Targeting Enhanced Production through Deep Carbonate Stimulation: Stabilized Acid Emulsions

Abstract

Abstract To meet rising global demands for energy, the oil and gas industry continuously strives to develop innovative oilfield technologies. A large portion of the world's oil and gas reserves are trapped in carbonate reservoirs, particularly in the Middle East. Well stimulation treatments for these highly heterogeneous formations have traditionally relied upon the use of strong mineral acids, e.g. hydrochloric acid (HCl). However, fast rock/acid reaction rates and corrosion issues pose a significant challenge to the longevity and practical application of such treatments. Alternative approaches have been proposed to slow down this process, among the most popular being emulsified acids. In this case, the release mechanism of live acid is governed by droplet contact with the surface of the formation in conjunction with suitable downhole conditions (i.e., temperature, pressure, pH). Upon release, the acid proceeds to rapidly react with the carbonate formation thereby unveiling a conductive wormhole network. This paper presents a systematic experimental study targeting deeper well stimulation using an improved emulsified acid system. The current study demonstrates the success of an emulsion-based system that is stable at elevated temperatures (e.g. up to 300°F/150°C) for a prolonged period of time. To gain a deeper understanding of the dissolution behavior and resultant wormhole morphology, a systematic coreflood study was conducted to identify key parameters that influence the overall performance of the newly developed stabilized emulsified acid. The parameters investigated include: (1) pore pressure, i.e. up to 7,000 psi, (2) flow rate, i.e. up to 5 cm3/min, (3) acid-rock contact time, and (4) fluid treatment composition. Indeed, the stabilized emulsified acid exhibited superior performance as confirmed from coreflood experiments performed at 3,000 psi and 300°F. In this study, 0.65 PV of fluid was injected into a 12″ core and shut-in for a period of 15h, thus giving the emulsion ample time to break in all cases. Interestingly, a 3-fold enhancement in carbonate permeability was observed for the stabilized emulsified acid as compared to the conventional acid-in-diesel emulsion. Moreover, the CT images for the treated core samples show that the stabilized acid system had less face dissolution and had the desired wormhole characteristics, i.e. narrow and directional propagation behavior with deeper penetration into the core sample. Finally, additives were defined and used to improve the performance of emulsified acid systems at high temperature, which in turn improved the acid penetration rate and wormhole propagation.