Title: State of the Basin: A Brief Summary of the Current Legal and Academic Interest in the Powder River Basin, Wyoming
Author: Catherine Campbell
Publication: The Outcrop, September 2008, P. 6, 17, 27-29
Objectives: The initial step I take when beginning a research project is to try Google, which after checking out the Wikipedia article, usually gives me a good perspective on the current state of my topic of interest. Googling the Powder River Basin is no exception, and the results demonstrate not only how large this petroleum system is, but also the enormity of issues surrounding it. After completing a Masters thesis on the topic and speaking with numerous people currently involved in the production, legal, and environmental aspects of the basin, the largest of these issues in relation to the coal bed natural gas (CBNG) production is the co-produced water.
The Powder River Basin, located in northeastern Wyoming and southeastern Montana (Fig. 1) produces 40% of the coal consumed nationally on an annual basis and is estimated to contain 25.2 TCF of natural gas. The CBNG is typically produced from shallow coal seams, at a depth less than 2,000 feet, with an average well producing 60 MCF gas and 4,500 gallons of water per day after an initial dewatering period. This produced water is typically discharged into surface drainages. Development of the CBNG has grown exponentially from 152 wells in 1999 to more than 22,000 in 2007, resulting in numerous lawsuits pertaining to the produced water and other environmental concerns (Fig. 2). This jump in legal activity around the basin has resulted in a slowing of development in recent years, and an increase in the academic and industry interest in the science of the resource.
The legal battles over CBNG range from cases involving ranching families to interstate large scale water quality standard debates. A recent smaller scale Powder River Basin case in Wyoming involving two Campbell County ranching families suing the Wyoming Board of Control and the State Engineer’s Office, was dismissed last month. The premise of the suit related to alleged lack of regulation for discharged CBNG co-produced water, which resulted in a lowering of the local water table and soil damage due to the high concentrations of minerals in the water. The court’s decision stated that the officials involved acted appropriately based on the Wyoming constitution, and issues relating to water discharge should be handled by the slate Legislature. The plaintiffs have appealed the court’s decision to the Wyoming Supreme Court.
The inter-state water quality standard debate between Wyoming and Montana is still ongoing, with a relatively inactive federal lawsuit pending in Cheyenne. Montana, with approval from the Environmental Protection Agency, adopted stricter water quality standards in 2006, an action that has faced heavy opposition by the energy industry end the state of Wyoming.
Last year, the Wyoming Outdoor Council took action against the Wyoming Department of Environmental Quality, citing that several of the Chapter One Rules are not legal. The main component of the lawsuit deals with effluent dependent water. Under the rule, water bodies that were ephemeral prior to CBNG co-produced water, which are now perennial or intermittent due to the addition of the discharge water, are considered to have background water quality levels based on the new composition of the water, including the co-production. According to the Wyoming Outdoor Council, this language violates the Clean Water Act. At this time, approximately one year has passed without any action on the lawsuit.
Along with the ramp up in legal issues in the basin, academic interest has also increased. Several Department of Energy grants have been awarded, along with money from numerous other funding sources, over the past few years to deal with the water and other environmental concerns related to the Powder Rive Basin. These studies include evaluations of water quality, and aquifer communication through fracture evaluations and isotopic tracing of water.
Lee (1981) demonstrates that water deeper than 200 feet is sodium-bicarbonate type, with greater variability seen in the shallower waters. Brink and others (2008) outline the evolution of this shallow water to the stable composition of the deeper water as a five step process involving dissolution of various salts, reduction reactions and cation exchange. The range in values of total dissolved solids (TDS) are highest along the Johnson-Campbell county line, some values are above 3,000 mg/L, with the lowest values along the eastern margin of the basin, typically around 1,000 mg/L (Fig.3) (Campbell and others, 2008; Rice and others, 2000). The strongest control on TDS seems to be residence time of water, with the highest values occurring away from the recharge, which for most of the samples considered is the eastern margin of the basin. The sodium absorption ratio (SAR) also follows this trend, increasing in value with increased distance from recharge (Campbell and others, in press).
Although the composition of CBNG co-produced water is a huge legal concern, the volume of this water compared to historical conditions in the basin is also a debated topic. In order to understand the processes, several studies involving evaluation of fracturing and isotopic tracing have been completed. Colmenares and Zoback (2007) evaluate hydraulic fracture completion techniques in relation to water and gas production. Hydraulic fracturing of coal seams, known in the Powder River Basin as water-enhancement, involves pumping approximately 60 barrels of water per minute into the coal seam for 15 minutes, resulting in fracture propagation and hopefully CBNG production enhancement. One of the issues with this practice is that fracture growth may be vertical causing communication between the coal aquifer and overlaying sandstone aquifers, resulting in increased water production and lack of necessary depressurization in the coal seam.
Colmenares and Zoback (2007) evaluate time pressure records from the water-enhancement process to determine the least principal stress in the coal seems completed. The least principal stress varies between overburden and minimal horizontal stress, resulting in both horizontal and vertical fracture propagation in the basin. Upon comparison to production, wells with horizontal fractures tend to produce less water and wells with vertical fractures produce the most water. The conclusion of this research is that injection rates in areas with known vertical fracture tendencies should be reduced to ensure that fractures do not propagate into overlying aquifers, ensuring adequate depressurization with minimal water production.
Campbell and others (in press) evaluate strontium isotopes as another means of identifying aquifer communication. The premise of the research is that the 87Sr/86Sr of groundwater is indicator of the water-rock interaction history, with different aquifers having unique signatures based on the rock matrix (Fig. 4). Frost and others (2002) show that coal and sandstone aquifers tend to have unique isotopic ratios, with intermediate values indicating aquifer mixing.
From this research, different coal seams throughout the basin are identified to have different aquifer characteristics including combined sand-coal aquifers in the area around and west of Gillette, mixing of sand and coal aquifers in the northeastern part of the Wyoming portion of the Wind River Basin due to faulting, and lower overall ratios In the Sheridan and Buffalo areas of the basin. One interesting correlation between isotopic ratio and production is seen in the northeastern section of the Wyoming portion of the basin where samples with higher Sr isotopic ratios were from higher gas producing wells than less radiogenic waters (Campbell and others, in press). This could be a useful tool in the future for producers who want to try and predict gas production in newly drilled wells.
I would like to acknowledge the Petroleum Association of Wyoming for their assistance in providing the information about the lawsuits summarized in this article.
Brinck, L.L., Drever, J.I., and Frost, C.D., 2008, The geochemical evolution of water co-produced with coal bed natural gas in the Powder River Basin,Wyoming, Environmental Geosciences, in press July 2008.
Campbell, C.C., Pearson, B.N., and Frost, C.D., in press, Strontium isotopes as indicators of aquifer communication in an area of coal bed natural gas production, Powder River Basin, Wyoming and Montana, Rocky Mountain Geology, v. 43, n. 2.
Colmenares, L.B., and Zoback, M.D., 2007, Hydraulic fracturing and wellbore completion of coalbed methane wells in the Powder River Basin, Wyoming: Implications for water and gas production, American Association of Petroleum Geologists Bulletin, v. 91, p. 51-67.
Lee, R.W., 1981, Geochemistry of water in the Fort Union Formation of the northern Powder River Basin, southeastern Montana, U.S. Geological Survey Water-Supply Paper 2076, 17 p.
Rice, C.A., Ellis, M.S., and Bullock, J.H., Jr., 2000, Water co-produced with coalbed methane in the Powder River Basin, Wyoming: preliminary compositional data, U.S. Geological Survey Open-File Report 00-372, 18 p.