Table 1. Rocky Mountain CBM Basins (GTI, 2001)

Regulatory and Analytical Activity Increasing

Table 2. Websites with Information on Rocky Mountain 
CBM Production and Water Management Issues

Technology Options for Handling Produced Water

Down-Hole Gas/Water Separation with Re-Injection

Conventional Surface Separation and Re-Injection

Surface Disposal Without Treatment

Figure 1—Stock Watering Tank at Tietjen Ranch Site (EPA, 2001)

Figure 2— Evaporation Pond Outflow at Tietjen Ranch (EPA, 2001)

Figure 3— Atomizer at Barrett Resources Site Near Gillette (EPA, 2001)

Surface Disposal With Treatment  

Real Cost Comparisons

Attention Will Drive Innovation

References  

Sidebar 1
Adapted from Colorado State University Cooperative Extension

Options for Coalbed Methane Water Management
by Karl Lang, Hart/IRI Fuels Information Services
Excerpts in PTTC Network News, 1st Quarter 2002

Demand for natural gas has led to a dramatic increase in coalbed methane (CBM) drilling and production since 1996, primarily in Rocky Mountain basins. During 2000, 1.4 Tcf of natural gas, about 7.5% of US demand, was supplied from coal seams. This has had a number of impacts, one of which is a dramatic increase in the amount of produced water. CBM wells must be dewatered to initiate methane desorption from the coal, resulting in significant volumes of water being pumped during a well’s early life. New technologies are being considered and tested to deal with the problem of how to effectively dispose of this water, but one thing is certain, there is no single solution to the question of how to best produce CBM gas and manage produced water. The circumstances, and therefore the economics that drive technology choices, are different in practically every situation.

Powder River Basin Center of Activity 

There are seven CBM basins in the Rockies: San Juan, Powder River, Raton, Uinta, Piceance, Greater Green River and Wind River. Each has its own character in terms of the CBM resource, and for those with significant production, its own producing behavior (Table 1). In many of these basins the produced water is of such poor quality that it must be re-injected. In the San Juan basin for example, high gas rates have justified the investment in deep well re-injection of produced water. Water rates have declined with time in the San Juan, one of the first of the country’s CBM development areas.

In the more recently developed Powder River (PRB) and Raton Basins a combination of lower per well gas rates (The PRB has shallower coals with considerably lower gas content than San Juan coals) and relatively fresh water, makes surface water disposal an environmentally viable and economically attractive option. However, the number of CBM wells in the PRB jumped from 1657 in 1999 to 5122 in 2000 (Environmental Protection Agency (EPA), 2001). Roughly 7500-8000 wells are now producing and many more await infrastructure and discharge permits. As many as 40,000 wells could ultimately be producing in Wyoming and another 10,000 or more in Montana, depending on spacing and access, by the end of the next decade. During 2000, 378 million barrels of water were produced in the PRB to obtain 151 Bcf of gas, more than the cumulative gas of all seven prior years of production. Today, the basin pumps roughly 1.4 million barrels of water per day to produce 800 Mcfd of gas. With estimates of the total CBM resource as high as 25 Tcf, only about 10% of the basin has been developed (DOE, 2002).

Currently the water and gas is produced in the eastern part of the PRB, but development is expanding toward the north and west at a rate of up to 10 wells per day. The produced water has sodium as a dominant cation and bicarbonate as the major anion, and the concentrations of these constituents appear to increase as well locations move northwest. Total dissolved solids (TDS) values range from 270 to 2010 mg/L with a mean of about 862 mg/L. Iron values range from 0.02 to 15.4 mg/L with a mean of about 0.8 mg/L (EPA, 2001). Depending on the water quality at a specific location, the soil at a particular location, and the landowner needs regarding water usage on the lease, the impact of the water’s disposal may be positive or negative. The degree of potential damage to a soil is a function of the chemical makeup of the soil as well as the chemical makeup of the water. One indicator of soil character is the sodium adsorption ratio (see sidebar).

Table 1—Rocky Mountain CBM Basins (GTI, 2001)

Regulatory and Analytical Activity Increasing

The Wyoming Department of Environmental Quality (DEQ) is issuing water discharge permits for surface discharge in the watershed drainage areas of the Belle Fourche and Cheyenne Rivers. The DEQ halted permits for discharges into the Tongue River and Powder River, both of which flow into Montana, due to protests by Montana groups. Subsequently, the states agreed to permit a number of discharges for the Powder and Little Powder Rivers as long as DEQ monitors the water quality (Shirley, 2002). The Tongue River remains closed to discharges however. With producing companies turning to other “of-channel” forms of surface containment, the DEQ is currently developing policies to regulate this as well.

