In hydraulic fracturing, or “fracking”, “One size fits all” is being replaced with a site-specific holistic approach which includes engineered fluids and smart materials.
Hydraulic fracturing, commonly called fracking, has been used by the oil and gas industry for over a half century. Many wells require stimulation in order to promote greater recovery of tightly held hydrocarbons from deep subterranean formations.
Key challenges in fracking today include:
Achieving even more production from each well while
Further minimizing environmental footprint and
Doing it for lower cost.
Innovations in hydraulic fracturing include availability or replacement of low cost chemicals such as guar gum, understanding and improving proppant properties , positioning them in the fracture, anchoring them but make them flushable when refracturing is necessary and replacing water as the primary fracturing fluid.
Hydraulic Fracturing Basics
Traditionally, in the process of fracking, water and sand are forced down the well hole under extreme pressure causing fissures and cracks to form (see Figure 1). The sand is left behind as a ‘prop’ or pillar to keep the crack from closing after the water is retracted. By keeping the fracture from fully closing, the proppant aids in forming conductive paths through which the hydrocarbons flow to the wellhead at a commercially viable rate.
If Only It Was That Simple
The recent pressure to extract more hydrocarbon from depleted fields as well as capturing trapped natural gas in tight gas sand and shale bed formations is moving fracking from an art to a high tech science. The science involves the physical chemistry of the water, the material properties of the sand and the nature of the surrounding rock formation.
A Quick Walk Through the Process
The primary chemical used in the fracking process is water. It comprises 99.2% of the fluid injected into the well. The remaining chemicals play specific roles. Figure 2 shows the array of other chemicals used.
The type of chemical is dependent on the oil or gas deposit of interest and the nature of the subterranean formation. If the rock formation contains clay, for example, a clay control agent is needed to mitigate swelling that may block a fracture. Once the well is drilled, a dilute acid solution, generally hydrochloric acid is added to clean out the pipe as the acid removes excess cement and carbonate build up. The fractures are formed by forcing water under extreme pressure into the well. Physics dominates this process since the harder the water is pushed the more resistance, or friction, is felt in response. To lower the friction of water, friction reducing agents such as polyacrylamide are added, making the water wetter and the composition is referred to as slickwater.
The challenge with slickwater is that the sand proppant can settle out making it difficult to place the props where they are most effective within the fracture. To better pack the fracture with proppant and to keep it from settling out in the wellbore, in the next stage, water is thickened so that the sand grains remain separated and suspended. This gel, generally guar gum, along with crosslinkers and surfactants to insure homogeneity of the suspended sand are injected into the well. Once in place the gel is broken and the sand settles into the cracks. The water is pumped out and the hydrocarbon can then begin to flow. Other chemical additives are used to protect the pipe from bacterial growth and corrosion. A summary list of the specific chemicals and their uses can be found here.
Pump What’s Available …Not!
The old saw about fracking technology simply “pump whatever you have available” is no longer seen as the best method of freeing the trapped hydrocarbon. It is economically desirable to extract the most oil and gas from each well over its lifetime. Regulations and public concern have also prodded the industry to look more carefully into what they use in the process.
The Guar Gum Issue
Guar gum is used as the gelling agent in the fracking process to keep the proppant from settling out before it reaches its intended target. Guar gum is a polysaccharide from the seed of the guar bean. It is also used in the food industry as a thickening agent for products such as ice cream. Primarily grown in India, it is estimated that 90% of the world production of this chemical is used for hydraulic fracking activities. The demand has doubled over the last few years and the price has increased 800%, significantly impacting the economics of using guar in fracking. The advantages of guar are that:
A little goes a long way since it has significantly greater gelling ability than cornstarch and
It is environmentally benign and biodegradable.
Alternatives for this material that can match the properties, work with existing crosslinkers and surfactants while being environmentally friendly and with long term cost stability makes this an attractive point of entry into oil and gas activities.
Sand has been the primary proppant and market analysts expect that to remain true for the next decade. However, as understanding of the nature of the fractures and proppants within various different subterranean formations becomes better understood, innovators are beginning to rethink the value of sand as a proppant.
There are three qualities for proppants that are critical: size, shape and crush strength. The size of the proppant used is dependent on the size of the fracture that needs to be propped. Particle shape is also a critical component. To promote maximum conductivity of the hydrocarbon through the fracture, spherical particles pack such that more open space is available. Mixed shape particles have the possibility of closing off pathways if the proppant packs tightly. This slows the rate the gas can escape and in some cases blocks the path completely. Frac sand is graded by its size and shape to minimize blockage formation. Since fractures vary in size, the trend is away from a ‘one size fits all’ to successive introduction of different size proppants into the fracture to ensure the entire fracture is filled with proppant.
