In the first part of this three part article series on hydraulic fracking we provided a brief overview of the process and discussed how it has changed and developed over time. This second part will get more in-depth about the process of hydraulic fracking itself, how it works, and what fluids and equipment are needed to make it recover oil and gas.
The Process of Hydraulic Fracturing
In order for fracturing to occur in rocks at deeper depths, the confining pressure caused by the immense load resting on top of the overlying layers of rock must be overcome. To overcome this pressure, the process of hydraulic fracturing introduces fluids until the pressure exerted by the added fluids overcomes the tensile strength of the rock causing fractures. These fractures are generally oriented in a perpendicular position from the wellbore since that is the origin point of the fracturing fluids.
Hydraulic fracturing is done using an encased wellbore, with the desired target zone for the fracturing set by perforating the wellbore at a given location. The fracturing fluids are pumped until the pressure exceeds what is known as the fracture gradient. The fracture gradient refers to the pressure increase per unit of depth needed to fracture the rock. This is generally measured in pounds per square inch within the United States. Using the metric system it would be measured in bars per meters.
The Role Proppants Play in Hydraulic Fracking
Once the rock cracks the fracturing fluid seeps into the cracks, adding more pressure to those cracks and thereby further extending the fracture. Once the pressure from the pumped fracturing fluids stops, there would be an inclination for the fractures in the rock to begin to reclose due to the compressive force from the overlying rock strata. In order to prevent this from occurring a proppant is added to the fracturing fluids. A proppant is a material that prevents the fracture from closing once the pressure of the fluid is removed.
Often grains of silica sand or resin-coated sand are used as proppants. However, synthetic ceramics are also very popular and it is believed that these may make better proppants. This is due to their uniform size and shape and is theorized to increase porosity by allowing more crude oil or natural gas to get through. Other types of proppants include various types of gels and foams.
The type of proppant used is selected based on several factors particular to the well. For instance the permeability of the proppant will come into play since this is what will allow the formation fluids to get through. The proppant’s strength is also very important, especially at deeper depths where the pressure on the fractures is higher. In general there is a tradeoff between the viscosity of the proppant and the energy required to maintain its pump rate or flow velocity. Greater viscosity allows for more concentrated proppant, but requires greater energy and pressure demands. Other factors at play may be the the cost and ready accessibility of the proppant, or its pH level.
Fracturing Fluids Commonly Used in Hydraulic Fracking
The fracturing fluid injected into the rock will vary and may consist of a slurry of fluids such as water, proppants, chemical additives, gels, foams, and compressed gases such as nitrogen, and carbon dioxide. Often fracturing fluid will be called “slickwater” because the chemical additives reduce its friction, making it more efficient. A common blend of fracturing fluid or slickwater might contain about 90% water, 9.5% sand or other proppants, and about 0.5% chemical additives. However, this may vary considerably from well to well and there have even been fracturing fluids developed which do not use water at all, instead relying on liquefied petroleum gas (LPG) and propane.
A typical composition of fracking fluid will generally use between 3 to 12 chemical additives. The most common chemical additive used in the United States between 2005-2009 was methanol. Isopropyl alcohol, ethylene glycol, and 2-butoxyethanol were also widely used. Other additives and their uses include the following:
Acids – Acids such as acetic acid, hydrochloric acid, and others are commonly used prior to the actual commencement of fracturing to clean out the perforations in the wellbore and to initiate fissures in the surrounding rock. Citric acid is used to help prevent corrosion.
Salts – Salts such as sodium chloride are commonly used to help delay the breakdown of the gel polymer chains in the proppants. Meanwhile, Borate salts are used to maintain fluid viscosity during temperature increases.
Carbonates – Sodium carbonate and potassium carbonate are commonly used to maintain the efficacy of crosslinkers (described in greater detail below).
Polyacrylamide – Polyacrylamide is a friction reducer which is used to create the “slickwater” commonly referred to in fracking fluid. The reduction in friction allows the fluids to flow more efficiently through the pipes and enables the fracking pumps to pump at a higher rate without having to increase pressure.
Glutaraldehyde – Glutaraldehyde helps eliminate bacteria in the water.
Ethylene Glycol – Ethylene glycol helps prevent scale deposits from forming in the pipe.
Isopropanol – Isopropanol is used to increase the viscosity of the fracture fluid.
Guar Gum – Guar gum, along with other water-soluble gels, also increases the viscosity of the fracking fluids and aid in more efficient delivery of proppants into the fractures.
Gels commonly used in fracking fluids include the following:
Aluminium Phosphate-Ester Oil Gels – These gels were one of the first-known gelling systems. They are comprised by slurrying Aluminium phosphate and ester oils together to form a cross-linked gel.
Conventional Linear Gels – Conventional linear gels are cellulose derivatives or guar derivatives with other chemicals added as desired for certain results.
Borate-Crosslinked Fluids – Borate-crosslinked fluids are guar-based fluids that are crosslinked with boron ions. At higher pHs (9 and up) they have greater viscosity, allowing effective proppant transport. Once the fracking operator wants to pump the fluids out the pH is reduced (4 and down) which causes them to become less viscous and facilitates easier pumping out.
Organometallic-Crosslinked Fluids – Organometallic-crosslinked fluids such as antimony, titanium salts, zirconium, or chromium can also be used to crosslink the guar-based gels. In order to break them down appropriate solvents must be used once the fracking is complete.
While these gel types are very common, others may also be used depending on the environment and geology of the particular wellsite.
Typical Equipment Used for Hydraulic Fracking
Fracturing equipment will be operated across a range of different pressures and injection rates which are specific to the well. On the high end of the spectrum, the pressure used for hydraulic fracking may as high as 15,000 psi and the injection rate could be as much as 100 barrels per minutes. Common equipment used for hydraulic fracking includes the following:
- Slurry blender to mix the fracking fluids.
- High-pressure, high-volume pumps such as triplex or quintuplex pumps.
- Monitoring equipment
- Fracturing fluid tanks
- Proppant storage tanks
- Chemical additive unit
- High-pressure treating iron
- Low-pressure flexible hoses
- Various meters and gauges
In this article we discussed the overall process of hydraulic fracking and how it works. This process can be adapted to different well types and optimized based on the particular situation. In our final part of this series we will discuss how hydraulic fracking differs with different well types as well as examine in more detail what benefits hydraulic fracking has to offer.