A Practical Approach from Design to Evaluation of Matrix Stimulation Operations in South Pars Gas Condensate Field
Authors: Panjalizadeh Hamed , Safari Alireza , Kamani Mehrdad , Jafari Hamed 
Published: November 2020

Since its discovery at 1971, numerous matrix stimulations were performed in South Pars field. However, there are still various challenges on stimulation job design and evaluation methods. In order to tackle these issues, fifteen matrix operations were selected to be analyzed from different phases of development project of the reservoir. The objective of this study is to introduce a new approach to figure out optimum acid volume (gal/ft), compare the effectiveness of diverters, calculate stimulation ratio (SR), and forecast post-acid production behavior from surface treating data.

Modified inverse injectivity (Iinv) method, which is fully discussed by Safari et al. (2020), is utilized in this study. Obtained data were analyzed in terms of Iinv decreasing trend, Iinv humps as well as pre/post-acid Iinv during 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 PLT data of two stimulation operations.

Obtained results indicate that optimum gal/ft of acid depends on well conditions. In this regard, wells with high initial formation damage require more volumes of acid. In this regard, wells treated with 27/27 gal/ft design (27 gal/ft 28%HCl and 27 gal/ft 15%VES, i.e. Visco Elastic Surfactant,) were under-stimulated. Although, 53/53 gal/ft design seems to be adequate for low-skin wells, higher gal/ft would further enhance productivity of highly damaged wells. This result was confirmed by stimulation of a damaged well with 60/60 gal/ft design. Finally, the most reliable design applied in the field is 70/70 gal/ft so far.

Iinv analyses depict that better diversion is observed in wells with lower injectivity and higher damage. Generally, the magnitude of Iinv humps, which represents as a supporting evidence for diversion, decreases as stimulation proceeds. Furthermore, fiber–laden VES diverter exhibited better diversion than VES alone.

Surface post acid drawdown at a specific production rate, e.g. 50 MMSCFD, is defined as the difference between shut-in and flowing wellhead pressures. Obtained results demonstrate 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. Ultimately, calculated SR values demonstrated average deviation of less than 10% with downhole PLT data.

The novelty of this work pertains to utilization of the Iinv method to evaluate performance of matrix stimulation operations. By applying this method, optimum volume of acid and diverter, diversion effectiveness, stimulation ratio as well as a prediction of post-acid surface drawdown can be obtained from surface treating data. The proposed methodology led to get better understanding of South Pars reservoir behavior during matrix stimulations. The same methodology can be applied to other carbonate reservoirs, too.