Properties Of Carbonate Rocks Research Articles

Exploring in-situ combustion effects on reservoir properties of heavy oil carbonate reservoir

Laboratory modeling of in-situ combustion is crucial for understanding the potential success of field trials in thermal enhanced oil recovery and is a vital precursor to scaling the technology for field applications. The high combustion temperatures, reaching up to 480°C, induce significant petrophysical alterations of the rock, an often overlooked aspect in thermal EOR projects. Quantifying these changes is essential for potentially repurposing thermally treated, depleted reservoirs for CO2 storage.In this study, we depart from conventional combustion experiments that use crushed core, opting instead to analyze the thermal effects on reservoir properties of carbonate rocks using consolidated samples. This technique maintains the intrinsic porosity and permeability, revealing combustion's impact on porosity and mineralogical alterations, with a comparative analysis of these properties pre- and post-combustion. We characterize porosity and pore geometry evolution using low-field nuclear magnetic resonance, X-ray micro-computed tomography, and low-temperature nitrogen adsorption. Mineral composition of the rock and grain-pore scale alterations are analyzed by scanning electron microscopy and X-ray diffraction.The analysis shows a significant increase in carbonate rocks’ porosity, pore size and mineral alterations, and a transition from mixed-wet to a strongly water-wet state. Total porosity of rock samples increased in average for 15%–20%, and formation of new pores is registered at the scale of 1–30 μm size. High-temperature exposure results in the calcite and dolomite decomposition, calcite dissolution and formation of new minerals – anhydrite and fluorite. Increased microporosity and the shift to strongly water-wet rock state improve the prospects for capillary and residual CO2 trapping with greater capacity. Consequently, these findings highlight the importance of laboratory in-situ combustion modeling on consolidated rock over tests that use crushed core, and indicate that depleted combustion stimulated reservoirs may prove to be viable candidates for CO2 storage.

Augmenting X-ray micro-CT data with MICP data for high resolution pore-scale simulations a of flow properties of carbonate rocks

Pore-scale modeling is essential in understanding and predicting flow and transport properties of rocks. Generally, pore-scale modeling is dependent on imaging technologies such as Micro Computed Tomography (micro-CT), which provides visual confirmation into the pore microstructures of rocks at a representative scale. However, this technique is limited in the ability to provide high resolution images showing the pore-throats connecting pore bodies. Pore scale simulations of flow and transport properties of rocks are generally done on a single 3D pore microstructure image. As such, the simulated properties are only representative of the simulated pore-scale rock volume. These are the technological and computational limitations which we address here by using a stochastic pore-scale simulation approach. This approach consists of constructing hundreds of 3D pore microstructures of the same pore size distribution and overall porosity but different pore connectivity. The construction of the 3D pore microstructures incorporates the use of Mercury Injection Capillary Pressure (MICP) data to account for pore throat size distribution, and micro-CT images to account for pore body size distribution. The approach requires a small micro-CT image volume (7-19 mm3) to reveal key pore microstructural features that control flow and transport properties of highly heterogeneous rocks at the core-scale. Four carbonate rock samples were used to test the proposed approach. Permeability calculations from the introduced approach were validated by comparing laboratory measured permeability of rock cores and permeability estimated using five well-known core-scale empirical model equations. The results show that accounting for the stochastic connectivity of pores results in a probabilistic distribution of flow properties which can be used to upscale pore-scale simulated flow properties to the core-scale. The use of the introduced stochastic pore-scale simulation approach is more beneficial when there is a higher degree of heterogeneity in pore size distribution. This is shown to be the case with permeability and hydraulic tortuosity which are key controls of flow and transport processes in rocks.

Experimental study of the mechanical properties and microscopic mechanism of carbonate rock subjected to high-temperature acid stimulation

