New Retarded Acid System for High Temperature Applications: An Efficient Alternative to Emulsified and Viscosified Acid Systems
Authors: Nazari Moghaddam Rasoul , Bokkers Albert , Aaldering Koos , Ferm Paul , Kooijman Cees 
Published: June 2023

Abstract
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.