Effect of Pore Fluids on Methane Adsorption in the Lower Bakken Shales, Williston Basin, U.S.A.

Abstract

Compositional variation and distribution of pore fluids (water, oil, and asphaltene) as a function of thermal maturity affect CH₄ adsorption on organic-rich shales. Five Lower Bakken organic-rich shale samples with varied thermal maturity from immature to over-mature were used in this study to assess this effect. Fluids were extracted from samples at four sequential stages: as received, dry, pentane extracted, and dichloromethane (DCM) extracted. Four aliquots from each sample were analyzed to determine presence/absence of water, mobile oil, and asphaltenes. CH₄ adsorption isotherms were measured at 35°C, 50°C, and 65°C. Presence of pore fluid (water, mobile oil, and asphaltene) reduces CH₄ adsorption capacity of shale. CH₄ adsorption capacity increases as follows: as received (water + mobile oil + asphaltene) < after drying (mobile oil + asphaltene) < after drying and pentane extraction (asphaltene presence) < after drying and DCM extraction (without pore fluids). The presence of asphaltenes has greater influence on the CH4 adsorption than mobile oil. As small as 2mg asphaltene per gram of high maturity rock samples almost can cause 50% affinity reduction of methane molecular on the surface of solid organic matter. Surfaces of OM-hosted pores probably are coated by asphaltenes, while mobile oil is distributed in the center of the pores. Surfaces of clay-mineral-dominated pores are covered mainly by water as shown by low heat of adsorption (q ~10kJ/mol) of dry samples with varied thermal maturity. It is difficult to accurately measure CH4 adsorption isotherms in the presence of water because of the varied amount of water loss during experimental procedure. Our preliminary results show that the presence of moisture can reduce gas adsorption capacity by about 30 %. The affinity of molecular CH₄ to the surface of solid organic matter is strongest in the absence of pore fluids. This is shown by the Langmuir constant value, which is larger for the dichloromethane-extracted samples than for the pentane-extracted samples. Surface chemistry of organic matter is altered significantly once oil cracking to gas starts, probably as a result of pyrobitumen formation. As a consequence, the heat of gas adsorption of CH4 on solid organic matter is around 14 kJ/mol at the peak of oil generation, and increases to 18 kJ/mol at dry gas generation. Pyrobitumen generated from oil cracking shows higher value of q than that of residual kerogen.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018