Impact of Viscous Acid-in-Oil Emulsion Formation on Well Productivity Following Acid Stimulation

Abstract

Acid stimulation techniques matrix acidizing and acid fracturing are commonly applied in carbonate reservoirs. The goal of such techniques is to increase well productivity by enhancing permeability in the near wellbore area. Frequently, these stimulation techniques can introduce formation damage such as sludge formation or viscous emulsions resulting in lower than expected well productivity enhancement or well productivity reduction, sometimes irreversible. This study will focus on the formation of viscous acid-in-crude emulsion formation and its effect on well productivity. Emulsion bottle testing experiments with 15% hydrochloric acid (HCl) and a North Texas Crude oil have been performed to study the effect of acid fraction and time on emulsion viscosity and stability. Also, acid coreflooding experiments have been conducted to study the in-situ formation of acid-in-crude emulsions and the effect on subsequent oil injection. Bottle testing results indicate that emulsion viscosity increases dramatically with increasing acid fraction; viscosities of over 1000 cP have been measured. Emulsion viscosity also increases with time as a separation of free oil and emulsion occurs; the separated emulsion has a higher acid fraction than the initial mix and is extremely viscous, nearly solid-like. Acid coreflooding results show direct evidence of acid-in-crude emulsion formation on crude-saturated cores. Viscosity of one such emulsion was measured at 860 cP and was shown to be extremely stable. Coreflooding tests with an acid injection period followed by an oil injection period could not indicate if in-situ emulsion formation during acid injection has a detrimental effect on subsequent oil production.