This invention relates to devices for anchoring a tubing string to a surrounding well casing. Tubing anchors have a variety of uses in oilfield operations. For example, when an oil well is produced with a downhole rod pump, during the pumping cycle the weight of the fluid in the tubing string shifts between a load on the rod string on the upstroke to a load on the tubing on the downstroke. For installations which do not have an anchor, during the downstroke the weight of the fluid in the tubing causes the tubing to stretch, but on the upstroke the load is transferred to the rod string so the tubing contracts. In an unanchored installation, this cycle causes wear in the rods and tubing because of the rubbing of the rods and rod boxes against the inner wall of the tubing. In unanchored installations the pump efficiency is decreased by the decrease in the effective stroke length.
A tubing anchor maintains the tubing in tension, thereby lengthening the effective stroke length of the pump and preventing the stretching/contracting cycle of the tubing string. Tubing anchors generally operate by the urging of slips against the interior wall of the production casing of the well by the application of mechanical or hydraulic force. Anchors which are activated hydraulically typically have a “live slip” or “floating slip” (hereinafter “live slip”) which is urged outwardly against the interior of the casing wall upon the application of hydraulic pressure to a piston which drives the live slip radially outward. The piston may have a relatively large diameter and is capable of providing a substantial applied force to the live slip thereby maintaining the position of the anchor in the casing and keeping the tubing string in tension.
When tubing needs to be pulled from the well, the tubing anchor needs to be released. The releasing procedures usually involve pulling up on the tubing string or rotating the tubing string. However, during this process, it is not uncommon for the live slip or other anchor components to separate from the tubing anchor and fall downhole, either to the well bottom or to be stopped by casing restrictions or by a smaller diameter casing/liner below the set point of the anchor. The loss of the live slip downhole is problematic. Anchor slips are typically manufactured from hardened steel and can form an obstruction requiring an expensive fishing job or, alternatively, can be left downhole with the potential for causing problems in the future.