Hydraulic fracturing is a technique that is used to increase the rate of flow of natural gas and oil (Yew and Weng 2014). This process is very productive and harmful to nature as well. A wellbore is first created that can be created using a seafloor drill. This wellbore can now readily be used for exploration purposes and extracting oil and natural gas form the sea bed. However, this process takes a lot of time and productivity is very low. Thus, hydraulic fracturing is used to boost the extraction process. Sand mixed with water is introduced into the wellbore at high pressure. Thickening agents are used in the mixture to keep it suspended in the cracks. This mixture thus helps to create faults in the rock formations located deep in the sea (Yew and Weng 2014). The cracks are expanded or kept constant depending on the requirement and the desired production level. However, this process was introduced in the recent times. The history of Hydraulic Fracturing contains very aggressive and dangerous procedures that were used to extract oil and natural gas, which would be discussed in the later part of this study. United States derives its almost 80% of its energy demands form fossil fuels (Eia.gov 2018). The annual energy report of 2016 by EIA shows that the United States produces almost 40% of the power from natural gas, about 30% of the power from petroleum and the rest of the energy is derived from coal (Eia.gov 2018). This shows that since the 18th century, the nation was highly dependent on fossil fuels for delivering its energy needs. With the increase in energy demands, the need for high quality petroleum and natural gas had aroused. Digging deeper into the bed rock and maintaining the developed cracks in it are necessary to keep the inflow of fuel.
Natural gas is being promoted as a cleaner source of energy. It emits low amounts of carbon dioxide and oxides of nitrogen into the atmosphere as biproduct to combustion. Thus, coal and petroleum got replaced wherever feasible. The calorific value of natural gas was comparable to that of petroleum but not greater (Howarth 2014). However, the concerns for the environmental impact of the greenhouse footprint of fossil fuels had led to the fast yet steady implementation of hydraulic fracturing. Natural gas is commonly found in small pores in a type of sedimentary rock formation known as ‘shale’ (Carroll 2014). These rocks are formed under high pressure and thus they are very high in density. Oil can easily be extracted from wellbores created by drilling on the sea bed. However, natural gas could not be extracted from this procedure. Exploration for natural gas can be conducted at almost every shale layer deposit in the world as these formations are guaranteed to contain deposits of natural gas (Väizene et al. 2014). Thus, the researchers started off by creating bigger cracks that would not collapse on itself to achieve the smooth extraction of natural gas and petroleum. Some areas are rich in natural gas and some are not. Exploring every single site is physically and economically impossible. Hydraulic fracturing thus provides an economical technique to extract natural gas from the shale rocks. However, various environmental impacts of using this technique was observed. Some of the impacts directly affected the living beings and some took its course over some years to affect them (Vinciguerra et al. 2015). Accidents at the extraction sites were common due to the nature of the natural gas and due to the procedures used for extracting it. Therefore, various regulations were developed to minimize the impact of hydraulic fracturing on the environment. These regulations were imposed on the Oil and Gas industries to standardize the process and reduce the risks to the workers and to the nearby residents as well (Ewen et al. 2015).
The history of hydraulic fracturing is filled with various accidents and tragedies as the earlier stages of this process involved more aggressive methods to produce cracks on the sea bed. There were various myths surrounding the technology. One such myth was that it is a new technology. However, that is not the true story. Fracking was used in 1862 by veteran Col. Edward A.L. Roberts during the civil war while fighting the battle of Fredericksburg VA (Montgomery and Smith 2010). He fired artillery filled with explosives into narrow canals to open up paths to the battlefield. He was later awarded a patent and this technique was soon recognized as “Exploding Torpedo” (Montgomery and Smith 2010). In this method, the gunpowder acting as the explosive was encased in iron and then it was slowly introduced into the oil well and the torpedo being as close to the oil as possible. The torpedo is then exploded and the void generated from it is filled with water. Such an invention was able increase the production of oil and natural gas. In the later years, the gunpowder was replaced with acid (Montgomery and Smith 2010). The use of acid was advantageous as now the holes did not close easily and thus it increased productivity even further. The first modern hydraulic fracturing experiment was conducted in 1947 at the Hugoton Gas Field by Floyd Farris who was employed at Stanolind Oil and Gas (Montgomery and Smith 2010). This experiment used a mixture of sand and gelled gasoline; however, the experiment was not successful in producing a substantial increase in oil production. In 1949, two experiments were conducted commercially by Halliburton, one at Archer County and the other was at Stephen County in Oklahoma (Montgomery and Smith 2010). These experiments were successful and thus they were able to commercialize the process of fracking. In 1970, this process of hydraulic fracturing for the oil and natural gas extraction was utilized in various basins across the United States such as the San Juan Basin, the Piceance Basin, the Green River Basin and the Denver Basin (Montgomery and Smith 2010).
