Mitigating Effects of Subsidence on Casing
Authors: JPT staff _ 
Published: February 2015

In producing wells, maintaining the straightest line possible becomes a challenge when subsidence from sinking and sliding formations shifts the casing beyond usability. In soft, unconsolidated formations, subsidence movement can be as much as 18 in./yr. These conditions can lead to high dogleg severity in short distances, shifting the casing so much that production is restricted or lost altogether.
In such cases, the operator’s objective is to reduce the severity of the dogleg sufficiently to run a remediation liner and restore production. Traditional solutions have had only limited success, in part because of extreme side-load stresses that can cause assembly connections to fail. A conventional milling process uses compression, which, when coupled with the shifted casing, creates a side load on the connections in the work string, causing failure. Another deterrent to success is loss of communication with the wellbore below the shifted section, which results in unintentional sidetracking.
These attempts to mitigate deformation by using traditional milling-assembly procedures can leave wells inaccessible. An alternative is to create a sidetrack above the shifted casing, allowing the well to be redrilled; although this solution has been successful, the expense of equipment and process make it cost-prohibitive in many wells. Too often these predicaments, coupled with inadequate mitigation procedures, force operators to plug and abandon the well.

Broach Milling—System and Procedure
The need to provide an efficient and affordable solution was the driver behind the development of Weatherford’s PowerStroke milling system. The system provides a single-run straightening and aligning solution for casing sections that have shifted or become deformed as a result of formation subsidence. In this system, 34-ft flexible broaching sections are dressed with a tungsten carbide cutting surface. To supplement the downhole configuration, the system is equipped with flexible, weighted, 30-ft sections that provide the broaching sections with weight. These standard section lengths can be shortened or lengthened if necessary. The flexible weighted sections are easily installed because of their modular design, and the variable-length broaching-mill assembly can be customized to address long or short casing-shift requirements.
The weighted sections hang freely in the well and supply pull-down weight. This configuration forces the broaching mill into the shifted-casing wall, putting pressure on the tungsten carbide and creating the cutting force. Broaching considerably reduces conventional milling time. Another feature of the system combines a right-left helix for quick cutting action of the carbide. The left lay cable also facilitates right rotation.
In addition to solving the problem of wellbore sinking and shifting, the system and procedure can also be used to remediate collapsed and buckled casing, as well as to address severe scale corrosion. Broach milling allows relief of side-load stresses before rotational milling begins. Side-load fatigue on the connections is eliminated by using tension, rather than compression, to implement the broaching process. The broaching procedure (Fig. 1) reciprocates the tool assembly to mill a slot in the shifted or offset casing section. After sufficient offset stresses are relieved, an integral stinger is lowered through the milled slot, into the wellbore. This stinger induces a side-load force for milling and serves as a guide for the system. Because the process minimizes side-load stresses to the stinger, the mill can continually open and align the shifted casing without causing fatigue failure of the assembly. The stinger acts as a lever to force the mill against the casing wall, and, because it contains no connections, none are subject to fatigue.