Environmental Aspects of Natural Gas Production

Only a few years ago, the price of natural gas was rapidly increasing and many people were struggling to heat their homes. The main reason that prices have fallen since then is that production of natural gas increased from two non-traditional sources: coal beds and organic shale deposits. [To learn more about how NETL’s research has contributed to this development, click on each link.] Yet, effective production from unconventional gas resources requires well drilling and completion technologies that are too new for their environmental footprint to be well established. NETL is conducting research that accurately assesses the impact of unconventional gas development on the ecosystems where gas plays are located to provide regulators with an impartial, scientific basis for possible rulemaking. NETL is also actively developing cost effective technologies and management strategies to mitigate identified environmental impacts.

Figure 1. Horizontal Marcellus shale gas wells on a multi-well drill pad in southwestern Pennsylvania (Photo courtesy of Range Resources-Appalachia).

NETL researchers have used geophysics to track saline water underground and to improve the way that saline water is treated and used at coal bed natural gas extraction sites in the western United States. They have also recently demonstrated that the water produced in the Powder River Basin of Wyoming can be used to irrigate crops using subsurface irrigation. In the eastern United States, NETL is leading a consortium of eight state and Federal agencies evaluating the environmental impacts of shale gas production that uses hydraulic fracturing and drill pads with multiple horizontal wells. Much of this research is briefly summarized below.

Environmental Study of a Sub-surface Drip Irrigation Site along the Powder River, Wyoming

The National Energy Technology Laboratory and the U.S. Geological Survey are collaborating with BeneTerra LLC to comprehensively monitor a sub-surface drip irrigation (SDI) system at a site in the Powder River Basin (PRB) of Wyoming. Irrigation water for the SDI system is coalbed natural gas (CBNG) co-produced water. The study is being conducted at the Headgate Draw area, located approximately 17 km south of Arvada, Wyoming at the confluence of Crazy Woman Creek and the Powder River. The study site encompasses six alfalfa fields and covers an approximate area of 1.2 km2.

The CBNG waters are applied to the root zones of agricultural land; this style of irrigation is capable of applying two to three times more water on a site than traditional surface irrigation, and is designed to minimize environmental impacts by parking potentially detrimental salts below the land surface, but above the water table. NETL is investigating the transport and fate of the water and salts from the injected CBNG produced waters at the SDI site.

This study has shown that subsurface irrigation with CBNG produced water is an environmentally acceptable practice in the Powder River Basin of Wyoming. The Ca- and Mg- minerals in the native soil dissolve in the applied CBNG produced water to maintain a sodium adsorption ratio that sustains soil permeability (the primary concern when using high-Na produced water for irrigation). Geochemical modeling based on three years of soil mineralogy, soil chemistry, and groundwater chemistry data from the prospective study indicates that CBNG produced water can be applied by sub-irrigation for about 10 years at current application rates before adverse impacts to soil permeability result. This study provided the Wyoming DEQ with the scientific basis to approve sub-surface irrigation for the management and beneficial use of CBNG produced waters.

On the left, a researcher using a geophysical tool to assess the effects of subsurface drip irrigation (SDI) on soil conductivity; on the right, alfalfa being harvested from land irrigated with produced water through SDI.

Research on the Air Quality Impacts of Marcellus Shale Natural Gas Production

The development of shale gas resources requires horizontal drilling and multi‐stage hydraulic fracturing; two processes that result in more air emissions than conventional natural gas. Horizontal drilling requires larger drill rigs with more horsepower that are operating longer to complete the long horizontal segments of shale gas wells. Similarly, hydraulic fracturing requires large volumes of water that often is hauled by truck to the well site. More than 500 truck trips are needed to transport enough water for the hydraulic fracturing of one well. The atmospheric emissions from trucks combined with the emissions from multiple, diesel powered, hydraulic fracturing pumps can cause a short-lived but significant impact to local air quality. Atmospheric emissions from the venting of condensate tanks, dehydrators, and the pneumatic valves are more constant sources that persist as long as natural gas is being produced. On July 28, 2011, EPA announced its intention to limit air emissions from natural gas drilling.

NETL is leading several field studies to determine air quality impacts resulting from development and production of gas resources in the Marcellus shale. A field monitoring trailer that measures multiple atmospheric signals (e.g., ozone, particulates, organics, and other gases) is being deployed at a range of sites to document temporal atmospheric signals as a function of geography, season, operation, etc. The mobile air monitoring laboratory was deployed to the Allegheny National Forest (ANF) in north-central Pennsylvania for approximately one year (July 2010 – June 2011). The forest has historically been a productive area for oil and gas wells but the number of wells has increased significantly in the past few years as Marcellus shale gas wells have been drilled. Ambient concentrations of pollutants and other air quality parameters were determined at three monitoring sites within the ANF, two of which were downwind of areas with heavy oil and gas exploration and production, and one site in an area relatively uninfluenced by emissions from oil and gas operations to serve as a background location for pollutant concentrations. Results of this study are still being analyzed.

