Methane in Water Well Systems

Transcription

1 Methane in Water Well Systems Water Well Construction and Service Practices for the Safety of the Water Well System Professional, the Property Owner, and Others

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3 Stray gas refers to any unwanted gaseous compound in water, occurring either in the free phase or the dissolved phase, or both. Stray gas can be naturally occurring or from human activities. Stray hydrocarbon gases can be either thermogenic (originating from a rock formation exposed to elevated burial pressures and temperatures), diagenetic (originating from the early abiotic decomposition of organic material in rocks), or biogenic (produced by microbes).

4 Methane, water wells, and Hollywood Private Investigator Jake Gittes in the motion picture, The Two Jakes Here s a scene Video clip for educational purposes only. The Two Jakes is copyrighted by Paramount Pictures, 1990.

5 Video clip for educational purposes only. The Two Jakes is copyrighted by Paramount Pictures, 1990.

6 NGWA Best Suggested Practices Best Suggested Practices A comprehensive overview in a short narrative aimed at the how to or methods of a very specific topic or activity. Typically less than 18 pages in length. Subject to approval by the NGWA Board of Directors. Examples deal with: Naturally occurring constituents in groundwater Flooded wells Well system inspection Wells and fire fighting

7 Methane BSP: History & Process Several drafts from staff and volunteers prior to formal task group review (December 2012 March 2013) Task group review: March 31, 2013 Legal review: April 2013 Public comment period: 60 days (April, May, June) Task group review (June) NGWA Board review (July) NGWA Board action (August 21)

8 Geologic Settings and Gases Addressing the presence of methane in water wells can be very complex as there are often multiple factors involved. Subsurface methane and other light gases may occur dissolved in groundwater or as free gas. Methane as both dissolved and free gas may be present in the well bore itself. The occurrence or manifestation of methane as dissolved or free gas in a well may vary through time due to pumping or seasonality. There may not be a relationship between the two depending on how the water well is constructed and where it is located.

9 Geologic Settings and Gases Gas migration paths depend on local geology and hydrology. Methane gas emerges in water wells because the cone of depression in the aquifer horizon formed by pumping the well creates a low pressure zone. Because natural gas is buoyant, free gas generally migrates in an up dip direction relative to the source depth and location. Dissolved gas typically migrates with the groundwater flow direction, however, pressure induced methane migration can influence gas movement along pathways not consistent with the hydraulic gradient.

10 Geologic Settings and Gases Methane is not limited to areas of oil and gas production.

11 Geologic Settings and Gases The occurrence of dissolved methane in groundwater is not unusual. Results of dissolved gas analyses from water wells throughout the U.S. show up to 60 percent of all water wells may contain detectable dissolved methane concentrations (Gorody, 2012). The presence of naturally occurring gases, such as methane, will vary due to the local hydrogeology and other conditions. Knowledge of local hydrogeologic conditions and local gas encounters during water well construction may help avoid new instances of gas intrusion. Additionally, the gas-impacted aquifer layer can be diluted seasonally; thus, over time, naturally occurring gas concentrations, including those from seeps, may have variations from measurement to another. The WWSP will benefit from knowing the local geologic setting, as well as from understanding human-related activities potentially contributing to methane presence in water well systems.

12 Geologic Settings and Gases Methane in groundwater can be: Thermogenic (typically originating from the geologic evolution of a rock column exposed to elevated burial pressures and temperatures) Diagnetic (originating from the early decomposition of organic material in rocks), or Biogenic (produced by microbes)

13 Well Location and Construction: Mitigating Gases County records, county agents, local well drillers and nearby landowners are all good sources of information. State or federal water offices. Some state agencies have even identified problematic areas and have implemented drilling depth restrictions. The WWSP should always be aware of the possibility of drilling into an unexpected pocket of overpressured gas, although this seems relatively rare.

14 Well Location and Construction: Mitigating Gases Generally, well design cannot mitigate the risks from gases unless the gas formation is cased off and water obtained from another aquifer

15 Well Location and Construction: Mitigating Gases Because pumping water wells create low pressure zones, and gas migrates along a pressure gradient from high pressure to low pressure, cones of depression gener-ated from well pumping tend to focus dissolved gas migration paths through an aquifer and into a well. For free gas to invade a water well, gas pressure must be greater than or equal to the hydrostatic head. Decreases in groundwater pressure caused by pumping extraction can be enough to initiate gas exsolution into the free phase. Since gases are compressible, as gas moves up a water column it will continually expand in volume as the water pressure decreases. Water well design, construction, and operation should be based on expectations of coming in contact with any of the mentioned problems.

16 Well Location and Construction: Mitigating Gases Methane may come from the formation producing the water, and thus the gas enters the well with the water. Water well casing grouted in place will not prevent gas from entering a water well. If dissolved gas from a well is a problem and the entry point into the well can be determined, the WWSP may be able to case off a zone to isolate the methane gas entry point and take water from a different depth to decrease the gas concentrations. Some types of logging tools or down-hole cameras can sometimes be utilized to help locate areas in the borehole where gases are exsolving. Mitigation of high gas levels from one formation by mixing the groundwater with groundwater of lower gas concentration from another formation in the same water well is not recommended. Some local and state regulations prohibit interconnection of multiple aquifers with the same well. When waters from multiple water-bearing zones are allowed to mix within a well, cross-contamination of the different water-bearing zones can occur.

