Mixing apparatus and method of using same

1. An apparatus ( 1 ) for mixing an additive for drilling fluid used in drilling a petroleum well, said apparatus ( 1 ) comprising:

a mixing tank ( 17 ), to which there is connected an inlet line ( 19 ) arranged to supply a drilling fluid which has been circulated out of the petroleum well to the mixing tank ( 17 );

a mixing chamber ( 3 ), to which there is connected an inlet line ( 5 ) arranged to receive the drilling fluid from the mixing tank ( 17 ) and an outlet line ( 7 ) arranged to return the drilling fluid to the mixing tank ( 17 );

at least one dosing device ( 9 , 9 ′) for an additive, the dosing device ( 9 , 9 ′) being a valve arranged for switching between a supplying position in which the additive is supplied from at least one container ( 11 , 11 ′) to the mixing chamber ( 3 ) and a non-supplying position in which the valve forms a pressure-tight barrier between the mixing chamber ( 3 ) and the at least one container ( 11 , 11 ′);

means for regulating a flow of the drilling fluid into the mixing chamber ( 3 ), the regulating means being arranged at the inlet or upstream of the mixing chamber ( 3 ); and

a volumetric pumping device ( 13 ), the volumetric pumping device ( 13 ) being arranged to create an underpressure in the mixing chamber ( 3 ) sufficient to suck the drilling fluid from the mixing tank ( 17 ) and also the additive from the at least one container ( 11 , 11 ′) into and through the mixing chamber ( 3 ) when the at least one dosing device ( 9 , 9 ′) is in the supplying position.

2. The apparatus in accordance with claim 1 , wherein the pumping device ( 13 ) is frequency-controlled.

3. The apparatus in accordance with claim 1 , wherein the apparatus ( 1 ) is provided with a device for automatically sampling, analyzing and evaluating the drilling fluid while flowing in lines ( 5 , 7 , 15 ) or in a mixing tank ( 17 ).

4. The apparatus in accordance with claim 3 , wherein the device for automatically at least sampling and analyzing the drilling fluid is arranged downstream of the mixing chamber ( 3 ).

5. The apparatus in accordance with claim 3 , wherein the device for automatically at least sampling and analyzing the drilling fluid is arranged upstream of the mixing chamber ( 3 ).

6. The apparatus in accordance with claim 3 , wherein the feeding of additive(s) by the dosing device is automatically controlled on basis of information from the sampling, analyzing and evaluating of the drilling fluid while flowing in lines ( 5 , 7 , 15 ) or in the mixing tank ( 17 ) and from information on flow through the pumping device ( 13 ).

7. A method for mixing an additive for drilling fluid used in drilling a petroleum well, said method comprising the steps of:

supplying a mixing tank ( 17 ) with a drilling fluid which has been circulated out of the petroleum well, wherein the mixing tank ( 17 ) is arranged to send the drilling fluid to a mixing chamber ( 3 ) and receive a mixed drilling fluid from the mixing chamber ( 3 );

providing an underpressure in the mixing chamber ( 3 ) by means of a volumetric pumping device ( 13 ) arranged downstream of the mixing chamber ( 3 ) and means for regulating a flow of fluid from an inlet line ( 5 ) into the mixing chamber ( 3 ), the regulating means being arranged upstream of the mixing chamber ( 3 );

sucking the drilling fluid from a mixing tank ( 17 ) through the mixing chamber ( 3 ) by means of the volumetric pumping device ( 13 ); and

controlling the supply of an additive from at least one container ( 11 , 11 ′) by means of a dosing device ( 9 , 9 ′) which is arranged to be operated between a non-supplying position, in which the dosing device ( 9 , 9 ′) forms a pressure-tight barrier between the container ( 11 , 11 ′) and the mixing chamber ( 3 ), and a supplying position, in which the additive is sucked from the container ( 11 , 11 ′) via the dosing device ( 9 , 9 ′) into the mixing chamber ( 3 ).

8. The method in accordance with claim 7 , wherein mixing of additive(s) is controlled automatically on basis of information selected from at least one member of the group consisting of: measurements and analysis of the drilling fluid upstream of the mixing chamber ( 3 ); measurements and analysis of the drilling fluid downstream of the mixing chamber ( 3 ); and information on flow through the apparatus ( 1 ).