To clarify the effect of acidification on the stress distribution around wells in deep carbonate reservoirs, it is necessary to study the changes in mechanical properties of carbonate rocks under different acid stimulation conditions. In this paper, a novel acid stimulation experiment is designed to investigate the mechanical properties of the carbonate rock through a self-built acid-rock reaction experimental system and triaxial rock mechanics apparatus. The changes in the core mechanical parameters with the acid stimulation time at different acid stimulation temperatures are obtained. Based on stress relaxation, the rheological properties of the core after acid stimulation are explored. The internal structural damage mechanism of the core after high-temperature acid stimulation is as analysed via by SEM. This research finds that the elastic modulus and compressive strength of the core exponentially decrease with increasing of acid stimulation time. The higher the acid stimulation temperature, the faster the core damage rate changes, and the earlier the damage amount tends to stabilize. The core has an obvious rheological mechanical property after acid stimulation. The stress relaxation amplitude exponentially increases with increasing strain during loading. Interestingly, when the acid stimulation temperature reaches 120 °C, a sudden stress drop occurs during stress relaxation. It is also found that the stress relaxation amplitude increases exponentially with increasing acid stimulation time under the same strain level. Moreover, a higher acid stimulation temperature results in greater stress relaxation under a constant acid stimulation time and strain. In addition, the core skeleton is destroyed through acid stimulation, and many complex damaged structures appear inside the core, which leads to a decrease in the core strength, which is the reason for the enhanced rheological characteristics of the rock after acid stimulation. These results can provide insights into the mechanical response mechanism of wellbore acidification.

Comparative analysis of the application of the Lichtenecker and Lichtenecker – Asaad models for the theoretical assessment of the rocks effective thermal conductivity

The analysis of the conditions and limitations of using the Lichtenecker and Lichtenecker – Asaad models for predictive assessment of the effective thermal conductivity of rocks have been carried out. For a comparative assessment, experimental data on the thermal conductivity of carbonate rocks (limestone, dolomite) from the South-West German Molasse basin, the Chicxulub impact structure (Yucatan Peninsula, Mexico) and the Bazhenov Formation (Western Siberia, Russia) have been considered. Comparative assessments were carried out based on the use of correlation relationships obtained by the authors of experimental works as a result of regression analysis of the petrophysical properties of carbonate rocks. The results of the computational and analytical assessment have shown that the highest accuracy of thermal conductivity prediction using the Lichtenecker – Asaad model can be achieved if experimental data are available that provide a highly accurate determination of the structure factor. The features of the influence of various parameters of the Lichtenecker – Asaad model on the error in estimating the effective thermal conductivity of the rock have been studied. It has been established that at the stage of pre-design research, the Lichtenecker model can be used for a fairly correct theoretical assessment of the thermal conductivity of water-saturated, slightly porous rocks.

Investigating the analytical relationship between pore geometry and other pore space properties in carbonate rocks

Although pore geometry plays an important role in carbonate rock physics modelling, few studies have been carried out on its analytic relationship with other pore space properties such as pore space stiffness. We propose an analytical workflow based on the differential effective medium (DEM) to estimate the elastic properties of carbonate rocks. The validity of our results is then cross-checked with the Xu and Payne model on a real carbonate dataset. This workflow establishes a direct and quantitative link between the pore geometry of carbonate rock and its other pore space properties such as the Biot coefficient and pore space stiffness. This relationship can, furthermore, be utilized in defining rock physics templates (RPTs) to investigate the role of pore geometry on the rock elastic properties. Furthermore, we extended the Biot–Gassmann–Krief (BGK) model through our proposed workflow by establishing a theoretical framework to relate the main components of the BGK model to the pore geometry usually estimated in the laboratory or empirically. This can help to investigate the impact of fluid substitution on each of these main components. Our investigation suggests that the higher the Biot and Gassmann coefficients, the more sensitive the rock is to fluid substitution. Moreover, this analytical workflow has been employed to examine the role of selecting different rotational spheroids (i.e. oblate and prolate) on the modelled velocities. Our results show that the modelled velocities depend on this selection in such a way that prolate pores are less sensitive to the variations in their pore aspect ratio compared with oblate pores.

Thermal influence of sills on the chemostratigraphy and mineralogy of carbonate rocks from the Irati formation, Paraná basin

