Viscoelastic surfactants (VES) are essential components in self-diverting acid systems. Their low thermal stability limits their application at elevated temperatures. The industry introduced new VES chemistries with modified hydrophilic functional groups, which enhances their thermal stability. These new chemistries are still challenged by the lack of compatibility with corrosion inhibitors (CI). This work aims to study the nature and the mechanism of the interaction between the VES and the corrosion inhibitors, which affects both the rheological and corrosion inhibition characteristics of the self-diverting acid system.
This study is based on rheology and corrosion inhibition tests, where combinations of VES and corrosion inhibitors are tested and complemented with chemical and microscopic analysis. Negatively charged thiourea and positively charged quaternary ammonium corrosion inhibitors were selected to study their impact on both cationic and zwitterionic VES systems. Each mixture of the corrosion inhibitor and the VES was blended in a 15 and 20 wt% HCl acid mixture, then assessed for its viscosity at different shear rates, CI concentrations, and temperatures up to 280°F in live and spent acid conditions. Each acid solution was assessed using Fourier-Transform-Infra-Red (FTIR) before and after each rheology and corrosion test to track the changes of the mixture functional groups. Each mixture was examined under a polarizing microscope to assess its colloidal nature. The corrosion inhibition effectiveness of selected acid mixtures was evaluated. N-80 steel coupons were immersed statically in the acid mixture for 6 hours at 150°F and 1,000 psi. The corrosion rate was evaluated by using metal coupon weight loss analysis followed by optical microscope examination for the metal surface.
The interaction between the CI and the VES surface charges and molecular geometries dictates both the rheological and the inhibitive properties of the acid mixtures. The use of a small molecular structure anionic CI with a cationic VES, results in a fine monodispersed CI particles in the VES-acid system. The opposite charges between the CI and the VES results in electrostatic attraction forces. Both the fine dispersion and the electrostatic attraction enhances the rheological performance of the mixture and packs the corrosion-inhibiting layer. The addition of a bulk and similarly charged CI with the VES results in a coarse polydispersed CI particles with repulsive nature with the VES. These properties increase the shear-induced structures and lower the packing of the inhibition layer deposited on the metal coupons, which decrease the rheological performance of the acid mixture and increase its corrosion rate. The FTIR analysis shows that there is no chemical reaction between the CIs and the VESs tested.
This work investigates the interactions between the corrosion inhibitors and the viscoelastic surfactants. It explains the impact of the surface charge of both corrosion inhibitors and VES on their rheological and corrosion inhibition characteristics. It adds a selection criterion for compatible VES and corrosion inhibitors.
In recent years, viscoelastic surfactants (VES) seemed like an optimum solution for fracturing fluids. The technology was introduced to replace heavily damaging polymers. VES low thermal stability, high cost, and incompatibility with acid additives limited its application in the field. This work aims to investigate the crosslinking of the VES micelles using different shapes of silica and iron oxide nanoparticles to reduce the VES loading and extend its thermal stability.
This work utilized surfactant templating and ultrasonicated co-precipitation methods to produce a specifically tailored mesoporous silica and magnetite nanorods respectively, which were mixed with an anionic VES using ultrasonic bathing. Both spherical and rod-shaped particles of silica and iron oxides were examined to investigate the particle size, shape, and surface charge impact on the degree and the strength of the VES micellization. The studied particles physical properties were assessed using zeta potential, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The rheological performance of the VES mixtures were evaluated at 280 and 350°F through various shearing and heating ramps. The mixture microstructure was investigated using a polarizing microscope before and after the heating process. The produced network between the VES micelles and the nanoparticles were examined using TEM to describe its nature.
The interaction between the nanoparticles and the anionic VES is controlled by the VES concentration, the particle shape, and the temperature range. Although the spherical particles failed to cross link the VES at a concentration of 2 wt%, it succeeded to extend the thermal stability of the VES at a concentration of 4 wt% up to 350°F. The nanorods succeeded to enhance and extend the thermal range of the VES system at only 2 wt% VES. Both shapes of particles performed similarly at 4 wt% VES and up to 280°F. The addition of 7 pptg of silica nanorods extended the thermal stability of the 4 wt% VES, which exhibited and held an apparent viscosity of 200 cp for 2 hours. The addition of rod-shaped particles contributed to stronger micelle to micelle entanglement, especially at VES concentration of 2 wt%. The nanoparticles resulted in secondary networking that contributed positively to the viscosity of the mixture. The rod-shaped particles showed lower thermal stability at 350°F. They maintained 50 cp compared to the total failure of the VES by itself with 10 cp at 350°F. The polarizing microscopy, the TEM, and the DLS analysis showed that the enhancement in the apparent viscosity comes from closely packed structures of nanoparticles in surfactant strings.
