Quartz dissolution in a single phase-high pH Berea sandstone via alkaline injection

Abstract

A common assertion is that alkaline solution aids oil mobilization by generating in situ soap, or by lowering interfacial tension (IFT) to ultra-low values in synergy with surfactants. This study takes a different approach that involves the alkaline dissolution of detrital quartz grains of sandstone reservoirs to create pathways for oil migration and accumulation. Quartz dissolution via alkaline injection will result in changes in permeability and porosity. This study performed high-pH core flooding on Berea sandstones using core displacement equipment. Silica molybdate spectrophotometry was applied to measure the amount of dissolved silica. Inlet and confining pressure variations were also observed. The molar concentration of NaOH varied at 0.5 M and 1.0 M. The results show higher initial silica dissolution for 0.5 M NaOH (˃200mg/L) compared to 1.0 M NaOH (20 mg/L), which can be attributed to the presence of pre-existing dissolved silica and precipitates in the system prior to the first injection phase. Nonetheless, a steady quartz dissolution rate of 0.4 mg/L/hr for 20 h was only achieved at 1.0 M. Conversely, an abrupt drop in quartz dissolution to below 0 mg/L was recorded for 0.5 M NaOH after 3 h of dissolution. At higher molar concentration of injected alkaline solution, both confining and inlet pressures increased from 8 and 5 bars to 12 and 11 bars as a result of the increased secondary phase of (hydr)oxides or precipitates in the pores. Thus, it can be inferred that the effect of alkaline solution on quartz dissolution is strongly dependent on molar concentration.

Keywords

• Alkaline injection
• Core flooding
• Dissolved silica
• Quartz
• Petroleum recovery