Single-Stage Stimulation of Anhydrite-Rich Carbonate Rocks Using Chelating Agent: An Experimental and Modeling Investigation

Summary Calcium sulfate (anhydrite) exists in most of the carbonate reservoirs, and its content can reach more than 20%. The high content of anhydrite affects the efficiency of the acidizing process because of the low solubility of calcium sulfate in different acids. Carbonate stimulation, matrix acidizing, or acid fracturing is carried out mostly using hydrochloric acid (HCl)-based fluids. The solubility of calcium sulfate in HCl or HCl-based fluids and organic acids is very low. In this study, a new formulation was developed to stimulate carbonate rocks with high anhydrite content. A formulation was developed to dissolve both anhydrite and carbonate at the same time. Anhydrite dissolution was achieved by converting anhydrite to calcite using the newly developed formulation. This treatment can be conducted in a single stage, and the formulation consists of a high pH chelating agent such as ethylenediaminetetraacetic acid (EDTA) in addition to potassium carbonate as a converter. Coreflooding experiments were conducted for carbonate rocks with varying anhydrite content. The effluent samples of the coreflooding experiments were analyzed for cations and anions concentration using inductively coupled (IC) and inductively coupled plasma (ICP) measurements. Computed tomography scans were conducted to show the wormholes generated in the samples. Rock mineralogical analysis using X-ray diffraction, thin section, and the QEMSCAN® automated mineralogy solution (FEI Company, Hillsboro, Oregon, USA) was conducted as well. The reaction kinetics were investigated using a rotating disk apparatus (RDA) using a disk that consists of almost 24% anhydrite and 71% dolomite. Also, a two-scale continuum model was built to simulate the reaction between the new formulation and carbonate rocks. The model could capture the experimental outcomes such as the number of pore volumes (PV) to break through and the shape of the wormholes. This provided confidence in the reaction parameters obtained because the model could reproduce the experimental outcomes. Coreflooding experiments showed that the new formulations were very effective in stimulating carbonate rocks with high anhydrite content in a single stage. The effluent analysis showed high sulfate concentration that indicates the dissolution of anhydrite compared to conventional acid treatments. Reaction kinetics results showed that the new formulation increased the reaction rate with anhydrite through a mass transfer reaction regime. The model reproduced the wormholing behavior obtained from the experiments, indicating that the branched wormhole was due to the anhydrite conversion.