After well hole is drilled into these shale formations, casings are installed to isolate the well bore. The bottom section of this casing is perforated so that gas can more easily be removed. Once the casing is in place in addition to concrete reinforcing in the upper sections, water and sand are injected under high pressure into the shale deposit. This creates small fractures in the shale and provides openings making the formation porous for gas removal. When the water pressure is relieved, the small cracks in the shale begin to close, but the sand or ceramic particles help to “prop’ them open. These materials are known as “Proppants”. In addition to the water and the sand, small amounts of various chemicals are added for various purposes. These include biocides, surfactants, corrosion inhibitors, thickeners, and other materials.
This water that is used to fracture the shale formation is called “frac water”. The process of fracturing is also known as completion since this is one of the final steps in preparing a well for production of oil or gas. Frac water can also be known as “completion fluid”. When the pressure of this water is relieved, the water flows back up out of the well casing. This is known as “Flowback water”. Some of the water will remain down in the hole and be absorbed into the more porous areas. The amount of water that flows back can vary a great deal and can be anywhere from 10% to 90% of the water that is injected as frac water.
The Flowback water can be reused to some extent, but as the water is injected into the shale formation, it absorbs minerals, salts, organics, and also brings sediment back out with it when it flows back. At a certain point, this water becomes too “dirty” to reuse. The conventional thing to do at that point is to haul this water off by truck for disposal. Many well sites are in remote areas and do not have established connections to municipal; sewers or to disposal wells for reinjection. This hauling can involve a high cost and the use of many trucks that can put a burden on the local community.
Some Potassium salts are added to the frac water to begin with. Some salt in the frac water is desired, but very high levels of mineral salts that can cause scaling and plugging of the formation would adversely affect the operation. Also injecting frac water that has a lot of sediment can also plug the formation. So before the Flowback water is reused, sediment and other fouling materials should be removed. Onsite water treatment can provide almost unlimited reuse of the Flowback water and greatly reduce the need for truck hauling. In addition to the benefits of less hauling, many locations have scarce supplies of fresh water that is available for use as frac water. So, reuse of the Flowback water can reduce the amount of local fresh water that is consumed as well as reuse the amount of wastewater hauled away.
For more details – Syngineering Water – Frac Water and Flowback
Oil and Gas Drill Rig