Decades ago, I never imagined that my precious collection of vinyl records would become essentially obsolete. But I was excited about the advent of 8-track tapes. I could then play music of my choice while driving! Then cassette tapes replaced 8-tracks. CDs, then DVDs, came along to make those conveniently smaller cassettes obsolete, too.
Today, the thought of 8-tracks and cassettes as advanced technologies is laughable, at least to those who even remember them. Now, all that is needed to listen to music or other audio recordings, or to watch movies or sports, is a cell phone, tablet, or laptop computer. So, could something as standard in the oil and gas industry as sand proppant for hydraulic fracturing—that seemingly has no need to be replaced—become obsolete in favor of an improved “technology?” It may seem unimaginable, and maybe it won’t happen anytime soon, and surely not all at once. But it is conceivable, and perhaps it should happen.
Sand has long since been the preferred and most abundantly used proppant in hydraulic fracturing because of its availability in large quantities and cheap cost. Its only application limitation is in wells with fracture closure stresses above about 6,000 psi. For higher closure stress wells, higher crush strength proppants, such as ceramics, are needed. There are also other specialty proppants that are alternatives to sand, but they cost substantially more. So, they are not realistic replacements on a significant scale. Sand really has no competition for applicable wells, and most wells can be fractured with sand as the proppant.
But sand has issues. Sand for proppant comes from mining, which is not an especially environmentally favorable operation, with respect to carbon emissions and the eventual need for land restoration. Sand mining also uses copious amounts of water, primarily for cleaning to remove contaminants from the sand. Such substantial water usage requirements can, at least in proximity of certain mining locations, impact water supplies for household use, farming, ranching and wildlife.
With the proliferation of hydraulic fracturing, especially in the exploitation of unconventional resources, which calls for multi-zone fracturing treatments in long horizontal wells, there are more frequent sand supply shortages and delivery delays, if not outright unavailability from time to time.
And not all sand is the same. Depending on the source, sand quality can vary. Roundness, crush strength, presence of fines, etc., are not all the same from one sand to another. Sand itself, regardless of quality, is abrasive and therefore rough on fracturing equipment. Handling of sand can also be problematic, given the potential for worker exposure to silica dust, which can cause silicosis.
Is there a realistic alternative to sand? If there is a realistic alternative, it would have to be competitive in cost, with equal or superior properties. It’s difficult to imagine, but there may be one on the horizon—oolitic aragonite. Oolitic aragonite, or oolites, are egg-shaped particles that form in agitated shallow-marine waters in tropical settings that are saturated with respect to calcium bicarbonate. Carbon dioxide is lost to the atmosphere through degassing as a result of agitation, through elevated temperatures from solar radiation, and the activity of photosynthetic organisms. The loss of CO2 results in precipitation of concentric layers of calcium carbonate in the form of microscopic layers of interlocking crystals on pre-existing skeletal or pelletoidal nuclei.
Oolitic particles formed this way are composed of highly pure calcium carbonate, with unique physical properties. When dried, these particles, comprising interlocking networks of needle-like crystals, form a highly porous substrate capable of imbibing liquids. Once many layers of calcium carbonate form, the oolitic particles become dense and fall out of suspension. In areas where conditions are right, a ridge of sand-like material will form and extend for more than 50 mi. For example, throughout the Bahamas, more than one billion metric tons of material accumulate through this process, making oolite aragonite actually sustainable, and one of the few renewable minerals in the marketplace today.
That explanation and description of oolite aragonite comes from the company Pisa Carolina, which supplies oolites for many different industrial applications, including agriculture, construction, aquarium and aquaculture, water treatment, horticulture, cosmetics, food, and pharmaceuticals.
In addition to those industries, and given its properties, oolite has application in the oil and gas industry as a proppant, and with the product name, EnviroProp. So, what are the advantages of oolite relative to sand?
- Oolite should be cost-competitive with sand, especially if, and when, usage is on a similarly large scale.
- Oolite is more uniformly rounded and much less angular than sand.
- Mining of oolite is a much lower carbon emission process than mining of sand.
- Oolite is not a consolidated material, as is mined sand, so crushers are not required to break up clumps of the material.
- Since oolite is a very high-purity calcium carbonate, wash plants are not necessary to remove impurities, such as clays and silts that are present in sand. This eliminates power and water requirements for cleaning.
- The extraction of oolite from the ocean does not require any environmental restoration when completed.
- Oolite does not contain silica, which eliminates worker exposure to silica dust and the possibility of silicosis. In addition, as a cost-savings, there is no need for silica dust control equipment at frac sites.
- Oolite is expected to be less erosive to frac equipment, resulting in longer life of pump fluid ends, blenders, and iron, and therefore reduced maintenance and equipment replacement frequency.
- Oolite is somewhat lighter (lower density) than sand, due to its natural porosity.
- The natural porosity may also aid crush resistance relative to sand. Fluid is imbibed by the oolite to equilibrium with the fluid pressure in the formation. This may provide resistance to closure stress that an essentially non-porous material, such as sand, would not provide. The porosity of oolite may additionally impart elasticity to some degree, which could withstand cyclic stresses better than sand.
- The natural porosity could enable infusion of treating chemistries such as scale inhibitor or surfactants, slowly releasing them into produced fluids for extended production enhancement. Utilizing the porosity to contain tracers is also a potential option for diagnosis of fracture properties and fluid entry assessment.
So, there are many potential advantages of oolite, as EnviroProp, if this naturally occurring and renewable mineral can be supplied on a scale large enough to serve as relatively low-cost proppant in hydraulic fracturing – and ultimately as a replacement for sand, the old standby. For this purpose, there is, or should be, an increasing desire for a superior alternative at a similar cost and overall lower cost when considering the associated supply and frac site expenses associated with sand.
About the Authors
Leonard Kalfayan
Contributing Editor
Leonard Kalfayan has 42 years of oil, gas and geothermal experience. He has worked for Hess, BJ Services, Unocal, and as a consultant. He is an SPE Distinguished Lecturer and Distinguished Member. He has authored numerous publications, and also holds 13 U.S. patents.
