Dissolution Channels Research Articles

Compaction Creep and Evolution of Transport Properties of Carbonate Fault Gouges During the Percolation of CO2‐Rich Fluids

AbstractTo investigate the impact of CO2‐rich fluids on compaction behaviors and transport properties in carbonate fault zones, we conducted compaction‐coupled fluid flow experiments with CO2‐rich fluids percolating precompacted calcite aggregates. Our findings reveal distinct responses among samples subjected to different fluid conditions. Specifically, samples exposed to dry conditions exhibited negligible compaction strain, while those under wet‐closed conditions displayed relatively minor strain. In contrast, samples subjected to flow‐through conditions demonstrated significant compaction strain, with strain rates higher by 2–3 orders of magnitude than closed conditions due to enhanced pressure solution, subcritical cracking, and chemical dissolution. Strain rate, permeability, and grain size distribution exhibited spontaneous variations in response to fluid flow and compaction. Microstructures and mechanical and transport data suggest that deformation during the initial infiltration of CO2‐rich fluids was dominated by subcritical cracking, followed by pressure solution as grain size evolved, which resulted in compaction and reduced permeability. The persistent infiltration of CO2‐rich fluids further enhanced inhomogeneous dissolution‐precipitation with preferred dissolution channels serving as fluid pathways. The re‐precipitations may cement fault rocks and form low permeability seals, resulting in anisotropic fluid flow and localized fluid pressure in fault zones. As applied to nature, our results provide experimental evidence for the evolution of internal structures and transport behaviors, shedding important light on the mechanisms and sealing potential of carbonate faults in response to the infiltration of CO2‐rich fluids during the post‐seismic and inter‐seismic processes.

General corrosion behavior and mechanism of ferritic/martensitic steel with and without TaWVCr coating in static oxygen-saturated lead bismuth eutectic at 550 °C

Here in present work, the ferritic/martensitic (F/M) steel HT9 and refractory TaWVCr high entropy alloy coated HT9 were tested in a static oxygen-saturated lead bismuth eutectic (LBE) at 550 °C through corrosion tests and slow strain rate tension tests to investigate the corrosion behavior and mechanism. The fracture pattern of HT9 and TaWVCr/HT9 both show the ductile fracture. Comprehensive microstructural characterizations from atomic-scale examinations to micro-scale fracture analyses were performed to illustrate the underlying mechanisms of the liquid LBE corrosion. It was revealed that the corrosion of HT9 is related to the surface oxide scale cracking, which propagates into the matrix but terminated in the Cr2O3 enriched zone at the crack tip. On the contrary, the stress concentration caused by local degradation of the TaWVCr coating accelerates the oxidation and dissolution of the coating in LBE, leading to the formation of a duplex oxide layer underneath the coating, which allows cracks initiate in the magnetite layer and annihilate in the chromite layer. The micrometer-scale penetrative oxide layer was found within the matrix, due to the inward diffusion of O through the diffusion channels such as dislocation channels of the matrix and the dissolution channels of the coating and this can accelerate the oxidation corrosion of the matrix.

Study on the quenching sensitivity of corrosion properties in 7199 aluminum alloy

This study used electrochemical corrosion, intergranular corrosion (IGC), and exfoliation corrosion tests, as well as microscopic observations of the microstructure and first-principles calculations, to investigate the effect of different quenching rates on the microstructure and corrosion properties of the 7199 alloy. The results show that as the quenching rate decreases, the size, Zn, and Mg content of precipitated phases at grain boundaries (GBs), as well as the width of the precipitation-free zone (PFZ), increase, resulting in a decrease in corrosion properties. During slow quenching, the η phase (MgZn2) at the GBs continuously absorbs surrounding Zn and Mg atoms, causing Zn and Mg atom depletion near the GBs. This results in the formation of a wide PFZ. Enlargement of the η phase and PFZ at the GBs leads to anodic dissolution channels, allowing for rapid corrosion expansion along the GBs. Additionally, at slow quenching rates, not only does the η phase precipitate at the subgrain boundaries (SGBs) but also the T and S phases. In corrosive environments, the η phase is more prone to corrosion than the T and S phases. Electron work function calculations show that the S phase has the highest corrosion potential, whereas the η phase has the lowest. As a result, SGBs containing T and S phases are unlikely to serve as corrosion pathways. Corrosion preferentially extends along SGBs containing η phases.

