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A closed-loop circulating system for drilling wells has control of the flow rates in and out of the wellbore. Kicks and fluid losses are quickly controlled by adjusting the backpressure. Kick tolerance and tripping margins are eliminated by real-time determination of pore and fracture pressure. The

A closed-loop circulating system for drilling wells has control of the flow rates in and out of the wellbore. Kicks and fluid losses are quickly controlled by adjusting the backpressure. Kick tolerance and tripping margins are eliminated by real-time determination of pore and fracture pressure. The system can incorporate a rotating BOP and can be used with underbalanced drilling.

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The invention claimed is: 1. In a drilling arrangement for drilling a well into a subterranean formation which includes, a tubular drill string (1) having an upper and lower end and with a drill bit (2) at its lower end, a drive mechanism arranged and designed to turn said drill bit (2) in a boreho

The invention claimed is: 1. In a drilling arrangement for drilling a well into a subterranean formation which includes, a tubular drill string (1) having an upper and lower end and with a drill bit (2) at its lower end, a drive mechanism arranged and designed to turn said drill bit (2) in a borehole where a borehole annulus (3) is defined between an outer diameter of said tubular drill string (1) and an inner diameter of said borehole, a drilling fluid pump (6) in fluid communication with a drilling fluid reservoir (5), a drilling fluid injection line (14) extending between said pump (6) and said upper end of said drill string (1) and providing fluid communication between said pump (6) and said drill string (1), a fluid return line (27) extending between an outlet of said borehole annulus (3) and said drilling fluid reservoir (5), a pressure containment device (26) arranged and designed to keep said borehole closed from the atmosphere at all times while said well is being drilled with said drill string (1) having drilling fluid circulating therethrough, said injection line (14), drill string (1), borehole annulus (3) and return line (27) defining a flow path, an output flow measurement device (10, 11) in said fluid return line (27) arranged and designed to generate an actual drilling signal Foutactual(t) representative of actual flow rate of fluid in said fluid return line (27) as a function of time (t), a pressure measurement device (9, 17, 24) disposed at a position in said flow path and arranged and designed for determining a downhole pressure signal Pactual(t) as a function of time (t), a central data acquisition and control system (18) arranged and designed to receive at least one of said actual drilling signals, a method for determining the fracture pressure of said well at said depth, said method comprising the steps of, employing said central data acquisition and control system (18), having software responsive to said Foutactual(t) signal and other drilling signals, to identify a loss event at a drilling time and depth of the well, and recording said Pactual(t) signal at said drilling time as the fracture pressure of said formation at said depth. 2. In a drilling arrangement for drilling a well into a subterranean formation which includes, a tubular drill string (1) having an upper and lower end and with a drill bit (2) at its lower end, a drive mechanism arranged and designed to turn said drill bit (2) in a borehole where a borehole annulus (3) is defined between an outer diameter of said tubular drill string (1) and an inner diameter of said borehole, a drilling fluid pump (6) in fluid communication with a drilling fluid reservoir (5), a drilling fluid injection line (14) extending between said pump (6) and said upper end of said drill string (1) and providing fluid communication between said pump (6) and said drill string (1), a fluid return line (27) extending between an outlet of said borehole annulus (3) and said drilling fluid reservoir (5), a pressure containment device (26) arranged and designed to keep said borehole closed from the atmosphere at all times while said well is being drilled with said drill string (1) having drilling fluid circulating therethrough, said injection line (14), drill string (1), borehole annulus (3) and return line (27) defining a flow path, an output flow measurement device (10, 11) in said fluid return line (27) arranged and designed to generate an actual drilling signal Foutactual(t) representative of actual flow rate of fluid in said fluid return line (27) as a function of time (t), a pressure measurement device (9, 17, 24) disposed at a position in said flow path and arranged and designed for determining a downhole pressure signal Pactual(t) as a function of time (t), a central data acquisition and control system (18) arranged and designed to receive at least one of said actual drilling signals, a method for determining the pore pressure of said well at said depth, said method comprising the steps of, employing said central data acquisition and control system (18), having software responsive to said Foutactaul(t) signal and other drilling signals, to identify an influx event at a drilling time and depth of the well, and recording said Pactual(t) signal at said drilling time as the pore pressure of said formation at said depth.