Sandstone Cores as Reaction Vessels: Synthesis of Calcium Carbonate Particles for Artificial Formation Damage in the Qualification of Matrix Acidizing Solutions

Abstract

Generally, matrix acidizing fluids for sandstone are executed in the field only after core tests qualify their ability to remove damage. However, most cores that are used come from sandstone quarries and the cores are largely clean and undamaged (and not representative of the sandstone conditions in actual producing wells). This study proposes novel applications of straightforward chemistry to synthesize calcium carbonate particles that damage the porosity of clean sandstone cores (in core flow tests); the study includes reactions carried out under controlled conditions as well as within the pore space of sandstone cores. In the controlled reactions, solutions of calcium chloride are mixed with solutions containing one of a variety of soluble inorganic or organic carbonates, and the reaction generates calcium carbonate particles. The resulting particles were characterized using both light scattering and scanning electron microscopy (SEM). The reaction rate and size distribution of calcium carbonate particles varies depending on the temperature, stir rate, chemistry of the carbonate species, and the concentrations (and concentration ratio) of calcium brine and carbonate species. This technique is also applied in actual core tests, where a sandstone core saturated in calcium brine is exposed to a solution of the soluble carbonate species and generates calcium carbonate solid (damage) throughout the length of the core from the injection face. The final permeabilities of cores subjected to this treatment exhibit damage compared to before the treatment, which is likely due to the precipitation of calcium carbonate solids. This method for damage generation has ramifications in the screening of a variety of stimulation fluids, including acids and scale inhibitors.