Carbonate rocks and shales are prominent lithologies in atypical petroleum systems. In these settings, the thermal effects of sills play a crucial role leading to mineralogical and geochemical alteration in the host rocks and in the maturation process of the associated organic matter. While extensive research has been conducted on shales within atypical systems, the understanding of the petrophysical and geochemical properties of carbonate rocks remains uncertain in numerous basins. In this study, we selected the Irati Formation (Paraná Basin) to investigate the thermal effects caused by diabase sills on the chemostratigraphic and lithologic features of carbonate rocks. We conducted mineralogical analyses, examined oxygen and carbon stable isotopes, and analyzed inorganic geochemical data from two distinct sections of the formation: one with diabase sills intruding the sequence and the other without identified intrusions in the area. Widespread recrystallization of minerals is observed throughout the studied rock wall, resulting in the formation of marbles and calc-silicate rocks, where dolomite is mainly replaced by calcite. The carbonate bank, located 5–7 m below the sill, is marked by brucite and chrysotile, while the rocks up to 1 m below and above the intrusion show the presence of tremolite, diopside, and antigorite. Substantial variations in elemental data are observed, including a significant increase in SiO2 and K2O below the sill, as well as elevated MgO levels within the carbonate bank. Additionally, stable isotopic signatures of oxygen and carbon display a negative shift influenced by decarbonation and intense fluid flow controlling the reactions. The contact aureole resulting from the intrusion is identified at least 7 m above and below the sill based on mineralogical and inorganic geochemical data.

Numerical Simulation of Electrical Properties of Carbonate Reservoirs Using Digital Rocks

Rock electrical experiments are essential means of researching the conductive properties of rocks and are fundamental to interpreting resistivity logging. Carbonate rocks have more complex pore structures than sandstone, which results in more complex conductive properties. However, conducting experiments on representative rock samples from carbonate reservoirs is difficult, making it challenging to study the micro factors affecting electrical properties. Therefore, researching the conductive properties of carbonate rocks is difficult. To address this, in this paper, three-dimensional (3D) digital rock models with different porosities are generated, and conductive simulations are carried out on these models using the finite element method (FEM). Firstly, a micro-computed tomography (μ-CT) 3D image of a carbonate rock is obtained. Secondly, mathematical morphology-based methods are used on the μ-CT image to generate cores with varying porosities and fluid distributions. Then, the electrical properties are simulated using the FEM method, and the results are analyzed. The results reveal that the formation factor of the reservoir is mainly influenced by the shape and structure of the pores. The Archie equation is more suitable for carbonate reservoirs with water saturation levels greater than 60%. The wettability of the rock can alter the distribution of fluid in the reservoir space under different water saturation conditions. In pure water-wet rocks, the water phase mainly occupies small pores, while the oil phase occupies larger pores. As a result, compared to pure oil-wet rocks, water-wet rocks have more conductive channels and better conductivity. Therefore, it is important to determine the wettability of the rock when calculating water saturation using the Archie equation. The saturation index value of water-wet carbonate rock is about 2, while that of oil-wet rock is around 3–4. This research lays a foundation for studying the electrical conductivity of carbonate reservoirs using digital rocks.

Experimental study of the influence of the degree of saturation on physical and mechanical properties of carbonate rocks in the Jurassic Tuwaiq Mountain Formation (Saudi Arabia)

Understanding the impact of saturation levels on the mechanical and physical properties of limestone and marly limestone deposits is very critical for geotechnical and mechanical studies in the energy and construction industries. This research aims to link the strength characteristics of carbonate rock (limestone and marly limestone) with physical and mechanical properties at saturation levels of 0, 0.25, 50%, 75% and 100%. To complete this work, 100 samples were collected and analysed employing an orderly research facility testing. According to experimental findings, the strength and durability of the limestone and marly limestone declined with rising saturation levels, while the P‐wave velocity Vp showed an upward trend from 0% to 100%. The uniaxial compressive strength (UCS) of the limestones employed in this research significantly decreased (up to 36%) between the dry and saturated stages for limestone samples. While marly limestone, there is a drop (up to 32%) in UCS. The findings of a straightforward regression study show that it is possible to predict the strength properties of rocks directly from various saturation levels. Additionally, a number of various estimate connections were created utilizing independent variables like Vp and saturation level. The experimental outcomes showed that Vp showed an upward tendency with an increasing degree of saturation. The deformability and strength parameters of the samples declined with increasing saturation degrees. The elastic modulus and strength both decrease as clay and water amounts increase. The quality index (QI) number for limestone is 45, which indicates that it has extremely good quality and can be used as a building material at 0% saturation. At 25%, 50%, 75% and 100% saturation, it is evaluated at 43, 40, 33 and 31, respectively. The marly limestone samples have high quality at 0% (QI = 36), but poor quality at 25%, 50%, 75% and 100% saturation (QI = 33, 30, 27 and 23). This research provides a better understanding of the relationship between saturation levels and the mechanical, physical and depositional characteristics of marly limestone and limestone, which might be helpful in industrial studies. Advance examination of distinctive sorts of carbonate rocks can be more supportive of broadening our understanding of this idea.

