  Porous silica materials offer wide range of possibilities for enhancement of
the productivity of oil reservoirs. However the mechanism of adsorption of
polar components of crude oil on silica surface is poorly understood that
hinders technological improvement of supports and oil extraction. We have
synthesized opal films with the silica microspheres size of about 360 nm,
specific surface area of 5.2 m2/g and pore size of 230 nm. We have fractionated
and characterized oil and oil asphaltenes from heavy (Ashalchinskoe) oil. By
pulsed electron paramagnetic resonance (EPR) and double electron nuclear
resonance (ENDOR) in the W band frequency range (microwave frequency of 94 GHz,
magnetic field of 3.4 T) we have studied the adsorption of oil asphaltenes on
the surface of opal samples using the intrinsic for asphaltenes paramagnetic
vanadylporphyrins (VO) complexes. 1H ENDOR spectra are found to be different
for initial and the adsorbed samples in their central parts (that is a sign of
asphaltenes VO disaggregation) whereas no significant changes in the W band EPR
spectra were detected. Contrasting to alumina support, no strong electron
proton interaction with the protons on the surface of SiO2 (presumably, silanol
groups) was found and infiltration of the oiled opal films with gasoline
changes the central part of 1H ENDOR spectra. It shows that the proton
containing groups on the surface of amorphous SiO2 sample can significantly
change the asphaltene adsorption properties and ENDOR of the intrinsic for oil
paramagnetic centers could be used for the characterization of surface state in
porous media in situ or operando.
