Amount Of Diesel Fuel Found In Fracking Fluids

Published Date: 02 Nov 2017

Fractures are formed when the pressure from the pumped fluid becomes great enough to force the rock to break, or fracture. In its natural condition, there is already tectonic stress acting on the rock, but the added fluid pressure is needed to finally create breaks in the reservoir. When the rock is saturated with the fluid, its criteria for breaking is substantially lowered so that the tectonic stress will be able to fracture the rock. The hydraulic fracturing fluid can be any type of fluid, including water. Most fluids contain other chemicals and substances that make the fluid more effective in lowering the breaking criteria. About 0.5% of the fracking fluid contains chemicals, but some chemicals are commonly used in daily life while others may be carcinogenic and dangerous to life. More fluid is pumped further down to extend the fracture into the pay zone, which is the production zone that can produce quantities of natural gas or oil. Fracturing wells are often drilled horizontally because the pay zone extends more horizontally than vertically. One new fluid is a thermo-setting gellable mixture that acts as an effective fluid for fracturing the rock and a stable proppant that helps the hydrocarbons flow towards the well. Hydraulic fracturing is centered around creating a better flow of hydrocarbons from shale reservoir to the well and up to the surface. It maximizes the production of hydrocarbons through interconnecting shale pores to yield large amounts of natural gas or oil.

The huge amount of hydrocarbons trapped in the shale reservoirs across many parts of the country has led to a booming fracking business that can benefit our economy and lessen our reliance on foreign countries for energy resources. The new industry boom has created thousands of jobs, such as construction workers and truckers. For example, one drilling site can create thousands jobs. By 2010, the natural gas industry had already created more than 600,000 jobs. The boom has reversed the decline of many small towns. Well production has increased substantially all across the country. For example, in North Dakota at the Bakken formation, production has increased by 150 times to producing 660,000 barrels of oil and natural gas a day. Also, the large and fast production of natural gas has substantially lowered gas prices, allowing more people to afford gas. This is critical because natural gas provides over 25% of our energy sources which can increase with the low, stable prices that come with increased production of natural gas through hydraulic fracturing. North America contains about 4.2 quadrillion cubic feet of natural gas that can amount to 175 years of natural gas usage at present consumption rates. An increase in the natural gas industry may reduce environmental costs by decreasing air pollution and health problems from the production of coal. At the same time, this energy boom has led to the development of safer and more reliable energy production. Lower gas prices, the increase in employment, and the potential gas production have an enormous impact on our economy, projecting around $18.85 billion and 212,000 jobs by the end of 2025.

However, there are also some controversies involved with hydraulic fracturing. Some problems include drinking water contamination, wastewater disposal, fluids containing diesel, and the usage of dangerous chemicals in the fracking fluid. The diesel fuel in the fracking fluid may have a potential negative impact on water quality. In all, over 30 million gallons of diesel fuel has been used in fracking fluid in all the states that do hydraulic fracturing from 2005-2009 as shown in Table 1.

Table 1. Amount of diesel fuel found in fracking fluids

People are afraid of the contaminated wastewater left over from the fracking process that may sometimes be radioactive and dangerous to humans. This water may leak into freshwater drinking reservoirs, endangering the lives of the people living around the freshwater source. Also, wastewater can sometimes contain methane, which burns when put to flame. Another risk of wastewater is inhalation hazards. When the wastewater is brought up from the fracture, it may contain toxic minerals and substances that are held deep underground in the shale reservoir. In wastewater disposal tanks, chemicals and toxic gases are present, such as hydrogen sulfide, or Hâ‚‚S. High enough concentrations can be poisonous, even lethal. Currently, the United States government has been working towards stricter regulations to minimize and prevent the environmental threats.

As discussed, there are both beneficial and negative impacts of hydraulic fracturing. It can help boost our economy and contribute to economic revival. But, it may also pose as an environmental threat.

Computer programming was used to figure out how much stress would be needed to fracture a shale reservoir, and how a fracture can be created that will yield the greatest conductivity, which means the greatest amount of gas or oil flowing from the well. This way, an ideal fracture can be created that will yield natural gas and/or oil successfully.

Mohr’s circles were used to figure out how much stress would be needed to fracture a shale reservoir. Mohr’s circles are 2-D geometric representations of stresses. For a plane surface in any direction in an object, the stresses acting on it could be divided into normal stress and shear stress. Shear stress is the stress that is acting parallel or tangential to the surface, while the normal stress is the stress that is acting perpendicular to it. On a 2D plot with the ordinate (y-axis) as the shear stress and the abscissa (x-axis) as the normal stress, the breaking criterion would be a straight line (Byerlee law), and the stress status could be represented by a Mohr’s circle. When any point of the circle touches the line of breaking, a fracture occurs. As the fluid is pumped into the reservoir, the Mohr circle is moved closer and closer to the line until the two intersect. The amount it moves towards the line is the amount of stress needed to fracture the rock.