This research shows the importance of the selected nanoparticle size, shape, and surface charge on the rheological performance of VES. It lays out a route to synthesize custom built nanoparticles to accommodate the chemistry of surfactants for higher performance and lower cost. This work has implementations in both self-diverting acid systems and fracturing fluids.
Viscoelastic surfactants (VES) have been used to replace polymer-based fluids as effective, cleaner, and non-damaging viscofying carriers in frac-packing, acid fracturing, and matrix acidizing. However, several limitations challenge the use of VES-based fluids including: thermal instability, incompatibility with alcohol-based corrosion inhibitor, and intolerance to the presence of contaminating iron. This work introduces a new VES-based acid system for diversion in matrix acidizing that exhibits excellent thermal stability and diversion performance in both low-and high-temperature conditions.
Rheology measurements were conducted on spent VES-acid system as a function of temperature (77- 300°F) at a pH of 4-5. The effect of acidizing additives on the VES viscosity was investigated. The additives included a corrosion inhibitor, non-emulsifier, iron-chelating agent, and iron-reducing agent. Single and dual coreflood experiments were performed using limestone core samples with an initial permeability range of 4-200 md and a permeability contrast of 1.5-55. Post CT-scan imaging was conducted to investigate the wormhole topography. The diversion characteristic of the new VES in the dual coreflood experiments was evaluated by the structure and the extent of wormhole propagation in the low-permeability core.
Rheological data for 15 wt% HCl spent VES-solutions showed a maximum viscosity of 200-800 over a temperature range of 150-170°F, depending on the VES concentration in the sample. Without acidizing additives, a minimum of 50 cP was obtained at 195, 230, 250, and 275°F at 4, 5, 6, and 8 vol% of the VES in solution, respectively. None of the tested acidizing additives had a negative impact on the VES viscosity. At 8% VES loading, the acidizing package was optimized such that a minimum of 75 cP was obtained at 300°F.
Dual coreflood experiments were conducted at 150 and 250°F, and the results proved the ability of the proposed VES to divert efficiently in limestone formations. Single coreflood experiments also confirmed these results. Coreflood data indicated that a range of permeability contrast of 4-10 is the optimum for diversion ability in terms of the final permeability enhancement of the low-permeability cores. The results revealed 18.6, 45.6, 82%, and infinity when the permeability contrast was 28.3, 14.4, 6. 3, 1.63, respectively. A dual coreflood experiment was conducted for two cores with a permeability contrast of 1.6 at 150°F. The VES-acid system in the presence of all acidizing additives exhibited divergent performance that exceeded the performance of the VES in the absence of additivies. These results prove the stable performance of the VES and the enhancement in viscosity response after addition of both the iron-control agent and the non-emulsfier, which resulted in less acid leakoff and better wormhole structure.
Restoring formation damage by matrix acidizing is one of the common ways to increase productivity in sandstone formations. While sandstone formations are typically composed of the same types of minerals, the concentration and composition of these minerals, as well as the permeability and porosity of the matrix, can vary widely. Such variable petrophysical and compositional properties of sandstone formations can affect the severity of fines migration and the performance of acid stimulation treatments.
To evaluate the effect of acid stimulation on petrophysical properties before and after fines migration, coreflood experiments were conducted on Bandera sandstone cores. Half of the treatments were conducted on cores that were damaged by triggering fines migration by injecting deionized water, and the other half were conducted on undamaged cores. These cores contain a wide range of clay content, mineral compositions, permeability, pore size distribution, and porosity. A series of characterizing tests and measurements were performed before and after acid treatments. The objectives of this work include: (a) analyzing X-ray diffraction (XRD) and X-ray fluorescence (XRF) measurements to determine the mineralogy and elemental composition of the rocks, (b) interpreting computational tomography (CT) scans to evaluate pore-scale heterogeneity and acid effect on cementing material, and (c) evaluating permeability by flooding the core samples with brine.
More than 20 coreflood experiments were conducted at constant temperatures of 72, 150, and 250°F and constant acid concentration on 6 in. x 1.5 in. core samples at different injection rates. The CT- scans proved, quantified, and localized the fines migration in the damaged cores. The effect of fines migration on the acid stimulation treatment success in terms of increasing permeability and ultimately productivity was determined. Additionally, acid reaction increases porosity and permeability. However, treatment of damaged cores yielded different pore-size distribution and permeability compared to undamaged cores. Furthermore, combining CT-scan and inductively coupled plasma (ICP) results proved an extremely high sensitivity of fines migration toward HCl at temperatures 150 and 250°F resulted in porosity deterioration toward the outlet compared to the gained porosity at the inlet of the treated cores.
The literature is rich with discussions about sandstone acidizing and acid systems. However, this comparative study provides a more comprehensive understanding of the effect of fines migration on the success of the stimulation treatment and its effect on petrophysical properties. The outcome of this work will lead to a reliable design of matrix acid treatments and, hence, increase the chances of successful acid stimulation treatment that optimizes well productivity.