Fluorescence evidence of polar hydrocarbon coating on quartz grains and implications to alteration of reservoir wettability

摘要

Petrographic analysis of reservoir rocks in apparent water-wet system under UV light indicates the ubiquitous presence of micro-sized 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 high sensitive fluorescence spectrophotometer after series of cleaning steps using solvents and oxidizing agents, it was revealed that tightly coated polar organic compounds are present on quartz grain surfaces after a series cleaning steps. Examination of the quartz grain surface using Scanning Electron Microscope (SEM) and an online 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 provides the pre-requisite for wettability alteration under reservoir condition, which may allow the direct contact between hydrocarbon and the water-wet quartz grains and the formation of micro-size oil inclusions in quartz grains.