipment receiving fluid and solids from the adjustable choke and removing the solids from the fluid; a fluid conduit for directing fluid from the solids control equipment to the reservoir; a processor receiving the measured pressure from the pressure transducer and calculating a set point backpressure; and a backpressure pump in fluid communication with the reservoir and applying a backpressure between the first flow meter and the adjustable choke based on the calculated set point.