Health and safety
The health and safety risks has been a matter of concern worldwide. Many experts have speculated that the huge growth in the figure of drilling sites would pose significant threat as additional people will be at risk from the harmful effects of the chemicals used in hydraulic fracturing (Reagan et al. 2015). Examination of the water bodies near the extraction sites showed evidence of contamination levels that would negatively impact the health of the living organisms consuming the water (Boudet et al. 2014). The emissions are also harmful for the workers at the site and the people living near those sites. Methane and other harmful emissions would produce various airborne diseases that would affect the health of the people inhaling the contaminated air (Vinciguerra et al. 2015). The safety of the workers at the sites is low as hydraulic fracture requires the use of various harmful chemicals and destructive procedures (Reagan et al. 2015). As discussed in the previous sections of the paper, gunpowder stuffed torpedoes were used to widen the cracks. Thus, the workforce often fell victim to accidental blasts that would result in casualties and loss of valuable life (Montgomery and Smith 2010).
Hydraulic Fracturing has increased the productivity of natural gas extraction at the cost of producing deep-etched adverse effects on the environment (Vinciguerra et al. 2015). Releasing harmful emissions into the air, contamination of the nearby water bodies, radionuclides and inducing seismicity are some of the harmful effects that the process has on the environment (Reagan et al. 2015). Air emissions are one of the prominent effects that can harm the living beings and the environment (Hyman et al. 2016). Emission of methane from the wellbores diesel fumes that can leak from the petroleum extraction process and various other hazardous pollutants might be responsible for ozone depletion and different health risks among living beings (Ryan et al. 2015). Studies has also showed that the well-to-burner ratio is significantly higher for natural gas that has been produced by the hydraulic fracturing process than the gas that has been produced by the conventional methods (Ferrer and Thurman 2015). Higher ratio means higher emissions from the oil wells. The water consumption at a standard hydraulic fracturing site is about 8 million US gallons (Burton et al. 2016). The amount of water required for smooth production increases exponentially with the increase in the size of the hydraulic wells (Hyman et al. 2016). The fluids injected into the cracks are highly carcinogenic chemicals and would contaminate the ground water and the nearby water bodies (Ferrer and Thurman 2015). The council responsible for the protection of ground water has participated in effectively launching a website called FracFocus.org that focuses on disclosing database of harmful fracturing fluids used by different oil and gas companies (Fracfocus.org 2018). This website can be used to spread awareness among the common masses about the drawbacks and limitations of hydraulic fracturing and the effects that it has on the environment. This initiative will aid in spreading the knowledge with the help of the right set of broadcasting tools like social media.
The countries utilizing hydraulic fracturing have developed various regulations that includes legislations and limitations to the local zone. Intensive public pressure has led France to ban the use of hydraulic fracturing in 2011 (Callies 2014). The ban was ruled as measures that were meant for implementing precaution as well as prevention and correction of environmental hazards. The ban was ruled as permanent in October 2013 (Elliott 2017). Countries like United Kingdom and South Africa imposed temporary bans and soon lifted those bans as they chose to focus in regulation rather than abolition. Germany has also drafted regulations where the hydraulic fracturing would be used to extract oil and natural gas, although the wetlands would be exempted from it to avoid contamination and accidental disasters (Callies 2014). Two types of approach were developed to regulate the use of hydraulic fracturing, risk-based and precaution-based approach. Various experiments and analysis are conducted in the risk-based approach. This approach poses severe risk to the environment as risks are accessed after the technology has been implemented (Elliott 2017). The approach based on precaution is however implements preventive and corrective measures that ensures environmental protection at acceptable costs (Callies 2014).
Modern hydraulic fracturing utilizes a huge number of equipment to make the process productive and economical yet safe for the workforce and the environment (Stewartandstevenson.com 2018). A list of such equipment are as follows:
Acidizing Units: These units serve as transportation for acid that can be pumped using onboard pumps of high horsepower. This machine also packs fluid mixing capabilities for on-site use.
Fracturing Blenders: The operators can use these blenders to create a mixture of fracturing slurries, which are complex and consists chemical of varying densities.
Chemical Additive Units: These chemicals increase the viscosity of the slurry and decrease waste.
Hydration Systems: These systems are used to hydrate the slurry that also minimized waste and increases the viscosity of the slurry.
Offshore Units: The units act as stimulation vessels that are fully integrated to independently operate using configuration cooled by seawater.
Fracturing Pumps: Used for pumping the oil.
Data and Control Centres: Used for information exchange and equipment control.
Support Equipment: Used for operation support.
Momentum, Continuity and Linear Elastic Fracture Mechanics (LEFM) are the three essential equations are primarily used for the hydraulic fracture modelling. PKN and KGD are 2D models that can be used to obtain controllable solutions but they are entirely dependent on the factors of assumption. A 2D model can be used by an engineer to make one of the dimensions of the fracture constant, preferably the height of the fracture. Thus, with the right amount of experience and data sets that are accurate, the 2D models can be derived and used with high precision. However, one assumption is still implemented that suggests that the engineer responsible for designing the 2D model can accurately estimate the height of the fracture created.
