Impact of geological complexity of the Fruitland Formation on combined CO2 enhanced recovery/sequestration at San Juan Basin pilot site

Abstract

In this study we present well log and 3D seismic interpretations of a coalbed methane reservoir in the Late Cretaceous Fruitland Formation in the area surrounding the Southwest Regional Partnership on carbon sequestration's San Juan Basin carbon sequestration pilot site in New Mexico, USA. The interpretation incorporated geophysical logs from 38 wells located in a 23km2 area surrounding the pilot site, which lies near the southern edge of the high rate Fruitland coal production fairway located along the axis of the San Juan Basin. Log interpretations revealed three coal-bearing zones (upper, middle and lower) within the Fruitland Formation that are approximately 7, 4.7 and 8.5m (23, 15.5 and 28ft) thick, respectively. The interpretations indicate that each of these coal zones is split by a parting into upper and lower beds making a total of six mappable coal beds in the area surrounding the CO2 injection well. The combined thickness of the upper two coal beds in the upper coal zone varies between 3.5 and 4.8m (11.5 to 15.7ft); and that of the middle beds, from 2 to 4.3m (6.6 to 14.1ft) thick. The thickness of the individual coal beds in the lower coal zone varies from about 2.7 to 4.25m (9 to 14ft) for the upper bed and from about 3 to 5.5m (10 to 18ft) for the basal coal bed. Partings separating the upper and lower coal beds in each zone are on average about 2m (7ft) in the upper and middle zones; and about 1m (3.5ft) in the lower zone. Seismic response of the upper, middle and lower coal zones was interpreted and correlated through the 23km2 area. The stratigraphic complexity of this coalbed methane reservoir is also accompanied by considerable structural disharmony between the three coal zones. The reservoir complexity revealed in this study may account for lower than expected CO2 injectivity and preferential injection into the lower coal zone during the pilot test.