Operators of cyclic steam stimulation (CSS) wells are required by Alberta’s Energy Resources Conservation Board to do periodic casing integrity pressure tests.

The idea is to ensure well casing hasn’t developed a leak during high-pressure steam injection over many years of thermal bitumen operations.

Each test costs about $50,000 — mainly because a service rig is used to remove the well production equipment and install a test string with an inflatable packer. The packer is inflated and the pressure test is done against the packer. Then the packer is deflated, the test string is pulled out, and the production equipment is put back into the well.

In the process about a day’s production time is lost. Service rigs are complex and have had their share of accidents over the years. In some areas thermally stimulated bitumen production projects are starting to encounter hydrogen sulphide, necessitating the use of self-contained breathing apparatus (SCBA). (In the case of CSS, exposure of bitumen to heat from high-temperature steam injected into the reservoir causes the breakout of H2S.)

Dale Kunz, a certified engineering technologist with several issued or pending patents to his name and a background in CSS, believes he has come up with a way to do the casing integrity pressure tests without using a service rig.

“We eliminate the need for a service rig by eliminating the need for an inflatable packer. We replace the inflatable packer with an annular eutectic salt plug,” Kunz explained at a Petroleum Technology Alliance Canada (PTAC) technology information session last week.

A eutectic system is a mixture of chemicals which, when two or more compounds are combined, will melt and solidify at a lower temperature than either chemical would do by itself.

In CSS wells, the plug would be placed by injection of a molten eutectic salt into the annulus through a permanently installed tubing string.

Kunz’s proposed design incorporates a heater, or melter, at the surface. When the system is pressurized with nitrogen and a rupture disc is broken, the melted salt is pushed down the injection tubing to a salt distribution nozzle located where a packer would currently be installed.

“A characteristic of eutectic salt is the larger the difference between the liquid temperature and the surrounding temperature, the faster

[the chemical] change occurs. Essentially it’s a liberation of latent heat,” said Kunz. “So the material we use melts at 325 C, freezes at 260 and we’ll introduce it into an atmosphere that’s probably going to be somewhere in the neighbourhood of under 100 degrees C.”

In centipoise, the viscosity of the liquid solution is in the low single digits — “very similar to a nice Chardonnay,” said Kunz, whose company is called Winterhawk Technologies Inc.

As the liquefied salt comes out of the nozzle, it freezes onto the wall of the casing, building up a layer of solid material that eventually closes the annulus off completely. The plug is then built up to the depth needed to hold the pressure.

As it freezes, the salt expands by about six per cent of volume. Kunz said the solid salt has a compressive strength greater than Class G cement.

He said the solidification process is likely to take roughly three or four minutes — from the time the salt sprays from the nozzle until the plug is frozen in place.

Once the plug is in place, treated boiler-feed water is run into the annulus above the plug to do the pressure test, which is typically done at 2,500 pounds per square inch and takes about 10 to 15 minutes.

When the test is done, nitrogen would be injected to create turbulence in the water that would completely dissolve the salt plug in five to six hours. The test water would flow to the bottom of the well, and the well could then be put back into production. Kunz said the volume of salt is estimated to be about 40 pounds, and once dissolved it would be distributed by the steam-injection process and then recovered with the steam condensate in the production flowback.

The eutectic material used in the Winterhawk system, often known as draw salt, is a commercially available mixture of sodium and potassium nitrates, Kunz said, adding that it is commonly used as the liquid heat exchange medium in gas-fired line heaters in certain settings. He said the manufacturer lists it as non-toxic and non-flammable.

He said reservoir engineers have opined that as it is essentially naturally occurring water soluble salt, it will not affect the reservoir.

Kunz said tests by Edmonton-based C-FER Technologies found the salt fails consistently at 3,700 pounds per square inch. “[That’s] an unconfined compressive strength test. Of course in the annulus it is confined. And so we speculate we’ll probably have a compressive strength of about 5,000 psi in the annulus.”

He said an advantage of the material is that it doesn’t cling to steel when wet. If steel is immersed in a bath of the solution, 99.99 per cent of the material drains off the steel once it’s removed from the bath. “But … if you … throw a spoonful up against a cold steel surface, it freezes and adheres to it vigorously.”

A service rig would be required to install the system initially, so the installation would cost about $50,000 for the service rig, maybe $10,000 for the tubing and possibly about $30,000 for the heater and control system, estimated Kunz.

But once the system is installed, each casing integrity pressure test would cost only about $4,000, he said. That includes the material, which costs between $1.50 and $2 a pound, depending on the volume purchased.

Having done the lab tests, Kunz is now looking for operators to provide financial support and CSS wells for prototype testing.

“The question we’re looking to answer is: can we reliably form a leak-tight salt plug in the annulus of a cyclic steam injection well? And we’re pretty convinced that we can,” said Kunz.

This proof-of-concept testing is expected to cost $500,000 — about half for C-FER testing and half for equipment and supplies for field trials.

CSS operators who may be interested in supporting the project can submit expressions of interest through PTAC by contacting Marc Godin ([email protected] / 403-218-7720).

By Pat Roche, Daily Oil Bulletin