Patent ID: 7395878

Claim:
A drilling system for drilling a bore hole into an earth formation, the bore hole having an inside wall, whereby the drilling system comprises: a drill string reaching into the bore hole leaving a drilling fluid return passage between the drill string and the bore hole inside wall; a drilling fluid discharge conduit in fluid communication with the drilling fluid return passage; pump means for pumping a drilling fluid through the drill string into the bore hole and to the drilling fluid discharge conduit via the drilling fluid return passage; back pressure means for controlling the drilling fluid back pressure; an injection fluid injection system comprising an injection fluid supply passage fluidly connecting an injection fluid supply with the drilling fluid return passage and further comprising an injection fluid pressure sensor arranged to provide a pressure signal in accordance with an injection fluid pressure in the injection fluid supply passage; back pressure control means for controlling the back pressure means whereby the back pressure control means is arranged to receive the pressure signal and to regulate the back pressure means in dependence of at least the pressure signal.