Abstract:

Mudrocks are fine grained low permeability sedimentary rocks. Due to this low permeability, they are difficult to characterize in terms of physical properties. The use of the reconsolidation technique allows us to determine mudrock properties while controlling mineralogy, brine salinity, and stress. Cation exchange capacity (CEC) makes these rocks water sensitive to double layer effects. Three properties were varied for this study: the cation exchange capacity, salinity, and silt content. To characterize the CEC we have performed conductometric titration and methylene blue colorimetry measurements. Our results are consistent with an external laboratory measured CEC using the cobalt hexamine technique. The methylene blue technique CEC measurements were ~25% higher for two of the three samples. The titratated CEC also correlates well with permeability and grain size. Salinity was observed to have a minimal effect on mudrock porosity. Double layer effects play a dominant role in determining permeability, based on the measured salinity effects as a function of CEC. Grain size however also has an influence. The effect of increasing silt content was to increase both porosity and permeability.

Introduction

This paper is to further improved understanding of mudrock rock properties. In particular the impact of brine salinity and sample CEC on the porosity and permeability is explored. Clay slurries were prepared with variable brine salinity and silt content and loaded into a reconsolidation oedometer cell. We used the reconsolidation method to fabricate the core samples used in the multistage triaxial tests (Germaine & Germaine, Geotechnical Laboratory Measurements For Engineers, 2009). We acquired mudrock materials from the Miocene section of the Gulf of Mexico, and Pierre Shale samples from South Dakota. To modify sand content we mixed the mudrock samples with Brazos sand sieved to mesh size 105-300 microns. The samples vary in mineralogy as well as cation exchange capacity.

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