Linking elastic properties of various carbonate rocks to their microstructure by coupling nanoindentation and SEM-EDS

Characterizing the elastic properties of carbonate rocks is a crucial parameter in geotechnical and reservoir engineering. However, these natural materials exhibit a wide variety of microstructures, even when they belong to the same facies, leading to a significant dispersion of their elastic properties. We focus on six selected rocks, for which global porosity is shown to be not the only factor controlling elasticity. This study aims to understand which additional microstructural parameters impact the rocks’ elastic properties by means of nanoindentation coupled with microscopic observations and analyses. A complete procedure is developed to combine nanoindentation measurements with Scanning Electron Microscopy (SEM) - Energy Dispersive Spectroscopy (EDS) analyses to identify volume fractions and elastic properties of the mineral phases in each rock. The macroscopic Young’s moduli are estimated using analytical homogenization and are finally compared with the values obtained from macroscopic experiments. We find that the mineral composition and mechanical properties are critical parameters involved in carbonate rocks’ elastic behavior. However, while capturing the overall trend, homogenization is found to overestimate Young’s modulus. A discussion on the possible impact of cracks and non-spherical pores is conducted to explain this overestimation. Besides, microindentation tests are carried out to explore higher scales, which remain too small, however, to characterize the macroscopic Young’s modulus of carbonate rocks.

Petrophysical characteristics of carbonate rocks of organogenic structures of late devonian age (Voronezh horizon) in Orenburg region

Background. The carbonate rocks represented by organogenic structures of the Voronezh horizon (D3vr) at the Volostnovsky, Vostochno-Volostnovsky and Yuzhno-Volostnovsky fields in the northern part of the East-Orenburg oil and gas bearing region of the Volga-Ural oil and gas bearing province are studied.Aim. To establish a connection between the structures of lithological types of carbonate rocks and their reservoir properties within the organogenic structures of the Voronezh horizon at the Volostnovsky, Vostochno-Volostnovsky and Yuzhno-Volostnovsky deposits.Materials and methods. Stock materials on the geological structure of the northern part of the Vostochno-Orenburg district were used. Histograms of the distribution of porosity and permeability values were constructed. A petrophysical model was proposed followed by its verification and analysis.Results. Data on the filtration-capacitance properties of carbonate rocks were analyzed. Histograms reflecting changes in porosity and permeability for individual zones of organogenic structures were constructed. A petrophysical model describing the relationship between the lithological characteristics of rocks of the selected zones and their reservoir properties was proposed. The model was verified based on the data on the Volostnovsky site and neighboring deposits.Conclusion. The carbonate reservoirs of the Voronezh horizon are characterized by a complex structure with degraded reservoir properties. This information should be taken into account when developing deposits to ensure an increase in geological oil reserves and an extended service life of wells.

Dissolution and Deformation Characteristics of Limestones Containing Different Calcite and Dolomite Content Induced by CO2‐Water‐Rock Interaction

AbstractTo investigate the impacts of mineral composition on physical and mechanical properties of carbonate rocks, limestone specimens containing different contents in calcite and dolomite are selected to perform CO2‐water‐rock reaction experiments. The X‐ray Diffraction (XRD) and Nuclear Magnetic Resonance (NMR) are carried out to examine the change characteristics of mineral dissolution and pore structure after reaction. The core flooding experiments with Fiber Bragg gratings are implemented to examine the stress sensitivity of carbonate rocks. The results show that the limestones containing pure calcite are more susceptible to acid dissolution compared to limestone containing impure dolomite. The calcite content in pure limestone decreases as the reaction undergoes. The dissolution of dolomite leads to the formation of calcite in impure limestone. Calcite dissolution leads to the formation of macropore and flow channels in pure limestone, while the effects of impure dolomite in impure limestone results in mesopore formation. When confining pressure is lower than 12 MPa, pure limestones demonstrate higher strain sensitivity coefficients compared to impure limestone containing dolomite after reaction. When confining pressure exceeds 12 MPa, the strain sensitivity coefficients of both pure and impure limestones become almost equal.

