ABSTRACT: Hydrology of the Upper Cretaceous Fruitland Formation and the Producibility of Coal-Bed Methane, San Juan Basin, Colorado and New Mexico
W. R. Kaiser, T. E. Swartz
Fruitland coal seams contain 49 tcf of methane, which is produced from abnormally pressured coals in a variety of hydrologic settings. In a study funded by the Gas Research Institute, we calculated bottom-hole pressures from wellhead shut-in pressures to map hydraulic head and pressure regime, and estimated vertical pressure gradients to evaluate Fruitland hydrology and its relation to methane producibility. We inferred relative permeability from hydraulic head, pressure regime, and hydrochemistry, and related these hydrologic elements to established production.
In the Fruitland Formation, coal seams are the primary aquifers, receiving recharge mainly from the elevated, wet, north and northwest margins of the basin. Formation waters in the north-central part of the basin have low chlorinities and high alkalinities, whereas those in the southern part of the basin are saline Na-Cl type similar to seawater. Regional discharge is to the San Juan River valley in the western part of the basin. Overpressuring in the north-central part of the basin is explained hydrodynamically and is attributed to artesian conditions.
We infer enhanced coal-bed permeability where the potentiometric surface is flat and reduced permeability where it is steep. Overpressuring indicates enhanced permeability because permeability in coal seams is stress dependent. In the overpressured region, groundwater is fresh, indicative of an active, dynamic flow system and of permeable pathways. In contrast, connate seawater in the underpressured southern part of the basin implies negligible permeability; strata are too tight to accept and transmit measurable recharge.
The basin's most productive coal bed-methane wells are overpressured and occur at hydrologic transitions from a flat to a steep potentiometric surface, overpressuring to underpressuring, and low- to high-chloride formation waters. Highly productive wells are more common in areas of upward flow than in areas of downward flow. Wells close to the northern recharge area may be difficult to dewater (depressurize) and may never produce gas at high rates. Coal-bed methane is produced from both overpressured and underpressured coal seams.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990