| CPC E21B 34/102 (2013.01) | 20 Claims |

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9. A method of converting a fluid flow valve from a bi-directional valve to a unidirectional valve in a wellbore, the method comprising:
in a subterranean wellbore, maintaining a valve assembly in a bi-directional configuration with a retainer device wherein both an upper plunger valve and a lower plunger valve of the valve assembly are in an open position, the upper and lower plunger valves connected together such that the upper and lower plunger valves move in unison;
while the valve assembly is maintained in the bi-directional configuration, sequentially flowing fluid through the throughbore in an uphole and a downhole direction;
releasing the retainer device and moving the valve assembly to a unidirectional configuration in which the upper and lower plunger valves are biased to a closed position by a biasing mechanism and;
flowing fluid unidirectionally through the valve assembly when fluid pressure exceeds the force of the biasing mechanism.
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