The study of produced fluids allows comparisons among tight-gas systems. This paper examines gas, oil, and water production data from vertical wells in 23 fields in five Rocky Mountain basins of the United States, mostly from wells completed before the year 2000. Average daily rates of gas, oil, and water production are determined two years and seven years after production begins in order to represent the interval in which gas production declines exponentially. In addition to the daily rates, results are also presented in terms of oil-to-gas and water-to-gas ratios, and in terms of the five-year decline in gas production rates and water-to-gas ratios. No attempt has been made to estimate the ultimate productivity of wells or fields. The ratio of gas production rates after seven years to gas production rates at two years is about one-half, with median ratios falling within a range of 0.4 to 0.6 in 16 fields. Oil-gas ratios show substantial variation among fields, ranging from dry gas (no oil) to wet gas to retrograde conditions. Among wells within fields, the oil-gas ratios vary by a factor of three to thirty, with the exception of the Lance Formation in Jonah and Pinedale fields, where the oil-gas ratios vary by less than a factor of two. One field produces water-free gas and a large fraction of wells in two other fields produce water-free gas, but most fields have water-gas ratios greater than 1 bbl/mmcf—greater than can be attributed to water dissolved in gas in the reservoir— and as high as 100 bbl/mmcf. The median water-gas ratio for fields increases moderately with time, but in individual wells water influx relative to gas is erratic, increasing greatly with time in many wells while remaining constant or decreasing in others.
Tight-gas accumulations are characterized by low porosity and low permeability and a lack of distinct geological boundaries. Tight-gas wells require stimulation to achieve satisfactory production rates (Holditch, 2006). Law (2002) enumerates the characteristics of tight gas (also called basin-centered gas), lists potential world-wide accumulations, and discusses aspects of sourcing, reservoir type, and the problematic issue of seals. A later review by Meckel and Thomasson (2008) chronicles the history of geologic concepts regarding tight-gas accumulations while discussing their attributes. As more production data have become available, it has become possible to create statistical distributions of the estimated ultimate recovery of tight-gas systems of the United States (USGS Oil and Gas Assessment Team, 2012). Early producers of tight gas reported little or no water, but as more accumulations were exploited, higher volumes of produced water were reported (Cluff and Shanley, 2005) so that avoidance of water-producing intervals became a major consideration in development strategy (Yurewicz et al., 2008).
URTeC 1580088