By Béla Lipták, PE, Columnist
In the United States, natural gas (NG) is the source of about 25% of the total energy consumption, and shale gas is the source of about 20% of the NG consumed. During the next years, hydraulic fracturing, or "fracking," to produce NG will continue to rise, and will increase from today's 20% to about 45% of American consumption by 2035. To date, just in Pennsylvania, there are some 4000 fracking wells in operation, and their number nationwide is projected to approach 100,000 within a few decades. The size of American recoverable shale gas deposits is debated. Until 2011, industry estimated the Marcellus Reserves (from Virginia to New York state) to be over 400 trillion cubic feet (tcf), while this year the U.S. Geological Survey reduced that estimate to 84 tcf.
Many of the natural gas wells in the United States use fracking to produce gas at economic rates. Large trucks, blenders, tanks and multistage pumps are used to inject millions of gallons of water at pressures of up to 20,000 psig into these wells that can be drilled to the depths up to 20,000 feet. Hydraulic fracturing can be performed in vertical or horizontal wells. In horizontal drilling, the terminal drill-hole is completed as a "lateral" that extends 1500 to 5000 feet parallel with the shale layer, while vertical wells extend only 50 ft to 300 ft into it. Horizontal drilling also reduces surface disruptions, as fewer wells are required.
After drilling the well, high-pressure liquids are injected into the shale rock or coal beds (Figure 1). When the "down-hole" pressure exceeds the fracture strength of the rock, it cracks, and the fracture fluid (FF) travels farther into the rock, extending the crack.
After cracks are formed, they have to be kept open. Proppants are solid particulates, such as grains of silica sand, resin-coated sand or harder materials such as ceramics. They serve to prevent the reclosing of the fractures when the injection phase is completed.
In the FF, sometimes, naturally radioactive minerals are also used in order to help to measure the depth of the fractures along the wellbore. Ninety-nine percent of the FF is usually water, while the rest consists of chemical additives used to protect the well and improve its operation. Initially, the injected FF is acidic to increase permeability. This phase is followed by injecting FF-containing proppants with gradually increased size solids, and the operation is completed by flushing the well with water.
When the fracking phase is over, most of the FF and drill cuttings are processed for reuse, trucked away, treated on-site before being released into the environment or stored on-site either in large tanks or in "frack ponds" that are several acres in size. Since these ponds are on the surface, and their wastewater can contaminate ground waters, wells and rivers, these ponds are sealed with plastic lining. Usually 30% to 40% of the FF can not be removed from the underground fractures and stays down in the shale, creating small and often toxic lakes.
During the removal of the FF, large amounts of NG, including methane, escape from the well or dissolve in the FF and enter the frack pond. After the removal of as much FF as possible, the actual production starts, and the drilling equipment is moved to drill another well.
Arguments Pro and Con
Some representatives of the gas industry and some politicians believe that the amount of recoverable fracking gas could meet the American energy needs for a century or more, while opponents argue that the NG that is recoverable will be exhausted in a few decades. Proponents argue that fracking creates jobs and reduces energy imports, while opponents argue that these jobs are temporary, and more permanent jobs could be created if the same investment was made in renewable energy. Industry representatives also argue that NG is inexpensive, while opponents say that the cost would be much higher if the value of the water used, reduced real estate values, increased mortgage costs, expenses associated with health effects, the cost of wastewater treatment and legal expenses were included.
The gas industry advertises NG as the cleanest fossil fuel. Opponents claim that during the lifetime of a well, 3% to 8% of the produced methane is leaked into the atmosphere and, because methane is such a potent greenhouse gas, the greenhouse-gas footprint of NG is worse than that of coal or oil. The tradeoff is that, while the burning of NG releases fewer allergy- and cancer-causing solids and other pollutants than coal, the released methane contributes several times more greenhouse gases.
Proponents argue that the forces generated by fracking are insufficient to cause earthquakes, even when applied to unstable geological formations. Opponents point to the tremors and small earthquakes that have already been caused and to the potential damage to buildings. Last year, nine quakes occurred, unclamping ancient faults (geophones) near the Mahoning River in Ohio and others were reported in Arkansas and Colorado.