Testing of Hydraulic Tubing Anchors

Abstract

Hydraulic tubing anchors, similar to the traditional mechanical tubing anchor (TAC), have been around for over 30 years. Unlike the TAC, the hydraulic tubing anchor does not require surface manipulation to set. Differential pressure between the tubing and casing anulus engages the anchor to the casing by way of a hydraulic piston. Technology and materials have greatly improved in recent years resulting in a vast improvement in reliability of this particular tool. To better understand the dynamics of hydraulic anchors, a test rig was constructed to approximate down-hole conditions in terms of depth and holding capacity. The test assembly allows for controlling the perceived depth by way of pressurizing the tubing, or internal bore of the hydraulic anchor. Varying the pressure in the “tubing” simulates the pressures seen at any depth. The holding capacity of the anchor is tested by a hydraulic jack, which is placed under the anchor. The jack, having a known bore, can easily correlate PSI to lifting force placed on the anchor. Numerous tests were conducted at varying depths to find the lifting force required to dislodge, or cause the anchor to slip. Anchor test data as well as analysis of the interface between the anchor and casing are presented.

Background

Tubing anchors are used to stabilize pump and tubing movement relative to the rod-string and casing. As the down-hole pump cycles, fluid loads are transferred from the rods onto the tubing. The resulting load differential causes the tubing to stretch and contract. In unanchored tubing, this tubing stretch reduces the net pump travel. In addition to lost production, cyclic stretching and movement can cause unnecessary wear on the casing and rod string in addition to the tubing string. In the 1950’s, it was shown that unanchored tubing actually buckles and can become helically coiled around the rod string, which remains essentially straight under tension (Lubinski).

Two basic types of anchors are considered in this paper. Mechanical type anchors engage serrated metal blocks, or “slips” onto the inner casing wall via a screw type, or positive engagement mechanism.  A second type of anchor, and primary focus of this analysis, utilizes the fluid pressure in the tubing string to energize a piston. This piston then pushes the serrated blocks onto the inner wall of the casing for a positive engagement. Mechanical anchors are well known and used throughout the industry, but are problematic in certain well conditions, in particular deep or deviated wells where their setting procedure can be difficult or impossible. Safety is also a concern as there are a series of potentially dangerous surface operations that take place in the placement and removal of the mechanical anchor. The hydraulic anchor does not require any surface manipulation in conjunction with installation or removal.