Large-scale scientific and engineering investigations into the natural gas potential of organic-rich shales began after the 1973–1974 oil embargo by the Organization of the Petroleum Exporting Countries. The Eastern Gas Shales Project (EGSP) was funded from 1977 to 1992 by the US Department of Energy with the goal of adapting engineered hydraulic fracturing treatments to create flowpaths from natural fracture networks within the shales to vertical wellbores. The EGSP field experiments showed that hydraulic fracturing alone was insufficient to produce economical amounts of hydrocarbons from vertical wells.
By the mid-1990s, technical advances in directional drilling for deepwater oil and gas, along with improvements in downhole bit navigation, enabled Mitchell Energy to bore long, horizontal wells called “laterals” into the Barnett Shale in the Fort Worth Basin of Texas. These laterals, which contacted a much greater volume of the shale formation than vertical wells, were stimulated with a series of staged hydraulic fractures carefully spaced into discrete zones along the lateral. The combination of horizontal drilling and staged hydraulic fracturing resulted in the production of economical quantities of natural gas from the Barnett Shale, initiating modern shale-gas and tight-oil development. Most estimates suggest that many decades of energy supplies are available from unconventional oil and gas (UOG) resources at current usage rates.
Fig. 1—Schematic diagram illustrating unconventional oil
and gas development activities relevant to research on
human-health and environmental impacts (not to scale):
well-pad construction (1); drilling (2); completion/stimulation (3, 4);
production of natural gas (5) and oil (6) with well casings designed
to protect drinking-water aquifers; ultimate closure (plug and
abandon), illustrating legacy well with leaking casing (7);
wastewater disposal (8); induced seismicity (9); landscape
disturbance (10); and potential for transport pathways from deep
to shallow formations (11). Also represented are water supply wells
in shallow and deep aquifers (12).
The commercial development of shale gas and tight oil requires drilling, fracturing, production, and transmission of oil/gas, management of waste streams, and well-closure (Fig. 1). The scale of development has raised questions about possible risks to air, water, landscapes, ecosystems, and human health. Large drill rigs are required to install the long, deep laterals. The land-clearing and pad construction activities needed to accommodate such equipment often modify landscapes and watersheds. hydraulic fracturing involves injection of large volumes of water (~0.1 to >10 million L) with sand to prop the fractures open and chemical additives such as friction reducers, corrosion inhibitors, anti-scale agents, and biocides. The water, sand, and additives are pumped into wells under pressures that exceed rock-strength to create fractures. Many of the risks at each step of UOG development are known while others remain poorly understood, and the overall combined risk is difficult to assess. For example, of 1,606 chemicals identified in wastewater from UOG wells, chronic toxicity values are only available for 173. The United States will continue to rely on the production of fossil fuel hydrocarbons for some time, and understanding of the risks must be improved.
Researchers at government agencies, universities, institutes, and industry have been investigating potential human-health and environmental impacts of UOG development. Herein we use “environmental impacts” to refer to impacts on aquatic and terrestrial organisms, communities, and ecosystems. This article aims to highlight the critical research questions in this area and to provide access to results of ongoing research.