Injection of liquid carbon dioxide with treating fluids has been used for many years to improve results and to eliminate some of the problems associated with the stimulation of oil and gas wells. One purpose is to promote faster cleanup without the need of swabbing. When the pressure is released at the wellhead after the treatment, the carbon dioxide vaporizes and forces the treating fluids from the formation. Presence of this gaseous carbon dioxide in these fluids reduces the weight of the fluid column so that normal reservoir drive can then help unload the fluids from the well. Rapid recovery of the stimulation fluids is normal.
Under certain conditions, dissolved carbon dioxide and calcium salts can react to form a calcium carbonate precipitate. This paper discusses the conditions of temperature, pressure, and pH under which calcium carbonate does not precipitate.
The most useful tools in helping to answer questions like "How much of a substance will dissolve?" and "Will precipitation occur?" are chemical equilibrium calculations.
Chemical equilibrium is the study of those factors that influence the extent to which a reaction takes place. The quantity of products formed in relation to the quantity of reactants used is one such factor. The equilibrium constant relates the concentration of the reactants to the products. Equilibrium constants are useful in determining how much of a certain reactant or product will be present at equilibrium. For slightly soluble substances that form precipitates, such as CaCO3, the equilibrium constant is simplified and called the solubility product constant (Ksp).
The following set of equations was used to study the CaCO3-CO2 equilibrium system to determine the amount of CO2 necessary to cause precipitation of calcium carbonate in spent acid solutions. The particular condition of interest was the equilibrium of CaCO3-CO2 at 400 °F under 12,000 psi. Due to lack of equilibrium data in the literature for these conditions, the following equilibrium equations and constants were used to obtain an approximate solubility of CO2 in spent acid.1,2
Chemical equation (1)(Available in full paper)
Chemical equation (2)(Available in full paper)
Chemical equation (3)(Available in full paper)
Chemical equation (4)(Available in full paper)
Chemical equation (5)(Available in full paper)
Chemical equation (6)(Available in full paper)
With the calcium ion concentration [Ca2 +] known, the equilibrium carbon dioxide concentration [CO2] was calculated for various pH values. Figure 1 shows the results of these calculations. The carbon dioxide concentration represents the amount of CO2 that could be present in solution without causing CaCO3 to precipitate. Once this concentracion is exceeded, however, precipitation of CaCO3 will occur. Solutions of 20-percent and 40-percent CaCl2 were considered since these would be equivalent to the concentrations of calcium chloride obtained upon spending of 15-percent and 28-percent HCl, respectively. on calcite. As the pH increases. the solubility of CO2 in both 20-percent and 40-percent calcium chloride solutions decreases. Also I at any specific value of pH, as the calcium chloride concentration increases the solubility of CO2 decreases.