API Proppant Test Not Looking So Strong
Authors: Rassenfoss Stephen 
Published: May 2024

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The short answer for how to judge proppant strength sounds simple—just ask for the K-value.

The K refers to the weight in thousands that a sand sample can withstand in a crush test defined by API STD 19C covering “testing sand used in hydraulic fracturing operations.” For example, a sample that passes an 8,000-lb crush test but fails at 9,000 lb is an 8-K sand.

The goal of the standard is to measure how much weight a sand sample can withstand when used to prop open fractures to allow oil and gas to keep flowing. But a recent paper argues that measuring sand strength by the number of fines—small grains created by crushing—is an erratic indicator.

The SPE paper delivered at the 2024 SPE Hydraulic Fracturing Technology Conference described a blind test where a Permian sand supplier, Atlas Energy Services, sent out 120 samples of variously sized sand grains to four testing labs. The results for identical samples varied widely, even within the testing labs (SPE 217767).

“We are seeing 2K to 3K variation internally at independent labs on split samples, and at least 3K variation between labs,” said Ian Renkes, the paper’s lead author who is regional continuous improvement manager for Atlas in the Permian, adding, “This makes it very difficult if not impossible to differentiate between sand providers.”

For Atlas and other sand mines competing for business by focusing on delivering quality sand, this is frustrating because the inconsistent testing undermines those efforts.

There was an interesting detail near the end of the paper suggesting a large operator also has concerns. The list of references began by crediting “Todd Cage of ConocoPhillips who performed the initial work that provided the basis of this paper.”

Cage, ConocoPhillips’ commercialization manger in Midland, did not comment, but his role in devising the test suggests more than a passing interest in sand testing.

Keeping It Secret

For the test, Atlas created large samples in three mesh-size ranges—30/50, 40/70, and 100-mesh (40/140), using sand mined and processed in Kermit, Texas.

Each of them was measured 30 times using the API 19C methodology. They then split them up into smaller packages. Each of the four testing companies ultimately tested 30 packages.

To conceal the fact this was a test, they sent out two to seven samples at a time to labs over a 6-month period. The paper said, “Care was taken when sending the samples out to label them, so the laboratories were not aware the samples in each batch were identical splits.” This method is referred to as “round-robin” testing.

Charts tracking the results show large variations in the test results. The critical number in the crush test is the percentage of the sample made up of fines after crushing. API rules define fines as grains smaller than the smallest grains in the original sample.

For example, when 40/70-mesh grains were tested using 8,000 lbs of force by the company identified as Lab 2, it reported measuring about 50% more fines than the other three labs. Its results, which were all around 15% of the whole, would mean those samples failed to pass at that crush weight. But the other three labs were mostly clustered just below the 10% level, suggesting most of those samples would pass.