Matrix Stimulation in Heterogenous Carbonate Reservoir: A Case Study from the UAE using Custom Acidizing and Self-Diverting Acid Treatment as an Alternative to Conventional 15% HCl Treatment

Abstract

Abstract This paper aims to highlight how important it is to apply the matrix acidizing engineering approach to achieve the best results for any well, which includes the custom chemistry selection and execution procedure to enhance well productivity in argillaceous limestone reservoirs. This study showcases the complexity of interaction between the various layers of the reservoir and the self-diverting agent, and how this optimized placement leads to increased treatment effectiveness. Using 15% Hydrochloric Acid (HCl) on carbonate reservoirs is challenging due to the speed and aggressiveness of the reaction. In addition, there is no doubt that in matrix stimulation the diversion and coverage of the treatment fluid plays a key role in driving the efficiency and final result of the operation. For this case study a customized acidizing blend combines primary and secondary reactions that provide longer reaction time and better etching. In addition, a pH-activated self-diverting acid was pumped in stages to provide dynamic diversion of the treatment fluid while being pumped. The well is a newly drilled, open-hole, single gas producer in a green field. Apart from the already listed benefits, other considerations were the natural fractures in the reservoir, the high percentage of swelling and migrating clays in some zones, and the long treatment interval. The original treatment was designed considering four (4) stages (2 acid and 2 self-diverting acid system stages) and 20 gal/ft of reservoir. After the 3rd stage, the well recorded a peak of 12 MMscfd of gas (the objective was 8 MMscfd), and the Wellhead Pressure (WHP) surpassed 4,500 psi, which created some operational concerns since the wellhead was only rated to 5,000 psi. Based on this, a decision was made to cancel the 4th stage of the treatment and open the well to reduce the pressure quickly. However, as the horizontal section is open hole and was recently treated with acid, this abrupt change in reservoir mechanics led to a suspected collapse of the formation. The stimulation approach proved successful during the operation as post-job analysis of the wellhead pressure response indicates a significant skin reduction. The results, however, brought conditions never seen before in the area, and the contingency steps taken proved detrimental to the reservoir. Following the correct matrix stimulation engineering approach helped to select a custom solution for the well that considered the proper reaction time of the treatment chemical systems with the reservoir. This improved the creation of the wormholes and determined the diversion technique to consider maximizing reservoir coverage over the length of the open hole section.