Natural gas production from hydrocarbon rich shale formations is currently one of the most interesting, talked about and rapidly increasing areas in oil and gas exploration and production. Unfortunately, some of the currently active areas in the United States never seen such developments in the past. According to Ground Water Protection Council (GWPC, 2009), new oil and gas exploration and production activities bring not only economic prosperity but also pose an enormous stress and bring a significant change to the environmental and socio-economic landscape, particularly in those areas where gas exploration is a new activity. Thus, oil and gas exploration and production face various challenges regarding potential environmental impacts and the ability of the current regulatory environment to deal with these developments (Centner and O'Connell, 2014).
The United States Environmental Protection Agency administers most of the federal laws, although the development on federally-owned land is managed by the Bureau of Land Management (Department of the Interior) and the United States Forest Service (Department of Agriculture). Also, each state in which the oil and gas activities are under way has one or more regulatory agencies that permit wells, including their design, location, spacing, operation and closure as well as environmental activities and reclamation / recycling of resources.
Currently, natural gas supplies about 22% of the total United States energy and it is expected to increase significantly over the next 20 years (GWPC, 2009). The Energy Information Administration estimates that the United States has more than 1,700 trillion cubic feet (tcf) of technically recoverable natural gas (about 637 tcf is shale gas, tight sands and coal-bed methane) including 211 trillion cubic feet of proved reserves with ~ 95 tcf of shale gas (US EIA, AEO2013 Assumptions report). It is predicted that shale gas resource is there for at least 120 years.
U.S. crude oil production increased by 847,000 barrels per day in 2012, compared to 2011, by far the largest growth in crude oil production in any country in the world. Production from shales and other tight plays accounted for nearly all of this increase, reflecting both the availability of recoverable resources and favorable above-the-ground conditions for production. Furthermore, increase in drilling activity in the Lower 48 shale formations has increased dry shale gas production in the United States from 0.3 trillion cubic feet in 2000 to 9.6 trillion cubic feet in 2012, or to 40 % of U.S. dry natural gas production. Dry shale gas reserves increased to 94.4 trillion cubic feet by year-end 2010, when they equaled 31 % of total natural gas reserves (U.S. Energy Information Administration | Technically Recoverable Shale Oil and Shale Gas Resources, 2013).
EIA/ARI World Shale Gas and Shale Oil Resource Assessment determined that Marcellus and Utica in Northeast shales has 369 and 111 tcf of remaining reserves and undeveloped resources, respectively. Furthermore, Southeast Haynesville, Bossier and Fayetteville has about 161, 57 and 48 tcf of remaining reserves, respectively. Also, Mid-continent Woodford has 77 tcf, Rockies Niobrara has 57 and Texas Eagle Ford, Barnett and Permian have 119, 72, 34 tcf of remaining reserves, respectively. Evidently, undeveloped resources will play a significant role in oil and gas exploration and production in the years to come.
Three factors and technical advances made shale gas production economical over the past several years: 1) horizontal drilling, 2) hydraulic fracturing and 3) rapid increase in natural gas prices.
Shale gas development is a technologically driven process and it includes drilling both, vertical and horizontal wells. In both wells, casing and cement are installed to protect drinking water aquifers. Current advances in horizontal drilling allows more operators to rely on horizontal wells which provide more exposure to a formation in comparison to a vertical one. Generally, six to eight horizontal wells from one pad have the same accessibility to a reservoir as sixteen vertical wells. The use of multi-well pads can also reduce the number of well pads, access roads, pipeline routes and production facilities, minimizing the habitat disturbance, impact on the public and leave smaller environmental footprint (GWPC, 2009). Another important technical discovery in the effective utilization of shale gas resources is hydraulic fracturing, which involves pumping of a fracturing fluid under high pressure into a shale formation to produce cracks or fractures in the rock formation underground. This allows the natural gas to be extracted from the shale into the well in large quantities. Groundwater is protected during the fracturing process by special casing and cement that is installed when the well is drilled. In majority of cases, the fracture zone and any drinking water aquifer are separated by at least several thousand feet of rock therefore minimizing any potential water contamination risks. Fracturing fluid is water based fluid mixed with additives that assist the water to carry sand proppant into the fractures. Water and sand constitutes about 98% of the fracture liquid with the rest containing various chemical additives to improve fracturing process. Each hydraulic fracturing treatment is specifically designed for each location taking into account rock formation, depth and phys-chem characteristics of that particular location. The amount of water necessary to drill and fracture one shale gas well generally ranges from 2 to 4 million gallons depending on the basin. As this is a very large amount of water required for successful operation of shale gas production, operators need to identify water resources prior drilling and fracturing as well as try to minimize the use whenever and however possible. Furthermore, the use of water resources for shale gas production should not interfere with water use of local communities and should not pose any stress on the environment. Another important factor to consider is that after the drilling and fracturing is complete, water is produced along with the natural gas. Some of it is called returned fracture fluid and some is natural formation water. These two types of water that move back through the well with gas must be managed using economic and environmental approaches. Currently, this water is managed through underground injections, treatment and discharge and recycling.
