GB1213281A - Well control - Google Patents

Abstract

1,213,281. Wells. CAMERON IRON WORKS Ltd. 29 Nov., 1967 [30 Dec., 1966 (2); 7 June, 1967], No. 54399/67. Heading E1F. A choke is positioned at the outlet for the drilling fluid of a well and has means to control it so as to make the difference (PD) between the bottom hole pressure of the drilling fluid, and the formation fluid pressure approach a predetermined value (P'D). When a "kick" is encountered in the well the blowout preventer is closed and the well static pressure (Ps) measured at the surface. On re-starting the pumps the inlet pressure P i of the fluid flowing into the well equals P S +P L +P D , where P L is the loss of pressure due to the fluid circulating to the bottom of the drill string. Hence:- P D -P' D =P d (say) = P i -P L -(P S +P D ) When P d is not zero the choke alters the rate at which the drilling fluid leaves the well until P d returns to zero. In one example (Figs. 9 and 10 not shown) a signal representing the inlet pressure is applied to one side of the piston 410 controlling the choke and a signal representing P L +P S +P' D is applied to the other side of the piston when it is desired to circulate heavier mud to the bottom of the well. When it is desired to circulate heavier mud to the bottom of the well signals representing the outlet pressure and the static pressure are instead applied to the two sides respectively so as to maintain the outlet pressure constant. The inlet and outlet pressures are automatically sensed and fed into a pneumatic controller for the choke. The product of the mud density times the square of the circulation (pV<SP>2</SP> say) is continuously sensed by an arm pivoted in the inlet conduit of the well and fed into the controller which produces a signal representing P L (which equals KDpV<SP>2</SP> where D is the length of the well and K is a constant). Values for P s , D and K are manually fed into the controller which also computes the increase in mud density (which equals P s /.052D) required to control the increase in formation pressure. In another embodiment a signal representing Pi-P L -P S -P' D is fed to one side of the piston of the choke and a signal representing the outlet pressure to the other side. The former signal may incorporate a feed-back to produce damping. The choke may be controlled pneumatically from the controller or hydraulically via a hydraulic relay. The choke described has a flow restricting member 33 movable by a piston 410 towards and away from valve seat formed by a ring 404. Sleeve 419 of the member 33 and ring 404 are reversible and have their inter engaging corners lined with tungsten carbide.