**7.1 Fracturing using CO2 and N2**

In the ordinary fracturing, large amounts of freshwater, sand, and chemicals are injected into the ground at high pressure. It has been reported that up to 9.6 million gallons of water on average are used for a single well fracturing; this lead to the use of more than 28 times the water for wells before fracturing, putting farming, and drinking sources at risk in arid regions, especially during drought [90]. Some of the water used for fracking is brought back to the surface and recycled, but the most of it is lost deep into the formations. Thus, fracking can increase demand for water by up to 30 percent, and this can be a major increase for groundwater consumption.

To solve the water scarcity problem, the fracturing using water, carbon dioxide, and nitrogen is commonly referred to the process in where substantial quantities of both nitrogen and carbon dioxide are incorporated into the fracturing fluid. Amounts of nitrogen and carbon dioxide are incorporated separately into an aqueous-based fracturing fluid to provide a volume ratio of nitrogen to carbon dioxide within an estimated range between 0.2 and 1.0 at wellhead conditions. The volume ratio for the total of both carbon dioxide and nitrogen to the aqueous phase of the aqueous fracturing fluid ranges between 1 and 4. The aqueous fracturing fluid that contains the nitrogen and carbon dioxide is injected in the well under conditions in which the pressure required is high enough to implement hydraulic fracturing of the subterranean formation undergoing treatment. In order to provide a viscous aqueous-based fracturing fluid, a thickening agent may be added into water. Additionally, a propping agent is to be incorporated into a portion of the fracturing fluid. Only then can carbon dioxide and nitrogen be added to the fluid. Carbon dioxide is incorporated in its liquid phase and the nitrogen in its gaseous phase. The use of carbon dioxide and nitrogen as fracturing fluids is discussed briefly in this essay.

Currently, carbon dioxide fracturing is one of the most effective and cleanest approaches available in order to increase oil and gas production. To produce the viscous aqueous-based fracturing fluid, carbon dioxide is injected in its liquid state using conventional frac pumps. Injection rates for it can be improved by

### *A Review of Fracturing Technologies Utilized in Shale Gas Resources DOI: http://dx.doi.org/10.5772/intechopen.92366*

incorporating booster capacity. An upside of using carbon dioxide in this process is that it can carry high concentrations of proppant in foam form due to its density and is compatible with all treating fluids (including acids). Because of that density, it is also not susceptible to gravity separation. Additionally, carbon dioxide can be pumped with synthetic and natural polymers, lease crude, or diesel as a foam or microemulsion, increasing the hydrostatic head to or greater than that of fresh water and decreasing the viscosity of the system. This feature of carbon dioxide results in vastly reducing horsepower costs and a decrease in the applied treating pressures. Another benefit of carbon dioxide is that it dissolves in water which causes it to form carbonic acid that dissolves the matrix in carbonate rocks. It buffers water-based systems to a pH of 3.2 which can also control clay swelling and iron and aluminum hydroxide precipitation. Known to act as a surfactant to significantly reduce interfacial tension and resultant capillary forces, carbon dioxide thus removes fracturing fluid, connate water, and emulsion blocks. In regard to it being one of the cleanest approaches in increasing gas and oil productions, carbon dioxide provides the energy to remove formations fines, crushed proppant, reaction products, and mud that is lost during drilling. In addition to that, swabbing of treating fluids can be greatly reduced which will allow for saving in associated treatment costs. Lastly, unlike other agents a carbon dioxide treatment with a 70 quality foam job allows low amounts of the water to contact the formation, roughly 30 percent compared to a gelled water fracturing. This decrease chances of clay swelling and inhibited production. All these benefits of using carbon dioxide as a fracturing fluid in wells with low bottomhole pressure or sensitivity to certain fluids make it a strong alternative candidate.

Although containing different properties, nitrogen similar to carbon dioxide comes with many benefits for fracturing fluids. Nitrogen for the fracturing fluids can be supplied by air products and provides both performance and cost advantages over certain formations of water-based fluids. Although water-based fracturing fluids are commonly used for hydraulic fracturing due to their advanced proppant transport into the fracture, they do also come with disadvantages. Because they can cause water saturation around the fracture and clay swelling which can result in hindering the mass transport of hydrocarbons from the fracture to the wellbore, water-based fluids are often unsuitable for water-sensitive formations. Nitrogen fracking fluids are an excellent alternative to water-based fluids in water-sensitive formations, depleted reservoirs, and shallow formations as they do not result in any water saturation.

Four main types of nitrogen fracturing fluids are used commercially: pure gas, foam, energized, and ultrahigh quality (mists). Foam fracturing fluids typically consist of a water-based system and a gas phase of nitrogen volume in the range of 53 to 95%. Below 53% nitrogen, the fracturing fluid is considered energized. Above 95 percent nitrogen, the fracturing fluid is considered a mist. Cryogenic liquid nitrogen fracking fluid is considered to be the fifth type of nitrogen fracturing fluids used. However, it is rarely employed for commercial operations due to material restrictions and equipment requirements.

### **7.2 Hydra-jet fracturing**

The process of hydra-jet fracturing combines hydra-jetting with hydraulic fracturing and involves running a specialized jetting tool on conventional or coiled tubing. Dynamic fluid energy jets form tunnels in the reservoir rock at precise locations to initiate the hydraulic fracture which is then extended from that point outwards. By repeating the process, one can create multiple hydraulic fractures along the horizontal wellbore [91–93]. The idea of hydra-jet fracturing is not a new one.

In fact, it was used a century ago with low-pressure jets [94] where waterjets with erosive materials were used to cut rock and glass. Because erosion does not involve a backflow hindering the sand cutting process, cutting steel plates, wellheads during the Iraqi war, and rock quarries tend to be easily be done. Hydra-jet cutting may be mistakenly claimed as a result of a perforating process which can be seen when used on the rocks sandstone and limestone.

For these two rocks, assume that the jet is used to perforate formation rock. Also assume that the jetting process creates a perforation with a larger inside diameter than the jet nozzle. The velocity of the fluid flowing into the perforation tunnel would be incredibly elevated. Near the bottom of the perforation, the velocity of the flowing fluid would dramatically decrease. If the flow area is sustained and there is no presence of friction, the fluid pressure will be equal to the original jet pressure per the example. However, this tends to be an unlikely happening because pressure losses are typically high. To further explain this, jet boundary friction works to convert kinetic energy to heat loss causing jet flaring. This drastically reduces jet velocity, which in turn reduces the pressure per unit area of impact. This results in a low-pressure transformation efficiency. More importantly, rocks can still be fractured when enough pressure is applied to the jets even at this low of a pressure efficiency rate. An important note is that laboratory tests have shown that rock fracturing is commonplace when jet pressures are high. However, when high-pressure and low-energy transformation efficiencies are used hand in hand, they are technically and economically impractical.