Fracking fluid recycling
Hydraulic fracturing is performed 5,000 to 10,000 feet underground. Water required for fracturing operations is mixed with various chemical additives such as acids (hydrochloric acid), corrosion and scale inhibitors (alcohols, organic acids, sodium salts and polymers), iron controls (sodium salts, citric acid), anti-bacterial agents (glutaraldehyde, sodium hydroxide, alcohols), friction reducers (polymers and hydrocarbons), surfactants (alcohols, glycol), cross-linked gelling agents (polyol and borax, guar gum) and friction reducers to increase the water flow and assist sand in better fracturing of a rock formation (He et al., 2014). Fracturing is performed using highly pressurized fluids between 2,000 and 8,000 psi at an average flow rate of 2000 gpm.
Composition of fracturing or flow back fluid is: total dissolved solids (TDS) may be up to 180,000 mg/L. Other parameters include chlorides up to 80,000 ppm (e.g. sea water has average 19,000 ppm Cl-), strontium up to 2,500 ppm, barium up to 2,500 ppm, boron 25 ppm, chromium 3 ppm, lead to 0.15 ppm, arsenic less than 0.2 ppm and radium-226 up to 5,000 pCi/L. Evidently, these chemicals need to be remove prior reuse or disposal. One way to get rid of such water is to inject into the deep geological formations. Unfortunately, this method already led to minor earthquakes that indicates severe disadvantages of this approach. Furthermore, recycling is another option which helps to preserve local water supply and minimize the costs associated with disposal. Unfortunately, it may also have its drawbacks such as the formation of highly concentrated (radioactive) sludge that is not regulated by agencies and therefore pose a risk to drinking water sources if disposed improperly. Thus, recycling technology that meets the high demands of shale gas operators and is safe for the public and environment needs to be developed. By reusing water onsite, shale gas operators could save nearly $2 per barrel of water used and with fracking industry estimated to produce more than 500 million barrels of water per year, this could lead to a $1 billion of savings if operators start recycling water (Freyberg, 2014). Unfortunately, only 14% of water used for drilling and fracking is recycled and reused onsite. This situation would change when regulatory agencies puts more stringent rules on produced water disposal. It already happened in Pennsylvania, where state regulatory agencies placed discharge limits on water in 2010, resulting in 90% increase in water recycling rates in Marcellus shale by 2014. In Texas, legislation was passed in 2013 that encouraged water recycling by limiting well operator’s liability after water has been sent to a third party for treatment. Sadly, these initiatives are still not enough to reduce the stress on water resources. Hopefully, new scientific and technical advances will soon allow economic recycling of water which is sustainable and easy to implement.
GWPC collected evidence on current produced water management practices by shale gas basin (GWPC, 2009).
State-of-the art recycling methods
One of the major obstacles in fracturing fluid recycling is the high total dissolved solids (TDS). Conventional options include removal (filtering out or depositing) boron, calcium and other minerals from water. Unfortunately, this is rather expensive approach that results in additional sludge formation that will need to be taken care of. Another option is to dilute it with fresh water, which may in turn result in secondary contamination or other environmental problems. None of these options are economically and environmentally feasible. As conventional methods do not reach the desired goals, novel approaches needs to be developed. For instance, low cost desalination, electrodialysis, electrokinetics or reverse/forward osmosis technologies have a potential to not only remove TDS but also to destroy microbial contamination and remove heavy metals and organic contaminants that are detrimental to the environment.
The selection of appropriate treatment technology is extremely challenging due to (Thiel and Lienhard, 2014): 1) the difficulty in characterizing mixed-electrolytes of high ionic strength; (2) the desirability of high recovery ratios; and (3) significant variations in water composition from formation to formation and even well to well.
Desalination includes distillation where the fracking fluid is heated or placed under partial vacuum to increase its vapor pressure and form water vapor that can be subsequently condensed and recovered as pure, high quality water. Vapor extraction can be carried out several times to get a higher purity water. Conventional commercially available methods include multi-effect distillation, multi-stage flash and vapor compression distillation. Desalination by distillation can minimize or eliminate physicochemical treatment and de-oiling of fracking fluid eliminating capital costs and minimizing the production of secondary chemical waste sludge (Coday et al., 2014). Another important advantage is that this method could be used to treat high TDS fluids, which makes it very attractive option for fracking fluid recycling. Unfortunately, energy demand is high and contributes to more than 95% total operating costs.
