Microstructural and Flowback Analysis Methodology in Matrix Acidized Carbonate Oil Reservoirs

The success of carbonate acidizing depends on the selection of proper fluid recipes, reservoir formation parameters, job design, and execution. Analysis of flowback spent acid will improve the acidizing process in future treatments, enhance the designed recipes and treatment design. The objective of this paper is to share the flowback analysis methodology following carbonate acidizing treatments with focus on solid analysis. Microstructural analysis with advanced microscopy and spectroscopy analytical techniques such as high-resolution environmental scanning electron microscopy (ESEM), energy dispersive X-ray microanalysis (EDX) and X-ray diffraction (XRD) techniques were utilized. Flowback samples were filtered through 0.45 µm filter paper. ICP was used to analyze the flowback samples. The injected acid recipes dissolved significant amount of calcite. The maximum calcium concentrations in flowback samples were 90,000-120,000 mg/L. Moreover, solid precipitates were found in flowback samples associated with pH values of 4.7-5.5. Gypsum was the dominant compound in the samples analyzed while the other compounds such as Lepidocrocite, Magnetite, Quartz, and Barite were detected in a single sample. The iron-based compounds were detected in the beginning of flowback period. Calcium and silicon rich compounds were identified in later flowback periods. The source of iron was identified to be most likely mill scale. Barite and Quartz were found to be associated with iron-based compounds. Gypsum and sodium chloride were detected with varying dominations between CaSO4 and NaCl compounds with a possible correlation as described by Dourba et al. (2017). Particles agglomerations were mainly associated with calcium, chloride and sulfate-based compounds. The rod-like and hexagonally-shaped particles were mainly found to be Si-based particles. Flower particles and dendrite structures were detected and probably associated with Gypsum precipitation amorphous and hemihydrate intermediates. The varying structures and agglomerations of sulfate compounds detected by the SEM indicated they were formed via different mechanisms and environments.