An Experimental Study of the Physical and Chemical Property Changes Occurring during the Smectite to Illite Reaction

Kevin M. Brown, Tina Fitts, Michael Tryon, and Barbara Ransom

Smectite dehydration and the formation of illite releases significant amounts of fluid deep within sedimentary basins. The release of water from smectite and the resulting physical property changes have been linked to the generation of significant overpressures and may also assist in the primary migration of hydrocarbons. Little quantitative data exists, however, as to how chemical and physical property changes may be coupled under in situ stress conditions. Presented here are the initial results from a one dimensional consolidometer system in which a K-smectite was reacted under differential stress conditions at temperatures up to 250°C. Sample volume, permeability and effluent fluid compositions are monitored through out the experiments. Partial collapse on the K-smectites occurred in all runs, with rapid dehydration apparently taking place at T close to 100° C. Initial samples were bilayer hydrates in the 0.5M KCl pore fluids used in the experiments. Dilution of the output fluid suggests the loss of two layers of water. Samples heated to < 115 degrees °C and were still subsequently expandable, while the samples run at 250 °C showed some irreversible collapse (illitization) after glycolation. Accelerated volumetric creep and permeability reductions occurring during heating increments, with over an order of magnitude reduction in permeability occuring during some runs.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California