What's new in production

	Vol. 227 No. 4
VICTOR SCHMIDT, DRILLING ENGINEERING EDITOR

Metallic squeeze. What do you do with a leaking well? Not one that just has a casing head or annular pressure leak, I mean one where well fluids have channeled up or through the exterior cement and are causing a surface problem around the conductor. Cement squeeze, right? Well, right, most of the time.

Occasionally, there are wells that don’t respond to traditional cement squeeze techniques. Multiple cement squeeze jobs later, you’ve spent thousands of dollars and still have a leaky, problem well. What do you do then?

This was the situation in Western Canada, where leaky wells with gas migration problems and vent gas flows were contaminating potable water reservoirs and creating fire hazards. Canadian regulators are very strict on fixing such wells. The problem can make an operator’s pocketbook very light from repeatedly squeezing an older, leaky well, according to Homer Spencer of Canitron Systems Inc. in Calgary, Alberta.

Fig. 1. An inductive heating system carries the billet down the casing to the well’s trouble zone, where the billet is melted then forced outward.

His company has developed a method of sealing leaking wells, using an alloy of two metals – bismuth and tin. The patented system uses a 58%/42% blend of the metals in billet form that can fit into casing. The alloy is similar to solder with a low melting point of 137°C (278.6°F).

The billet is solid, so it is easily delivered to the source of the well’s leak, and only becomes liquid when heated. Delivered directly to the problem site by wireline, an inductive, electrical heating element carries the billet to the well’s trouble zone. There the alloy is melted and forced outward under pressure, Fig. 1.

The procedure requires a water pad above the tool and uses normal pump pressure to squeeze the molten metal outward across the trouble zone. This action produces a metallic, impermeable seal that fills small channels in the cement outside of the well’s casing. Billets can be staged to seal larger vertical zones.

The company has developed electrical induction tools that can melt the alloy out to three casing strings. This provides wellsite flexibility for reheating and liquefying a pre-installed seal, if re-squeezing is needed.

In an interesting historical note, Spencer located a 1944 US patent that described a process similar to the one he devised. While researching for suitable alloys, he came across a patent, number 2,366,269 (for the curious), that was granted to Marcel Schlumberger of Paris, France, (yes, the very one!) for a “Method for Sealing Borehole Casings.”

In the patent, Schlumberger mentions low melting point alloys, such as lead-based mixtures, bitumen and sulfur as possible sealing materials. He preferred sulfur, because of its availability, low cost and low melting temperature.

The patent describes methods for delivering sulfur downhole. The sulfur could be melted at the surface and delivered by a ladle device. It explains how to use an exothermic reaction alternative that would melt the sulfur downhole using an ignitable material. It also describes how to use solid sulfur disks melted with a resistance heating element. In all cases, the melted sulfur would be squeezed outward to seal channeling in the cement behind the casing.

According to Spencer, the molten alloy he uses has a very low surface tension and viscosity that fills small fissures efficiently. At 8.7 g/cc, the metal is dense, so it displaces any formation fluids that might block the micro-channels in the annular cement. Also, unlike cement, the metal does not go through a weak transitional phase during solidification. The alloy also expands about 1% as it cools, which increases the seal’s tightness. Any metal that remains in the casing’s interior is easily milled, because the alloy’s hardness is similar to aluminum.

The company’s Casing Annulus Plugging System well remediation process evolved from its work on induction heating tools, which was originally directed toward downhole, heavy oil stimulation.

The company’s process is being used by several operators, including Talisman Energy Canada, which was the first company to use the sealing system in 2004, and Encana, which uses the system routinely. The sealing system is being used on two to three wells per month, says Spencer. Operators are also finding new uses for the sealing technique. In perforated zones that are producing water, this system can seal off the water in the lower part of the perfs. Casing collar leaks from improperly made-up strings can also be sealed without having to pull and remake the casing. Even water wells have been sealed by isolating the top of the reservoir zones, thus preventing vertical channeling.

I don’t have any personal experience with this technology, but across the globe, there are many older, leaking wells that could benefit from this simple approach to surface leak problems. Even older offshore wells, like those in the mature Gulf of Mexico and similar regions, could benefit from this sealing technology.

Canadian oil shale. Royal Dutch Shell is taking a full ownership position in an “oil sand” trend of Canada. Using Shell Exploration, its US subsidiary, the company bought heavy oil leases for 10 land parcels west of Fort McMurray, Alberta, for $465 million. RDS is setting up a new Canadian subsidiary to begin extracting oil trapped in the Grosmont limestone. The formation holds an estimated 300 billion barrels of oil, but the new acreage controls only a part of the trend.

Producing the oil will be difficult. The limestone cannot be easily mined and dissolves if water is used, which negates the use of steam methods. Some other type of enhanced thermal recovery technology will be used to develop the resource. The project will begin production in the next decade, according to the company.