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

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

Molecular dynamics and energy distribution of methane gas adsorption in shales

This study uses simulations to explore the energy distributions involved in the adsorption of methane gas in shales. Molecular mechanics calculations were carried out using the Forcite module in BIOVIA material studio software. The critical challenge in molecular-scale simulations remains the need to improve the description of the gas adsorption prior to up-scaling to a realistic scenario. Resolving this challenge requires the implementation of multi-scale techniques that employ atomistic/molecular-level results as input. Thus, it is pertinent that the appropriate molecular data on CH4 gas interaction with shale is obtained. This study provides empirical data on CH4 sorption/adsorption in shale at the molecular level to confirm the CH4 storage potential of shales. The effect of pressure on the CH4 sorption/adsorption was also investigated. A vital aspect of this study is elucidating the energy distribution and dominant energy that controls CH4 sorption/adsorption to serve as a basis for the exploitation of CH4 in productive shales. Following the intensive simulation exercise, the average total energy of CH4 sorption varied from approximately −30 to −120 kcal/mol with increase in pressure from 500 to 2500 psi, suggesting increasing thermodynamic stability. The results indicated that van der Waals energy is the major sorption energy with values ranging from 60 to −250 kcal/mol as the sorption pressure increased, while electrostatic energy recorded the least contribution. The total adsorption energy increased from −5 to −16 kcal/mol for reservoir pressure range of 1–15 MPa. This energy distribution data confirmed the possibility of CH4 adsorption on shale under reservoir pressure conditions