Wellhead isolation tool and method of use

A wellhead isolation tool to permit the injection of fluids, solid particles or mixtures thereof through a wellhead having a vertical passage therethrough including at least one valve and into a well having a production tubing or a well casing aligned with the vertical passage, the wellhead isolation tool comprising:

• a mandrel positioning assembly which includes a hollow cylinder having a closed top end and an open bottom end that is adapted for fluid tight connection with a top end of a high pressure valve assembly, a piston which is forcibly reciprocatable within the cylinder, a valve located adjacent the closed top end of the cylinder for injecting pressurized fluids between the closed top end and the piston and evacuating the injected fluids, and a pull rod affixed to a top side of the piston and extending through a fluid seal in the closed top end of the cylinder;
• the high pressure valve assembly including at least one high pressure valve for selectively closing at least one high pressure bore which communicates with a sealed bore defined by the tool and having a bottom end adapted for a fluid tight connection with a top end of a pressure relief valve assembly;
• the pressure relief valve assembly including at least one valve for selectively closing at least one bore which communicates with the vertical passage defined by the tool, and a bottom end adapted for fluid-tight connection with a top end of the wellhead;
• packing means for engaging a periphery of a mandrel portion of a mandrel assembly in a fluid tight sea1 which permits reciprocal movement of the mandrel, the packing means being located between the high pressure valve and the pressure relief valve in the sealed bore defined by the tool;
• the mandrel assembly including an injection head adapted to be attached to a bottom side of the piston, the mandrel being connected to the injection head, and a pack-off nipple assembly connected to a bottom end of the mandrel, the injection head including at least one injection port in fluid communication with an axial bore through the mandrel and the pack-off nipple assembly;
• and
• the at least one injection port being located in the injection head so that the at least one injection port is in fluid communication with the high pressure bore when the mandrel is extended through the wellhead and the pack-off nipple assembly sealingly engages the well tubing or casing.

A method of isolating a wellhead located on an oil or gas well from the effects of high pressure or corrosion due to a stimulation treatment of the well, comprising the steps of:

• a) connecting to the wellhead an isolation tool which includes a high pressure valve that selectively closes a high pressure bore which communicates with a sealed bore defined by the tool, and a hollow mandrel having no direct connection with the high pressure bore, the mandrel being forcibly reciprocatable in the sealed bore and including an injection port which aligns with the high pressure bore when the mandrel is stroked into the wellhead, and a valve for injecting pressurized fluid to forcibly stroke the mandrel into the wellhead;
• b) opening a valve in the wellhead to open a vertical passage through the wellhead, and injecting pressurized fluid into the tool to forcibly stroke the mandrel through the wellhead until a pack-off nipple assembly connected to a bottom end of the mandrel sealingly engages a production tubing or well casing of the well and the injection port of the mandrel aligns with the high pressure bore;
• c) connecting a high pressure line to the high pressure valve and pumping well stimulation fluids into the well;
• d) closing the high pressure valve and disconnecting the high pressure lines;
• e) relieving fluid pressure from the tool to permit wellbore pressure to stroke the mandrel from the well;
• and
• f) closing the valve in the wellhead and removing the tool from the wellhead.