Targeting Enhanced Production through Deep Carbonate Stimulation: Stabilized Acid Emulsions

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.

Bromate Oxidation of Ammonium Salts: In Situ Acid Formation for Reservoir Stimulation

“…To address these limitations, the oil and gas industry has identified various strategies to delay acid release. These include but are not limited to acid-in-diesel emulsions (i.e., emulsified acids) − or encapsulation methods such as polymer core–shell and nanoparticle-based systems . Additionally, uses of polymers and gelling agents, organic acids, acid-generating systems, and delay additives have been extensively reported. − Regardless of the chemical approach, the objective is the same, that is, to provide a means of minimizing the reaction between acid and the metal tubulars or slowing the reaction between acid and rock to promote deeper penetration of live acid into the desired pay zone and restore/increase permeability.…”

Synthesis and evaluation of Jatropha oil-based emulsified acids for matrix acidizing of carbonate rocks

“…All the three emulsion types were tested for near zero electrical conductivity (Nasr-El-Din et al 1999Xiong et al 2010;Zakaria and Nasr-El-Din 2015;Cairns et al 2016). If a slight electrical conduction was observed, then the emulsion was mixed for another 30 min, after which the emulsion prepared showed no electrical conduction (Sayed et al 2012).…”

Optimization of Fracturing Fluid and Retarded Acid for Stimulating Tight Naturally Fractured Bedrock Reservoirs

“… 28 − 30 After carefully designing the surfactant formulation, the emulsified acid can stabilize at 150 °C for 5 h and enhance oil production by 3 times compared to other retarded acid systems. 31 However, because the oil phase is the main phase that wraps the acid, the emulsified acid requires large amounts of oil and surfactants, which are typically over 40%. Therefore, it is not economical to apply it in the target reservoir.…”

“…In the gelled acid, acid (like hydrochloric acid, HCl) is mixed with polymers (like polyacrylamide), which increases the viscosity of acid, reduces the diffusion rate of hydrogen ions, and thus reduces the reaction rate of acid with the reservoir rock . In the emulsified acid, surfactants and solvents are used to generate acid-in-oil emulsions; because the oil phase separates the acid from the reservoir rock and can slowly release the hydrogen ions, the acidizing rate can be significantly reduced that allows the acid to penetrate deep into the reservoir and extends the stimulation area. − After carefully designing the surfactant formulation, the emulsified acid can stabilize at 150 °C for 5 h and enhance oil production by 3 times compared to other retarded acid systems . However, because the oil phase is the main phase that wraps the acid, the emulsified acid requires large amounts of oil and surfactants, which are typically over 40%.…”