Facies control of the reservoir properties distribution in the carbonate rocks of epicontinental platforms

Background: It is difficult to imagine the modern world without hydrocarbon raw materials. Oil and gas production is not only the basis of a powerful fuel and energy complex but also the main source of foreign exchange earnings and a leading component of the budget for many countries. Carbonate rocks strike the imagination by their diversity, but at the same time they cause many problems: they are extremely variable from the point of view of reservoirs. The main problems of productive carbonate members are heterogeneity; distribution of reservoirs in the form of lenses with different configurations and strikes as well as hydrodynamic connections between the lenses is often chaotic. All of the listed problems of carbonate reservoirs are associated with secondary processes.
 Aim: The project aims to assess the reservoir properties of carbonate rocks from the point of view of facies zoning on the example of Ovinparm rocks.
 Materials and methods: In this work, we used: rock material from 8 wells (475 m), 14 wells with hydrodynamic data and a 3D seismic cube.
 Results: Based on the data obtained, the principles of facies diagnostics of the epicontinental platform deposits of were developed, facies of epicontinental platforms were identified, rock material was linked to logging data, a facies model and a facies map were built, a sequence stratigraphic analysis was carried out, and the regularity of the selective nature of the manifestation of secondary processes depending on the characteristics of the conditions for the accumulation of facies of epicontinental platforms was revealed, promising zones were identified taking into account the facies zoning and the intensity of the passage of secondary processes in the study area.
 Conclusion: By understanding how the reservoir properties are related to carbonate depositional conditions, we can predict the most interesting zones and determine the main secondary processes leading to such improved or degraded reservoir properties.

Exploring carbonate rock wettability across scales: Role of (bio)minerals

HypothesisThe wettability of carbonate rocks is expected to be affected by the organic components of biominerals which are complex, nanostructured organo-mineral assemblages. Elucidating the nanoscale mechanisms driving the wettability of solid surfaces will enable a better understanding of the role of biominerals in the wetting properties of carbonate rocks to control various geological, environmental and industrial processes. ExperimentsUsing Atomic Force Microscopy and Spectroscopy (AFM/AFS) we probed the wettability properties of carbonate rocks with different amounts of organic material. The adhesion properties of two types of limestones were determined in liquid environments at different length scales (nm to mm) using functionalized tips with different chemical groups to determine the extent of surface hydrophobic and hydrophilic organo-mineral interactions. FindingsWe observed homogeneous hydrophobic areas at length scales below < 5 µm. The origin of this hydrophobicity is linked to the presence of organics, whose amount and spatial distribution depend on the rock composition. Specifically, our results reveal that the biogenic vs non-biogenic origin of the mineral grains is the main rock property controlling the wettability of the solid surface. Overall, our methodology offers a multi-scale approach to unravel the role that organic moieties and biominerals play in controlling the wettability of rock-water interfaces.

Mechanical and hydraulic properties of carbonate rock: The critical role of porosity

Carbonate rocks are extensively used in civil infrastructure and play a critical role in geoenergy geoengineering, either as hydrocarbon reservoirs or potential repositories for CO 2 geological storage. Carbonate genesis and diagenetic overprint determine the properties of carbonate rocks. This study combines recent data gathered from Madison Limestone and an extensive dataset compiled from published sources to analyze the hydraulic and mechanical properties of limestone carbonate rocks. Physical models and data analyses recognize the inherently granular genesis of carbonate rocks and explain the strong dependency of physical properties on porosity. The asymptotically-correct power model in terms of (1- ϕ / ϕ ∗) α is a good approximation to global trends of unconfined stiffness E and unconfined compressive strength UCS , cohesive intercept in Mohr-Coulomb failure envelopes, and the brittle-to-ductile transition stress. This power model is the analytical solution for the mechanical properties of percolating granular structures. We adopted a limiting granular porosity ϕ ∗ = 0.5 for all models, which was consistent with the loosest packing of monosize spheres. The fitted power model has exponent ( α = 2) in agreement with percolation theory and highlights the sensitivity of mechanical properties to porosity. Data and models confirm a porosity-independent ratio between unconfined stiffness and strength, and the ratio follows a log-normal distribution with mean ( E/UCS ) ≈ 300. The high angle of internal shear strength measured for carbonate rocks reflects delayed contact failure with increased confinement, and it is not sensitive to porosity. Permeability spans more than six orders of magnitude. Grain size controls pore size and determines the reference permeability k ∗ at the limiting porosity ϕ ∗ = 0.5. For a given grain size from fine to coarse-grained dominant carbonates, permeability is very sensitive to changes in porosity, suggesting preferential changes in the internal pore network during compaction.

