Fracture permeability is inferred to be the most important influence on productivity of natural gas from commercial Fruitland Formation coalbed methane reservoirs in the Cedar Hill, Northeast Blanco Unit and Ignacio Blanco Fields of the northern San Juan Basin of Colorado and New Mexico. Reservoir deliverability in these areas with significant coalbed methane resources and reserves is less influenced by thickness, depth, completion technique, and measured and inferred pressure and formation damage.
Cleats (coal fractures) in cores from four northern San Juan Basin Gas Research Institute Fruitland evaluation well in these fields formed during coalification in strain fields imparted as a result of Early Tertiary Laramide and Late Tertiary-Recent Rio Grande Rift regional tectonic episodes. These cleats are not the result of sedimentary compaction folding. The presence of discrete, very thin inter-bedded layers of Fruitland coal and non-coal ("ash???") litho types with different respective rock mechanical properties favored brittle deformation of coal layers. Many commercial Fruitland wells are located in areas with extremely minor true structural flexure (folds), which indicates cleat permeability is largely a function of coal composition.
Knowledge concerning fracture permeability will influence further delineation and development of northern San Juan Basin Fruitland coalbed methane resources and reserves. The following procedure for qualitative recognition of areas with greater Fruitland permeability prior to drilling is suggested. 1. Preliminary collection and analysis of reservoir description and supporting geological and engineering data from in-house files and public literature should be completed. 2. The geologic setting of available acreage should be determined through interpretation of stratigraphic and structural maps and cross sections. 3. Favorable locations within acreage blocks should be delimited through interpretation of - a) available drill stem test data, and b) gas and water production rate maps and data with respect to completion techniques and formation damage. 4. These areas of interest should be analyzed to find potential drilling locations by finding specific sites that a) required higher Fruitland drilling mud density to prevent blowouts using mud logs, drilling records and mud density maps, and b) have Fruitland coal waters with relatively low total dissolved solids (salinity) and chlorine ion concentrations. 5. Once a drilling location bas been selected, potentially producible coal zones can be determined a) through identification of coal intervals with densities between 12 and 1.75 g/cc on open hole high resolution density logs, and b) by identification of coal intervals with drilling mud invasion through interpretation of micrologs and resistivity log suites from offset wells.
The purpose of this paper is to discuss the importance, genesis, and a suggested procedure for qualitative recognition of fracture permeability in Fruitland Formation (Upper Cretaceous) coalbed methane reservoirs in the Cedar Hill, Northeast Blanco Unit, and Ignacio Blanco Fields of the Northern San Juan Basin of Colorado and New Mexico. Coals that are natural gas reservoirs are termed "coalbed methane" reservoirs because the chemistry of produced gases from such wells is usually methane-rich. Fracture permeability is a crucial component of commercial coalbed methane reservoirs.