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A flow stop valve (200, 300, 400) for placement in a downhole tubular operating in a dual fluid density system, wherein the flow stop valve is arranged such that it is in communication with a pressure difference between one of: fluid outside the downhole tubular and inside the downhole tubular at th

A flow stop valve (200, 300, 400) for placement in a downhole tubular operating in a dual fluid density system, wherein the flow stop valve is arranged such that it is in communication with a pressure difference between one of: fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve; and fluid above and below the flow stop valve inside the downhole tubular, wherein the flow stop valve comprises a first valve element (226′, 326′, 424) arranged such that the pressure difference acts across at least a portion of the first valve element and that the first valve element is movable between open and closed positions under action of said pressure difference so as to selectively permit flow through the downhole tubular, wherein the first valve element comprises a first passage (212, 312, 446) arranged so as to transmit fluid from a first port (213, 313, 447) in a first side of the first valve element to a second side of the first valve element, the first port being positioned such that it is adjacent to a low pressure flow region (290) when the flow stop valve is in an open position.

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1. A flow stop valve for placement in a downhole tubular operating in a dual fluid density system, wherein the flow stop valve is in communication with a pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve, or between fluid above and

1. A flow stop valve for placement in a downhole tubular operating in a dual fluid density system, wherein the flow stop valve is in communication with a pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve, or between fluid above and below the flow stop valve inside the downhole tubular, wherein the flow stop valve comprises: a first valve element arranged such that the pressure difference acts across at least a portion of the first valve element, wherein the first valve element comprises a first passage to transmit fluid from a first port in a first side of the first valve element through a second side of the first valve element to a first chamber, the first port being positioned adjacent to a low pressure flow region when the flow stop valve is in an open position such that the low pressure flow region is in fluidic communication with the first chamber via the first port and the first passage;a second valve element, wherein the low pressure flow region is at least partially defined in a gap between the second valve element and the first valve element when the flow stop valve is in the open position, and wherein the first valve element is configured to move in a downhole direction with respect to the second valve element to actuate the flow stop valve from a closed position to the open position; anda resilient member disposed in the first chamber, the resilient member applying a biasing force on the first valve element, the biasing force being oriented so as to force the first valve element toward the second valve element, such that the biasing force causes the flow stop valve to actuate to the closed position when the pressure difference is below a threshold value. 2. The flow stop valve of claim 1, wherein the flow stop valve selectively permits flow between the first and second valve elements and thereby through the downhole tubular when in the open position. 3. The flow stop valve of claim 2, wherein the first port is arranged such that it is not in fluidic communication with fluid in the downhole tubular and below the flow stop valve by the interaction between the first and second valve elements when the flow stop valve is in the closed position and the first port is in fluidic communication with the fluid in the downhole tubular and below the flow stop valve when the flow stop valve is in the open position. 4. The flow stop valve of claim 3, wherein the second valve element is substantially spherical and the first valve element comprises a corresponding valve seat portion adapted to receive the second valve element, the first port being provided within the valve seat portion. 5. The flow stop valve of claim 2, wherein the flow stop valve further comprises a third valve element, wherein the third valve element is disposed so as to limit the movement of the second valve element. 6. The flow stop valve of claim 1, further comprising another resilient member, the another resilient member applying a biasing force on the second valve element in a downhole direction, toward the first valve element. 7. The flow stop valve of claim 1, wherein the flow stop valve is biased so as to prevent flow of fluid from an uphole side of the flow stop valve to a downhole side of the flow stop valve. 8. A method of controlling flow in a downhole tubular operating in a dual fluid density system, the method comprising: restricting flow through the downhole tubular by closing a flow stop valve when a pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve, or between fluid above and below the flow stop valve inside the downhole tubular, is below a threshold value, wherein the flow stop valve comprises: a first valve element arranged such that the pressure difference acts across at least a portion of the first valve element, wherein the first valve element comprises a first passage to transmit fluid from a first port in a first side of the first valve element through a second side of the first valve element to a first chamber;a second valve element, wherein a low pressure flow region is defined at least partially in a gap between the first and second valve elements when the flow stop valve is in an open position, and wherein the first valve element is configured to move in a downhole direction with respect to the second valve element to actuate the flow stop valve from a closed position to the open position; anda resilient member disposed in the first chamber, the resilient member applying a biasing force on the first valve element, the biasing force being oriented so as to force the first valve element toward the second valve element, such that the biasing force causes the flow stop valve to actuate to the closed position when the pressure difference is below the threshold value;permitting flow through the downhole tubular by opening the flow stop valve when the pressure difference is above the threshold value, wherein the flow stop valve opens in response to a first valve element moving in a downhole direction with respect to a second valve element; andtransmitting fluid from the first port of the first valve element to the first chamber, the first port being positioned adjacent to a low pressure flow region when the flow stop valve is in the open position such that the low pressure flow region is in fluidic communication with the first chamber via the first port. 9. The method of claim 8, wherein the method further comprises: arranging the first port such that it is not in fluidic communication with fluid in the downhole tubular and below the flow stop valve by the interaction between the first and second valve elements when the flow stop valve is in the closed position and the first port is in fluidic communication with the fluid in the downhole tubular and below the flow stop valve when the flow stop valve is in the open position. 