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 the design of more effective acid treatments.

Coreflood experiments at flow rates ranging from 0.5 to 20 cm3/min were performed to determine the optimum acid injection rate for wormhole propagation when acidizing homogeneous limestone reservoir cores, low permeability Indiana limestone cores, and homogeneous dolomite cores with dimensions 3 and 6 in. of length and 1.5 in. of diameter. The study involved acidizing cores saturated with water, oil, and waterflood residual oil using 15 wt% HCl. The viscosity of the crude oil used was 3.8 cp at 200°F. CT scans were used to characterize wormholes through the cores. The concentrations of the dissolved calcium and magnesium ions were measured using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and the effluent samples were titrated to determine the concentration of the acid.

At injection rates 0.5 to 20 cm3/min, 15 wt% HCl was effective in creating wormholes with minimal branches for cores with residual oil (Sor=0.4-0.5). Compared to brine and oil saturated cores, waterflood residual oil cores took less acid volume to breakthrough. Additionally, the wormholing efficiency of regular acid improved with increased acid injection rates in the presence of residual oil. A decrease in acid pore volumes to breakthrough for oil saturated cores was noted at high acid injection rates, which could be attributed to viscous fingering of acid through oil. Unlike brine saturated and oil saturated cores, waterflood residual oil cores showed no face dissolution at low acid injection rates. Conclusions from this work will aid in the design of better acid jobs by highlighting the impact of oil saturation on the wormholing characteristics of acid while acidizing carbonate rocks.

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