MANAGEMENT OF PRODUCED WATER IN COALBED METHANE OPERATIONS
| Jurisdiction | United States |
(Nov 2002)
MANAGEMENT OF PRODUCED WATER IN COALBED METHANE OPERATIONS
Arthur P. O'Hayre
Applied Hydrology Associates, Inc.
Denver, Colorado
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Introduction
While the development of coal bed methane (CBM)1 has been prevalent since the early 1980s, the past four to five years have seen a rapid increase in the development of this important energy resource. One of the most controversial aspects of CBM development is the co-production and subsequent management of water pumped from the coal seam. The majority of the methane gas that is contained in the coals is adsorbed in coal pores under hydrostatic pressure. In order to develop the methane gas resource, the hydraulic pressure in the coal is reduced by pumping groundwater from production wells completed in the target coal. This reduction n hydraulic pressure allows the methane gas to desorb from the coal pores and be collected.
The major concerns associated with co-produced water are (1) depletion of water resources, and (2) proper management of water discharge or disposal. This paper focuses on the management of produced water using examples from the Powder River Basin (PRB) in Wyoming and Montana; the San Juan Basin (SJB) of Colorado and New Mexico; and the Raton Basin (RB) of Colorado and New Mexico.
CBM Water Production Characteristics
Water production from coal seams is influenced by the thickness and continuity of he coal seam, and by the properties of the coal and adjacent geologic units. Coal seams often split and merge over distances of a few miles, resulting in considerable variation in water production over these distances. Initial water yields from CBM wells vary widely, depending upon the transmissivity2 of the coal and the reduction in pressure due to pumping. The transmissivity of the coal varies with coal thickness and with patterns and degrees of fracturing and cleating3 in the coal. The transmissivity of the coal can also vary as a function of pressure, as cleat apertures decrease with reduction in pressure. Stress reduction will be most evident around the wellbore and may lead to significant reduction in well pumping rate with greater drawdown. Free gas in the coal fractures will also result in decreased permeability to water, due to relative permeability effects. Water production rates can also be altered by well completion and stimulation techniques. Variation in water production also occurs due to operational practices, such as shutting-in wells or changing target depressurization levels.
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There is a general misperception about the amount of water production from CBM operations in the Powder River Basin. For instance, a recent report by the Powder River Basin Resource Council and the Wyoming Outdoor Council states, "In many places in the Basin, produced water can reach astounding levels of 80 to 100 gpm! At these rates, one well can produce over 100,000 gallons of water each day." While initial water production from a few CBM wells in the Powder River Basin may approach rates of 80 or 100 gpm, water production from wells varies considerably, even over relatively short distances within a given coal, and in almost all cases, water production declines relatively quickly with time.
Water production rates from individual wells typically decline with time. The water production declines are due to the low specific storage4 of most coal units. Fractures and cleats within the coal tend to occupy a relatively small amount of the total coal volume, typically from 0.1 to 0.4%. Figure 1 shows historic water and gas production for representative wells in the Powder River, Raton. and San Juan Basins.
During the early period of well production, most of the produced water is derived from groundwater that is stored in the coal cleats and fractures. Later in the well production life, leakage of groundwater from underlying and overlying units into the coal aquifer, induced by the hydraulic head reduction in the coal, may contribute an increasingly large proportion of the produced water. The extent of leakage is largely a function of the vertical permeability of the geologic units that separate these aquifer units from the coal, and the vertical hydraulic gradient5 that develops within these separating units as a result of coal depressurization. Figure 2 shows the decline in pumping rates from coal, and the influence of storage and leakage.
Potential Water Resource Impacts from Water Extraction
The primary groundwater impact associated with development of CBM involves removal of groundwater in storage within the target coal seams and loss in available hydraulic head in these coal seams. If the coal is a viable water supply aquifer, as is the case in the Powder River Basin, this head loss (drawdown) could impact water wells completed in the coal seams, in the form of reduced well yields and potential methane production.
Other potential effects of groundwater withdrawals on groundwater resources include potential changes in groundwater chemistry or in the nature of groundwater discharge to the surface in the form of springs, seeps, or base flows to surface drainages. Natural discharge of springs can potentially be impacted by reduction in hydraulic head in the source aquifer unit.
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Figure 1
Historical Water and Gas Production in the Powder River, Raton, and San Juan Basins.
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Figure 2
Influence of Storage and Leakage on Production Rates
After CBM operations cease, and water removal ends, water levels in the coal are expected to recover over time. The rate of recovery initially would be rapid due to re-equilibration of pressure heads within the coal and then be sustained at a much slower rate by leakage from overlying and underlying units.
To evaluate and monitor the effect of depressurization due to CBM development in the Powder River Basin, the Bureau of Land Management (BLM) have established several "nests" of monitoring wells, with wells completed in different zones. One such nest is in an area that has had active CBM development for about eight years, so that data from this monitoring well nest yields very good information about the potential impacts on overlying aquifers resulting from extended CBM operations. The nest has a well completed in the coal, a well completed in a sand about 40 feet above the coal, and wells completed in two shallow sands. Monitoring over the past eight years shows that the water level in the coal has been drawn down about 300 feet as a result of pumping of water and production of gas in the coal. Over the same time period, the well completed in the overlying sand shows only a slight decrease in water level of about 10 to 20 feet starting in about 1999.
This is not an isolated example. The monitoring conducted in the Powder River Basin by he BLM to date has not shown any evidence that CBM depressurization of the coal has significantly affected overlying or underlying sand aquifer water levels. There is leakage into the depressurized coals from these sands, but not to the extent that major impacts are seen in terms of loss of resources in the sands
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Produced Water Management Considerations
Pumped water is managed in several ways including: (1) discharge to local surface drainages, (2) infiltration via shallow impoundments, (3) injection into or recharge of shallow aquifers (4) storage and evaporation in reservoirs (containment), (5) deep-well injection, end (6) land application. In a CBM operation, a combination of options may be appropriate, and some options may only be feasible if combined with water treatment. Each water handling option presents different technical and regulatory challenges that are briefly described in this paper. It is critical to the emerging coalbed methane industry in the west to work through these technical and regulatory issues to develop cost-effective and environmentally acceptable methods for management of produced water from CBM operations.
The choice of management option largely depends on the quality and quantity of produced water. However, topographic or hydrogeological conditions may constrain the choice of water management option. Limitations may also imposed by regulatory and landowner issues. The cost of the various water management options must also be considered, because these costs can significantly affect the economics of CBM development, particularly in marginal fields.
The depletion of groundwater resources due to extraction of coal water (Section 9.03) can be offset somewhat if the produced water management option includes a component that recharges the aquifer system. Surface discharge of extracted groundwater from CBM operations into surface drainages, flow-through stock reservoirs, upland or bottomland infiltration impoundments, or upland containment impoundments enhances recharge of shallow aquifers below creek and impoundment areas. Infiltration of CBM-produced water may cause new-springs or seeps to develop downstream. Injection of CBM produced water would recharge the aquifer units in which the injection wells are completed.
[1] Water Quality Considerations
The quality of produced water from CBM operations will largely determine whether it can be used for various beneficial uses. A good indication of beneficial use possibilities can be made by comparing the water quality of CBM produced water to the water quality criteria or standards that have been developed by various Federal and State agencies for drinking water, stock water, irrigation and aquatic6 water. In addition to numerical standards, discharges of CBM water to surface streams also have to meet narrative standards for protection of agricultural water use and "antidegradation" criteria promulgated by several western States. Discharges to tributaries of the Colorado River are subject to salinity control issues.
The amount and quality of...
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