Hydraulic fracturing, commonly known as fracking, develops oil and gas wells by injecting water, sand, and chemicals under high pressure into the bedrock. This fractures the rock, increasing the size of existing fractures and connecting one or more fractures together.
The theory behind hydraulic fracturing is that this process allows access to natural gas which would be inaccessible by other means. Despite this clear benefit, opponents of hydraulic fracking say that the number of chemicals used during the process contaminates soil and groundwater sources to dangerous levels and could also reduce air quality.
Fracturing fluid typically contains 90% water, 9.5% proppants (usually sand), and 0.5% of a mixture of what could be more than 750 different chemical additives.
The list of chemicals that potentially could be used is long, but can generally be grouped into certain functions, including:
- Gelling agents
- Pressure loss additives
- Thermal stabilisers
- Clay control additives
Gelling agents and pressure loss additives
The core function of gelling agents and pressure loss additives is to increase fluid viscosity for improved proppant activity. This helps to promote transport into perforations and along fissures which creates and maintains the desired fracture geometry. These agents also limit pressure loss when applying fracturing fluid to the well, in particular in slickwater applications. Common products used include natural guar, cellulose, synthetic polymers, and surfactants. These chemicals usually represent less than 1% of the overall composition of the fracturing fluid.
Cross-linkers, breakers, and thermal stabilisers
Cross-linkages are used to increase effective molecular weight by connecting the gelling agent to a long polymer chain. These connections create a 3D structure and increase elasticity and suspension properties. Borate is the most well-known cross-linker, including borate salts and esters or polyborates, which can also work as thermal stabilisers. This is a reversible process and, once the proppant is delivered to the fracture, the pH is reduced, the cross-links are dissolved and a more watery solution can be pumped out.
In contrast, alternatives to borate-based compounds include metallic cross-linkers, most likely with titanium or zirconium. These form chelated compounds to retard oxide formation and subsequent gel degradation. As this linkage is not reversible, additional chemicals such as oxidisers (for example, persulfate, perborate, hypochlorite, and peroxide), acids or enzymes need to be added to break down cross-links, reduce viscosity and promote flow-back, while at the same time ensuring that the proppant remains in the fracture.
Other functions of chemical additives in fracking
The list of additives continues, with many specific compounds added for different reasons. Examples include glutaraldehyde to disinfect the water, ethylene glycol to maintain clean injection pipes and prevent the build-up of scale deposits, polyacrylamide to reduce friction between the pipe and fluid and maintain a stable pressure throughout the process, and citric acid to prevent corrosion. Sometimes, radioactive tracers are injected alongside the fracturing fluid to determine the injection profile and fractures created by this process.
Fortunately, most of the chemicals used are harmless, but some are known contaminants or are carcinogenic. The problem is that many companies do not disclose the list of chemicals used and therefore it is difficult to fully assess the health and environmental impact of this type of gas extraction.
Although most companies deny any risks to public health, researchers recommend full disclosure with strict environmental testing to ensure no contamination takes place. Only when this happens can we have a complete picture of the impact of hydraulic fracturing.
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