Casing Leaks
In some circumstances, Polymer Gels can be used successfully as an alternative to cement, or in combination with cement, to squeeze casing leaks. A number of processes have been developed for this application. The type of polymer, and the process used depends on the location and severity of the leak, and whether or not the squeeze will be required to hold a solid pressure, or simply block the encroachment of foreign water in a producing well. The advantage of using polymer in these applications is two-fold. Polymer can be washed out of the wellbore after a leak is squeezed, preventing the costly rig time involved in drilling out cement. Secondly, since polymer solution exerts a much lower hydrostatic weight than a cement slurry, there is less possibility of breaking down the formation and losing the squeeze. On difficult leaks, such as the salt section, where multiple cement jobs are often attempted before the leak is successfully squeezed off, a small slug of polymer can be run ahead of the cement as a buffer to prevent the cement from "running away" or washing out the section you are trying to squeeze. Since the polymer continues to adsorb or bond to the formation, and the bulk gel fills the larger voids, it is often enough to slow down the coasting of the cement, and give it something to squeeze against.
Four basic Polymer Gel Systems are in use today in casing leak squeeze operations, as follows:
Acrylic Monomer Grout
This non-toxic system is most effective on tight casing leaks and pressure leak-off situations such as leaks which bleed-off pressure but cannot be pumped into. This system pumps as a water-thin fluid, then sets up into a tough, ringing gel. Gel times can be controlled from 10 minutes to 2 hours, depending on temperature. Treatment sizes typically range form 10-25 bbl. This is an excellent application for disposal and injection wells which fail MIT's because of slight pressure leakoff.
HCLM
(High Concentration Low Molecular Weight Polymer) This system is useful for a variety of leaks ranging from tight pressure leakoff situations, to moderate leaks which can be pumped into under pressure. This system can be crosslinked using standard metallic crosslinkers, or a low-toxicity organic crosslinking system can be used in environmentally sensitive areas, or leak intervals.
HMW
(High Molecular Weight Polymers) This system is most effective in larger leaks in producing wells, to correct channeling behind pipe, and for some list circulation applications. The primary benefit of using this system is the ability to economically block the flow of foreign water into the wellbore, or block the outflow of produced fluids to thief zones. This system is most effective to block foreign water encroachment in producing wells.
CPC
(Cement / Polymer Combination Squeeze) Severe casing leaks which require mechanical integrity and are unlikely to be successfully sealed using either cement or polymer alone are candidates for this process. In most cases, a small (25-50 bbl) slug of High Molecular Weight Crosslinked Polymer is injected ahead of 50-100sx Cement. In this application the polymer acts as a filler/buffer, filling larger voids and coating formation surfaces, preventing water loss and cement contamination by formation fluids. The polymer also acts as a pad, holding the cement in the near wellbore area, where it is most effective. This process blocks the foreign water away from the wellbore, and allows pressure integrity to be obtained more cost-effectively than would be possible with cement alone or polymer alone.