Acid matrix treatments in carbonate reservoirs create a network of passages in the near-wellbore zone. The acid volume is a key variable that determines the effectiveness of acid treatment. For matrix treatments, the acid volume ranges from 20 to 100 gal/ft. Acid concentration ranges from 15 to 20 wt%. Selection of acid volume and concentration varies based on engineering experience and judgment.
This paper presents an analytical model for calculating time-lapsed injectivity index (II) in order to verify the impact of acid volume with all other contributing variables being fixed. The model integrated matrix acid treatments; injection profile surveys, injection data, completion configurations, and reservoir parameters. The model has been applied on six stimulation treatments where all contributing variables varied, except the acid volume. A normalized performance indicator was characterized based on the II's initial improvement, decline rate, and cumulative improvement.
The calculated IIs by the model successfully matched the measured IIs by the fall-off tests. The model evaluated the treatments contributing variables and established clear correlations between acid volume and the post stimulation performance.