Acidizing of high-temperature carbonate reservoirs faces many challenges and requires a superior retarded acid system with high thermal stability, controlled reaction rate, and acceptable corrosion profile as compared to lower-temperature formations. In this study, a novel retarded acid system is introduced to address the shortcomings of the available retarded acid systems in the market. The proposed retarded acid system is based on a unique formulation of HCl and the sodium salt of monochloroacetic acid and does not require gelation by a polymer or surfactant or emulsification in diesel. The proposed acid system combines the use of a strong mineral acid (i.e., hydrochloric acid) with sodium monochloroacetate (HCl/SMCA). The acid system benefits from two mechanisms: 1) hindering the fast reaction of HCl and 2) in-situ acid generation by hydrolysis of SMCA towards glycolic acid which provides dissolution capacity for deeper penetration. The hydrolysis of SMCA occurs over time as acid penetrates through the formation. The HCl/SMCA system has an initial pH of 2-3, which significantly reduces corrosion rates at high temperatures. In this study, the dissolution capacity of the acid system was first measured. Then the potential risk of unwanted precipitation of the reaction products was investigated. Finally, the performances of the SMCA system at various formulations were investigated by performing coreflood experiments at high temperatures. The coreflood experiments were conducted at different injection rates to obtain the acid efficiency curve or pore-volume-to-breakthrough (PVbt) curve. Finally, corrosion experiments were conducted at high temperatures using three SMCA formulations. From the dissolution experiments, it was found that the dissolution capacity of the HCl/SMCA acid system, containing only 6 wt% HCl, can be as high as 1 lb CaCO3 scale/gal. It was shown that the reaction products from the calcite dissolution are fully soluble and the chelation by sodium gluconate is the main responsible mechanism. From the coreflood results, it was found that the new HCl/SMCA system can efficiently stimulate limestone formations with no face dissolution. It improves the wormholing performance significantly over HCl acid only and the PVbt decreases from 2.6 to 1 at 130°C. Benefiting from the gentle nature of the acid/SMCA system, tighter formations can be treated at much lower injection rates. CT scan images confirm the favorable wormhole propagation characteristics of the SMCA formulations. It was shown that 60% of the acid capacity remained unused even at very low injection rate, showing the retardation properties of the proposed system. According to the corrosion data, when SMCA used as retarding agent the corrosivity of HCl is decreased and much lower inhibitor concentrations are needed. The new HCl/SMCA system effectively retards initial non-uniform HCl acidizing and adds in-situ acid generation, thereby improving overall the uniformity of the formation acidizing process. This slow-release HCl/SMCA acid system has a low viscosity and less aggressive initial pH, making its use attractive in a broad range of stimulation applications and offering the oilfield industry a high performing and a cost-effective alternative to acid retardation via polymers/surfactants or emulsification in diesel.

This work presents a matrix acidizing formulation which comprises a salt of monochloroacetic acid giving a delayed acidification and a chelating agent to prevent precipitation of a calcium salt. Results of dissolution capacity, core flood test and corrosion inhibition are presented and are compared to performance of 15 wt% emulsified HCl. Dissolution capacity tests were performed in a stirred reactor at atmospheric pressure using equimolar amounts of the crushed limestone and dolomites. Four different chelating agents were added to test the calcium ion sequestering power. Corrosion tests were executed using an autoclave reactor under nitrogen atmosphere at 10 barg. Core flood tests were performed to simulate carbonate matrix stimulation using limestone cores. It was found that the half-life time of the hydrolysis reaction is 77 min at a temperature of 100 °C. Sodium gluconate and the sodium salt of D-glucoheptonic acid were identified to successfully prevent the precipitation of the reaction product calcium glycolate at a temperature of 40 °C. Computed Tomography (CT) scans of the treated cores at optimum injection rate showed a single wormhole formed. At 150 °C an optimum injection rate of 1 ml/min was found which corresponds to a minimum PVBT of 6. In addition, no face dissolution was observed after coreflooding. Furthermore, the corrosion rates of different metallurgies (L80 and J55) were measured which are significantly less than data reported in literature for 15wt% emulsified HCl. The novelty of this formulation is that it slowly releases an organic acid in the well allowing deeper penetration in the formation and sodium gluconate prevents precipitation of the reaction product. The corrosivity of this formulation is relatively low saving maintenance costs to installations and pipe work. The active ingredient in the formulation is a solid, allowing onsite preparation of the acidizing fluid.

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.