The third quality, crush strength, represents a challenge when sand is used. If the particle has a low crush strength and shatters under the pressure generating a wide range of particle sizes it potentially blocks passages within the fracture. Proppant breakage can occur during the introduction phase (gel phase) or over time under the extreme pressure conditions found within the fractures. The crystalline and polycrystalline nature of the sands mined from different locations also contribute to its ability to withstand the pressures encountered in a mile deep well.
Shifting is another issue with proppant packs. Like marbles in a jar, there is little adhesive force between particles within the pack or between the pack and the fracture surface thus over time they can shift, settle or migrate out of the fracture into the wellbore.
To increase the integrity of the proppants within the fractures, resin coatings can be used to help them to stick together and tackifiers added to modify the surface of the fracture. The coating also solves the problem of shattered sand proppant since the shattered proppant can remain encased in the coating material. Presently curable and pre-cured resins are used as coatings. Smart resins that do not completely cure or cross-link until after they are in place help hold the prop pack in place. Too slow a cure and the longer it takes to remove the fracking fluid, too fast and the proppants fuse before their time.
A Balancing Act
The nature of these coatings is a balance between allowing the proppants to stick together as well as adhere to the rock but not cause a disruption in the other components of the fracking fluid. If the coated surface is too hydrophilic it risks collecting water within the pores causing water blocks, too oleophilic, it will collect the hydrocarbon. If the tackifier is too sticky a traffic-jam of proppant at the entrance of the fracture will produce insufficient diffusion to where it is needed. The same holds true for the gellant, crosslinkers and surfactants. The coatings and tackifiers must not disrupt their function. If it does the whole system must be re-thought.
The value of an engineered surface where the tackifier and proppant coating are chosen based on individual well specification to maximize conductivity of the hydrocarbon through a specific subterranean formation is only beginning to be explored. Two new wrinkles in the coating and tackifier are to encapsulate the chemicals for slow release and to make the process of packing a fracture reversible on command. By removing the proppant after the well slows in production, the system could then be refracked thus reenergizing the well for decades more production. This reuse of hydraulically fracked wells has the potential to take production of oil and gas to the next quantum level.
Spherical ceramic and sintered bauxite beads represent a shift away from sand as these materials can achieve the ideal spherical shape necessary for increased hydrocarbon conductivity. Additionally the density and strength of the proppant can be tailored to improve suspension and crush strength. Like sand, these proppants can be coated.
Water Everywhere and Water Nowhere
The greatest challenge today in hydraulic fracking is securing and dealing with the produced water which is the fracking water that has been removed after so the well can begin its function. Produced water contains high levels of salt leached from the well. Water is cheap and effective and the fracking fluid system has been developed around it. However, depending on the location of the well relative to a water source and water disposal or treatment option, it could represent a very large expense.
Innovators are considering alternatives to hydraulic fracturing. These include liquid nitrogen injection, meta-phase natural gas and even ultrasonic waves. There are challenges as well as benefits by not dealing with water sourcing and clean-up. Cost is the primary factor. However, shifting the paradigm by rethinking the base material of the fracking fluid frees the innovator to consider a whole system approach to oil and gas recovery.
Sandstone, coal and carbonate rock all can contain hydrocarbon deposits. Fracturing the rock increases the rate of flow of the hydrocarbon allowing the trapped hydrocarbon to flow at a rate significant enough to be practical. This process has significantly increased production in the United States. There are similar formations around the world that have yet to be tapped. As oil and gas concerns turn to science and technology to fine tune the process, opportunities open up for new ideas in this rapidly expanding area. New thinking, holistically and in the physical, chemical and material sciences is poised to take fracking over the top.
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About the Analyst
Rosemarie Szostak, Ph.D.
Rosemarie Szostak, Ph.D., advises companies on technology, patents, innovation and disruptive technology. She has 20 plus years of experience as a thought leader and analyst with broad technical knowledge in chemistry, materials and chemical engineering.
Post Doctoral Fellow, Chemical Engineering Department, Worcester Polytechnic Institute
D., Chemistry, University of California Los Angeles
S., Chemistry/Physics, Georgetown University
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