Investigating the Acid Erosion Characteristics of Carbonate Rocks under Hydrodynamic Action.

In carbonate areas, the unique dissolution features bring a lot of resistance to engineering constructions. The acidic filtrate will definitely cause accelerated dissolution of the surrounding rocks, and the mechanism of accelerated dissolution of such rocks in acid is not clear. In order to explore the dissolution pattern of carbonate rocks after the alteration of their primary environments, a self-made rotating reaction device was used to conduct laboratory dissolution experiments on carbonate cores under three conditions. The mass loss, the change of pH, the molar concentration of Ca2+ and Mg2+, and the morphological changes before and after acid erosion were obtained. The results of correlation analysis show that the dissolution characteristics of carbonate are significantly related to dissolution time, composition of rocks, liquid flow rate, and acid concentration. Segmented characteristics were recorded between CaO/MgO and the reaction sequence (m). When 2 < CaO/MgO < 30, the increase of CaO/MgO has a significant contribution to the chemical dissolution rate; however, when 30 < CaO/MgO < 66, the increase of CaO/MgO does not contribute significantly to the chemical dissolution rate. The dissolution rate is positively correlated with the liquid flow rate. Also, liquid flow rate changes affect dolomite more than they do limestone. The mass loss rate (Rc) order of the rocks of the five carbonate formations was Maocaopu > Qingyan > Falang > Anshun > Dengying. Differences in dissolution induced by the acidic fluids in different formations will eventually form complex dissolution channels.

Control of brine composition over reactive transport processes in calcium carbonate rock dissolution: Time-lapse imaging of evolving dissolution patterns

This study investigates the impact of brine composition—specifically calcium ions and NaCl-based salinity—on the development of dissolution features in Ketton, a porous calcium carbonate rock. Utilizing a laboratory XMT (X-ray microtomography) scanner, we captured time-lapse in situ images of Ketton samples throughout various dissolution experiments, conducting four distinct flow-through experiments with differing brine solutions at a flow rate of 0.26 ml min⁻1. The scans yielded a voxel size of 6 μm, enabling the assessment of the temporal evolution of porosity and pore structure through image analysis and permeability evaluations via single-phase fluid flow simulations employing direct numerical solutions and network modeling, as opposed to direct measurement.Time-lapse imaging technique has delineated the extent to which the concentrations of CaCl₂ and NaCl in the injecting solution control the structural evolution of dissolution patterns, subsequently triggering the development of characteristic dissolution pattern. The inflow solution with no Ca2+ ions and with the minimal salt content manifested maximum dissolution near the sample inlet, coupled with the formation of numerous dissolution channels, i.e., wormholes. Conversely, solutions with a trace amount of Ca2⁺ ions induced focused dissolution, resulting in the formation of sparsely located channels. Inflow solutions with high concentrations of both Ca2⁺ ions and salt facilitated uniformly dispersed dissolution, primarily within microporous domains, initiating particle detachment and displacement and leading to localized pore-clogging. The relative increase in permeability, in each experiment, was correlated with the developed dissolution pattern. It was discerned that varying ratios of salt and calcium concentrations in the injected solution systematically influenced image-based permeability simulations and porosity, allowing for the depiction of an empirical porosity-permeability relationship.

Effects of Mixing at Pore Intersections on Large‐Scale Dissolution Patterns and Solute Transport

AbstractThe flow‐induced dissolution of porous rocks governs many important subsurface processes and applications. Solute mixing, which determines pore‐scale concentration fields, is a key process that affects dissolution. Despite its importance, the effects of pore‐scale mixing on large‐scale dissolution patterns have not been investigated. Here, we use a pore network model to elucidate the mixing effects on macroscopic dissolution patterns and solute transport. We consider two mixing rules at pore intersections that represent two end members in terms of the mixing intensity. We observe that the mixing effect on dissolution is the strongest at moderate Damköhler number, when the reactive and advective time scales are comparable. This is the regime where wormholes spontaneously appear. Incomplete mixing is shown to enhance flow focusing at the tips of the dissolution channels, which results in thinner wormholes and shorter breakthrough times. These effects on passive solute transport are evident independent of initial network heterogeneity.

High-cycle fatigue behaviour and crack growth mechanism of Ni-base single crystal superalloy under intermediate temperature

This study investigates the high cycle fatigue behaviour of a Ni-base single crystal superalloy under 760 °C and 850 °C. At 760 °C, the crack initially grows in the crystallographic shearing mode and the crack tip propagation mechanism is non-crystallographic mode. Furthermore, near the crack tip propagated at 760 °C, the oxide film distributes along the γ/γʹ interface and the Ni-rich γʹ phase dissolution channels pass through the matrix in different ways, resulting in the wavy-like and dendritic cracks. When the temperature rises to 850 °C, recrystallization grains are observed in the fatigue initiation region. Moreover, the stress reduction leads to the increment of accumulative strain and the aggravation of the recrystallization. This will enhance the crack growth driving factor, promoting the crack to grow in crystallographic shearing mode. Finally, the mechanisms of high cycle fatigue crack growth influenced by temperatures and stresses are obtained and enhancement conditions of crack growth driving factor are revealed.

Alteration of trioctahedral micas in the presence of inorganic and organic acids

The alteration of two trioctahedral micas, biotite and phlogopite, was investigated at the meso, micro, and nanoscale using three complementary microscopy techniques to better understand mica surface reactivity. In situ and ex situ experiments were performed to monitor the mineral interface during dissolution in acidic solutions (nitric and oxalic acid, pH ∼ 1–2), over a temperature range of 25–100°C. The inorganic acid was used as a benchmark condition to elucidate the effect of the organic acid on the dissolution behavior. The observed topographical changes that arose during mineral alteration revealed the simultaneous occurrence of different processes that heterogeneously shaped the mica surface: 1) the retreat of pre-existing and newly formed steps (edge surface reactivity). In the case of biotite, layer curling and peeling-off occurred in the presence of nitric acid whereas dendritic-shaped step edges resulted from the effect of oxalic acid; 2) the nucleation of etch pits and the formation of dissolution channels on the (001) surface. Oxalic acid promoted the growth of the pits to such an extent that they were discernible at each scale and resolution investigated; and 3) precipitation of secondary phases. Overall, a multi-scale approach offers new insights into the dissolution behavior of biotite versus phlogopite and provides and enhances understanding of the effect that oxalic acid has on the surface reactivity of mica.

Optimization and Uncertainty Quantification Method for Reservoir Stimulation through Carbonate Acidizing.

Reservoir stimulation is a widely used technique in the oil and gas industry for increasing the productivity of hydrocarbon reservoirs, most notably in carbonate formations. This work aims to develop an optimization workflow under uncertainty for matrix acidizing. A reactive transport model is implemented in a finite-element framework to simulate the initiation and propagation of dissolution channels in porous carbonate rock. The model is verified using an analytical solution. We utilize surrogate modeling based on polynomial chaos expansion (PCE) and Sobol indices to identify the most significant parameters. We investigate the effect of varying 12 identified parameters on the efficiency of the stimulation process using dimensionless groups, including the Damköhler, Peclet, and acid capacity numbers. Furthermore, the surrogate model reproduces the physics-based results accurately, including the dissolution channels, the pore volume to breakthrough, and the effective permeability of the stimulated rock. The developed workflow assesses how uncertainties propagate to the model's response, where the surrogate model is used to calculate the univariate effect. The global sensitivity analysis shows that the acid capacity number is the most significant parameter for the pore volume to breakthrough with the highest Sobol index. The marginal effect calculated for the individual parameter confirms the results from Sobol indices. This work provides a systematic workflow for uncertainty analysis and optimization applied to the processes of rock stimulation. Characterizing the impact of uncertainty provides physical insights and a better understanding of the matrix acidizing process.

Hydrothermal fluorite solubility experiments and mobility of REE in acidic to alkaline solutions from 100 to 250 °C

Fluorite is a common vein mineral in critical mineral deposits that are overprinted by hydrothermal processes. Its chemistry potentially reflects the mobilization conditions of rare earth elements (REE) in these hydrothermal fluids. However, the controls of temperature and fluid chemistry (i.e., pH and salinity) on fluorite solubility and coupled REE mobilization is still uncertain. In this study, batch-type experiments were conducted to investigate REE mobility in aqueous fluids controlled by the solubility of fluorite at temperatures between 100 and 250 °C. Natural fluorite crystals from Cooke’s Peak (New Mexico) were equilibrated with aqueous solutions of variable starting pH (2–10), salinities (0 and 15 wt.% NaCl), and initial REE concentrations (0 and 0.5 ppm). Reacted fluorite crystals display dissolution-controlled reaction textures (i.e., etch pits and dissolution channels along fluorite cleavage planes), which are most pronounced in experiments conducted in acidic fluids and at elevated temperature. Addition of NaCl considerably enhances fluorite dissolution, in both acidic and alkaline solutions, which is also reflected by the compositions of the reacted solutions displaying an increase in Ca, F, and REE concentrations. The overall fluorite solubility is controlled by the association behavior of HF0 at acidic conditions, and by the formation of Ca chloride, Ca hydroxyl, and Na fluoride complexes, which become enhanced by addition of NaCl, including in mildly acidic and alkaline solutions. Mass balance calculations were used to compare the REE stoichiometry of unreacted fluorite crystals with measured REE concentrations in the reacted solutions, which reveal several possible controls on REE mobility: i) stoichiometric release of REE upon fluorite dissolution; ii) precipitation of secondary REE-depleted fluorite; and/or iii) precipitation of secondary light REE fluorides, which were observed to form on fluorite crystals surfaces in the REE-doped experiments. Fluorite solubility constants (Ksp) were regressed with available low temperature solubility data using the equation logKsp = A + BT + C/T + Dlog(T) (T, temperature in K) valid between 25–250 °C. Fitted coefficients yield: A = 171.180, B = 0.01477, C = −7794.10, D = −64.6634. The experimental results have important implications for geochemical modeling of natural systems, where fluorine controls the mobility of REE, and can be enhanced by addition of NaCl in both acidic and alkaline solutions. Therefore, the salinity of magmatic-hydrothermal fluids plays a crucial role in mobilizing REE during the formation of hydrothermal fluorite veins.

Experimental study of apatite-fluid interaction and partitioning of rare earth elements at 150 and 250 °C

Abstract Apatite is a common accessory phase in igneous and metamorphic rocks. Its stability in magmatichydrothermal and hydrothermal systems is known to be a key control on the mobility of rare earth elements (REE). To better constrain how apatite is altered during fluid-rock interaction at comparably low temperatures, batch-type apatite dissolution experiments were conducted at 150 and 250 °C at saturated water vapor pressure in acidic to mildly acidic (pH of 2–4) aqueous fluids having variable salinities (0, 0.5, and 5 wt% NaCl). The study reveals the dominance of apatite dissolution textures with the formation of micrometer-scale etch pits and dissolution channels developing prominently along the c-axis of the apatite crystals. Backscattered electron imaging shows an increase in apatite dissolution with increasing temperature and upon reacting the crystals with more acidic and higher salinity starting fluids. This study also demonstrates an increase in dissolved REE in the experimental fluids corroborating with the observed apatite dissolution behavior. Backscattered electron imaging of secondary minerals formed during apatite dissolution and scanning electron microscopy-based energy dispersive spectrometry peaks for Ca, P, and REE support the formation of monazite-(Ce) and minor secondary apatite as deduced from fluid chemistry (i.e., dissolved P and REE concentrations). The studied apatite reaction textures and chemistry of the reacted fluids both indicate that the mobility of REE is controlled by the dissolution of apatite coupled with precipitation of monazite-(Ce), which are enhanced by the addition of NaCl in the starting fluids. This coupled process can be traced by comparing the REE to P ratios in the reacted fluids with the stoichiometry of the unreacted apatite crystals. Apatite metasomatized at temperatures &amp;lt;300 °C is therefore controlled by dissolution rather than dissolution-reprecipitation reactions commonly observed in previous experiments conducted above 300 °C. Furthermore, this study demonstrates that the presence of NaCl plays a crucial role in increasing the solubility of apatite, which controls the availability of REE to form secondary phosphates even in mildly acidic aqueous fluids. This implies that both the effects of acidity/alkalinity of the fluids and the role of dissolved alkalis (NaCl and KCl), need to be considered for understanding the controls on REE in magmatic-hydrothermal systems. Lastly, the experiments of this study expand the known conditions at which apatite is susceptible to be overprinted by hydrothermal alteration from 900 °C down to 150 °C and highlights the necessity of appropriately screening apatite grains using backscattered electron and cathodoluminescence imaging for signs of hydrothermal alteration textures in igneous apatite.

Morphology, composition and dissolution of chromite in the Goro lateritic nickel deposit, New Caledonia: Insight into ophiolite and laterite genesis

The morphological and chemical composition of chromite in the partially-serpentinized harzburgite and oxide zone of two laterite profiles at the Goro lateritic nickel deposit, New Caledonia, were investigated to: (1) establish the melt conditions at the mantle-crust boundary during ophiolite formation, and (2) characterize the weathering of chromite and its role in the goethite maturation model in formation of the oxide zone at the Goro Ni-deposit. Chromite in the Goro harzburgite, despite being highly fractured, is relatively fresh. Dendritic branching of skeletal outgrowths from poikiloblastic chromite, encapsulate partially serpentinised harzburgite and is indicative of a parent melt under changing cooling conditions, varying from supersaturated to undersaturated with localised melt mixing. Compositionally, the refractory nature (Cr#>0.6) of chromite, low (predominantly < 0.05 wt%) TiO2 content and high logFe2+:Fe3+ ratio values of 14–16 indicate a highly depleted and reduced peridotite melt with boninite-type affinity of suprasubduction provenance. Furthermore, the high Cr# of Goro chromite may define a compositional end-member along a continuum of Al2O3-Cr2O3 exchange with a relatively consistent (i.e., 25–35 wt%) FeO content, from the Goro deposit (Cr > Al) in the south-east through to high-Al chromite (Cr < Al) at Poum in the north-west of New Caledonia. In the oxide horizon, extensive surface etching and development of solution channels throughout chromite grains, supports both their abiotic and biogenically mediated dissolution. Abiotic dissolution is evidenced by the tessellation of surface etching features and epitactic oxidation of chromite to Cr-substituted hematite. Extensive development of meandering dissolution channels through the grain interior and the precipitation of locally elevated Mn (to 3–4 wt%) that juxtapose areas depleted in Cr but elevated in Al, Mg and Fe support the inferred role of Mn-oxidizing bacteria in the preferential dissolution of Cr from chromite. Preferential leaching of Cr from chromite, whether by abiotic and/or biogenic means, feeds into the mineralogical evolution of the oxide zone at Goro and supports an alternative crystallisation model via a ‘green-rust-like’ precursor for the formation of poly-metallic-substituted goethite.

Transitions of Dissolution Patterns in Rough Fractures

AbstractRock fractures are ubiquitous in geological systems and usually provide dominant pathways for fluid flow in fractured reservoirs. When the flowing fluid is reactive, fracture dissolution expands the aperture and forms various dissolution patterns that amplify the pathways. Previous works focused on the dissolution processes in Hele‐Shaw cells (parallel‐plate) and porous media, but the transitions of dissolution patterns in radial rough fractures are not well understood. Here we combine flow‐visualization experiments with theoretical analysis to elucidate the transitions of dissolution patterns under various flow‐rate and reaction‐rate conditions. We observe and quantify three distinct dissolution morphologies as compact, wormhole, and uniform patterns. We show that the critical Péclet numbers, corresponding to the transitions from compact to wormhole and to uniform patterns, increase with the reaction rate. Based on the growth of dissolution channels in the flow and transverse directions, we establish a theoretical model that describes the transitions of these three distinct dissolution patterns. The phase diagram predicted by the model exhibits good agreement with our experimental results and also well captures the pattern transitions reported in the previous studies. This work improves the understanding on how fracture aperture expands in the dissolution processes that lead to various dissolution channels. Our work is also critical for controlling the fluid flow behavior in dissolving natural rock fractures in subsurface flow systems.

Hyphal tips actively develop strong adhesion with nutrient-bearing silicate to promote mineral weathering and nutrient acquisition

Fungi actively enhance the local dissolution of nutrient-bearing minerals through the combined biomechanical and biochemical actions of their hyphal tips to obtain mineral-bound inorganic nutrients (MINs). However, little is known about the dynamic processes underlying hyphal tip-mineral interactions. Here, we assess the adhesive force between a single hypha of the common fungus Talaromyces flavus and the Fe-bearing silicate lizardite and quartz (as a control), as well as hyphal tip-induced lizardite weathering and hyphal Fe uptake. We showed that T. flavus hyphae formed their maximal adhesive force with lizardite at the growing tips, reaching 6.11 ± 0.69 nN after a contact time of one minute. The adhesive forces of the tip-lizardite interface within two minutes were >2.65 times stronger than those of the tip-quartz interface. Examination of the hyphal tip-lizardite interface after 18 h indicated the formation of dissolution channels with a depth of 27.7 ± 8.0 nm. Furthermore, the hyphal tips resulted in an altered lizardite up to 46 nm. The thickness of the altered lizardite increased to ∼130 nm after contact with the mature regions of the hyphae for ∼173 min. And the altered lizardite was found to have depleted Fe levels that increased with increasing contact time. The total content of Fe in T. flavus associated with the lizardite surface after 18 h was 52.98 ± 12.20 nmol mg−1, which was 6 times greater than the total amount of Fe in quartz surface-associated T. flavus after 24 h of culture. These results demonstrate that fungi access MINs by the active development of a strong adhesive force with target minerals through their hyphal tips, effectively enabling fungi to flourish in heterogeneous environments and be major geological agents for biogeochemical transformation.

Mineral foraging and etching by the fungus Talaromyces flavus to obtain structurally bound iron

Fungi possess remarkable abilities to acquire mineral nutrients through foraging for and weathering minerals, and these fungal behaviors are influenced by the physiochemical properties of the minerals. Here, we investigated the dynamic processes underlying these behaviors in the interactions between the common fungus Talaromyces flavus and two serpentine minerals (antigorite and lizardite) and quartz as a control. In the culture of T. flavus on solid Czapek medium with these minerals, we found that the elongation rates of T. flavus hyphae toward antigorite and lizardite particles were similar, and both were much higher than those toward quartz particles. After reaching the minerals, the growth of T. flavus was promoted significantly by antigorite and lizardite. Atomic force microscopy measurements uncovered the production of distinct dissolution channels with a depth of 29.34 ± 6.18 nm on the lizardite surface, but no local dissolution was found on the antigorite surface. Laser ablation multicollector inductively coupled plasma mass spectrometric analysis indicated that the total Fe in T. flavus hyphae grown on the surfaces of lizardite and antigorite flakes was ~8.3 and 2.3 times, respectively of those grown on the surface of medium. Whewellite was ubiquitously found at the hyphal tips and their vicinity during T. flavus interactions with lizardite. These results demonstrate that fungal hyphae have a remarkable capacity to sense and respond to minerals and to extract structurally bound inorganic nutrients, and highlight that the instant weathering of minerals by fungi represents an important but unassessed contributor to the short-term release of mineral nutrients and should be incorporated into biogeochemical models.

Creation of high-conductive filtration channels in core samples when simulating acid exposure at a filtering unit

Background. Hydrochloric acid treatment is currently one of the main methods used for recovering and improving the reservoir properties of bottom-hole formation zones. In the process of acid treatment, during the reaction of the acid composition and the rock, highly conductive filtration channels are formed. The structure and shape of such channels characterise the treatment efficiency. As a result, much research attention is currently paid to predicting the formation of filtration channels and changes in the filtration characteristics of reservoirs with different properties and types of pore space.Aim. To study the factors that directly affect the formation of dissolution channels in core samples when simulating hydrochloric acid treatment of the bottom-hole zone of carbonate reservoirs on a filtration unit. The main objectives are to determine the significance of these factors and to establish dependencies reflecting their effect on the efficiency of technologies aimed at stimulating oil inflow.Materials and methods. We used the results of filtration and X-ray tomographic studies on core samples taken from the scientific base of the “Geology and Development of Oil and Gas Fields” Scientific and Educational Centre. The collection of rock samples is represented by various deposits confined to the oil and gas complexes of the Perm Territory.Results. The conducted analysis allowed us to identify the effect of various factors on the formation of highly conductive filtration channels during acid treatment. These factors were found to include the lithological and mineralogical composition and initial filtration parameters of core samples, as well as the type of pore structure. Dependences that characterise the efficiency of acid treatment were determined.Conclusions. The obtained results can be used when developing measures for the intensification of oil production, taking into account the revealed factors.

Fracture orientation and fluid flow direction recognition in carbonates using diffusion-weighted nuclear magnetic resonance imaging: An example from Permian

The vertical permeability of carbonate reservoir rocks bearing complex, diagenetically altered pore systems may neglect fluid flow possibilities in remaining directions. Therefore, the main aim of this article was to present an alternative method for the potential examination of 3D fluid flow possibilities using diffusion-weighted NMR imaging (DWI). A cylindrical plug sample was withdrawn from the Permian Zechstein Limestone's drill core (West Poland). The plug was selected upon microscopic research, according to which it contained numerous, bendy dissolution channels formed after former foraminifer encrustations surrounding the bryozoans (ideal for DWI tests). Firstly, the diffusion coefficients (D) were calculated by comparing the signal components with (S) and without (S0) the diffusion weighting. The former was obtained under the presence of three, separately activated gradients, acting perpendicularly to each other (x, y, z). The diffusion coefficients were examined within xy, xz and yz planes (in 3 mm-thick slices) and utilized to construct 3D models, presenting their values in space. “Diffusion vectors” were constructed based on the average D values and helped to resolve channels' orientation. The spatially presented values of diffusion coefficients themselves allowed for drawing preliminary conclusions regarding the fluid flow possibilities. For comparison, this kind of information was supported by Kozeny-Carman's formula-based and nitrogen (N2) permeability derivation. In case of the former, the surface to volume ratio (S/V) was derived via PGSTE experiments, through the linear fitting to the normalized, time-dependent diffusion coefficients, in the short diffusion time regime. The effective NMR porosity (12.1%) was measured using a low-field, 0.05 T spectrometer with the echo time of 100 μs. The T2 cut-off value was assigned by comparing the 70 °C-dried and saturated sample's response. The potential fluid flow through the sample was found to be chiefly controlled by the z-oriented (nearly vertical) structures, with a significant support of the x-oriented pores, generally confirmed by the vertical permeability of the sample, ranging between 100 (N2) and 169 mD (DWI). It was also concluded that DWI allows for spatial, quantitative analysis of flow possibilities through fractured or highly dissolved carbonate samples. Despite their relative expensiveness, the 3D models of diffusion coefficients could be considered as an alternative or addition to the standard permeability tests in the case of complex pore systems. It also seems that combining DWI and X-ray microtomography in future work could help to bridge the gap between resolution limitations and dynamically resolved fluid mobility, which can be translated into pore system connectivity. Finally, the construction of diffusion vectors appeared to facilitate magnetic field gradient's direction calibration, which should depend on the trend of pore space elongation, thus giving more reliable, diffusion-based permeability values.

Thermo-mechanical treatment regulation of microstructure and comprehensive performance in Al-Zn-Mg-Cu alloy

The comprehensive performance of Al-Zn-Mg-Cu alloy can be significantly improved by a proposed novel thermo-mechanical treatment (NTMT). The influence of the NTMT on the properties and microstructure was investigated by tensile test, corrosion resistance test, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Results show that Al-Zn-Mg-Cu alloy treated by the NTMT can obtain an excellent combination of strength and ductility. The highest yield strength and ultimate tensile strength reached 643 MPa and 664 MPa respectively, and the elongation was 9.7%. Meanwhile, electrochemical corrosion resistance and intergranular corrosion resistance in the aluminum alloy can be improved after the NTMT. The mechanism of the excellent combination of strength and ductility is thought to be the synergistic effect of dislocations substructures, texture configuration, and nanoprecipitates. The improvement of intergranular corrosion resistance of the aluminum alloy is caused by changes in the micro-morphology of grain boundary precipitates after the NTMT, which can block anodic dissolution channels along grain boundaries to reduce the rate of anodic dissolution and avoid hydrogen embrittlement.

INFLUENCE OF ACID SOLUTIONS THICKENERS ON FILTRATION IN THE MODEL OF THE CARBONATE LAYER OF THE SEBORUKO OIL FIELD (REPUBLIC OF CUBA)

Background In the process of extracting oil from carbonate reservoirs, the permeability of the bottomhole formation zone (BFZ) strata decreases. The deterioration is due to the hydromechanical contamination of the filtering surface of the BFZ by mechanical impurities and hydrocarbon compounds, insoluble deposits, the loss of inorganic salts, the quality of opening of the productive layer, the increase in the saturation of BFZ and the decrease in phase permeability for oil. In order to increase the permeability of BFZ strata, various methods are used: mechanical, chemical and thermal. For carbonate collectors, the treatment of the BFZ with hydrochloric acid is one of the main methods for the intensification of oil production. The priority for the treatment of watered carbonate reservoirs is the creation of highly permeable channels with a wormhole structure. A large number of theoretical and experimental studies are devoted to the determination of parameters of effective acid exposure and the creation of conditions for wormholes formation in the carbonate reservoir. In recent years, acid thickeners are used to increase the acid treatment efficiency of fractured reservoirs. Increasing acidic solution viscosity allows increasing the penetration depth of the acid in the strata in the active state, thus forming channels of filtration of different geometry, which improve the permeability coefficient of BFZ. Aims and Objectives To carry out a comparative analysis of the acid exposure effectiveness with the use of acidic solutions thickeners and without thickeners. Estimate of the thickened acid filtration capacity in the strata and its dissolving ability according to the increase in the permeability coefficient of the sample from the Seboruko carbonate formation (Republic of Cuba). Results According to the research results, the increase in the permeability coefficient of the strata sample was determined when exposed to a traditional acid solution and thickened. The geometry of the formed dissolution channels was determined. Under the action of a conventional acid, a conical shape channel with low branching was formed, and when a thickened acid was applied to the sample, a branched channel formed with a wormhole geometry. Thus, this study shows that the treatment efficiency of carbonate formations increases with the addition of thickeners to acidic solutions. Thickeners increase the viscosity of the acidic solution, reducing the rate of acid reaction with carbonate minerals.

Multiscale modeling of CO2-induced carbonate dissolution: From core to meter scale

Reactive transport models can be used to predict the long-term storage capacity of CO2 in carbonate formations if the parameters that couple physical and chemical interactions can be calibrated from centimeters (laboratory-scale) to kilometers (field-scale). However, calibration across length scales is challenging in highly reactive and heterogeneous carbonate reservoirs. In this study we translated simulations of CO2-induced carbonate dissolution and transport processes of centimeter-sized cores to meter-sized rock blocks. The meter-sized rocks represent an intermediate scale between laboratory experiments and field demonstrations.We conducted brute-force, meter-sized simulations maintaining the same grid resolution used to successfully model rock heterogeneities and physical behaviors observed in laboratory experiments. To do this, we applied multi-point statistics to generate rock heterogeneity that honors characterization data, and adopted the reaction and transport parameters (e.g., reaction rate kinetics, porosity-permeability relationship) from models calibrated by core-flood experiments.We used the high-resolution simulation data sets to understand the interplay between carbonate reactivity, flow velocity, and rock heterogeneity on the development and consequences of dissolution fronts at a meter scale, and in particular how parameters that couple chemistry and transport change at variable length scales. The modeling results reveal scale dependence of porosity-permeability relationship and carbonate reactivity. We found that upscaling reactive transport processes from centimeters to meters requires: (1) Effective representation of initial rock heterogeneity and related preferential flow field. In more permeable rocks, several dissolution fingers or wormholes form and grow along the flow direction, competing for reactive solution and leading to fast breakthrough. In less permeable rocks, a single dissolution channel forms with a dramatic increase in lateral branching due to the resistance from low-permeability regions along the flow path. (2) Higher exponential values for porosity-permeability power-law correlations. The value of the power parameter n increases by about two times as we scale the dissolution process from centimeters to meters. (3) Lower carbonate reaction rates. Carbonate reaction rates obtained for meter-sized models are about ten times slower than those for the smaller core-flood experiments. We also used the high-resolution meter-sized simulations to upscale carbonate dissolution by coarsening the representative elementary volumes (or grids). The calibrated reactive transport parameters are consistent with those derived from scaling analysis.The upscaling analysis developed in this study improves our understanding of carbonate dissolution (especially wormhole development) and associated reactive transport across time and length scales, providing a useful basis for establishing a direct relationship between core-scale and field-scale models. Additional work is needed to see if relationships developed from single-phase reactive transport processes shown in this work are directly applicable to in-situ reservoir multiphase flow conditions arising from geological storage of CO2.