Prolific Coalbed Methane Production from Multiple Thin Coal Seams in the Raton Basin - a Low Net-to-Gross Success Story
The Raton Basin gas field is a continuous-type coalbed methane gas accumulation covering more than 700 mi2 located in Colorado and New Mexico. Reported cumulative production through 2008 is >800 BCFG from >3700 wells. Field development by three primary operators continues on a pace of about 400 wells per year and production from the field has increased to over 400 MMCFD in 2008. Estimated ultimate gas recovery is >3 TCFG. The Raton Basin field ranks 3rd in producing gas volumes for CBM production in Rocky Mountain Basins, behind the San Juan and Powder River Basins. The field was also ranked 9th largest by proved gas reserves in the USA by the EIA in 2006.
The coal seam reservoirs in both the San Juan and Powder River Basins can be described as high net to gross, consisting of a few relatively thick individual coal seams distributed over a relatively thin gross interval. In contrast, the Raton Basin reservoirs can be described as low net to gross, consisting of many thin coal seams distributed over a large gross interval. Productive coal seams in the Raton and Vermejo Formations are found in a gross interval of >1500 feet (500 meters), at depths ranging from 250 to >3400 feet. Individual wells can have 30+ individual coal seams, each averaging less than 3 feet (1 meter) thick, distributed over the >1500 feet gross interval. This pattern of pay distribution requires that well completions include multiple frac stages in order to stimulate production from the many individual coal seams in a well. A typical well producing from both the Raton and Vermejo intervals will be stimulated with up to 7 frac stages distributed over the 1500 feet gross interval. Gelled water and sand fracs are generally preferred.
Mapping of production history data is used as a method of illustrating variations in the quality of production across the field. Peak production rates are one measure of the quality of production. The average Raton Basin gas well produces at peak rates of 100-200 mcfd but some outstanding individual wells have reached peak production rates well over 1000 mcfd. These high quality wells are found distributed across the field in a somewhat random pattern of “sweet spots” where several high quality wells are clustered near one another. Detailed studies of the geology of some of these “sweet spots” will be illustrated.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009