In response to their perceived need for additional study of this issue, EPA is currently undertaking an analysis to develop a “Best Professional Judgement” determination of effluent limitations that represents the “Best Available Technology Economically Achievable” (BAT) for coalbed methane activities throughout Region 8. A draft report to have been completed last November has been delayed.  According to Mike Reed, with the EPA Region 8 Water Program in Denver, “We are hoping to have our model runs completed and a draft report by the end of April, although that may be optimistic. Following the draft’s publication there will be a public meeting where we will take comments. We hope to have a final guidance document by the end of June or July, 2002.” While this guidance document is primarily for use by the EPA in permitting activity on Indian Lands, the EPA hopes that it will provide information that the states can use in their own permitting decisions. Depending on the study results and follow-on actions, beneficial use of produced water may be restricted. This in turn could increase the costs of production and limit development across multiple Rocky Mountain basins.

More recently, the Strategic Center for Natural Gas at the U.S. Department of Energy’s National Energy Technology Laboratory (NETL) is sponsoring a comprehensive analysis that will examine: (1) the magnitude and character of the CBM resource in the PRB, (2) estimates of future CBM and water production, and (3) impacts of alternative produced water management practices in the PRB (DOE, 2002). A complementary review of available subsurface water disposal zones in the basin is also being supported by NETL through its National Petroleum Technology Office. The results of the SCNG study, to be carried out by Advanced Resources International, are expected in mid-2002.

Concurrently, The Bureau of Land Management (BLM) published a Powder River Basin Oil and Gas Draft Environmental Impact Statement (EIS) on January 18^th. This draft is now in the public comment period until April 18th, 2002. This draft envisions the construction of 39,400 new gas wells in the PRB. The final version is expected to be completed in the summer of 2002.

Table 2—Websites with Information on Rocky Mountain 
CBM Production and Water Management Issues

Technology Options for Handling Produced Water 

There are three basic options for dealing with produced water: (1) utilize a down-hole pump to re-inject the water within the well bore or pump the water to the surface and either (2) re-inject it into a disposal well or (3) discharge the water at the surface, treated or untreated. Option three encompasses a number of approaches, depending largely on whether or not the water is treated. We discuss each of these options as they relate to coalbed methane water, below.

Down-Hole Gas/Water Separation with Re-Injection

This option can be active, where a down-hole pump injects the water into a convenient formation either above or below the producing coal seam, or passive, where gravity drains the water into the disposal zone below the coal. The latter situation requires such a rare combination of circumstances (suitable high kh zone below the coal with pore pressure such that the water head between coal and disposal zone is sufficient to maintain injection rate) that it is not a common option. Active down-hole injection, either below production disposal (BPD) or above production disposal (APD), is an option if there is a suitable disposal zone available. The zone must exhibit sufficient kh and pore volume, low skin factor, hydrologic isolation, and water compatibility.

Two examples of active down-hole re-injection of CBM water in East Central Oklahoma were recently described (Phelps, 2002). In each case, an up-hole re-completion of a coal zone at about 1200 feet was made in a depleted oil well. A 1 1/2 inch downhole pump was used to pump produced water from the casing/tubing annulus into the depleted oil reservoir disposal zone below a packer set below the coal seam. In each case the wells went from shut in to producing 50-65 Mcfd.

An economic comparison of the down-hole gas/water separation option to conventional water disposal by re-injection from the surface reveals that the main economic factors are: well depth, proximity to disposal zone, and the number of producers per disposal well. In a comparison shown by Phelps for a Cherokee Basin field (Oklahoma), the water handling cost (CAPEX + OPEX) for a conventional re-injection scenario varied from $1.10/Bbl for 4 producers/disposal well, to $0.55/Bbl for 12 producers/disposal well. Down-hole separation and disposal cost in the same field was estimated at $0.60/Bbl to $0.85/Bbl, depending on whether or not the disposal zone was completed as part of a new CBM well drilling plan or during reentry into an existing well. The incremental cost of drilling a new well slightly deeper is less expensive than re-entering an old well. This exercise showed that while down-hole gas/water separation and disposal can be competitive with conventional surface re-injection into disposal wells, planning for this approach in advance when developing a new area provides the best economics.

About 20 CBM applications of these systems have been installed in the Cherokee Basin, injecting between 50 and 250 Bwpd of down-hole separated CBM water. Unfortunately, due to a change in ownership of the properties, quantitative information regarding the performance of these wells is lacking. According to Craig Phelps, a manager in the Produced Water Management group at Gas Technology Institute, GTI is funding a data collection effort by the equipment manufacturer (DHI Tools) to provide a better picture of well performance. “The equipment provider has developed a technique that allows the operator to determine the volume of produced water being injected downhole from a pump dynamometer card,” says Phelps. “This is important for permitting purposes. While the EPA Region 5 authorities are satisfied with it, we hope to be able to satisfy the regulators in Region 8 as well.”

Another idea being considered for areas where all water disposal alternatives with the exception of downhole separation and re-injection are cost-prohibitive, is the possibility that this technology could be considered an improved recovery technique and thereby qualify for some form of tax incentive.

Conventional Surface Separation and Re-Injection

Water disposal wells are the most common choice in areas where the produced water quality precludes surface disposal. The complexity and cost of such systems depend primarily on the depth of the disposal zone. Shallow zones often mean that gravity flow can be utilized to dump water from a holding tank to a down-gradient disposal well. Deeper injection zones mean higher drilling, pump equipment and operating costs. The number of CBM wells that can be served by a disposal well also is an important factor in the overall economic attractiveness of this option. And importantly, the disposal well re-injection option must contend with the fact that if the disposal well goes down for some reason, gas production from all of the wells is immediately curtailed.

Surface Disposal Without Treatment

If the produced water meets National Pollutant Discharge Elimination System standards, it can be disposed on the surface without treatment. Options here include: (1) discharge to drainage systems (creeks, rivers), (2) storage in a pond from where it infiltrates into shallow aquifers or evaporates, or (3) atomization, where the water is sprayed from a high standing pipe to accelerate dispersal and promote vegetative growth . Along the way, the water can be put to beneficial use for irrigating crops or watering livestock.

One example of the surface disposal option is Pennaco Energy’s operations at Tietjen Ranch in the Powder River Basin (PRB) near Gillette, Wyoming. A portion of the produced water is piped to a tire tank for livestock (Figure 1) and the rest is piped to an evaporation pond (EPA, 2001). The water is discharged to the pond across limestone rocks (rip rap), which help precipitate out dissolved iron. These rocks can then be replaced, eliminating any permanent staining of the surrounding landscape (Figure 2).

Figure 1—Stock Watering Tank at Tietjen Ranch Site (EPA, 2001)

Figure 2—Evaporation Pond Outflow at Tietjen Ranch (EPA, 2001)

Barrett Resources Corp is conducting a pilot operation to test the effectiveness of atomizers, also in the PRB (EPA, 2001). At the Scooner Road site, 15 CBM wells produce approximately 28,000 Bwpd, which is pumped to a holding pond. Overflow from the pond is then piped to 8 atomizers (Figure 3) that together can spray more than 16,000 Bwpd. 

Figure 3—Atomizer at Barrett Resources Site Near Gillette (EPA, 2001)

An alternative is the use of center pivot sprinklers typically used for irrigation of crops. With the high percolation and evaporation rates found in the Rocky Mountains, a significant amount of water can be handled in this manner.

Surface Disposal With Treatment

A growing number of treatment technologies exist or are being contemplated/tested for removing the ions, metals and organics that exceed established limits in certain areas. These include: reverse osmosis, freeze-thaw evaporation and artificial wetlands.

Reverse osmosis (RO) is a synthetic membrane process that removes most dissolved solids, resulting in a clean water stream and a concentrated waste stream. The waste stream must be further processed and eventually re-injected. RO applications are found worldwide in the semiconductor, pharmaceutical, petrochemical, and nuclear power industries, and in certain public water supply situations. Although several companies are apparently investigating RO for CBM applications, it has not been proven commercially.

Freeze-thaw evaporation (FTE^®) utilizes the natural contaminant-concentrating action of the water freezing process to create separate streams of clean and high-salinity brine. Combining the freeze-thaw cycle with conventional evaporative technology allows treatment on a year-round basis. During the winter season, a produced water stream fed to an FTE facility in the San Juan Basin yielded 53% treated water, 27% evaporated water, and 20% brine. Total dissolved solids in the treated water dropped from 12,800 to 1,010 mg/L and total alkalinity dropped from 9380 to 700 mg/L (Harju, 2002). Like RO, this approach requires disposal of a concentrated waste stream. It is somewhat climate specific.

The creation of artificial wetlands that rely on the natural water cleansing capabilities of plant and microbial life are another approach that is being considered for CBM produced water. However, here there are a lot of issues: effectiveness that varies with contaminant, suitability of native plants, large areal requirements, and regulatory changes that may be required. Successful examples of artificial wetland treatment of produced water have shown an ability to reduce hydrocarbon contaminants, but not total dissolved solids (Myers, et al., 2001).

A number of other treatment options are possible alternatives, but their applicability to the volumes and ionic character of CBM produced water has not been proven commercially. Some of these include electro-dialysis reversal, ion exchange, capacitive desalination, and rapid spray distillation. While all of these technologies are theoretically applicable, their reliability under field conditions and overall economics are open to debate.

