Three-Phase Carbonate Acidizing: Quantification and Analysis of Evolved CO2 in the Presence of Oil and Water

Abstract

Recent times have seen an advancement in the area of carbonate acidizing, moving forward from single-phase to two-phase analyses, in an effort to account for the presence of the oil-phase during stimulation treatments. Yet, a lack of a complete capability to understand this complex subsurface process still exists. Characterizing the effect of CO2 (carbon dioxide), a byproduct of the chemical reaction between carbonates & HCl (hydrochloric acid) has been ignored till date, under the pretext of using high pore pressures to keep CO2 dissolved in surrounding solution. The presence of CO2 in porous media changes the dynamics of fluid flow. A three-phase two-scale simulation model is described toward the purpose of accurately modeling the physics of carbonate acidizing. A validation of the model, is conducted using published literature experiments and conducted laboratory corefloods in the area of carbonate acidizing. The acid efficiency curve for a single phase scenario from literature is matched, with the effects of the evolved CO2 being modeled. Two Indiana limestone core, 6 in. by length and 1.5 in. by diameter, are used for the purpose of a tracer injection study using 5 wt% KCl (potassium chloride) solution, and acid injection study using 15 wt% HCl solution. The experiments were conducted at 71.6°F, and 1,180 psi pore pressures. The Indiana limestone cores are characterized via CT (computed tomography) scans, and a detailed, accurate porosity profile of the core is used as input to the simulation model. The tracer fluid was used to characterize the porous environment and effective dispersion coefficients, and for subsequent calibration of the simulation model. From the conducted single phase acidizing coreflood, the experimental parameters such as pressure drop curves are closely monitored to assess acid breakthrough, and the effluents from the acid coreflood are analyzed for determining the concentrations of CaCl2 (calcium chloride) and HCl with time. CT scans of the core post acidizing describes the wormhole pattern. These parameters are accurately matched using the simulation model, and subsequent sensitivity studies with the presence of oil are performed thereof. The modeling of CO2 as a separate phase for mimicking the acid coreflood played a major role in acquiring a better match with all experimental parameters, with limited dependency on empirical pore-scale parameters. It is shown that the rock-wettability for an oil-water system has a large degree of influence on the acid PVbt (pore volumes of acid required to breakthrough), with oil-wet systems requiring higher volumes. An approximate of 30% recovery of the residual oil in place is predicted, purely based on capability of the evolved CO2 to swell the surrounding oil.