Abstract: NMR Characterization of the Water Adsorbed by Montmorillonite Impact on the Analysis of Porosity Logs

CHITALE, DATTATRAYA, Halliburton Energy Services

Reservoir rocks contain interstitial pore water as well as the water adsorbed by the clay minerals. Chitale, et al. (SPWLA 41st Annual Logging Symposium, June, 2000) have demonstrated for montmorillonite that NMR measurements show the adsorbed clay water is 500 mg per g of dry clay, a value in close agreement with values reported in the literature. This paper highlights the impact of the water adsorbed by montmorillonite on the interpretation of conventional density and neutron logs which require knowledge of the exact bulk density and the neutron porosity for wet clay end-members for interpretation.

The wet clay density values for montmorillonite presented in the standard charts (Schlumberger, 1994 and Halliburton, 1994) range between 2.0 to 2.7 g/cc. The adsorbed water associated with montmorillonite is the minimum water volume that can be contained in an aggregate of montmorillonite particles. Consequently, this volume is also the minimum contained in wet montmorillonite. For a dry density of 2.6 to 2.7 g/cc, the wet clay density must then be less than 1.7g/cc. The value of 500 mg per g of dry clay results in a minimum porosity of wet montmorillonite of 0.57. Since the neutron log measures both water and the hydroxyls, the neutron porosity for water-saturated montmorillonite must be greater than 0.57. As the volume of montmorillonite in a reservoir decreases, the fractional NMR porosity and neutron porosity representing montmorillonite must also decrease, whereas the bulk density will increase proportionately.

Rationalization of the wet clay-end-member values for the density and the neutron logs based on the adsorbed water in montmorillonite enhances the quality and accuracy of the overall log interpretation. This enhancement will be illustrated with a shaly sand log analysis as an example.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California