Real Cost Comparisons

When comparing the costs of produced water treatment alternatives, and in particular when evaluating vendor quotes, operators should be careful to consider the following costs in addition to the basic capital and operating costs:

1. Engineering design.
2. Permitting for facilities and/or devices.
3. Posting a bond for facilities.
4. Pilot testing technologies that are untested in specific situations.
5. Storage capacity for inflow and outflow to and from treatment facility.
6. Pretreatment equipment (to remove iron, coal fines, organics, etc. …this can be critical for membrane-based treatment technologies).
7. Delivery of power to site.
8. Heated buildings (some technologies require a minimum operating temperature), and related onsite wiring and plumbing.
9. Land and fencing.
10. Realistic labor and training costs, particularly for technically sophisticated systems that require daily monitoring.
11. Realistic maintenance costs, including things like membrane replacement and chemical pretreatment consumables.
12. Realistic electric power costs.
13. Contingency costs that recognize possibility of disruptions.

Ignoring these elements can lead to misleading or inaccurate evaluations of technology options.

Attention Will Drive Innovation

Increased attention on the need to manage produced water from CBM operations will act to accelerate the development of innovative technologies, increasing the number of economic options available. These treatment technologies could find application across the whole spectrum of oil and gas operations, well beyond the Powder River Basin.

References

Phelps, C.; 2002. “Down-hole Gas/Water Separation with Re-injection in Coal-Bed Methane Plays,” SRI Conference on CBM Water Management Strategies, Durango, CO., February.

EPA, 2001. “Draft Site Visit Report Coalbed Methane Operations, Gillette, WY.” August 21. (www.epa.gov/region08/water/wastewater/npdeshome/cbm/cbm.html).

Harju, J.; 2002. “The FTE Process for Produced Water Handling,” SRI Conference on CBM Water Management Strategies, Durango, CO., February.

Myers, J.E., et al.; 2001. “An Evaluation of the DOE Naval Petroleum Reserve No. 3 Produced Water Bio-Treatment Facility,” SPE 66522, Presented at the SPE/EPA/DOE San Antonio Environmental Conference, February 26-28.

Hodgson, B.; 2002. “Profit By Reviewing Your Current Options and Costs for Treating CBM Produced Water,” SRI Conference on CBM Water Management Strategies, Durango, CO., February.

GTI, 2001. “North American Coalbed Methane Resource Map,” GTI 01/0165.

EPA, 2001. “Economic Impact Analysis of Disposal Options for Produced Waters from Coalbed Methane Operations in EPA Region 8,” Public presentation, Billings MT, September 25. (www.epa.gov/region08/water/wastewater/npdeshome/cbm/cbm.html).

DOE, 2002. Policy Facts: Impacts of Alternative Management Practices for Produced Water in the Powder River CBM Play (study announcement, www.netl.doe.gov/scng).

Shirley, K., 2002. “Operators Continue to Expand Coalbed Methane’s Geographic Diversity.” The American Oil and Gas Reporter, March, p.94.

Sidebar 1
Adapted from Colorado State University Cooperative Extension

Sodium Adsorption Ratio (SAR)

Soils with an excess of sodium ions, compared to calcium and magnesium ions, remain extremely sticky when wet, tending to crust and become very hard and cloddy when dry. Water and air do not readily move through such soil and they can be almost impermeable to rain or irrigation water.

Elevated concentrations of sodium ions are measured by one of two methods. The more common method, the Sodium Adsorption Ratio (SAR), is the proportion of sodium (Na) ions compared to the concentration of calcium (Ca) plus magnesium (Mg). It is defined as the milliequivalent weight of sodium divided by the square root of the total of the milliequivalent weight of calcium plus the milliequivalent weight of magnesium divided by 2. The second method of estimating the sodium hazard is called the exchangeable sodium percentage (ESP). ESP refers to the concentration of sodium ions on cation exchange sites.

When the SAR rises above 12 to 15, serious soil problems occur and plants have difficulty absorbing water. An SAR “value of 15 or greater indicates an excess of sodium will be adsorbed by the soil clay particles. Excess sodium can cause soil to be hard and cloddy when dry, to crust badly, and to take water very slowly.” Extracted from: Soil Test Explanation. December, 1999. Clangor, P.M., and Fillet, R.C. Colorado State Cooperative Extension Publication No. 0.502.

Author: Karl Lang is Director of Custom Publishing at Hart/IRI Fuels Information Services. He edits GasTIPS, a technical journal produced by Hart for Gas Technology Institute and the Strategic Center for Natural Gas within DOE's National Energy Technology Laboratory.. He also writes for a number of Hart energy publications. E-mail: klang@chemweek.com