Method and apparatus for maintaining bottom hole pressure during connections

A method to maintain a bottom hole fluid pressure control to a wellbore of a drilling rig that utilizes a drillstring and to facilitate adding a new drill stand, said method comprising:

• providing a gas reservoir equipped with a gas compressor connected to a computer via a connection wherein a user operates the computer via a human-machine interface and wherein the computer monitors pressure inside the wellbore with the use of a pressure sensor and further wherein the computer controls a volume of gas injected into the wellbore;
• providing a drilling fluid reservoir connected via a drilling fluid line to a managed pressure drilling choke and a valve connecting the drilling fluid to a primary flow line;
• injecting drilling mud in the wellbore by a mud injection device adapted to maintain fluid pressure control;
• providing a managed pressure drilling manifold with a flowline connected to said gas compressor;
• conducting drilling by the drilling rig;
• executing a ramp-schedule computation;
• halting drilling to add a new stand to the drillstring, wherein the mud injection device is stopped;
• injecting the compressible gas directly into the managed pressure drilling manifold from said flowline to maintain borehole pressure within the annulus of the wellbore at the desired constant pressure;
• in response to said executing step, ramping down pumping of the drilling mud by ramping down revolutions per minute (RPM) of the mud injection device and adjusting the managed pressure drilling (MPD) choke following the execution of the ramp-schedule computation;
• wherein injecting the compressible gas into the wellbore occurs following execution of the ramp-schedule to maintain the borehole pressure;
• wherein the gas compressor is activated simultaneously to inject gas from the gas reservoir following execution of the ramp-schedule;
• breaking connection of the drill rig to add a new drill stand and then adding the new drill stand to the drill rig;
• ramping up RPM of the mud injection device while adjusting the MPD choke and bleeding down the gas compressor;
• executing the ramp-schedule computation comprising parameters obtained from a pressure monitoring system, said parameters enabling an operator to maintain a constant desired bottom hole pressure during a managed pressure drilling connection;
• and
• wherein the parameters comprise consideration of at least one of:
• drilling fluid weight, drilling fluid flow rates, drill string rates of penetration and drill string rotation rate.