Using a Synthetic Borehole Model to Determine the Pore Aspect Ratio Dependence of Velocities from Acoustic Well-Logging Data

Revealing the elastic wave properties of carbonate rocks with complex pore structures and improving the reliability of carbonate reservoir descriptions have always been a global challenge in the field of carbonate geophysical exploration. In this study, we established a synthetic borehole model by selecting different particle sizes of cement, carbonate cuttings, and micro-silicon as the matrix, and silicon disks as the pores in carbonate rocks. We conducted four sets of low-porosity (0-3%) borehole models with different pore aspect ratios (ARs) and measured the P- and S-wave velocities ( V P and V S ) at the well-logging scale obtained from an acoustic logging system with one source and two receivers. The results indicate that the relationship between velocities and porosities in these borehole models follows a linear relation, with the pore AR significantly influencing the velocities at any given porosity. The velocity variation caused by pore AR reaches 560 m/s and 410 m/s at 3% porosity for the P-wave and S-wave within the AR range of 0.017-0.13. The theoretical DEM models provide a high and broad estimation of V P and V S at the well-logging scale in our measurement. They could perform better in fractured formation than in dissolved porous formation in carbonate reservoirs. The linear relation of V P and V S is independent of the pore AR and is effective for both fractured and dissolved porous formations. The change of V P / V S in different pore AR is more responsive to porosity and nonlinear dependent on the pore AR. The relationship between the defined normalized V P and V S indicates the pore AR has a more significant effect on V S than V P in our model. The constructed borehole models provide a unique opportunity for evaluating the availability of rock physics models at an acoustic logging scale. The study’s findings have significant implications for improving the reliability of carbonate reservoir descriptions and enhancing the accuracy of geophysical exploration in carbonate rocks with complex pore structures.

The Role of Clay Minerals in the Petrophysical Properties of Carbonate Rocks from the Cotinguiba Formation (Sergipe-Alagoas Basin, Brazil)

The Cotinguiba Formation, of Cretaceous age, located in the Sergipe-Alagoas Basin, Brazilian Northeastern coast, comprises carbonate breccias and carbonate shales interlayered with fine to coarse clastic sediments (mudstones, marls), deposited in the maximum sea level episode during the Cenomanian-Coniacian transgressive event. Core sections were selected from the formation materials, from which 20 plugs were obtained, representing the main carbonate horizons present in Cotinguiba Formation. The selected samples were analyzed with petrophysical and physical methods (air porosimetry, mercury injection, and acoustic wave velocity) and mineralogical methods (optical microscopic analysis, x-ray diffraction (XRD) and scanning electron microscopy (SEM)). Results showed that the clay minerals’ content in the carbonates varies from 2% to 20%, with illites/micas and interstratified illite/smectite as the most abundant clay minerals, containing also traces of chlorite and kaolinite, and presence of palygorskite varying from trace to 8%. According to the percentages of calcite, dolomite, and siliciclastic + clay minerals obtained in the XRD analysis, the carbonates were lithologically identified as limestone, impure limestone, dolomitic limestone, impure dolomitic limestone, impure calcitic dolostone, and impure dolostone. The rocks have a good porosity, varying from 6% to 20% in the carbonates, predominantly related to microporosity. Clay minerals’ content influenced the grain density and acoustic properties, albeit not having the same role with porosity and permeability. SEM images, though, show that they have an important role in microporosity. Grain density variation related to phyllosilicate content was observed more significantly in impure limestones with a clay content higher than 8%, and a more pronounced decrease was observed in samples with palygorskite. Microporosity is the main factor for reducing Vp and Vs velocities in those carbonates, with values not exceeding 5341 m/s for Vp and 3026 m/s for Vs but, when clay content is higher than 4%, the Vp and Vs wave velocities do not exceed 4100 m/s and 2480 m/s, respectively. Therefore, the research allowed evaluating the influence of mineralogical and textural properties in the petrophysical properties of Cotinguiba Formation.

