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Quantifying Shale Porosity—A Thermodynamically Based, Predictive Model Which Includes the Effects of Mechanical Compaction, Temperature, Mineralogy, and Chemical Diagenesis

James T. Krushin
Consultant, 13102 Fallsview Lane, #4904-F, Houston, Texas 77077

Historically, geologists have borrowed the model of exponential loss of water from shale due to increasing net overburden pressure from soil scientists and civil engineers. This method does not extrapolate well to the pressures, temperatures, and diagenesis encountered while exploring for hydrocarbons. Study of sorption isotherms of clays illustrate the defining variables that control the water content in mudrocks. These variables are: cation exchange capacity, the specific exchangeable cation, effective stress (mechanical compaction), and temperature. Sorption isotherms are simply a measure of the mass of water per gram of dry clay, as relative humidity (p/po) is varied between 0 and 100%. This water includes interlamellar, intraaggregate, surface adsorbed, and interaggregate. Published desorption isotherms of Na-exchanged clays, the most abundant species in the subsurface, show that the amount of water per gram of dry clay increases with increasing cation exchange capacity values for a given p/po. A plot of p/po vs. mass of water per meq (a unit of cation exchange capacity measurement) for selected clays and clay mixtures results in a detailed compaction curve for the depths encountered in exploration. Thermodynamic equations enable the conversion of p/po to effective overburden stress, as well as quantify the temperature effects of thermal compaction. The effects of chemical diagenesis (i.e. smectite to illite) and variable mineralogy are incorporated in the compaction model via the bulk cation exchange capacity parameter, calculated from well logs using a published algorithm. Density log derived porosity and porosity from the model described in this paper show excellent agreement. The application of this model results in better understanding and quantification of pore pressure prediction, drilling fluid/wellbore interaction, and seismic modeling.


AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana