by regular means or had steam injected
into it.
amount of gas needed, except that we are
pretty sure that the more, the better.”
of low viscosity oils,” he explains. “As the
gas flows to the wellbore in the formation,
it is in a foamy state with the oil and the
water, and perhaps a small amount of
entrained sand.”
“Steam injection knocks out solution gas, The gas helps the mixture of heavy oil and
and depletion drains it. CHOPS requires gas sand move through the reservoir to the
in solution to keep the sand flowing to the production well through what Dusseault
wellbore. If there is no gas in solution, calls the “foamy oil” mechanism. In While gas must be present in a CHOPS
CHOPS does not work because sand influx conventional low-viscosity oil, gas mixed in reservoir, free water must not be. If water is
cannot be sustained,” Dusseault says. “How comes out very quickly, while in heavy oil, it present even 500 metres away laterally, it
much gas is required? We know that a lot of forms microbubbles that grow slowly and could have a significant impact on the
gas is better than some gas, but CHOPS have difficulty connecting with each other. operation.
apparently can work effectively in shallower
reservoirs where there is less gas in solution. “The gas stays in a bubble phase far longer “It will likely channel into the well and the
There are not rigorous, fixed limits to the than one would expect from the behavior water production will rise, largely at the
expense of oil production.”
As well, Dusseault explains that the ideal
CHOPS reservoir should be neither “too
shallow” nor “too deep,” but adds that the
industry does not yet fully understand the
depth limits in specific cases. Industry does,
however, understand very well the concept
of CHOPS and how to use it effectively.
“The objective is to produce the reservoir at
as high a rate as possible without damaging
the well or reservoir cap rock,” says CNRL.
“To maximize overall recovery from a pool,
development should occur on relatively
tight [well] spacing and over as short a
period of time as possible.”
Once produced, the sand is separated using
large vertical stocktanks that are heated to
90 degrees Celsius, Dusseault says, designed
to lower the viscosity and accelerate the
settling rate of the sand in the hot oil.
Operators then have three options for
disposal of the waste sand: landfilling, salt
cavern disposal and slurried solids injection.
“The development of salt caverns for
disposal of produced sand has proven to be
important to keep costs down,” CNRL says.
In Alberta, CHOPS production generally
occurs in the area around Lloydminster and
Cold Lake and not in the
oil-sands-bearing regions. Dusseault says
the technology is not suitable for the oil
sands areas because of viscosity issues. The
upper limit for the viscosity of a successful
CHOPS operation is between 10,000 and
20,000 centipoise (the common unit for
expressing absolute viscosity), while the oil
sands regions have viscosities exceeding
50,000 cP. In the Athabasca region, viscosity
is generally more than 500,000 cP.
“The gas does not come out of solution and
form bubbles fast enough, so the benefits of
the foamy oil phase are reduced. Also, if the
gas bubbles do not form fast enough and
the oil is too viscous, instead of the CHOPS
process propagating far into the reservoir