NETL’s mobile air monitoring laboratory deployed in the Allegheny National Forest

Baseline Environmental Monitoring at a Marcellus Shale Gas Well Site

NETL is leading a consortium of eight state and Federal agencies evaluating the environmental impacts of shale gas production that uses hydraulic fracturing and drill pads with multiple horizontal wells. This effort was selected by EPA (one of the participating agencies) to be a “prospective case study” for their congressionally mandated investigation of the impact of hydraulic fracturing on sources of drinking water. The Marcellus Test Site differs from the other EPA case study sites (where only impacts to drinking water are being monitored) in that impacts to: 1) air quality, 2) terrestrial and aquatic wildlife, 3) soil properties, 4) vegetation, and 5) landscapes (future land use) will also be considered.

The multidisciplinary team will assess changes to water quality, air quality, wildlife and aquatic habitat, agricultural and forestry resources, and land use attributable to the development of the shale gas resource. NETL’s mobile air monitoring station has already been moved downwind of the site to collect four seasons of background (baseline) environmental data before well construction begins. Monitoring of environmental parameters will continue through well drilling and completion, and for at least one year of well production. By providing a more complete understanding of the impacts of Marcellus Shale natural gas production on the local and regional environment, NETL can help ensure that development proceeds at a rate that protects the environment while ensuring an adequate domestic supply.

Large pond below proposed Marcellus shale gas well pad that currently supports an abundant and diverse fish population

Chemical and Microbiological Characterization of Water Co-produced with Marcellus Shale Gas

NETL microbiologists have determined that the microbial ecology of Marcellus Shale production water changes significantly with time as the water is stored in on-site pits and centralized impoundments. Changes in bacterial populations have been linked to the consumption of labile organic compounds used in hydraulic fracturing (particularly friction reducers) and to the amount of dissolved oxygen. Understanding the complex, ever-changing ecology of these waters is important when the water is to be re-used for additional well completions. Furtherance of this work is expected to lead to more environmentally acceptable biocides and new methods to manage the adverse impacts of bacteria on shale gas development and production.

Lined impoundments provide temporary storage for: water needed for hydraulic fracturing and the water that rapidly flows back to the surface when the well is being prepared for production. (Photo courtesy of Energy Corp. of America)

Finally, collaboration between researchers at the University of Pittsburgh and NETL has identified a unique isotopic signature for Marcellus Shale productions waters that permits these waters to be distinguished from other sources of contaminated water in western Pennsylvania (e.g. water from coal mines). This methodology should theoretically allow researchers to calculate the contribution of Marcellus Shale production waters to the overall volume of flowback water from completed gas wells.

Other Environmental Aspects of Marcellus Shale Natural Gas Production

Another NETL study has examined the influence of unpaved roads built to service the oil and gas industry in the Allegheny National Forest on stream sedimentation and ecology. Two adjacent watersheds, similar in size and topography, but having either low or very high road density, were studied. The primary stream in each watershed was instrumented with water flow and quality monitors and directly monitored via satellite telemetry, in cooperation with the U.S. Geological Survey. A rainfall simulation device provided by Penn State’s Center for Dirt and Gravel Roads was used to create repeatable simulated rainfall events, and runoff was collected from 14 different road sites. An average of 370 kg of sediment runoff per kilometer of road was measured for the 30 min rainfall events. Macroinvertebrate populations indicative of stream health were collected by Clarion University of Pennsylvania personnel during the early summer, late summer, and fall from two sites in each watershed. Preliminary data analysis indicates that macroinvertebrate populations in both watersheds appear similar in terms of overall community richness and diversity, despite large differences in sediment runoff. This implies that the greater sediment loads of the oil and gas impacted watershed may be below the threshold needed to adversely impact macroinvertebrate populations. These results can potentially be extended to brook trout and some bird species that rely on macroinvertebrates as their primary food. The results also suggest that the presence or absence of specific indicator species may be a better indication of road-generated sediment impact than the more costly and time-consuming analysis of the total population.

The deciduous forests of the central Appalachians provide habitat for many bird species, including raptors and songbirds. Another NETL study, being carried out by West Virginia University, has established an important baseline of bird population data in an area impacted by both historic and current oil and natural gas industry activities. In addition to complete bird population counts, the study focuses on two forest songbirds that are of particular regional conservation concern: the cerulean warbler (a ridge top species) and the Louisiana waterthrush (a stream valley-dwelling species). The field-intensive research examines the distribution and abundance of these species relative to energy extraction-related activities in these two habitats. Birds that live at the forest edge have been found to be more abundant in areas disturbed by oil and gas activity due to forest clearing and road building. Long-term field studies such as this are required to fully understand the complex annual variations that occur in natural bird populations and to develop better management plans that optimize habitat protection and species diversity.

A cerulean warbler (a ridge top species) captured, tagged, and released in a disturbed area of the Wetzel Wildlife Management Area, Wetzel Co., WV

A Louisiana waterthrush (a stream valley-dwelling species) captured, tagged, and released in a disturbed area of the Wetzel Wildlife Management Area, Wetzel Co., WV