The exploration for new petroleum resources is venturing into deeper formations as most of the conventional reservoirs are less productive over time. These deeper, high-pressure high-temperature (HPHT) reservoirs create major challenges for various operations, such as acidizing, fracturing, and sand control. Especially for acidizing operations, the formation of wormholes at high temperatures is a challenge because the acid reaction rate is higher and the viscosity of the fluid is lower. Therefore, acid emulsified in diesel, using diesel as the external phase, is used in acidizing applications. This is anticipated to perform better compared to neat acids at higher temperatures when it remains in an emulsified form. Hence, the emulsion stability of the system is a major concern, especially for high-temperature applications. The stability depends on various factors, such as temperature, corrosion inhibitors, and intensifiers, among others. This paper presents a recent study including the effect of different corrosion inhibitor intensifiers, acid strengths (20 to 28%), and temperatures (275 to 300°F) on the emulsion stability and corrosion rate. The instability of the emulsified acid system and corrosion were found to be positively correlated. The addition of a high amount of corrosion inhibitor also destabilizes the emulsion. Hence, it needs to be used at an optimum concentration.

The effect of two different corrosion inhibitor intensifiers on the stability of the system was also studied. One inhibitor intensifier was found to affect the stability of the emulsion only at higher temperatures (300°F and above). The effect was a result of a faster release of the degradation products of the additive at higher temperatures. The amount of this intensifier should be limited to 5 gal/1,000 gal for higher temperatures (300°F and above) for 28% HCl acid at pressures less than or equal to 1,000 psi. The stability of the emulsion was enhanced by the addition of another intensifier at higher temperatures. Based on these observations, an optimized emulsified acid recipe was designed that was stable at 300°F.

This system can be used for acid stimulation of carbonate reservoirs with bottomhole static temperatures (BHSTs) of 300°F, meeting the current market demand for acidizing operations. In addition, the findings reveal the effect of corrosion inhibitor intensifiers on emulsified acid systems. The results of this study are very useful for designing an emulsified acid system for use in high-temperature applications.

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