Category Archive: Frac

K-Value and Microproppants

It’s 4:00 am, there is a test in four hours. You still have to shower, make coffee, feed the dog, get the kids to school, and drive 45 minutes to the testing facility. That leaves you with 13 minutes and 27 seconds to understand proppants.

Don’t worry, there is only two things that you must know about proppants and why they work.

  1. Proppants’ first job is to open the earth below and keep it open (“Prop” it open).
  2. After the proppants accomplish the first job, their second job is to allow the gases and liquids to flow back to well head. They need to keep the well permeable.

Done with at least 6 minutes to spare.

Bad news, the world of proppants can be a lot more complicated than that. Good news, you don’t actually have to take that test.

One of the more difficult things about hydraulic fracking is understanding what is actually happening several thousand feet under the ground. Cameras do not work under those harsh conditions, and you definitely cannot send someone down to watch.

The easiest and most cost effective way of understanding things we cannot see is to model key aspects of the environment below the surface of the Earth.

One test mimics the pressures of 2-3 miles of dirt, rocks and water, ISO/API’s Proppant Crush-Resistance Test. This puts the proppants under different pressures and checks proppants’ resilience to these forces.

The test starts by prepping the proppants to make sure there are no particles smaller than a 200 US mesh screen. Then the sample of the proppant is placed onto a smooth steel plate within a cylinder. A piston is used to apply and hold specific pressures for two minutes. These pressures start at 2,000 PSI and step up every 1,000 PSI.

After performing the test, the proppants are sieved to see how many particles pass through the 200 mesh.

What does this show? Before the pressure was applied, the proppant sample was sieved, removing all particles smaller than 200 mesh. If any new particles pass through the 200 mesh sieve after the test, it was due to the proppant literally crumbling under the pressure. When the crushed, pulverized, and destroyed particles surpass 10% of total weight of the sample, the proppant is considered unusable at those pressures.

When the individual grains of the proppant crumble, break, and fail, they leave behind small broken glass-like pieces. These shards block the passage of the oil and gas through the proppant, not allowing the well to produce as much of the commodity as it could. Also, the irregular shape of the shards do not add any strength to the other proppants in the area. Meaning the other particles have to hold more weight. With the increased weight, the greater chance the other particles also crumble, break and fail. It becomes a feedback loop that causes less output in the real world.

Now enter microproppants. Microproppants are particles smaller than US Sieve 140 mesh. What else is smaller than US Sieve 140 mesh? US Sieve 200 mesh.

The ISO/API crush-resistance test removes would be microproppants before it can be evaluated. Making this test as-is, irrelevant to evaluate microproppants. The solution to adjusting ISO/API crush-resistance test would be to lower the sieve size to a US Sieve 270 mesh or 325 mesh depending on the microproppant.

Unfortunately, not everyone that makes decisions on the best proppant for their wells understands the ins and outs of this process. Those that depend heavily on ISO/API’s method to discern what is a good or bad proppant when looking at microproppants might miss out because of the bureaucratic nature of depending on specification sheets.

Another quagmire with microproppants as an emerging technology is silica flour. Silica flour is being pushed as a microproppant.

What is silica flour? Silica flour is where silica sand is crushed, pulverized, and destroyed in a milling process. Silica flour is the exact particle that ISO/API’s crush-resistance test looks for.

Silica flour fails at proppants’ first two jobs of holding open the shale and allowing the petroleum to pass through it.

When evaluating possible microproppants, look for wholegrain products. Ask for ISO/API results for the microproppant. If there are no results, it shows that the microproppant cannot pass the tests at any capacity.

Microproppants are a new and exciting development in the oil and gas world, but be careful when deciding what to use for the horizontal wells.

Roundness, Sphericity and Microproppants

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.

API Frac Sand Standards and Microproppants

There has been a slowly developing trend in proppants in the oil and gas markets. That trend, the size of the silica sand. The market has demanded a smaller and smaller size of silica sand as the industry has grown and evolved. The size of the sand used in fracking started with mesh sizes like 16-30, 20-40, 30-50 standard sieve size mesh. Eventually, 40-70 and 70-140 mesh size silica sands became the norm when propping open a well.

At each shift to smaller sized silica, oil and gas wells increased production in both barrels per day and total life of the well.

Does this make sense? Let me ask you another question, that might help understand this trend. Would you rather lie on the tips of three landscaping spikes or bed of thousands of nails? The same reason that a bed of thousands of nails is much preferred over the few spikes, the smaller silica sand grains work better to hold open the cracks in the shale after the fracking process. The smaller silica can distribute weight of the cracks more evenly over each grain of sand. This increases the amount of pressure the silica sand can withstand before crumbling to dust.

The mesh sizes like 70-140 grain size silica can penetrate smaller cracks that happen during the fracking process. This increases the total open surface area of the shale for oil and gas to flow to the well head. The more surface area, the higher production of each well. Halliburton has marketed a product service known as MicroScout which has shown improvements in well production.

Another benefit of using smaller sand is the distance the sand can be carried further away from the well head. This is known as the “Settling Velocity Rate” (“SVR”). The larger and heavier the sand, the quicker the sand settles in slurry. The smaller sand stays in suspension and can travel further, thus leaving more surface area propped open. With the understanding of these ideas, the fracking world has started to develop the concept of microproppants. These are particles smaller than a 140 mesh grain. This idea makes sense, but do the normal material testing methods for frac sand work with the smallest of silica?

There will be a series of articles that explain and breakdown each American Petroleum Institute test methods for frac sand. Then compare to see if those tests correlate to smaller grain size silica (also known as microproppants).