Reservoir Rock Properties of the Cretaceous Niobrara Chalk, Northwest Kansas and Northeast Colorado
by Alan P. Byrnes, Kansas Geological Survey; Geoff Ice, Rosewood Resources, Inc.; Mark Malinowsky, Rosewood Resources, Inc.; and David Eby, Eby Petrography & Consulting, Inc.
Optimum resource assessment, reservoir stimulation and management require a quantitative understanding of the nature of reservoir rock properties and heterogeneities at many scales. The gas-productive interval at the top of the Smokey Hill Chalk member of the Niobrara Chalk can be broadly characterized as exhibiting high porosity (PHI=28-44%) and moderate permeability (k=0.01-5 md). Data from cores in KS and CO indicate that the ratio of in situ to routine porosity can be predicted using PHIinsitu/PHIroutine (%) = 0.236 PHIroutine (%)+86 and that organic matter content influences matix density and therefore density log interpretation. In situ Klinkenberg gas permeability (kik) can be predicted using kik (md) = 1.77x10^-11 PHI^6.69.
Capillary pressure curves differ significantly between lower permeability and higher permeability rocks with minimum gas column heights necessary for gas to displace water exhibiting values of 750 ft, 250 ft, 100 ft for chalks of 0.01 md, 0.1 md, and 1.0 md respectively. Relative permeability properties also vary significantly with porosity and permeability with critical water saturation (immobile water saturation) able to be predicted using Swc (fraction) = -0.137 ln(kik) + 0.52. Gas relative permeabilities at Swc can be generally predicted using krg (fraction) = 11.9 e^(-7.85Swc). They can also be modeled accurately using Brooks-Corey type equations. Analysis of pre- and post-fracture well performance indicates that fractures play a role in gas production. However, production from many wells can be explained by the presence of a single horizontal parting within the 30-50 ft productive interval.