| dc.description.abstract | Limited studies have been conducted in understanding the mechanics of pre-flush stages in sandstone acidizing processes. Amongst those conducted in this area, all efforts have been directed toward singular aqueous-phase scenarios. Encountering 100% water saturation (Sw) in the near-wellbore region is seldom the case, as hydrocarbons at residual or higher saturations can exist. Carbonate mineral dissolution, being the primary objective of the pre-flush stage, results in carbon-di-oxide (CO2) evolution. This can lead to a multi-phase presence depending on the conditions in the porous medium, and this factor has been unaccounted for in previous studies under the assumption that all the evolved CO2 is dissolved in the surrounding solutions. The performance of a pre-flush stage changes in the presence of multi-phase environments in the porous media.
A detailed study is presented on the effects of evolved CO2 due to carbonate mineral dissolution, and its ensuing activity during the pre-flush stages in matrix acidizing of sandstone reservoirs. Carbon Tan sandstone cores were used toward the purpose of this study, of which two were fully water saturated, the remaining two were brought to initial water saturation (Swi) and residual oil saturation to waterfloods (Sorw), prior to conducting pre-flush stage experiments. The pre-flush stage fluid, 15 wt% hydrochloric acid (HCl), was injected in the concerning cores while maintaining initial pore pressures of 1,200 psi and constant temperatures of 150°F.
In fully water saturated cores, a major fraction of un-reacted carbonate minerals still existed even after 40 pore volumes (PV) of pre-flush acid injection. Heterogeneity is induced as carbonate mineral dissolution progresses within the core, creating paths of least resistance, leading to preferential flow of the incoming fresh acid. This leads to regions of carbonate minerals being untouched during the pre-flush stimulation stage. An average of 25% recovery of the oil in place was seen from pre-flush experiments conducted on cores with Sorw. In cores with Swi, the oil saturation was reduced during the pre-flush stage to a similar value as in the cores with Sorw.
The potential of evolved CO2, a by-product of the sandstone acidizing pre-flush stage, towards its contribution in swelling the surrounding oil and thus mobilizing the trapped oil has been depicted in this study. | |
