A Low-Viscosity Retarded Acid System for Stimulation of High-Temperature Deep Wells

Abstract

To enhance the efficiency of acid stimulation treatments in high-temperature carbonate reservoirs, the industry requires a superior retarded acid system that inherently displays high thermal stability, controlled reaction rate, and acceptable corrosion losses. One of the earliest means of acid retardation was through the addition of polymer gelling agents; however, these sytems commonly lead to formation damage. Alternatively, a commonly used approach to slow down the acid/rock reaction rate is through the use of acid-in-oil emulsions (i.e., emulsified acids). The viscous nature of these fluids, however, leads to unfavorably high friction pressures during the pumping process. Additionally, the thermal stability of emulsified acids is questionable at temperatures exceeding 300°F. Herein, we introduce a novel engineered acid system for stimulation of high-temperature carbonate reservoirs. The proposed technology reduces the free water in the system and consequently prevents full dissociation of the acid. Reducing the free water in the acid system was successfully achieved by engineering the acid formulation in the presence of another organic compound that is readily soluble in the acid. A series of benchtop carbonate dissolution experiments were performed under static conditions at room temperature to identify the optimal engineered formulation needed to achieve the desired retardation effect. The retardation behavior of this system was further supported by reaction rate studies, coreflood measurements and corrosion loss data. The proposed acid system was evaluated at temperatures and pore pressures up to 350°F and 3000 psi, respectively. Experimental results confirmed that the engineered acid system requires only 0.28 PV to achieve a breakthrough (12 inch core), which represents a significant reduction as compared to some reported emulsified acid systems. Importantly, the fluid formulation was systematically fine-tuned in order to impart the desired fluid-property characteristics: low-viscosity (est. 4 cp), high dissolving power (4.16 lb acid/gal), fast wormhole propagation rate, low corrosion loss (<0.05 lb/ft2, 275°F) and high thermal stability (>300°F). The acid system can be readily mixed on-the-fly with existing equipment. This is a desirable feature particularly when it comes to offshore operations as no special preparation methods are required. It is the opinion of the authors that this novel acid system provides the industry with a suitable fluid alternative that may offer the potential to enhance oil and gas production in carbonate reservoirs. The fluid is stable at high temperatures and may have the potential to be used in a broad range of stimulation applications. Importantly, this low-viscosity retarded acid system presents the industry with a novel acid alternative where acid retardation by polymer addition or emulsification in diesel is no longer required.

Review of Acid Diffusion Measurement Methods in Porous Media

“…The use of cross-linkers, emulsifiers, and other additives has gained tremendous attention in the past couple of decades. Even though these additives retard the acid, leading to better wormhole propagation, the effect of high viscosity could lead to formation damage . This section contains a review of publications addressing the effects of additives such as cross-linkers and emulsifiers on acid diffusion measurement.…”

A High Effective Acid System to Stimulate Sandstone Reservoirs

“…Diverting agents are often added to the acid system to promote acid flowing into the entire perforated interval homogenereously. Diverting is a technique designed to alter the fluid injection profile in formation, and it can be classified as mechanical (straddle packer or ball sealers), solid (sand, oil soluble resins, graded rock salt, and benzonic acid fines), and chemical (viscous agents, natural gums, emulsions, acid swellable polymers, solid organic acids, foam, and other particulate diverters) [23,24]. Chemical diverters are one of the most commonly utilized diverting techniques, and they are also the most difficult systems to use properly.…”

Specific-Ion Effects Unveil a Route for Modulating Oxidatively Triggered Acid Systems for Reservoir Applications

“…Often regarded as a “green acid”, HOMs is readily biodegradable and considered less toxic and corrosive than mineral acids . Its universal aqueous metal salt solubility, coupled with its high conductivity and stability, has made it a beneficial acid electrolyte substitute for metal surface treatments in the electronics industry. , Additionally, like the aforementioned acids, many organic reactions rely on HOMs as an acid catalyst to promote processes such as polymerizations, benzene alkylations, biodiesel production, etc. − In recent years, it has also gained the attention of the oil and gas industry for its potential to replace mineral acid systems for use in subterranean reservoir stimulation applications, e.g., scale removal, matrix acidizing, and acid fracturing applications. − In fact, in our quest to develop a polymer and emulsifier-free slow reacting acid system for stimulation of carbonate reservoirs, we became interested in combining HOMs with HCl to yield a single-phase low-viscosity acid system with high dissolving power and suitable reaction kinetics to induce an attenuated behavior with respect to calcite dissolution . In the context of high-temperature well stimulation, delivery of HOMs precursors from readily available materials that, when triggered, distribute acid in a facile manner to the zone of interest would provide an opportunity to alleviate the need for costly corrosion inhibitor packages.…”

“…33−36 In fact, in our quest to develop a polymer and emulsifier-free slow reacting acid system for stimulation of carbonate reservoirs, we became interested in combining HOMs with HCl to yield a single-phase low-viscosity acid system with high dissolving power and suitable reaction kinetics to induce an attenuated behavior with respect to calcite dissolution. 35 In the context of high-temperature well stimulation, delivery of HOMs precursors from readily available materials that, when triggered, distribute acid in a facile manner to the zone of interest would provide an opportunity to alleviate the need for costly corrosion inhibitor packages. In situ generation of the acid could also extend the depth of treatment in the reservoir by preventing premature reaction with the rock.…”