The breaking criterion line was set up by using the equation τ= σ*tan(φ), where φ is the angle of internal friction. The actual Mohr’s circle was created by the equation, σn = 0.5(σ1+σ3)+0.5(σ1-σ3)cos2θ, where σ1 represents the maximum principal stress and σ3 represents minimum principal stress. The second equation involved was τ=0.5(σ1-σ3)sin2θ. Sigma, or σn, which represents the principal stress, is on the x-axis while tau, or τ, which represents the shear stress, is on the y-axis. These two equations create the Mohr’s circle that can be constantly manipulated. Figuring out how much stress is required to fracture a shale reservoir depends on manipulating both sigma 1 and sigma 3 until a point of the Mohr’s circle intersects the criterion line. The increased pore pressure reduces both the sigma 1 and sigma 3 values. This explains why Mohr’s circles move left towards the line. Sigma 1 prime, or σ1’, is equal to σ1 - P. Likewise, σ3’ is equal to σ3 - P. P represents the pore pressure. The radius of the original circle and the new circle should be the same, which is equal to σ1-σ3.

For example, as shown in Figure 1a, when sigma 1, the maximum principal stress, is equal to 6000 psi and sigma 3, the minimum principal stress, is equal to 4000 psi, not one point of the Mohr’s circle is touching any point on the criterion line, therefore a fracture is not formed. But, on Figure 1b, when sigma 1 is reduced to 3000 psi, and sigma 3 is reduced to 1000 psi, a point of the circle intersects a point on the criterion line, creating a fracture. Notice how the the radius of both circles remain the same.

Figure 1a. Notice that the Mohr’s circle is not touching the criterion line, therefore no fracture is formed.

Figure 1b. The Mohr’s circle intersected the criterion line, creating a fracture in the reservoir.

The sigma values were altered to move the Mohr’s circle to intersect the criterion line. Sigma 1 and sigma 3 are reduced because once the fluid is pumped into the shale reservoir, the breaking criteria for the rock is substantially lowered until the tectonic stress finally fractures the rock.

The second part of this project was to use the equation, q=k*h*(pe-pwf)/141.2/B/μ/log(re/rw) to determine the greatest productivity rate of a shale reservoir. This equation is derived Darcy’s Law and is commonly used for reservoir simulation. The q represents production rate of the reservoir, k is the permeability of the reservoir, h is the height of the reservoir, pₑ represents the reservoir pressure, pwf represents the wellbore pressure, μ is the viscosity of the reservoir fluid, rₑ is the distance from the reservoir boundary to the wellbore, and rw is the radius of the wellbore . This equation is fundamental to reservoir productivity, and reservoir exploitation can be made possible by manipulating the different variables in the equation. So, computer programming was used to calculate which values of permeability, height, reservoir pressure, wellbore pressure, fluid viscosity, wellbore radius, and distance between the wellbore and reservoir boundary will yield the greatest production rate for the reservoir.

All the values chosen were reasonable, real-life values that would be used in the professional world. For reservoir permeability, the values ranged between 1 md to 20 md. For reservoir height, the values ranged from 50 ft to 300 ft. The values of reservoir pressure ranged from 4000 psi to 6000 psi while the values of wellbore pressure ranged from 1000 psi to 3000 psi. The fluid viscosity ranged from 0.5 cp to 2 cp. Wellbore radius ranged from 0.2 feet to 0.5 feet, and the distance between the wellbore and reservoir boundary ranged from 500 ft to 1500 ft.

To find the values for the greatest hydrocarbon flow, I calculated the flow rate for each individual variable at a time. The other variables would have a fixed value that would be used in the equation. The fixed value for permeability was 5 md, height was 75 ft, reservoir pressure was 5000 psi, wellbore pressure was 2000 psi, viscosity was 0.7 cp, distance was 1500 ft, and wellbore radius was 0.328 ft. With the end values, relationships and patterns between variables can be observed. After, I calculated the greatest possible hydrocarbon flow rate to compare with the least possible hydrocarbon flow rate. The results are shown in Table 2:

Table 2: The least and greatest possible hydrocarbon flow rate based on a range of reservoir properties

In conclusion, how much stress needed to fracture shale reservoirs can be easily calculated by using Mohr’s circles, which are very effective. For example, the original stresses, 6000 psi and 4000 psi, were reduced to 3000 psi and 1000 psi when fluid was pumped into the reservoir, creating a fracture. In creating the most successful drilling well with the highest hydrocarbon flow rate, the greater the permeability in md, the greater the reservoir production rate, which is shown in Figure 2. For a greater reservoir production rate, the reservoir height should be the greatest value (Figure 3), the reservoir pressure the greatest (Figure 4), the wellbore pressure the lowest (Figure 5), the viscosity the lowest (Figure 6), the wellbore radius the greatest (Figure 7), and the distance from the wellbore to the reservoir boundary the lowest (Figure 8). The values used for reservoir exploitation are very important in hydrocarbon flow because it can determine whether the shale reservoir will be successful or not.