SCANNING ELECTRON MICROSCOPY AND ENERGY DISPERSIVE SPECTROSCOPY PARAMETERS: ANALYZING SURFACE CHEMISTRY OF EKOFISK TOR FORMATION TO PREDICT PROPER WETTABILITY ALTERING IONIC COMPOSITIONS

Microporosity in carbonates is responsible for complicating oil recovery, causing inaccurate recoverable reserve estimates, and producing high-water saturation measurements on wireline logs. The microcrystals that occlude micropores play a critical role in fluid flow dynamics during hydrocarbon recovery. Bulk geochemical and petrophysical analyses imply progressive calcite cementation with burial.

Microcrystal growth causes an increase in surface area, which will increase the rate of chemical reactions between mineral and fluid, altering reservoir properties, like wettability. Most carbonate reservoirs are moderately oil-wet which can cause complications in oil recovery. However, water composed of the correct ionic composition can improve water-wetness of carbonate reservoirs through both spontaneous imbibition and forced displacement at high temperature. It is therefore critical to understand fluid and surface chemistry of the carbonate reservoir.

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) provide the ability to qualitatively and quantitatively analyze the chemical composition of microcrystals within a carbonate reservoir. When the proper sample preparation, electron gun, accelerating voltage, and electron detectors are employed, sub-micron spatial resolution enables the analysis of microcrystal surface chemistry.

SEM-EDS obtained the chemical composition of calcite microcrystals from Tor Formation (Late Campanian to Maastrichtian) core samples. Mg/Ca ratios in mmol/mol shift from the core to the rim of the crystal. Large crystals, 5-10 microns in diameter, tend to have low-Mg content core (0-7 mmol/mol). Small crystals, 1-5 microns in diameter, tend to have higher Mg cores (6-34 mmol/mol). Some samples exhibit large core-to-rim variation (22.2 to 5.7 mmol/mol Mg/Ca), while others less so (9.7 to 7.6 mmol/mol Mg/Ca). We find that magnitude and type of zonation is correlated with crystal diameter.

SEM-EDS characterization of microcrystal chemistry therefore may lead to more effective water flood chemical formulations because they can be based on surface chemistry, not bulk chemistry. This finding could not have been made by using bulk geochemistry because such measurements average many discrete zones across ~300,000 microcrystals.