10. The method of claim 8, wherein the method further comprises permitting the second valve element to be movably disposed with respect to the first valve element. 11. The method of claim 10, wherein the method further comprises biasing the second valve element towards the closed position by virtue of another resilient member. 12. The method of claim 8, wherein the method further comprises providing a third valve element disposed so as to limit the movement of the second valve element. 13. The method of claim 12, wherein the method further comprises: permitting the first and second valve elements to move together under action of the fluid above the flow stop valve and in the downhole tubular until the second valve element abuts the third valve element; andpermitting the first and second valve elements to move apart upon further movement of the first valve element so as to allow fluid to flow between the first and second valve elements, thereby permitting flow through the flow stop valve and placing the flow stop valve in an open position. 14. The method of claim 8, wherein the method further comprises drilling in the dual fluid density system with the flow stop valve disposed in a drill string. 15. The method of claim 8, wherein the method further comprises cementing in the dual fluid density system with the flow stop valve connected to a casing section to be cemented into place. 16. A flow stop valve for a downhole tubular, comprising: a housing defining a first chamber;a first valve element disposed in the housing and defining a central flow passage, a first port, and a second port, the first and second ports being in fluid communication with the first chamber, wherein the first valve element is movable by exposure to a pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve, or between fluid above and below the flow stop valve inside the downhole tubular; anda second valve element disposed in the housing and engageable with the first valve element to block the central flow passage,wherein, when the flow stop valve is in an open position, the second valve element is at least partially spaced apart from the first valve element such that a high velocity, low pressure flow region is defined in a gap between the first and second valve elements, and a first pressure of the high velocity, low pressure flow region is communicated to the first chamber via the first port, andwherein, when the flow stop valve is in a closed position, the second valve element engages the first valve element and blocks the central flow passage, and a second pressure is communicated from a point within the housing and uphole of the first valve element, at least through the second port, to the first chamber. 17. The flow stop valve of claim 16, wherein the first valve element further defines a first passage extending from the first port, and a second passage extending from the second port, the first and second passages being in fluid communication with the first chamber. 18. The flow stop valve of claim 17, wherein the first and second passages intersect within the first valve element. 19. The flow stop valve of claim 16, wherein the first valve element comprises a valve seat that engages the second valve element when the flow stop valve is in the closed position, the first port being defined in the valve seat. 20. The flow stop valve of claim 16, further comprising a resilient member disposed in the first chamber, engaging the first valve element, and applying a biasing force thereto, the biasing force being oriented toward the second valve element such that the biasing force causes the flow stop valve to actuate to the closed position when the pressure difference is below a threshold value. 21. The flow stop valve of claim 16, wherein the first valve element further comprises: a sleeve that is slidable relative to the housing, the sleeve comprising a valve seat that engages the second valve element when the flow stop valve is in the closed position; anda tubular section disposed at least partially radially within the sleeve, wherein the tubular section is prevented from sliding with the slidable sleeve, and wherein the tubular section comprises a contact portion that disengages the second valve element from the valve seat when the sleeve slides downhole relative to the tubular section, the central flow passage being at least partially defined within the tubular section. 22. The flow stop valve of claim 21, wherein first and second ports are disposed on a first side of the sleeve, and the first chamber is disposed on a second side of the sleeve. 23. The flow stop valve of claim 21, wherein the first chamber is positioned radially between the housing and the tubular section. 24. The flow stop valve of claim 16, wherein, when the flow stop valve is in the open position, the second pressure is communicated to the first chamber via at least the second port. 25. A method for dual gradient drilling, comprising: deploying a tubular string comprising a flow stop valve in a closed position into a well, the flow stop valve comprising:a housing defining a first chamber;a first valve element disposed in the housing and defining a central flow passage, a first port, and a second port, the first and second ports being in fluid communication with the first chamber; anda second valve element disposed in the housing and engageable with the first valve element to block the central flow passage,wherein, when the flow stop valve is in the closed position, the second valve element engages the first valve element and blocks the central flow passage, and a pressure is communicated from uphole of the first valve element and in the housing, through the second port, to the first chamber; andafter at least partially deploying the tubular string, increasing a fluid pressure in the tubular string, such that the flow stop valve is actuated to an open position, wherein, when the flow stop valve is in the open position, the second valve element is at least partially spaced apart from the first valve element such that a high velocity, low pressure flow region is defined between the first and second valve elements, and a pressure of the high velocity, low pressure flow region is communicated to the first chamber via the first port. 26. The method of claim 25, wherein deploying the tubular string comprising the flow stop valve into the well causes the first and second valve elements to move relative to the housing in response to a hydrostatic pressure increase, without the first and second valve elements separating, and wherein increasing the fluid pressure causes the second valve element to be disengaged from the first valve element, to open the flow stop valve. 27. The method of claim 25, wherein, when the flow stop valve is in the open position, the pressure is communicated from uphole of the first valve element and in the housing, at least through the second port, and to the first chamber.