The existence of an optimum injection rate for wormhole propagation, and face dissolution at low injection rates during matrix acidizing are well established. However, little has been documented that describes how the presence of residual oil affects carbonate acidizing. This study demonstrates the impact of oil saturation on wormholing characteristics while acidizing field and outcrop cores under reservoir conditions (200°F). Knowledge of the effect of different saturation conditions on acid performance will contribute towards designing more effective acid treatments.
Coreflood experiments at flow rates ranging 0.5 to 20 cm3/min were performed to determine the optimum injection rate for wormhole propagation when acidizing homogeneous carbonate and dolomite reservoir cores, and low permeability Indiana limestone cores of dimensions 3, 6, and 20 in. length and 1.5 in. diameter. Absolute permeability of the cores ranged from 1 to 78 md. The study involved acidizing cores saturated with water, oil, and water flood residual oil using 15 wt% HCl. The viscosity of the crude oil used was 3.8 cP @ 200°F. CAT scans were used to characterize wormholes through the cores. The concentrations of the dissolved calcium and magnesium ions were measured using Inductively Coupled Plasma-OES and the effluent samples were titrated to determine the concentration of the acid.
HCl was effective in creating wormholes with minimal branches for cores with residual oil (Sor=0.4-0.5) at injection rates 0.5 to 20 cm3/min. Compared to brine saturated cores, water flood residual oil cores took lesser acid volume to break through. Besides, the wormholing efficiency of regular acid improved with increasing acid injection rates in the presence of residual oil. Cores with residual oil after water flood showed no face dissolution at low acid injection rates. This is evident from the fact that at low injection rate, brine saturated cores measured the maximum calcium concentration in the effluent samples while cores with residual oil the least. Conclusions from this work aids better designing of acid jobs by highlighting the impact of oil saturation on wormholing characteristics of acid while acidizing carbonate rocks.