In some shale deposits, there are significant reserves of natural gas that cannot naturally leave because of the rock’s low permeability. The practice of hydraulic fracturing (often called fracking) shatters the shale, opening up cracks through which the natural gas can flow into wells and then be brought to the surface. Figure 1 illustrates the process.
The fracturing of the shale is initiated by pumping fluids into the rock at very high pressures. The fluid is mostly water but also includes other chemicals that aid in the fracturing process. Some of these chemicals may be toxic, and there are concerns about fracking fluids leaking into freshwater aquifers that supply people with drinking water. The injection fluid also includes sand, so once fractures open up in the shale, the sand grains can keep them propped open and permit the gas to continue to flow. Once the fracturing has been accomplished, the fracking fluid is brought back to the surface. This wastewater is then injected into deep disposal wells. In some locations, these injections appear to trigger numerous minor earthquakes. Due to concerns about potential groundwater contamination and induced seismicity, hydraulic fracturing remains a controversial practice. Its environmental effects remain a focus of continuing research.
Author: E.J.Tarbuck, F.K. Lutgens, illustrated by D.Tasa