The figure 1 below demonstrates the Perkins-Kern-Nordgren (PKN) geometry that can be used when a certain condition is fulfilled. The condition states that the fracture height has to be much greater than the fracture length. In figure 2, the geometry of Khristianovic-Geertsma-de Klerk (KGD) is demonstrated that is primarily used if only fulfils the condition that the height of thee fracture is more than that of the length of the fracture. Either PKN or KGD can be successfully used in some rock formation to design the hydraulic fracture. The design must be always used to compare the calculation obtained from the model with the actual results that are obtained after implementing the model. The field results can thus be sued to calibrate the 2D models to and make certain changes to the design that will contribute to successful stimulation of the wells. The 2D models will give accurate estimates of the length and the width if the fracture length is the height of the fracture is given. Some other parameters are also needed in the calculation that contribute to producing accurate predictions such as Young’s modulus, the permeability of the formation, the coefficient denoting to the total leak off and the stress level that the layers of rock can withstand.
Figure 1: PKN Geometry and KGD Geometry for calculating a 2D fracture
(Source: Fokker, Peter A and Pizzocolo, Francesco 2017, p.4)
Figure 2: PKN fracture equation
(Source: Tian et al. 2016)
The above equation represents a relationship that can be used to calculate the distribution of pressure in the fracture, taking fixed values of rate of injection, fluid velocity used in the fracture, the height of the fracture along with its width. Thus, this equation aids in calculating the pressure distribution of the fracture obtained. However, specific values of constraints and physical dimensions must be provided in the equation.
Natural gas can be found in reservoir rocks that has low permeability. This gas is called tight gas and the reservoir rock is called tight gas reservoir. Immense hydraulic fracturing is required to create a well that would be economical to operate. These reservoirs consisting if tight gas have very low permeability. The measure of permeability is as low 0.1 millidarcy (mD) and the porosity level is ten percent and lower (Sanei et al. 2016). Shales also have very low porosity and permeability. However, tight gas is considered different from shale gas as tight gas is frequently found in sandstone and very rarely found in limestone. Therefore, tight gas is deemed to be a source of natural gas that is unconventional in nature. The permeability of the rock of a tight gas reservoir is one nanodarcy and sometimes even lower (Sanei et al. 2016). Thus, stimulation of the reservoir may be effective, productive and economical to maximize the extraction of natural gas in regard of the cost incurred in the whole process. Tight gas reservoirs are unconventional resources of natural gas and they offer substantial production of natural gas that can offer significant growth to the oil and natural gas industry. The production can be enhanced by hydraulic fracturing, similar to the process of shale gas extraction. However, the use of this technology has serious limitations as the layers of rock are hundreds and thousands of layers thick. Thus, the drilling and extraction becomes very complex and the outcome is often unpredictable.
The Oil and Gas Authority of the United Kingdom have produced an estimation that there might be about 3.8 trillion cubic feet (tcf) natural gas remaining within the Southern North Sea that can be accessed and extracted (Song et al. 2014). These resources are yet to be discovered and tapped for use. The oil companies sometimes consider extracting tight gas from its reservoirs as high risk along with high cost. Thus, these companies often focus on less complex opportunities that would require less infrastructural but give high return on investment. Strategies are being developed that could be utilized to tap into the potential of tight gas that is still undiscovered in the Southern North Sea. A special interest group was commissioned named the SNS Rejuvenation by the East of England Energy Group (EEEGR) in collaboration with the OGA. This group will be responsible for creating a budget and a programme that would prioritize the strategy of tight gas exploration.
Porosity can be defined as the void space found in between rocks measured in percentage. The porosity of a rock can be calculated by dividing the volume of the pore space by the total volume of the rock [Porosity(n)= Vpore space/Vtotal] (Busch et al. 2016). Studying the porosity of different types of rocks is important to identify and analyse the rocks that would contain oil and natural gas deposits. Geologists study the porosity of different rock beds for discovering new sources of oil and natural gas. Dolomite, Shale and Sandstone are the most common types of rock beds that contain such deposits. Natural gas deposits are harder to extract from these layers of rock than oil deposits.
Figure 3: Lithology
(Source: Busch et al. 2016)
It is evident from analyzing the above figure that the porosity of the shale consists of a very wide range of 8% to 29% (Aguilera and Lopez 2013). However, shale has low permeability and due to this reason, the rock formation is ideal for extraction of natural gas but not oil through conventional methods. The uses of Hydraulic Fracturing and Horizontal Drilling thus became vital for the production of oil and natural gas from various shale deposits locate around the world.
References
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