Carbonate reservoir matrix acidizing is commonly conducted with HCl. In these treatments, HCl acid is used to create conductive channels (wormholes) to enhance well productivity/injectivity. However, its use has been limited due to associated rapid tubulars corrosion and formation face dissolution, especially in deep hot reservoirs. Emulsified acid was used as an effective alternative to HCl, but it is associated with drawbacks such high friction losses and emulsion stability. In this paper, an aqueous single-phase retarded HCl alternative system was evaluated as an alternative to straight and emulsified acid fluids.
Coreflood experiments were conducted using Indiana limestone core plugs at 180 and 270°F. Computerized Tomography (CT) scan analysis was conducted on the core plugs before/after coreflood testing. Compatibility testing was conducted on prepared retarder acid recipes. ESEM, TGA, and ICP analysis was used to analyze prepared retarder acid recipes and associated solids. Turbiscan LAB was used to assess the stability of the retarded acid recipes.
The low pore volume to breakthrough (PVBT) values (i.e., 0.9-1.6) obtained from coreflood testing at 180 and 270°F, confirmed the retarded HCl acid recipes were effective to stimulate carbonate reservoirs. Compatibility testing showed presence of significant white precipitate. ESEM analysis showed the precipitates were rod-like crystals composed of mainly of Cl and high C with small amounts of N, O, Al and Mg. TGA results showed the major constituent of precipitate were organic-based materials. The precipitate was mainly H4EDTA and chloride. Despite presence of white precipitate at the core inlet, the effect on the performance of the retarded acid system was insignificant. CT scanning analysis of the plug samples before/after the coreflooding experiments showed that wormholes along the plug length with multiple branches were formed in all cases indicating the compatibility of the selected acid recipe.