Effects of Oil and Water on Mudrock Pore Size Distribution and Pore-Scale Fluid Distribution

Abstract

The presence of oil and water pore fluids can greatly affect the measurement of porosity and pore size distribution in oil-bearing mudrocks. We used a sequential pore fluid removal method to remove water, mobile oil and asphaltane-dominated residual oil from clay- mineral and organic matter-rich mudrocks from Triassic lacustrine Yanchang Fm., Ordos Basin, China. 300-850μm sized oil-bearing samples were prepared from core crushing and first dried at 110°C, then treated with pentane to remove mobile oil comprising saturate and aromatic hydrocarbon fractions, and subsequently with CH2Cl2 for asphalt removal. The pore size distribution (PSD) of as-received, dried, pentane-extracted and CH2Cl2-extracted aliquots of the sample splits was determined using N2 adsorption isotherms. Comparison of PSD of the four sample types revealed differences related to pore size. Total pore volume and BET surface area are lowest for water-bearing sample, however, both values significantly increased after samples were dried. The increase in pore volume is mainly attributed to the pores with less than 10nm in width when compared the PSD before and after sample being dried. The pores prior to drying could be partially filled with water, and those pores could be also storage spaces of gas and light oil volatiles which were escaped after coring. Our results suggest that pores smaller than 10nm, mainly hosted in clay minerals, are important to gas/connate water storage in these mudstones. Total pore volume and BET surface area increase after oil removal of with solvent extraction, and greater than 10nm pores made a great contribution to the increased pore volume. About 2nm sized pores were examined, in particular after dichloromethane extraction, indicating that heavy molecular asphaltanes can be adsorbed on the surface of tiny pores with the width equivalent to asphaltane molecular size (0.5-3nm). Our results suggest that oil is mainly stored in pores greater than 10nm in these mudstones. This observation was validated with the amount of extracted oil by mass balance calculation. Pore-scale fluid distribution provides important insight for gas and oil storage in mudrocks.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016