Microencapsulation of Acids by Nanoparticles for Acid Treatment of Shales

Abstract

Acid treatment is one of the common well-stimulation techniques to improve the production in tight carbonates or calcite-rich shale reservoirs. Because the reaction rates are often too large at the reservoir temperatures, retarded acid systems such as emulsified acids, foamed acids, and polymer-gelled acids are used. The retarded acids also minimize damage to the wellbore and propagate acids long-distance from the wellbore in fractured shales. In this work, a novel encapsulation method of highly concentrated acid (∼10 wt % HCl) is reported as an alternative retarded acid system. Microencapsulation of these acids is performed using highly hydrophobic silica nanoparticles. The mixing of these particles with acid under high shear rates results in the formation of acid-in-air powders. The release of acid from these powders could be triggered by external stimuli such as mechanical pressure or surfactant addition. The thermal stability, corrosion inhibition efficiency, and shale surface reactivity are compared to conventional acid-in-oil emulsions, which are typically used for shale acidization processes. The encapsulation is found to be quite robust at 80 °C, and half-life (time taken to release 50% of encapsulated acid) is increased by more than 2 orders of magnitude as compared to acid-in-oil emulsions. A gravimetric measurement technique is used to quantify the iron corrosion rate for these two systems. The corrosion rate for the case of powders is found to be lower than that of emulsions at all of the studied temperatures (25, 60, and 80 °C). The shale surface topography is analyzed using an optical profilometer before and after the surface treatment with these two encapsulated-acid systems. The release of acid from powders, triggered via mechanical pressure, results in surface etching, which could potentially increase the fracture conductivity in shales. The release of acid can also be enhanced by the addition of anionic surfactants in the carrier fluid. The simplicity of the acid-in-powder generation process along with excellent thermal stability and corrosion inhibition efficiency makes the proposed acidencapsulation technique lucrative for acid treatment applications.

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.…”

A State-of-the-Art Review of Nanoparticles Application in Petroleum with a Focus on Enhanced Oil Recovery

“…NPs have been studied for improving hydraulic fracturing and acidizing jobs [72]. Huang et al [73] introduced nanoparticle-coated proppant which has the ability to maintain viscosity at high temperature, control fluid loss, and reduce fines migration.…”

Development of Acid Nanocapsules with Tailored Breaking Reservoir Temperature for the Removal of Formation Damage by Fines Migration

“…However, the authors did not recommend the encapsulated treatment due to a decrease in the productivity of the target well associated with unspecified precipitation problems. Recently, Singh et al 20 implemented a Pickering emulsion method to stabilize colloidal systems for microencapsulated acid (MEA) and hydrophobic silica nanoparticles to stabilize the colloidal system, prompting the formation of acid-in-air powder. According to the authors, the system shows an average particle size of 27 ± 1 μm and high thermal stability at 80 °C in comparison with conventional acid-in-oil emulsion.…”

“…Nowadays, the academy and industry have focused on developing new technologies to improve acid stimulation in sandstone formations to mitigate the critical issues associated with the high reactivity of the acid solutions. − Most studies are focused on acid reformulation with chelating agents, solvents, and dispersants to increase the performance for fines removal . However, these new molecules increase costs, could lead to corrosion issues in the production system, and cause the formation of emulsions. − One innovative alternative is the encapsulation of acid solutions, which shows a huge potential due to the increase in the acid penetration, the control over undesirable reactions, and the reduction of the corrosion issues without lowering the acid efficiency. − …”

“…The encapsulated systems reduced the acid release half-life by two orders of magnitude regarding the control and showed a faint reduction in the iron corrosion rate. It is worth mentioning that, within our knowledge, all studies of acid encapsulation reported in the scientific literature were of micrometric size, which must be contrasted with the distribution of throat pore size for determining the possibilities of formation damage. − Also, there are no reports of HF encapsulation with a focus on sandstone formation. Hence, more efforts are required to develop nanosized acid capsules with high thermal stability to guarantee deeper acid penetration and avoid the formation damage for blockage, considering the properties of the fluids of the oilfield of interest.…”