Determining the Quality of Your Proppants

What is fracking?

Hydraulic fracturing (fracking) is a process for drilling and extracting oil or gas from shale deposits. This process has been growing for several decades, but especially rapidly in the last 5 – 10 years. In the past, fracking was the second most expensive extraction method, behind Canadian oil sands. Recent downward pressure on oil prices forced many operations to be closed or temporarily suspended over the last year. This encouraged many major oil/gas producers in the US and Europe to invest heavily in lowering fracking costs by advancing the technology. This progress has reduced break-even costs from about $60 per oil barrel (bbl) to the low $50s. Lower costs have made fracking competitive again under the current stabilizing cost of oil. The race in the industry to open and staff fracking operations, and, in preparation, to enter into long-term contracts for frac sands, or proppants, as the industry refers to them.

The fracking process injects water containing small amounts of chemicals and large quantities of proppants, under very high pressure, into shale deposits which end up being fractured to be able release the oil or gas back to the surface, as depicted in the diagram below.

Diagram of Fracking a Shale Deposit
Fracking a Shale Deposit

What are Proppants?

The fissures produced are propped open by the “proppants” to keep the fissures from collapsing and to ensure and maintain good convective flow to the surface. To optimize that convection, proppants must pass two specifications of shape factors and have a narrow size distribution. The proppants must have smooth surfaces and must be much more spherical than oblong in shape. The first shape characteristic is known as roundness and the second as sphericity in particle image analysis nomenclature.

Roundness and sphericity have been determined in this industry since the 1960’s by visually comparing a small number of proppant particles with 2-dimensional images on a chart displaying different values of those parameters. This chart was developed by Krumbien and Sloss, early pioneers in setting the existing specifications. See illustration below.

Krumbien-Sloss Chart for proppant shape parameters

The Krumbien-Sloss Chart for proppant shape parameters. The image at the top right represents the best particle shape, in both roundness and sphericity, for optimum convection of oil or gas through a bed of proppants. The current API (American Petroleum Institute) Standard calls for the particles of the sample examined under a microscope to show at least as good images as the image in the chart at 0.7 roundness and sphericity.

Size distribution measurements required by the API for proppants

The size distribution measurements required by the API for proppants (frac sands) in the drilling and extraction processes from shale-based oil and gas deposits. These sizes and more than 15 different shape parameters, including roundness and sphericity illustrated in the above chart, can be measured in very large sample sizes in minutes.

Problems with the existing standard are:

  1. The improbability of such a small sample being representative of an entire batch of proppants
  2. different analysts having different subjective opinions of which particles pass the spec
  3. the length of time it takes to make the measurement – physical sampling, manual microscopic examination, making the judgement on each particle, and reporting the results

There is now a commercially available automated analytical instrument which measures size distribution, and many shape parameters, including roundness and sphericity, of very large samples, hundreds of thousands or more, in a matter of minutes. Looking for detailed information on this 3-dimensional dynamic image analyzer?

Click here for info

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