Frac sand attrition scrubbing 101
Published: Mon, 11/17/14
cemented to each other, and then often cemented to a large sand
particle. These clusters are commonly formed when in a sandstone
formation, but even loose sand deposits can have some cementing and
clusters.
Frac sand has to perform downhole under tremendous pressure, and
under pressure the clusters "blow up," breaking apart into
the smaller particles and fines. This results in poor performance.
We have seen it time and time again in the lab with crush resistance
testing - the frac sand crush test performance goes up dramatically
as the number of clusters approaches 0%.
Go here to see a picture of a sand sample (otherwise high quality
sand!) with many, many, many clusters:
http://www.globalenergylaboratories.com/frac-sand-attrition-scrubbing-101/
So, when hiring a lab to test your sand, it is of utmost importance
for your sand sample to be cleaned of clusters. The American
Petroleum Institute recommends that "the sand should not be
considered suitable if it contains 1 percent or more by count of
clusters of multiple sand grains." (Reference: API
Recommended Practice 56 - Recommended Practices for Testing Sand
Used in Hydraulic Fracturing Operations).
If you are headed towards frac sand production, you can acquire
large industrial scale attrition scrubbers. The key to attrition
scrubbing is creating a "pulp on pulp action" where the
flows of sand and water (or in some cases acid-water) flow against
each other, and the sand is in a way sandblasting itself and
removing the clusters.
How do you create this counter opposed flow?
You can think of two blender blades next to each other - or with one
inverted over the other - operating in reverse directions. Then
there will be a "collision" of sand and water streams as
the blades each send their own mixtures at the other. It's not
really the blade itself removing the clusters, but the sand-on-sand
action!
We recreate the industrial scrubbing action as best we can here in
the lab - on a much smaller scale.
Instead of tons of sand, we typically deal with 400-500 grams of
sand per batch, and put our scrubbers on high. We sometimes add
acid to the mix to help break down the cement.
Then we wash your sand again (removes any smaller than 200 mesh
fines), dry it, and then take a good luck under the microscope.
If clusters remain, we do the process again. By now we have the
process fine tuned, though and can estimate our scrubbing times
accurately.
Sure, this scrubbing trial and error might cost you a bit more in
testing (at least $150 per sample, at the time this post is
written), but your material will perform much better.
And that's a main tenet of our approach to client work here at
the lab - enhance your material as best we can (using good
engineering and laboratory practices) so your material performs as
best as it can.