Abstract:
Fines migration is a common issue causing formation damage in sandstone reservoirs. Many experimental studies have observed a decline in permeability due to pore blockage with fine particles, which strongly depends on the attractive and repulsive forces between the pore surface and fine particles. However, another part of the researchers found fines migration as a mechanism for enhanced oil recovery (EOR). Therefore, this research aims to investigate the impact of rock wettability on critical salt concentration (CSC) in the presence of oil and determine whether fines migration can serve as a mechanism for oil recovery.
Core samples were initially saturated with high salinity water (HSW) containing 0.2 M NaCl. Experiments were conducted in several series, starting with sequential injection of low salinity water (LSW) with decreasing salinity into the water-wet sandstone core to determine CSC in the absence of oil in the system. Then into another water-wet core, brine with salinity less than CSC was injected right after the HSW to study a change in the core permeability after a gradual and sharp decrease in salinity to see the effect of fines migration on formation damage. Next, to assess the effect of oil on CSC, crude oil was injected into other core samples, and the sequential injection of LSW was repeated to observe changes in the CSC. Finally, the same procedure was repeated for the oil-wet core, and the recovery factor (RF) from the cores containing oil was measured to study the role of fines migration in additional oil recovery.
The results indicated that the CSC in the presence of oil in the water-wet core is similar to the case without oil, and CSC was found to be at 0.09 M NaCl. However, the CSC was obtained earlier in the oil-wet core at 0.12 M NaCl. These findings suggest that the oil-wet system is more sensitive to LSW than a water-wet core, regardless of the presence or absence of oil in the system. Also, the abrupt decrease in injection brine salinity resulted in permeability decline, which indicated the fines migration as a cause of formation damage in this scenario, while in water-wet and oil-wet cores containing oil, the impact of migration of fine particles on EOR was almost in the same order, approximately 10% of original oil in place (OOIP).