An Efficient Interpretation Method for Matrix Acidizing Evaluation and Optimization in Long Heterogeneous Carbonate Reservoirs
Authors: Panjalizadeh H. , Safari A. R. , Kamani M. 
Published: January 2021

Summary
Since its discovery in 1971, numerous matrix stimulations have been performed in South Pars field. However, there are still various challenges surrounding stimulation job design and evaluation methods. To tackle these issues, 16 matrix operations were selected to be analyzed from different phases of the development project of the reservoir. The objective of this study is to introduce an efficient interpretation method to determine optimum treatment volume (gal/ft), compare the effectiveness of diverters, calculate stimulation ratio (SR), and forecast post-acid production behavior from surface treating data.
The modified inverse injectivity (Iinv) method, which is fully discussed by Safari et al. (2021), is used in this study. The obtained data were analyzed in terms of Iinv decreasing trend, Iinv humps, and pre-/post-acid Iinv during the stimulation process. In addition, pre-/post-stimulation surface testing data are gathered and analyzed. These data are coupled with post-acid Iinv to find a correlation to predict production behavior of treated wells. SR is defined as the ratio of pre-acid Iinv to post-acid Iinv of a treated well. Finally, SR values are validated with available production logging tool (PLT) data from two stimulation operations.
First, the obtained results indicated that optimum treatment volume (gal/ft) of acid depends on well conditions. It means that wells with high initial formation damage require more volumes of stimulation fluids. In this regard, wells treated with 27/27 gal/ft treatment volume design [27 gal/ft 28% hydrochloric acid (HCl) and 27 gal/ft 15% viscoelastic surfactant (VES)] were understimulated. Although treatment volume design of 53/53 gal/ft seems to be adequate for low-skin wells, higher treatment volume (gal/ft) would further enhance productivity of highly damaged wells. This result was confirmed by stimulation of a damaged well with treatment volume of 60/60 gal/ft. Finally, the most reliable design applied in the field so far is the 70/70 gal/ft treatment volume. Second, Iinv analyses depicted that better diversion is observed in wells with lower injectivity and higher damage. At the next step, the calculated SR values showed an average deviation of less than 10% from downhole PLT data. Ultimately, the produced results demonstrated that there is a direct relation between the post-acid Iinv and surface drawdown in this field. Therefore, production behavior of treated wells can be correlated by having access to post-acid Iinv.
The novelty of this work pertains to use of surface treating data recorded during a stimulation operation to generate Iinv and its associated analysis curves to evaluate performance of matrix stimulation operations. By applying this method, optimum volume of acid and diverter, diversion effectiveness, SR, and an estimation of post-acid surface drawdown can be obtained from the simple surface treating data. The secondary-produced data could lead to a better understanding of carbonate reservoir behavior during matrix stimulations such as in the South Pars field.