Fluorescence evidence of polar hydrocarbon interaction on mineral surfaces and implications to alteration of reservoir wettability

Petrographic analysis of reservoir rocks in an apparently water-wet system under UV light indicates the ubiquitous presence of microscopic oil inclusions within quartz grains. The micro-sized inclusions are often trapped along healed micro-fractures or along quartz overgrowth boundaries. The apertures of the micro-fractures are usually a few microns in width, an order of magnitude smaller than the typical pore aperture (throat) of reservoir rocks. When these same reservoir rocks were analysed using a highly sensitive fluorescence spectrophotometer after series of cleaning steps using solvents and oxidizing agents, it was revealed that polar organic compounds are present on quartz grain surfaces. Examination of the quartz grain surface using scanning electron microscope (SEM) and an energy dispersive X-ray (EDAX) system confirmed the presence of fine residual hydrocarbon particles associated with clay minerals coated on the highly irregular quartz grain surface. The presence of polar organic compounds and/or asphaltenes on water-wet quartz surfaces results in wettability alteration under reservoir conditions, which may allow direct contact between hydrocarbon and the water-wet quartz grains, resulting in the formation of micro-size oil inclusions in quartz grains. (C) 2003 Elsevier Science B.V. All rights reserved.