--> Abstract: Using Temporal Changes In Produced Water Geochemistry As A Guide To Assess Reservoir Compartmentation In Coal Bed Methane Reservoirs, by A. W. Gorody and T. Casey; #90937 (1998)

Datapages, Inc.Print this page

Abstract: Using Temporal Changes In Produced Water Geochemistry As A Guide To Assess Reservoir Compartmentation In Coal Bed Methane Reservoirs

GORODY, ANTHONY W., Universal Geoscience Consulting, Inc., and TOMANN CASEY, Enervest San Juan Producing Co.

The Fruitland Coal Formation of the San Juan Basin, Colorado, is a confined aquifer. This prolific coalbed methane-producing formation is recharged at outcrop exposures along the northern and northwestern margins of the basin. Recharging surface waters are fresh Ca-Mg bicarbonate type waters, whereas within 100 feet from the surface, formation fluids are Na-bicarbonate type waters. Stable carbon isotopic evidence indicates that bicarbonate is derived from the reduction of CO2 to produce bacterial methane; Na is derived from cation exchange reactions in the subsurface. Within five miles of the outcrop belt, 98% of the ionic strength of Fruitland waters is accounted for by sodiuum bicarbonate ions.

The major ion chemistry of produced waters from seventeen wells in the Valencia Canyon field and from three nearby springs on outcrop has been determined monthly since 1993 This field, at the northwest margin of the basin, has wells that produce from both sides of a linear, NE-trending topographic valley: Valencia Canyon. Geological evidence suggests this lineament may be the surface expression of a thrust fault. Gas to water production ratios are much lower among wells updip of the lineament than among those downdip. Histograms of bicarbonate concentrations indicate fresher waters updip. Temporal changes show that updip wells are becoming fresher with time, whereas downdip wells are becoming saltier with time. When the rate of change in bicarbonate concentrations among all wells is plotted, distinct patterns emerge that identify reservoir flow boundaries. Fairly constant bicarbonate/chloride ratios and low ionic concentrations rule out chemical reactions as source of changing bicarbonate content.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah