Cleat Property Trends in San Juan Basin Fruitland Formation Coalbed Reservoirs
Charles R. Nelson
TICORA Geosciences, Inc, Arvada, CO
The major exploration risk in most coalbed methane plays is generally not the drilling of a dry hole; rather it is not being able to obtain an economically viable gas production rate since coalbed reservoirs typically have very low (millidarcy level) bulk permeabilities. Naturally occurring microfractures, called cleats, are the primary plumbing system that controls the permeability essential for bulk fluid flow in coalbed reservoirs. Cleat porosity strongly affects both gas and water production rates. Because cleat porosity and permeability vary inversely with increasing effective stress, geologic models commonly assume that since effective stress increases with depth that, in turn, coalbed reservoir permeability also decreases with increasing depth. However, other properties such as the cleat spacing also affect porosity and permeability. Because cleat spacing commonly decreases with increasing coal rank, porosity and permeability should increase as a function of increasing coal rank. This paper presents results from an analysis of the effects of rank on the porosity and permeability of Fruitland Formation coal in the San Juan Basin. As coal rank increases cleat spacing decreases, which, in turn, increases the effective cleat porosity and permeability. Since cleat spacing, porosity and permeability are strongly linked to coal rank, which, in turn, commonly varies systematically as a function of depth, a predictive geologic model has been developed wherein rank and depth can be used as parameters for estimating cleat property values and conducting regional scale assessments of cleat property trends in coalbed reservoirs.
AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming