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Matrix acidizing is vital to enhance the production of tight carbonate formations. In weak carbonate rocks or acid over-dissolution, the rock is prone to failure upon developing wormholes. Strengthening agents can play a significant role in mitigating failure and supporting more extended productivity. In this study, we tested 0.1 M and 0.2 M zinc sulfate (ZnSO4) and 0.5 M diammonium phosphate ((NH4)2HPO4) as strengthening agents, which change the dominant host mineral into harder ones. Both agents were tested on two different types of acidized (i.e., wormholed) carbonate rocks, specifically Austin chalk and Indiana limestone. The impulse hammer and acoustics measurements evaluated the carbonate rocks' mechanical properties before and after the treatment. In addition, scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM–EDS) and scanning electron microscopy (SEM) were used to affirm the change of rock mineralogy into harder minerals after the treatment. The chemical treatment caused no noticeable change in the wormhole size and shape, as revealed by the scanned micro-computed tomography (micro-CT) images. The chemical treatment of the samples was conducted by core flooding the plugs at high pressure and temperature with 4–5 pore volumes of the chemicals and leaving them in the treatment solution for 72 h. As a result of the formation of harder minerals, the mechanical properties after the treatment showed improvement. Zinc sulfate raised the surface rock hardness of limestone by 16%, while diammonium phosphate showed a 30% increase when evaluated with the same technique. The most noticeable improvement happened to chalk after treatment with diammonium phosphate. Chalk impulse hammer hardness almost quadrupled with the diammonium phosphate. The acoustics measurements showed a similar result to the impulse hammer through an increase in the dynamic Young's modulus and a decrease in Poisson's ratio. [ABSTRACT FROM AUTHOR]

• A dissolution reaction mechanism near the wellbore is expected by injecting CO2 in saline water aquifers. • The injection velocity of CO 2 -saturated brine in limestone significantly impacts the dissolution rate. • At the optimum velocity, the CO 2 -saturated brine creates the thinnest and fastest wormhole. • An optimum Damkholer number is needed to determine the generalized CO2-saturated brine/limestone dissolution mechanism. Deep limestone aquifers are potential CO 2 storage sites, but CO 2 -saturated brine reacts with the carbonate rock, changing its transport and storage properties. This study provided a preliminary investigation of the optimal injection rate of CO 2 -saturated brine in carbonate rocks. Indiana limestone cores were subjected to CO 2 -saturated brine injection at varied rates using an HPHT coreflooding setup with X-ray CT monitoring. The samples were characterized pre- and post-treatment in terms of porosity and pore size distribution using a gas porosimeter and NMR T 2 measurements. Moreover, the reaction was evaluated by measuring the aqueous effluent calcium ions concentration as a function of throughput using ICP-OES analysis. A high-resolution micro-CT scan was used to capture the dissolution post-treatments and characterize the wormhole's size and patterns. Results showed that the wormholes broke through to the sample exit face after injecting 160, 48, and 36 pore volumes at 0.5, 1, and 2 cm3/min, respectively thereby revealing the importance of injection velocity. The ICP-OES analysis revealed that a larger dissolution rate was achieved at 2 cm3/min, which explained the fast wormhole propagation. An increase in rock porosity and the pore-size distributions was observed after coreflooding on all samples with minimum precipitation, as concluded from the NMR T 2 relaxation time. A universal optimum Damköhler number can be obtained that enables calculating the optimum injection rate of CO 2 -saturated brine at different rock and fluid conditions. We speculated that the optimum Damköhler number could be different from the value of 0.29 proposed by Fredd and Fogler (1998). This study provides a preliminary understanding of the optimal CO 2 -saturated brine injection velocity that has an application for CO 2 storage, water alternating gas (WAG) operations, and acid stimulation of carbonate formations. [ABSTRACT FROM AUTHOR]

Carbonate rock strengthening techniques are widely used in cultural heritage preservations. Recently, there has been a growing interest in their application in the oil and gas sector to improve the hardness and stability of fracture surfaces compared to the predominant host mineral. In this study, two rock lithologies were treated (chalk and limestone) at ambient temperature using 0.9 M NaF and 0.1 M ZnSO4 solutions with the aim of improving the hardness of the rock surfaces by transforming their mineralogy. The carbonate rocks were characterized before and after treatment using various techniques to establish changes in their mineralogical compositions (using x-ray diffraction (XRD), scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM–EDS), and X-ray photoelectron spectroscopy (XPS) techniques) and petrophysical/mechanical properties (using Brinell test, impulse hammer, scratch test, steady-state gas injection). The reaction rates were monitored using inductively coupled plasma optical emission spectrometry and ion chromatography (ICP-OES) over a period of 2–5 days at 4–6 h intervals. It was found that the NaF treatment yielded a 4% and 6% improvement in indentation resistance of limestone and chalk, respectively. The ZnSO4 treatment produced the most notable improvement in rock hardness (presented by Young's modulus), with a 21% improvement for chalk and a 17% improvement for limestone, respectively. The novel findings in this study suggest that NaF and ZnSO4 are potential carbonate rock strengthening agents that can be utilized in various industrial applications, such as, for example, solving conductivity reduction problems arising in hydraulic fractures due to rock softening. [ABSTRACT FROM AUTHOR]

The temperature has a significant impact on acid stimulation efficiency in carbonate reservoirs. To fully understand its effect, this study utilizes a two-scale continuum model coupled with heat transfer. The study investigated the importance of the heat of reaction, injected acid temperature, and reservoir temperature on wormhole propagation and treatment efficiency. This research's novelty stems from its application to limestone, dolomite, and combined mineralogy in radial and linear flow patterns. It also investigated the role of vugs on acidizing efficiency in dolomite formations, showing that vugs improve wormhole penetration distance. The model reproduced the outcomes from the literature, showing that the heat of reaction has a negligible effect on wormhole propagation. On the contrary, increasing the injected acid temperature reduced the wormhole radius in limestone but improved its efficiency in dolomite. The acid could not create efficient wormholes at low acid temperatures in dolomite formation. It diverted itself towards the limestone in sequence mineralogy cases. Nevertheless, it equally stimulated the limestone and dolomite sections at elevated acid temperatures. The reservoir temperature was less critical than the acid temperature in determining the wormhole size and treatment efficiency. The model can be used to select the optimum temperature at which the most efficient wormhole is created. [Display omitted] • A two-scale continuum was used to investigate the impact of temperature on wormhole efficiency. • The model was applied to various mineralogy under linear and radial flow patterns. • The injected acid temperature impact was more significant than the reservoir temperature. • The heat of reaction has no significant impact on wormhole propagation. • Higher acid temperature has a positive impact in dolomite but negative in limestone. [ABSTRACT FROM AUTHOR]

Fracture conductivity is a measure of the fracture ability to transfer fluids from a hydrocarbon-bearing zone to a well. Proppant embedment in soft formations and rock weakening due to the acid/rock reaction limit the stimulation success resulting in sharp fracture conductivity decline. The focus of this study is to increase the rock strength by mineralogical alteration aimed at sustaining long-term fracture conductivity. To achieve these, several standard cylindrical Indian limestone plugs with diameters of 1.5 and 2.5 in and length of 3 in were treated with different types of zinc solutions to substitute calcium with zinc in calcite crystals. The goal of these mineral alterations is to change the principal component of limestone rocks surface, calcite (CaCO 3), into smithsonite (ZnCO 3). Smithsonite is harder than calcite and its lattice system is similar to calcite. The treatment was carried out by full immersion of samples into zinc solutions at a predetermined time intervals. The rock strength was measured from samples surface (Young's modulus) before and after the treatment by a non-destructive technique using an impulse hammer. Zinc sulfate solution was found to enhance the rock hardness by values ranging from 25% to 35% via the formation of a layer of smithsonite on the sample's surface. SEM, XRF, XPS and FIB-SEM experiments were conducted to understand the exchange mechanism that eventually led to the hardening. Also, effective porosity and permeability were measured before and after the chemical treatment to investigate its impact. A field-scale modeling study of rock hardness impact on productivity is provided as a preliminary proof of concept. • Improving hydraulic fracture conductivity in carbonate minerals by forming harder ones was proved to be possible. • Rock surface Young's modulus improved significantly when calcite minerals were replaced with smithsonite. • Hi-Res XPS, XRF, SEM, and FIB-SEM experiments were conducted to understand the exchange mechanism. • Automated impulse hammer measurements were taken to prove the increase in rock hardness. • Numerical simulations proved that enhancing the rock hardness could improve the fractured well productivity. [ABSTRACT FROM AUTHOR]

Fracture conductivity is a fracture's capacity to deliver fluids from the hydrocarbon reservoir to the wellbore. Sustaining long-term fracture conductivity results in better well productivity and reserve recovery. Harder rocks tend to sustain conductivity better than do weak and ductile rocks because they are less likely to suffer from proppant embedment and the crushing and creeping of asperities. This study shows that the hardness of carbonate rocks can be improved significantly when they are treated with carbonate-consolidating chemicals. Ideally, this should be done without damaging the matrix permeability. This research tested the capacity of microemulsion, calcium hydroxide (Ca(OH) 2) nanoparticles, and tetraethyl orthosilicate (TEOS) to consolidate carbonate rocks. These are all believed to improve rock hardness by enhancing carbonate grain attachment. The treatments were applied to cores and slabs of Mississippian Indiana limestone and permo-carboniferous Khuff carbonate rocks. These chemicals were tested by either impregnation or complete immersion of the rock sample in a solution. SEM, X-ray fluorescence, porosity, permeability, and rock hardness were all measured before after the treatment. It was observed that the rock surface Young's modulus could be improved by 10%–38%. Matrix permeability was reduced by 50% when Ca(OH) 2 nanoparticles were tested. Nevertheless, the TEOS improved the carbonate rock's hardness and permeability 18% and 44%, respectively. A modeling study has also been provided to explain the impact of fractured rock strengthening on productivity improvement. • TEOS is an efficient chemical for strengthening of carbonates. • Ca(OH) 2 nanocrystals consolidate carbonates. • Treatments lead to an increase in Youngs modulus of up to 38%. • Permeability is improved in the case of TEOS treatments. [ABSTRACT FROM AUTHOR]

The exploitation of unconventional formations requires propped hydraulic fracturing treatments. Propped fracturing is an expensive process that usually suffers from operational challenges. In this study, a new technology is proposed which targets implementing thermochemical fluids to stimulate unconventional formations. These fluids release large pressure pulses upon a reaction that creates networks of cracks along the fracture. Triggering thermochemical fluids with acid creates differential etching along the fracture surfaces due to the acid/rock dissolution. The new technology was tested experimentally through coreflooding on Indiana limestone and Kentucky sandstone samples and through breakdown pressure experiments on Eagle Ford shale samples. The breakdown pressure of the Eagle Ford shale samples was reduced from 2400 to 900 pisa using thermochemical fluids triggered with acid. It was also observed that the acid triggered thermochemical fluids could maintain the permeability of the fractures at high closure stresses due to the acid/rock dissolution. A laboratory and field-scale models were also developed in this study to understand thermochemical reactions. The laboratory-scale model could capture the pressure pulses generated experimentally and the system temperature. The field-scale model was then used to understand the thermochemical reactive transport in a hydraulic fracture. The model showed that thermochemical concentration is the most significant parameter in controlling the temperature and pressure magnitudes in the field. • Thermochemical fluids triggered with acid generate high conductivity and network of cracks. • Thermochemical fluids triggered with acid reduce the breakdown pressure. • A model is created to match the pressure pulses and temperature magnitudes obtained experimentally. • Sensitivity analysis was conducted at field-scale to investigate the importance of different design parameters. • Thermochemical fluids concentration was found to be the most important design parameter. [ABSTRACT FROM AUTHOR]

Matrix acidizing is a well stimulation method that is applied to enhance productivity. The success of a stimulation job depends mainly on the efficiency of the acid systems that are employed. This study investigated a new and efficient acid system that is biodegradable, not corrosive, and safe to handle. The system efficiency was compared to that of HCl and chelating agents. Acid coreflooding has been applied to chalk, dolomite, and limestone carbonate rock samples to measure the efficiency in terms of injected pore volume to breakthrough (PVBT). The rock samples were characterized in terms of porosity, permeability, composition using X-ray diffraction (XRD), and pore distribution using nuclear magnetic resonance (NMR). The effluent was collected at different times and analyzed using induced coupled plasma (ICP). Medical computed tomography (MCT) was used to visualize the interior of the rock samples after acidizing. The new acid system was more efficient in terms of acid spending than straight HCl acid and chelating agents in Indiana limestone. At the optimum injection rate, chelating agents had an optimum PVBT value of about 15 compared to 0.85 for plain HCl acid. Using the new acid system resulted in an optimum PVBT value of only 0.41, which is less than half of HCl acid. A numerical wormhole simulator was implemented to match the experimental PVBT and assess the reactivity of the acid system. • A novel acid system has been identified to be better than aqueous HCl. • A modelling approach reproduces the wormholing. • Different lithologies react (systematically) different. [ABSTRACT FROM AUTHOR]