The Impact of Initial Oil Saturation on Wormhole Propagation in Carbonate Reservoirs

Abstract Carbonate acidizing has been used for nearly a century in the oil and gas industry. Despite the numerous research efforts to understand the acid effect on carbonate reservoirs, the majority of the work only considered fully water saturated samples, which does not accurately duplicate the situations encountered in the field. The objective of this work is to investigate the effect of oil saturation on the acid volume requirements and the optimum injection rate. Coreflood experiments were used to examine the effect of the presence of oil on both the pore volume to breakthrough and the optimum injection rate. Three saturations were considered: fully water-saturated, residual oil saturated, and connate water saturated cores. Six different flow rates were tested: 1, 2, 3, 5, 10, and 15 cm3/min. Cores were tested under both aged and unaged conditions to investigate the wettability effect. The pore volumes of acid injected to reach breakthrough were used to identify the optimum injection rate for each set of experiments. The wormhole shape was inspected using computed tomography. Experimental results show that the oil saturation improves the efficiency of limestone acidizing by decreasing the volume requirements to achieve breakthrough. The pore volume to breakthrough for all flow rates decreased from the fully saturated conditions, with the connate water saturation condition showing the lowest acid volume required. A correlation between the oil saturation, the pore volume to breakthrough for fully water saturated cores, and the pore volume to breakthrough for oil saturated cores is proposed. This correlation can predict the acid requirements for oil saturated conditions, in agreement with the current literature. Additionally, the oil saturation decreased the optimum injection flow rates due to the change in the area available to flow, and thus, the effective acid velocity in the pores. Lastly, the rock wettability effect was minimal on both the volume and the optimum injection rate. This work explains the contradicting observation in the literature and determines the effect of the initial oil saturation on the wormhole process. In addition, a new correlation is suggested that can help industry professionals better design acid field treatments to account for the effect of oil present in the near-wellbore area.