Coal Bed Methane development in the
San Juan Basin of Colorado has raised concerns as to whether water produced
from the gas-producing coal formation results in stream depletion necessitating
regulation. Accordingly, the Colorado Oil and Gas Conservation Commission and
the Colorado Division of Water Resources commissioned a study to evaluate stream
depletions, specifying that the Glover- Balmer model (1954) be used. Given the
geologic configuration of the basin and the fluid withdrawal characteristics,
the question was also posed as to whether this methodology, given its simplicity,
could be reasonably applied to assess the general magnitude of stream depletion
within a regulatory framework.
Key elements of the hydrogeologic
setting were incorporated into a simplified conceptual model that could be simulated
using the Theis analytical model. Pressure changes simulated with the Theis
model, given historic water production rates, were compared to pressure change
observations to evaluate whether a simplified model could capture the general
nature of system behavior. For regions where the comparison suggested that the
method was reasonable, parameters derived using an automated parameter estimation
procedure (PEST, Watermark, 2004) were utilized in the Glover-Balmer analysis
to obtain a first-order approximation of surface water depletions at the coal
formation surface outcrop. The outcrop was considered as the “stream”
boundary, capturing the combined depletion to streams traversing the outcrop,
springs, seeps and water-table storage within the outcrop.
This approach produced good results, both in terms of fit to observed data, and in comparison with results from far more complex numerical models. The reasons for this include:
• Using PEST to optimize parameter
selection removed user bias from the process;
• The recharge boundary was configured to capture outcrop features relevant
to the stream depletion analysis.
This work reminds us that simple techniques can provide useful results in complex
systems, as long as
key system factors are identified and represented.