Only In Canada, Eh?
EARLY TRIALS SUGGEST A NEW LIGHTWEIGHT cement containing unique mineral mined
in Western Canada may facilitate well-completions for Canadian producers,
including those working on coalbed methane (CBM) plays.
And the product's initial success has its developer, Halliburton, planning to
export the technology worldwide.
VariSeal, which incorporates zeolite, is designed to minimize formation
damage during cementing, while allowing well operators to adjust cement
slurries for changing well conditions without using other blends or
additives.
"You can take one blend out to the wellsite, [rather than] two," says
Halliburton's James Griffith, technical manager for Canada, who considers
VariSeal's ability to circulate to surface its main advantage over other
lightweight cements. Many blends use Portland cement, with such lighteners as
fly-ash often added.
Getting cement returns to surface is more than a practical advantage under
Alberta Energy and Utilities Board rules for cementing. For the EUB, a
primary goal of cementing well casing is isolating shallow freshwater
supplies from lower zones that might otherwise leak hydrocarbons or brackish
water. Poorly cemented wells encourage such leakage, potentially
contaminating freshwater supplies.
Griffith says VeriSeal, available in five different blends, was tested in
some 300 Canadian wells approximately one year before becoming commercially
available in October. Of these wells, roughly 75% were conventional oil and
gas wells, according to Griffith, with a number of CBM wells.
Of the five blends, only VariSeal CBM, as the name suggests, is tailored to
coalbed methane wells. In particular, it accommodates the fragile nature of
many coals, a quality especially apparent during cementing, when slurry that
was meant to travel up the annulus to surface has in the past often entered
the formation.
In some cases, the leaked cement damages the coals, plugging pores and
channels that would otherwise carry methane from formation to wellbore. As
well, cement that does not circulate back to the surface often indicates that
remedial cementing will be necessary later, further adding to the operator's
expense.
Since entering the CBM field in 2003, APF Energy Trust has had a chance to
try VariSeal. Last Year, APF's drilling superintendent, Ed Chapin, supervised
four CBM wells around Innisfail, Alberta. VariSeal CBM was used on just the
last well, drilled to roughly 850 metres, penetrating a coal seam of three to
four metres thickness at a depth of 375 metres.
The seam was "quite a bit higher in the well and [we] couldn't get cement to
surface because of it," says Chapin. "The bottom would drop out of our
[cement] column," something he attributes to the easily-fractured coals of
the Edmonton Group. "We tried other lightweight cements and VariSeal, on our
last well, was the first one we got cement to the surface on."
While Chapin says Halliburton's blend is more expensive than other
lightweight cements, he still considers it economical if all costs, including
the cost of remedial cementing - work that could later prove necessary under
other circumstances - is taken into account.
UNIQUE MINERAL. While lightweight cements have been used in well operations
for years, the downhole use of naturally-occurring zeolites is more recent.
The mineral's absorptive properties are key to VariSeal's ability to contain
water in the slurry, avoiding the formation of free-water channels that could
later become conduits for leaks.
Discovered by Swedish scientist AxelCronstedt in the 18th century, zeolites
drew little attention in North America for many years. Today the minerals,
named for the Greek word for "boiling stones," are mined from high-quality
deposits in British Columbia and Alberta, where they are also processed.
Three-dimensional. microporus crystalline solids with well-defined
structures, zeolites both occur naturally and can be produced synthetically.
Containing void space in their structure that can host water, cations or
other molecules, zeolites have a high-affinity for water. They can hold more
than 25% of their weight in water and have the ability to absorb and loose
water without damage to their crystal structures.
According to Halliburton's Griffith, zeolites give VariSeal properties which
conventional lighteners, such as fly-ash and silica, do not possess. Also,
compared to a conventional cement slurry, the product has a lower density and
viscosity, traits that explain its ability to facilitate circulation to
surface.
Griffith credits zeolites for another VariSeal characteristic. In
environments where downhole temperatures exceed 100 C, the compressive
strength of Portland cement declines and operators often use silica to
counter the problem. Yet, VariSeal Thermal, another of the blends, resists
"strength retrogression" at high temperatures according to Griffith, making
steam-assisted gravity drainage (SAGD) and heavily oil applications a natural
fit.
VariSeal's compressive strength can range from seven to 21 megapascals,
exceeding the EUB minimum within the required setting time, he says.
The product can also be used when mercury falls. At temperatures of five to
10 C, a range that presents challenges for conventional slurries, Griffith
says VariSeal functions well, helped by downhole geothermal heat that
counteracts the cold, and by Halliburton's practice of heating its water
before mixing cement.
"We normally do cement jobs in harsh conditions, [and] we don't treat
VariSeal any different when it's 25 below as opposed to plus 25 [C],"
Griffith adds.
A third VariSeal blend, designed for conventional oil and gas wells, may have
the broadest application of all, if recent interviews are any guide. A
consultant who would not be named discussed a client's use of the product in
remedial cementing for groundwater protection. Cement squeezes were done at
approximately 600 metres depth on a total of 20 oil wells, ranging in depth
from 1500 to 5000 metres.
"When we go [to a wellsite], we don't know whether we're going to do a
circulation squeeze or a block squeeze," the engineer says. "But the density
of the cement is variable just by changing the water-mix ratio. We can use
the same blend regardless of [the] job, so there are cost savings there."
Also the opinion that VariSeal works well at cooler temperatures, the
consultant says it shows up better on cement bond logs, something attributed
to its having more compressive strength than comparable lightweight cements.
Yet, cost-wise, the engineer feels the product is, all in all, cheaper than
competing lightweight cements, due to a perceived higher yield and fewer
additives.
Like certain tea blends, VariSeal is currently only available in Canada. That could change soon, however, according to Halliburton's Griffith.
"It is such a step-change in technology, we are going to be offering it around the world. It will be aggressively moved into other places where it's applicable," he says.
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