17 WWSP Safety Issues Methane is not a health problem; but a safety problem because it is explosive and flammable Water well system professionals have reported employees being burned or having equipment damaged Methane gas venting from the well ignited as they uncapped a well, or were doing work that created a spark in the well or at the wellhead

18 WWSP Safety Issues The human symptoms of excessive methane exposure can include headache, dizziness, nausea, or unconsciousness Methane detection meters should be on the job site at all times Workers entering a well pit must be equipped with self-contained air packs and clothed with static-free garments

19 Well Operations and Gases When free gas under pressure enters a water well, which can happen during decreased pressures at depth due to initial well drilling, or decreased pressures resulting from pump operation, the gas can impact the well in several ways. In many cases, gas invasion merely induces effervescence in the water column, which can cause fine sediment in the bottom of the well to become suspended and generate turbid water. Under the most extreme circumstances, pressure may be great enough to dislodge the entire well casing and pump assembly. At lower pressure, the water column can be gas lifted, dislodge the well seal, and pro-mote artesian flow. It is not unusual to detect significant, yet short-lived, temporal changes in water quality during such events resulting from the invasion of deeper aquifer fluids into shallow aquifer regimes.

20 Well Operations and Methane Where there is a potential for gas to move outside the well casing and towards structures through preferential pathways, methane survey techniques may need to be employed to determine if a hazard exists, and what mitigation or treatment measures should be employed to ensure the safety of persons and property.

21 Well Operations and Methane Any water well with methane present should have a gas venting system installed to vent the gas to the atmosphere.

22 Well Operations and Methane Venting the casing through the well cap may allow a portion of methane gas to escape before it can accumu-late in the water distribution lines, pressure tanks, water heaters, water treatment equipment, or well houses. Venting the casing through the well cap is the only practical means and in some states, this is the only option. This option is most effective for dealing with free gas bubbling though water. Venting will not remove gas present in the pump system s riser or drop pipe. Vented well cap image courtesy of Baker Monitor Manufacturing

23 Well Operations and Methane With a captive air tank there is no practical means by which to vent the released methane. Captive air tanks should not be used if methane is not first vented from the water.

24 Well Operations and Methane Submersible pump shroud may help with water well systems with methane presence.

25 Well Operations and Methane Submersible pump motor shrouds may help with water well systems with methane presence. Shrouds are not practical for jet pumps or positive pressure pumps used in water well systems.

26 Well Operations and Methane Even when a shroud is used, the submersible pump can experience socalled gas lock. Gas lock is where the gas bubbles adhere to the impeller and impede the water flow. Gas lock is not cavitation. A solution to eliminate the condition is to turn off the power to the pumping unit, allowing the gas to migrate from the impeller and out of the groundwater. The pump head s check valve is removed to create a space to which the gas evaporates. A check valve is then installed one pipe joint, a minimum of 10 feet, above the pump head. The gas evaporating from the impellers and accumulated in the drop pipe will be pumped up and into the gas venting system.

27 Methane Treatment Choices Only appropriately qualified personnel should select, install, and maintain methane treatment and mitigation options. This information is provided here to the WWSP so as to provide effective counsel to the well owner.

28 Methane Treatment Choices Aeration at the point of entry (POE) can also help remove methane gas. Aeration removes gas from suspension in the water into the air space where it can be liberated to the atmosphere through a vent pipe.

29 Methane Treatment Choices Aeration equipment must be specifically designed or specially modified by the manufacturer to safely mitigate methane gas. A totally enclosed system should be provided. Systems not designed to safely mitigate methane may permit dissolved gases to seep out of the water and into the atmosphere of the room or building where the aeration equipment is stored. Because methane is flammable, any spark or gas accumulation in an aeration system or an enclosed atmosphere where an aeration system is stored has the potential to result in an explosion. It may be advisable to design in pressure relief blowout features at tanks where explosive limits of gas are routinely found inside. Outdoor installation of this equipment would be preferred to basement or elsewhere indoors.

30 BSP Documents

31 References Cadwallader, R Invisible yet Deadly. Water Well Journal, March: Westerville, OH. National Ground Water Association. Gorody, A.W., 2012, Factors affecting the variability of stray gas concentration and composition in groundwater: Environmental Geosciences, v. 19, no. 1, p , doi: /eg Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, Ground-water data-collection protocols and procedures for the National Water-Quality Assessment Program Collec-tion and documentation of water-quality samples and related data: U.S. Geological Survey Open-File Report , 113 p. Methane LEL and UEL found at February 14, National Ground Water Association Reduce and Mitigate Problematic Concentrations of Stray Gases in Water Well Systems. Westerville, OH. National Ground Water Association. Swanson, G Natural Gas and Other Phenomena: How Some Contractors Manage, Water Well Journal, April: Westerville, OH. National Ground Water Association. U.S. Environmental Protection Agency, 2004, Sample preparation and calculations for dissolved gas analysis in water samples using a GC headspace equilibration technique, found at ( ), Feb 19, U.S. Geological Survey National Field Manual for the Collection of Water-Quality Data, Chapter A4. Collection of Water Samples, found at ( February 12, 2013.

32 Contact NGWA at: National Ground Water Association 601 Dempsey Road Westerville, Ohio U.S.A Fax: Internet: