More than just sand – the importance of proppants
Today we talk to Unimin’s Vice President of Strategy & Development, Sameer Rupani, to find out more about the role proppants play in the hydraulic fracturing process. Unimin is the world’s leading producer of quartz proppants (frac sand) and a major producer of resin coated sand proppants for oil and natural gas stimulation and recovery.
Monica Thomas (Shale Gas International): Why are proppants – frac sand for example, critical for hydraulic fracturing operations?
Sameer Rupani (Unimin): Nature abhors a vacuum and therefore fissures created underground during hydraulic fracturing would close up if there wasn’t anything to keep them “propped” open, hence the term “Proppants”, because they literally prop open the fractures enabling the oil or gas to flow out. They play an important role because without proppants oil and/or gas production from shales would not be feasible.
MT: What are the pros and cons of using sand as a proppant? Why would you choose to use sand rather than, say, ceramic proppants?
SR: The most important reasons are that it’s cheap and abundant. It does the job and it is available in the quantities that are required. This does not mean, obviously, that all of the sand that is available is suitable for use as proppant – otherwise we wouldn’t be shipping frac sand the distances that we do – but its low cost, fit-for-purpose and availability certainly go a long way to account for its popularity.
Ceramic proppants, on the other hand, are much more expensive. The difference in cost of ceramics versus sand varies but it is somewhere between 3:1 and 6:1.
MT: You mentioned that not all sand is suitable for fracking. Could you tell us more about what makes a good proppant?
SR: One thing to understand is that when E&P (exploration and production) companies purchase proppant what they are really buying is conductivity. To better appreciate this concept, you need to understand the difference between conventional and unconventional reservoirs. A key difference being the natural flow rates between the two types of reservoirs.
In simplified terms, a conventional oil and gas reservoir like in Saudi Arabia for example – has a lake of hydrocarbons underneath the surface of the earth where you just drill down into the reservoir and oil or gas starts flowing (with the aid of a pump aka artificial lift).
In the case of unconventional deposits – like shales – the situation is very different. Rather than deposits that look like an underground lake you’ve got something that looks more like Swiss cheese. What I mean by that is you have a body of low permeability rock with pockets that contain oil and gas. These pockets are small and randomly dispersed in the matrix of the rock. This is why horizontal drilling and hydraulic fracturing are such great innovations. By the way, people think that we’ve had the shale revolution in the last five years, but in fact the first frack in the U.S. was done in the Barnett Shale in the mid-50s. That technology has actually evolved over the last 50 plus years.
Horizontal drilling allows you to access the pockets of oil and gas that would otherwise be difficult and expensive to reach – simply uneconomic. If the pockets of hydrocarbon are not where the wellbore is, we can access them by using hydraulic fracturing – by creating trees of fissures – called perforation clusters which are further expanded by a technique called stimulation. In this process proppant is pumped through the well-bore into the fissures to keep them open – to enable flow of the oil or gas which would otherwise be trapped in the low permeability rock.
So E&P (exploration and production) companies are really buying conductivity when buying proppant. Using the right kind of proppant facilitates the flow of hydrocarbons from these dispersed pockets through a more conductive proppant pack and into the wellbore.
The “right kind of proppant” needs to have several characteristics that are crucial to its effectiveness.
One of these characteristics is sphericity. When we talk of hydrocarbons flowing in the formation, they are actually flowing through the interstitial space between the spheres. If the proppant is angular in shape, it would not create the spaces through which oil and gas could flow.
Another thing that makes sphericity important is the fact that a sphere tends to withstand and distribute stress better than any other shape since it doesn’t have any weak points such as the edges in say a triangle. A suitable proppant must also possess the crush strength to withstand the closure stresses applied by the reservoir, and hence maintain conductivity.
MT: Can I ask you about other characteristics that make for an effective proppant?
SR: Acid solubility is another important feature. Given other materials used in frac fluids it tends to be acidic. So obviously the last thing we want is for the proppant to dissolve in the ‘acidic’ frac fluid. And if you have something on the actual proppant grains – say, clay deposits or something else that will disintegrate – it may actually block up the interstitial spaces through which oil and gas would flow.
Another characteristic that is similar is turbidity. Turbidity indirectly measures clay content of the proppant via the optical clarity of a solution into which proppant is added. Essentially what you’re looking for is a clean proppant particle (sand grain or otherwise) of a specific size, with nothing else on its surface that can dissolve or breakaway and in the process contaminate either the frac fluid or reduce the conductivity of the proppant pack.
MT: With so many requirements that frac sand should meet, can you tell us a little about how this sand is sourced?
SR: The best fracking sands in the world are found in the Upper Mid-West region of the USA. This is because Wisconsin and Minnesota were beach front property around 500 million years ago. Silica sands in the region were rolled over millions of years by wave action resulting in round, spherical grains – this is ideal for proppants. These are what we call Northern White sands, which are known for high purity, sphericity and strength.
As a global mining company, Sibelco, Unimin’s parent company has explored and located fracturing sands in Europe (UK, Poland), Latin America and Australia. But the global benchmark remains the Northern White frac sands found in the states of Minnesota, Wisconsin and Illinois in the USA.