A Delayed In-Situ Generated Acid System to Enhance Carbonate Acidizing

Abstract

Conventional acidizing of high temperature reservoirs is currently facing different challenges including high corrosion rate, face dissolution, rapid and uncontrolled reaction and formation damage. In this study, a new in-situ generated acid system is introduced to address the shortcomings of the conventional acid systems. This paper presents an acidizing fluid with delayed acidification properties that can be tailored to different reservoir conditions. A new acidizing system based on sodium salt of monochloroacetic acid (SMCA) is developed and introduced to resolve the technical challenges that conventional acid systems usually fail to tackle appropriately. The hydrolysis rate (i.e., acidification rate) of SMCA, which brings the delayed properties to the acid system, was further optimized by addition of halogen salts. Furthermore, the dissolution capacities of the acid system were measured for different minerals using the main formulation as well as boosted acid by addition of an organic/inorganic acid. Also, the precipitation potentials of the by-products were also investigated after the acid was fully spent and cooled down. Several additives were tested to improve the solubility of the calcium salt. Finally, the performance of the acid system was investigated by performing several coreflooding experiments using Indiana limestone cores. The coreflood experiments were conducted at different injection rates and at various degrees of hydrolysis. From the hydrolyzation experiments, it is found that addition of 1 wt% sodium iodide increases the SMCA hydrolyzation rate with a factor 1.7. It proved that fast hydrolysis can even be obtained at lower temperatures. It was shown that the reaction products from the calcite dissolution are fully soluble and chelation is the main responsible mechanism. In addition, it was found that scale inhibitors made from polymaleic acids, polyacrylic acids and copolymers thereof can also efficiently prevent solid precipitation. It was shown that the addition of hydrochloric acid in the range between 0.5 and 5 wt% can increase the dissolution capacity by at least 30%. From the coreflooding results, it was found that the new acid system can efficiently stimulate the limestone formations with no face dissolution under varying conditions in terms of temperature and injection rate. The CT-scan images confirmed the favourable wormhole propagation characteristics of the new acid system. The new acid system introduced in this paper is an in-situ acid generator which releases an organic acid over time and as function of temperature allowing deeper penetration. The main ingredients in the acid system are solid which allow safe handling and onsite preparation for offshore applications.