Membrane separation technologies are based on a pressure driven separation processes that separates dissolved and suspended matter from aqueous solutions. These technologies include microfiltration, ultrafiltration, nanofiltration, reverse osmosis and electrodialysis. Electrodialysis is a membrane process, during which ions are transported through semi-permeable cationic and/or anionic membranes, under the electric potential. Cationic membranes are polyelectrolytes that has negatively charged materials, which reject negatively charged ions (e.g. Cl-) and allows positively charged ions to pass through. Anionic membranes contain positively charged materials that reject positively charged ions (Ba2+, Ca2+, etc) and allows negatively charged ions to flow through. Unfortunately, substances that do not carry an electrical charge cannot be removed using this approach. Thus, pre-treatment methods are used prior electrodialysis that include activated carbon filtration or advanced oxidation (organic matter), flocculation (colloids) and filtration techniques. Unfortunately, electrodialysis may not be the best choice if TDS is < 12,000 ppm.
Electrokinetics is a technique where application of low level direct current induces the destruction and migration of contaminants towards electrodes depending to their charge or along with the water flow. The principle of electrokinetics is similar to a conventional battery. Upon introduction of electric current, positively charged ions and water move towards the cathode and negatively charged ions move towards the anode. Destruction of contaminants may occur especially when the oxidant solution is added to enhance the process. Advantages include a relatively straight forward reactor design, ease of operation, low OPEX costs (with exception of electricity costs in some areas) and high efficiency in destroying/removing TDS, ions and uncharged particles. Disadvantages of this approach include the selection of electrodes that are cheap and long-lasting, difficult optimization of the reaction conditions especially when TDS is above 150,000ppm and high electricity costs in some areas. Electricity costs could be mitigated by the solar panels in areas where sunshine is abundant. However, this technology for recycling of fracking fluid is in its infancy therefore technical and scientific advances are to follow.
Reverse osmosis involves pumping of water through a semipermeable membrane at extremely high pressure. This pressure forces water to flow through the membrane leaving the salt behind and producing a clean stream of water containing no or very small amounts of TDS. Unfortunately, challenges include fouling of a membrane and high OPEX costs due to pressure. However, these challenges may be overcome if reverse osmosis is used together with electrokinetics and/or advanced oxidation processes to destroy contaminants prior their contact with a membrane.
Forward osmosis an emerging technology that can potentially overcome drawbacks of pressure driven membrane processes. It has several advantages including no fouling limitations allowing proper filtration and concentration of difficult products and waste streams (Valladares Linares et al., 2014). No problems associated with high TDS concentrations. This process is highly selective and requires little or no electricity or external power source. It can be used as standalone or in combination with reverse osmosis or nanofiltration.
Recycling of fracking fluid is still in its early years, therefore it is very difficult to tell whether recycling is economically feasible or its commercially attractive option. However, it is clear that in order to keep our environment clean, something needs to be done. And with scientific and technical advanced, recycling may become a viable option to significantly reduce the costs of fracking and reduce the burden on environment.
J. Centner, L. K. O'Connell. 2014. Unfinished business in the regulation of shale gas production in the United States. Science of the Total Environment, 476-477: 359-367.
D. Coday, P. Xub, E. G. Beaudry, J. Herron, K. Lampi, N. T. Hancock, T. Y. Cath. 2014. The sweet spot of forward osmosis: Treatment of produced water, drilling wastewater, and other complex and difficult liquid streams. Desalination, 333: 23-35.
Freyberg. 2014. Fracking: Time to end US “wild west” wastewater treatment, WWi magazine.
He, X. Wang, W. Liu, E. Barbot, R. D. Vidic. 2014. Microfiltration in recycling of Marcellus Shale flowback water: Solids removal and potential fouling of polymeric microfiltration membranes. Journal of Membrane Science, 462: 88-95.
Modern Shale Gas Development in the United States: A Primer. Ground Water Protection Council Oklahoma City, OK 73142 405-516-4972 www.gwpc.org and ALL Consulting Tulsa, OK 74119 918-382-7581 www.all-llc.com April 2009. G. P. Thiel and J. H. Lienhard. 2014. Treating produced water from hydraulic fracturing: Composition effects on scale formation and desalination system selection. Desalination 346: 54-69
Valladares Linares, Z. Li, S. Sarp, Sz.S. Bucs, G. Amy, J.S. Vrouwenvelder. 2014. Forward osmosis niches in seawater desalination and wastewater reuse. Water Research, 66: 122-139.
US Energy Information Administration (EIA), AEO2013 Assumptions report,http://www.eia.gov/forecasts/aeo/assumptions/ (last accessed August 31, 2016).