The Hydraulic Fracturing Technique: Things You Should Know
When it comes to hydraulic fracturing, you need to understand fracture orientation, fracture length and the hydraulic fluid used in the procedure.
There are many misconceptions and facts about hydraulic fracturing that the layman may not fully comprehend. Hydraulic fracturing, also known as fracking, is not actually a drilling process. Instead, it is a procedure used after workers complete the drill. Workers then use fluid to create small fractures in the formation. To better understand hydraulic fracturing, here are a few facts you should know.
The hydraulic fluid is made from 98% to 99.5% water and sand. It also contains 0.5% to 2% additives. These additives have specific functionality. The exact formula of water, sand and additives will vary from job to job.
Additives are frequently used as part of the hydraulic fluid water and sand combination. However, additives are not found in every fracking site. There is a wide variety of additives used. One additive is dilute acid solution; this is generally used in the first stage of fracking. It helps to clean out debris from the drill site and helps to facilitate the hydraulic fracturing solution.
In some drilling sites, workers use biocide additives. This additive is a disinfectant which kills germs and bacteria which may interfere with hydraulic fracturing operations. Also used is a scale inhibitor to control the formation of sulfate minerals and carbonates. Alongside a scale inhibitor, an iron control additive is often part of the hydraulic fluid. The iron stabilizing agent reduces the precipitation of iron in the site. Some sites use a friction-reducing agent such as potassium chloride to reduce the constant pressure of the fluid. It also reduces the tubular friction within the wellbore.
Another additive is an N: N-dimethylformamide. This agent combined with other additives such as ammonium bisulfate keeps the steel casing from degrading during fracking procedures. These agents are also often used with a gelling agent like guar gum. This agent helps to thicken the solution and improve transport material.
Uses for Hydraulic Fracturing
There are a couple of different uses for hydraulic fracturing. The first is to test for in situ stress at a new construction site. The other is the most common usage: in oil and gas well sites to keep them operational.
In Situ Stress Test
With in situ stress testing, workers use geological surveying before drilling. Once they drill a pilot hole, they fill a section with a straddle pack. The pack has two inflatable packer elements that seal off the area for testing. The packer fills with fluid, and the resulting cracks are analyzed. (For more on geological surveys, check out Why a Detailed Geotechnical Report Means Success for Your Trenchless Project.)
Hydraulic fracturing stimulates the rock around the borehole, causing stress fractures. The stress state can change dramatically with excavation and construction. The gathered information can help to determine the stress field of the surrounding soil. The stress field data helps project engineers plan the work appropriately.
Hydraulic fracturing is mainly used by the oil and gas industry as a way to keep oil/gas wells operational and extend the life of the unit. Field experts believe that anywhere from 60% to 80% of all oil/gas wells currently drilled within the United States will need fracking at some point over the next ten years. If these wells do not receive the necessary hydraulic fracturing, they will cease to be operational.
While fracking may not end our dependence on foreign oil, it can help to access oil reserves that were once thought to be impossible to tap. Formations such as tight shale that geologists once believed were impossible to obtain oil or natural gas from are now accessible due to hydraulic fracturing techniques. These formations are located not just in well-known oil-rich areas, such as Oklahoma and Texas, but also in the Northeast and much of the West.
For hydraulic fracturing performed between 0 and 2,000 feet below ground, the fractures form horizontally based on the direction perpendicular to the stress area. This formation is due to the pressure located at the center of the structure. The stress makes it easier for the cracks to form in a perpendicular direction, resulting in breaks across the horizontal plane.
As the fractures are related to the path of most considerable stress, once you go below 2,000 feet into the Earth’s surface, the pressure changes. Once you get beyond 2,000 feet, the overburden stress increases by 1 pound per square inch for each foot. Instead of the horizontal stress being the confining stress point, it is now the overburden stress that is the greatest. Thus, hydraulic fracturing results in a vertical crack instead of a horizontal one.
Fractures sometimes run through multiple planes of directional stress. In that case, you may see a crack reorient itself as it hits a new threshold. A vertical break from the depths of the Earth will change to horizontal as it approaches the Earth’s crust. (For more on the oil and gas industry, see Why the Oil and Gas Pipeline Industries are Eyeing Horizontal Directional Drilling.)
Fracture length is a point of contention for those who worry about the effects fracking may have on the water table. Fracture length formation is limited by the confining zone, as well as the volume, pressure and rate of the fluid pumped from the well site. This confining zone limits the growth of fractures because there is enough elasticity to absorb the force of the added hydraulic fluid.
Fracture length is also affected by natural faults or fractures in the area. A natural attenuation occurs over a short distance (relatively). The limitation is due to the minimum volume of fluid pumped into the area.
The oil and gas industry uses hydraulic fracturing to keep their wells working. Construction companies also use this method as a way to check the in situ stress in work area. By understanding fracture orientation, length, and what the fluid is composed of, companies can make the best choices for their needs.