In oil fields, serious difficulties with acid-induced sludge, a major source of well damage, can vary from a viscous, stable emulsion. A chemical analysis of sludge shows that hydrochloric acid (HCl) is present in the sludge structure. Also, Fe(III) ions in HCl formulations can cause a large amount of sludge. Even though ferric ion could not be detected in the oil phase or in the sludge, it induces sludge mainly because it works as a phase-transport catalyst for HCl. Therefore, the objective of this study is to determine the effect of residual oil saturation on the performance of regular, gelled, and in-situ gelled acids, that based on Fe(III) as crosslinker agent, inside the carbonate cores.
A core flood study was conducted using Indiana limestone cores (1.5" diameter & 6" long) at 200 °F. Propagation of the acid and crosslinker (Fe(III)) inside the long cores was examined. Analysis of the CT measurements was used to determine the locations of residual polymer inside the core. Acid injection rate was the main parameters that were investigated in this study.
Experimental results show that there was no compatibility problem between the used oil and the different acid systems. During injection of pre-flushed water, Pressure drop across the core initially increased because of creating oil bank in the zone behind the water. After reaching the maximum value, the pressure drops across the cores decreased by increasing the volume of water injection due to oil displacement. Increasing the injection rate during the pre-flush stage increases the amount of oil remained inside the core. At any injection rate, regular acid required more pore volume to breakthrough from the core that contains residual oil saturation. At 2.5 cm3/min, gelled acid took less volume to breakthrough from the core that contains residual oil saturation. However at injection rate of 10 cm3/min, gelled acid took higher volume to breakthrough from the core that contains residual oil saturation. At 2.5 cm3/min, damage due to gel was higher inside the core that contains residual oil saturation. However at injection rate of 10 cm3/min, damage due to gel was higher inside the core that contains residual oil saturation.