Patent ID: 8393403

Claim:
A flow stop valve positioned along a downhole tubular string, said string characterized as having an inside and an outside, said flow stop valve disposed to permit fluid to pass therethrough and to control fluid flow in said tubular string, wherein a first fluid pressure exists inside said tubular string and a second fluid pressure exists outside said tubular string, said flow stop valve comprising: a valve housing having a first end and a second end, wherein the valve housing has a housing flow path from a housing flow inlet to a housing outlet, wherein the first end is attached to an upper sub characterized by an interior and forming part of the string; an internal flow port defined between the first and second ends of the valve housing; a hollow tubular section disposed at least partially in the valve housing, wherein the hollow tubular section is axially movable within the valve housing from a valve closed position to a valve open position such that fluid flow from the housing inlet through the internal flow port is substantially impeded when the hollow tubular section is in a first position and allowing substantial fluid flow from the housing inlet through the internal flow port when in a second position; a first pressure chamber defined within the valve housing; a second pressure chamber defined within the valve housing; a first pressure port in fluid communication with the first pressure chamber; and a second pressure port in fluid communication with the second pressure chamber; a first biasing element, wherein the first biasing element biases towards an end of the valve housing and into the valve closed position; wherein one of the pressure chambers is in fluid communication with the inside of the downhole tubular and wherein the other pressure chamber is in fluid communication with the outside of the downhole tubular; wherein the hollow tubular section is in the closed position when a pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve is below a threshold value, thereby preventing fluid flow through the valve and downhole tubular; and wherein the hollow tubular section is in the open position when the pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve is above a threshold value, thereby permitting fluid flow through the valve and downhole tubular, wherein one of the chambers is in fluid communication with the interior of the upper sub; wherein the pressure difference between fluid outside the downhole tubular string and inside the tubular string biases the valve towards the open position, such that when the pressure difference exceeds the threshold value, the valve is in the open position and drilling fluid is permitted to flow through the downhole tubular string.