Shallow Coal Aquifer Water Chemistry

Alberta's exploration for natural gas from coal reservoirs (also called coalbed methane or CBM) is increasing as new technology and economic factors attract exploration companies to this unconventional gas resource. Although Alberta's extensive development of coalbed methane (also spelt coal-bed methane and coal bed methane) is just beginning, CBM production in Montana, Wyoming, Colorado, Utah, New Mexico and Alabama has been growing for many years and now accounts for over 10 per cent of U.S. domestic gas production. Other countries also produce signficant amounts of natural gas from coals, notably China. All of the CBM-producing American states regulate their coalbed methane development regarding well locations, well spacing, dewatering effects and produced water. Likewise, the Alberta CBM industry is regulated by the Energy Resources Conservation Board (ERCB).

Production of coalbed methane generally requires dewatering of fractured coal seams, though dry gas production from coal reservoirs is not unheard of. Reduction in water pressure through dewatering results in desorption of the methane from the coal matrix. The methane flows toward the well bore via the fractures and is captured at the surface.

Produced water from CBM wells is disposed of in many different ways around the world, including deep well injection, evaporation and surface disposal. Produced water is also used for irrigation, livestock watering or other industrial purposes, where suitable. The appropriate disposal option for produced water depends, first, on its chemical quality and, second, on flow rate and timing. In Alberta, all produced water from conventional oil and gas reservoirs require an ERCB-approved disposal scheme, usually deep well disposal, as per ERCB Regulation 8.040 under the Oil and Gas Conservation Act. However, it may yet arise in the Alberta CBM industry that the produced water from shallow coal seams will be of such chemical quality that alternative disposal methods to deep well disposal could be considered, or that other beneficial uses for the produced water will exist.

To evaluate the possibility that alternative disposal or beneficial uses of CBM-produced waters may exist for shallow coal seams in Alberta, high-quality chemical quality data for shallow coal beds are needed. Chemical analyses for formation waters captured in drillstem tests and production tests are on file with the ERCB, and chemical analyses for potable groundwater from domestic water wells are on file with Alberta Environment. However, these analyses generally report only six major ions: sodium, calcium, magnesium, bicarbonate, sulfate and chloride. They are of limited use when pre-screening water disposal or alternative beneficial use options that require knowledge of trace element concentrations, particularly when those trace elements are regulated for the protection of human health, soils, livestock or the aquatic environment.

To provide such data to government and industry, Alberta Geological Survey, with support from the Alberta Energy Research Institute, has created a public-domain database of high-quality chemical analyses of groundwater from water wells known or strongly suspected to be completed in coal seams. These analyses came from a fall 2002 AGS field sampling program. AGS staff collected groundwater samples from domestic water wells with high-quality completions and coal in their screened or perforated intervals. Wells were selected based on their location and known or suspected stratigraphic interval, as determined from drillers’ reports on file with Alberta Environment. Water wells with only coal in their screened intervals are very rare because Alberta plains coals tend to be thin and interbedded with other rock types. However, it is reasonable to conclude that some portion of the water in these wells will come from coal seams. Details of the AGS sampling program will be forthcoming on this Web site in the near future.

The water samples were analyzed in the laboratory to determine major-, minor- and trace-element concentrations, polycyclic aromatic hydrocarbon (PAH) concentrations, nonhalogenated aromatic hydrocarbon (BTEX) concentrations, hydrocarbon concentrations, phenol concentrations, adsorbable organic halide concentrations, stable isotopic composition (focusing on the O, H, C, B and S stable isotopes), and radiogenic isotopic composition (focusing on ⁸⁷Sr/⁸⁶Sr, ¹²⁹I, ³⁶Cl, ¹⁴C and naturally occurring radioactive materials).

At this time, only a portion of the data will be presented here. Additional information will be added to this site on a regular basis. The knowledge generated by the interpretation of this data will be of use not only in understanding the nature of coal water chemistry in the Alberta Basin, but may also provide information on CBM potential within the Alberta Basin and highlight areas as development targets.

The AGS water sampling focused on three coal-bearing geological intervals. They are:
1) The Paskapoo-Scollard formations
2) The Horseshoe Canyon Formation
3) The Belly River Group

The stratigraphic relationships between these units, throughout the Alberta Plains region, are presented in the stratigraphic table below. Within each of the intervals, water samples were collected from coal, mixed coal-sandstone or sandstone aquifers. Click on the links above, or within the stratigraphic table, to see maps of the locations of the water samples and to access the available chemistry information for that geological interval.