Abstract

Gas, oil, and water production data were compiled from selected wells in two tight gas reservoirs—the Codell-Niobrara interval, comprised of the Codell Sandstone Member of the Carlile Shale and the Niobrara Formation; and the Dakota J interval, comprised mostly of the Muddy (J) Sandstone of the Dakota Group; both intervals are of Cretaceous age—in the Wattenberg field in the Denver Basin of Colorado. Production from each well is represented by two samples spaced five years apart, the first sample typically taken two years after production commenced, which generally was in the 1990s. For each producing interval, summary diagrams and tables of oil-versus-gas production and water-versus-gas production are shown with fluid-production rates, the change in production over five years, the water-gas and oil-gas ratios, and the fluid type. These diagrams and tables permit well-to-well and field-to-field comparisons. Fields producing water at low rates (water dissolved in gas in the reservoir) can be distinguished from fields producing water at moderate or high rates, and the water-gas ratios are quantified.

The Dakota J interval produces gas on a per-well basis at roughly three times the rate of the Codell-Niobrara interval. After five years of production, gas data from the second samples show that both intervals produce gas, on average, at about one-half the rate as the first sample. Oil-gas ratios in the Codell-Niobrara interval are characteristic of a retrograde gas and are considerably higher than oil-gas ratios in the Dakota J interval, which are characteristic of a wet gas. Water production from both intervals is low, and records in many wells are discontinuous, particularly in the Codell-Niobrara interval. Water-gas ratios are broadly variable, with some of the variability possibly due to the difficulty of measuring small production rates. Most wells for which water is reported have water-gas ratios exceeding the amount that could exist dissolved in gas at reservoir pressure and temperature.

The Codell-Niobrara interval is reported to be overpressured (that is, pressure greater than hydrostatic) whereas the underlying Dakota J interval is underpressured (less than hydrostatic), demonstrating a lack of hydraulic communication between the two intervals despite their proximity over a broad geographical area. The underpressuring in the Dakota J interval has been attributed by others to outcropping strata east of the basin. We agree with this interpretation and postulate that the gas accumulation also may contribute to hydraulic isolation from outcrops immediately west of the basin.

First posted August 9, 2011