**6.2. Problem 2: keeping the fractures open**

Hydrofracking generally is used on deep targets, usually at least a kilometer below the ground surface. If hydrofracking was done with water alone, the newly created fractures would collapse as soon as the applied pressure decreased due to the weight of the overlying rock. Hydrofrackers use proppants to prop the fractures open. Proppants need to be strong enough not to fracture or be crushed during the fracking process or during the producing life of the well. By far the most common proppant used in the USA to date is so called "white" sand. White sand is high-purity silica sand with few other minerals. This gives the sand its light color and relatively uniform chemical and physical properties. Prior to use the sand is washed and sieved to produce a more uniform size distribution. Multiple sizes are used, with smaller particles injected first to infiltrate farthest into the newly fractured bedrock and larger sand particles used near the end of the process to better match the larger aperture near the well. The volume of sand used in the hydrofracking industry is considerable. The US Geological Survey reports that sand and gravel production in the USA more than doubled between 2010 and 2014, with more than 70% of the total 2014 sand production being used by the hydrofracking industry! [23].

**6.6. Problem 6: preventing bacterial proliferation**

to fracking fluid to prevent this counterproductive effect.

could be a threat to shallow aquifers.

ties for pollution of surface water or aquifers.

Many of the additives in hydrofracking solution, including the breaker, surfactant, gelling agent, cross-linkers, surfactants, and friction reducers, are organic chemicals. All of these are substrates for microorganisms to feed on. Proliferation of microorganisms creates biomass. The biofilm on surfaces made up of living and dead microbes can lower porosity and gas permeability by lowering the fracture aperture. This is the same effect seen in bioremediation of aquifers where the stimulation of the native flora choke off conductivity. Biocide is added

Effect of Hydrofracking on Aquifers http://dx.doi.org/10.5772/intechopen.72327 47

Among all the chemicals used to formulate hydrofracking fluid, it is the biocides that are of most concern for water quality. Fewer than 20 biocides have been identified as more or less commonly used in the hydrofracking industry. Among the most common are glutaraldehyde, dibromonitrilopropionamide (DBNPA), tetrakis(hydroxymethy)phosphonium sulfate, and chlorine dioxide [25]. These biocides are toxic to microorganism, and some are quite toxic to aquatic fauna. They have low toxicity for mammals. Although they are not acutely toxic to mammals, some are toxic during long-term exposure or possess carcinogenicity or mutagenicity. How do these hydrofracking chemicals get into the hydrosphere, and how do they become a threat to water resource quality? Chemicals must be transported to the site by rail or truck so accidents are of course a threat to surface water quality. Hydrofracking fluids are generally mixed on site and stored in railroad tank cars or in lined storage lagoons. It is not uncommon for lagoons lined with geotextile to leak at the geotextile seams or from punctures, so this

Once fully mixed hydrofracking fluid is injected into the target formation. Fluids travel first down through the vertical well and then into the horizontal portion being hydrofracked. Improperly cased and grouted wells could leak into shallow aquifers during high-pressure injection of the fracking fluid. Hydrofracking opponents in the USA have claimed that hydrofracking of shale gas formations can cause fractures to extend upward from the target shale formation, allowing fracking fluid to reach freshwater aquifers above. This is likely not a realistic fear. Freshwater aquifers are generally found within a few hundreds of meters of the ground surface. Shale gas formations that could be subject to hydrofracking are generally found thousands of meters below the ground, and the pressures used to hydrofrack are incapable of creating fractures of the length that would be required to reach a freshwater aquifer. It is possible however for hydrofracking fluid to flow upward through existing faults or abandoned wells. There have been wide ranges reported, but between a quarter and half of the hydrofracking fluid injected to break the shale returns to the surface as flow-back subsequent to the frack. This hydrofracking fluid wastewater returns to the surface to the wellhead where it is collected. Wastewater can be treated onsite, treated offsite, treated and reused as hydrofracking fluid, or disposed of in a deep brine aquifer. This presents a number of additional opportuni-

Hydrofracking wastewater that is treated offsite or treated and reused as hydrofracking fluid must be transported and so again poses the threats associated with transporting chemical-laden water. Wastewater treated onsite and then disposed of to a surface water body may not be sufficiently treated and could still contain some chemicals not removed by the

Sand is not the only proppant. Ceramic proppants of various formulations allow for a more uniform manufactured proppant with specific beneficial properties. Ceramic proppant can be manufactured with properties that make them better than sand, such as higher sphericity, more uniformity of size, and more crush resistant. Formations hydrofracked with ceramic proppants have higher conductivity than those propped with sand. Many other materials have been used as proppants including resin-coated sand, resin-impregnated crushed walnut shell, and thermoplastics.

Proppants by their nature are inert and nonpolluting, but not without environmental impact. The landscape of rural Wisconsin is being transformed by mines that provide roughly 9000 truckloads of fracking sand per day [24].
