Imagine a world where rocks are just rocks, or dirt is just dirt. Now jump forward to today where the name “biatite muscovite hornblende porphyritic granodiorite” exists. Why did the science of studying the earth get to a nomenclature. Simple answer, so two people discuss what they are observing at two different places. How did the geology get a to five-word, forty-nine letter description?
One step in that process of classifying rocks and minerals is called Sphericity and Roundness. This helps describe the shape of gravel, sand and other aggregates.
The purpose of using sphericity and roundness is to describe the shape in two different dimensions. Sphericity measures how close the shape of the particle is to a sphere.
Roundness describes the edges of the particle. It is a way of distinguishing a rhombus and football, or an octagon and circle without actually counting the different sides.
There is also a mathematical way of determining both the roundness and sphericity of an object by measuring different dimensions of the particles. Those equations can be found here.
An easier and more commonly used method is a visual test. The Krumbein roundness and sphericity chart has been widely adopted as a visual method of determining the shape of an aggregate. That chart can be seen in Figure 1.
Why is this so important for fracking oil and gas wells? Two reasons: Strength and permeability.
Silica sand’s ability to withstand high compression stresses is very dependent on both its roundness and sphericity. If the sand is low on the sphericity and roundness chart, this particle will have a lot of sharp edges. If the compression force is directly applied to an edge, the stress concentrations increase exponentially. When stress gets too high, the sand crushes like a glass bottle under a steam roller. When the sand shatters, it increases the amount of particles that are jagged and not round. A domino effect of sand breaking and crushing happens until the only thing that is left, is a vain of silica dust deep under the earth’s crust.
If the purpose of the silica is to hold open or prop open the cracks in the shale, does it matter if the silica is dust?
The first purpose of a proppant is to prop open the cracks in the shale. However, there is a more important property of the sand; to allow the oil and gas to flow back to the well head. To do that, there needs to be gaps between each grain of sand to allow for the fluid to flow back to the opening.
Imagine stacking basketballs in the shape of a pyramid. You will notice a significant gap between each ball. Instead of basketballs, imagine stacking Lego blocks in that same shape. There would be no gaps between the Legos. The same thing happens with sand, but at a much smaller scale.
As discussed in the article entitled API Testing Standards and Microproppants, sand particles smaller than US Sieve size 140 mesh are referred to as “microproppants”. There are silica products smaller than 140 mesh, but the silica is “ground” or “milled” in most instances. These ground silica products are referred to as “silica flour” and can be found in 200 and 325 mesh sizes. But is this what is needed?
Silica flour is made by crushing silica sand to a dust. The particle shapes of silica flour is very low on both sphericity and roundness chart. The particles are flat and jagged, just like broken glass. If silica flour was used in a well, it would have a extremely low crush strength. More importantly, it would have almost no permeability. The oil and gas would not be able to flow to the well head.
When looking for a microproppant, the silica sand needs to be a whole-grain silica. Below is a picture of a 200 mesh whole-grain silica. The dark column is a 0.7mm pencil lead for size reference.
Whole-grain microproppants have roundness and sphericity and therefore have a higher crush resistance than ground silica flour. This adds to the productivity and the life of the well.