Permeability of Mudrocks Effects: Lithology, Texture, and Pore Fluid Salinity

Abstract

We use a reconsolidation technique to vary mudrock properties, controlling mineralogy, brine salinity, and stress. We have performed conductometric titration and methylene blue colorimetry to measure the samples cation exchange capacity (CEC). Our results are consistent with externally measured CEC performed using the cobalt hexamine technique. CEC values obtained by using conductometric titration and cobalt hexamine are very similar for the measured samples; however the methylene blue technique resulted in increased measured values of CEC values. Samples were also characterized in terms of their grain size and XRD measurements. The resedimentation method is used to calibrate modified cam clay model. CT scans and thin sections are incorporated to study the effect of salinity, grain size distribution, and mineralogy on mudrocks compaction and pore structure. Porosity and permeability were then measured in one dimensional consolidation experiments as a function of axial stress and the equilibrating NaCl salinity. Porosity is calculated from the measured axial strains, permeability is calculated using the logarithm of time fitting method. Changing the saturating brine salinity has a minimal effect on porosity which is therefore interpreted to be dominated by the mineralogy and texture of the matrix. A large salinity dependence on permeability is observed. Increasing salinity increases the permeability for all the samples. This effect increases with the measured CEC. This effect is attributed to the increasing tortuosity and smaller channels of flow as the CEC increases. Increasing the silt content increased both porosity and permeability. Future work will include NMR studies to quantify the bound water effects, expanding the data set to include additional clay minerals and calibrating permeability models to the data.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017