Impact of diagenesis and pore aspects on the petrophysical and elastic properties of carbonate rocks from southern Lebanon

Impact of diagenesis and pore aspects on the petrophysical and elastic properties of carbonate rocks from southern Lebanon

Comparative Study on Hydraulic and Electrical Transport Properties of Carbonate Rocks Based on Rock Typing

The permeability (k) and electrical resistivity of carbonate rocks with similar porosity (ϕ) may exhibit multiple orders of magnitude differences due to the diversity of pore types and the multiscale characteristics of pore size. As a result, traditional permeability-porosity and resistivity-porosity models usually have strong limitations when applied to carbonate reservoirs, let alone evaluate permeability from electrical resistivity. In this study, a new rock typing parameter named relative transport quality index (RTQI) is proposed to categorize rock samples using permeability and formation resistivity factor (F). Samples with similar RTQI values tend to have similar relative capacities of hydraulic and electrical transport. Subsequently, the classification performance and microscopic differences between RTQI and other rock typing indices including flow zone indicator (FZI), current zone indicator (CZI), and electrical quality index (EQI) were compared and discussed based on the reported petrophysical data of 316 carbonate rock samples. The results indicate that the RTQI method significantly reduces the scattering of k and F by dividing the samples into distinct groups, and each RTQI group corresponds to a special k–F correlation with a high determination coefficient. However, the number of rock types identified by different rock typing indices is different, meaning that samples belonging to the same rock type in the k–F plot of the RTQI method may be classified into different rock types in the k–ϕ plot of the FZI method and the F–ϕ plots of the CZI and EQI methods, which may be caused by the microscopic differences of rock typing parameters. Different from the strong dependence of FZI on hydraulic tortuosity and CZI/EQI on electrical tortuosity, RTQI is mainly dominated by the ratio of electrical tortuosity to hydraulic tortuosity. Additionally, in view of the good correlation between petrophysical data after rock typing, two workflows are proposed to estimate the hydraulic and electrical parameters of carbonate rocks from log data.

Hydrothermal silicification and hypogene dissolution of an exhumed Neoproterozoic carbonate sequence in Brazil: Insights from fluid inclusion microthermometry and silicon‐oxygen isotopes

AbstractHypogene dissolution‐precipitation processes strongly affect the petrophysical properties of carbonate rocks and fluid migration pathways in sedimentary basins. In many deep carbonate reservoirs, hypogene cavernous voids are often associated with silicified horizons. The diagenesis of silica in carbonate sequences is still a poorly‐investigated research topic. Studies exploring the complexity of silica dissolution‐precipitation patterns in hypogene cave analogues are therefore fundamental to unravel the diagenetic and speleogenetic processes that may affect this kind of reservoir. In this work, we investigated an exhumed and silicified Neoproterozoic carbonate sequence in Brazil hosting a 1.4 km‐long cave. Quartz mineralization and silicified textures were analyzed with a multidisciplinary approach combining petrography, fluid inclusion microthermometry, silicon‐oxygen stable isotope analyses and U‐Th‐Pb dating of monazite crystals. We found that an early silicification event caused the replacement of the dolostone layers with micro‐crystalline quartz forming chert nodules. This event was likely associated with mixing fluids (ancient Neoproterozoic seawater and hydrothermal solutions sourced from the underlying Mesoproterozoic basement) at relatively low temperatures (ca. 50–100°C) and shallow depth. After the tectonic deformation produced by the Brasiliano orogeny, silica dissolution was promoted by high temperature and alkaline hydrothermal solutions rising from the quartzite basement along deep‐rooted structures. Hypogene hydrothermal alteration promoted the dissolution of the cherty layers and the precipitation of chalcedony and megaquartz. Homogenization temperatures from primary fluid inclusions in megaquartz cement indicate minimum formation temperatures of 165–210°C. Similar temperature estimates (110–200°C) were obtained from the δ30Si and δ18O isotope systematics of quartz precipitated from hydrothermal solutions. The dissolved salts in the fluid inclusions were evaluated as NaCl + CaCl2 from microthermometric data combined with cryogenic Raman spectroscopy, corresponding to salinity ranging between 17 and 25 wt.%. No reliable age constraints for hydrothermal silica dissolution‐precipitation phases were obtained from monazite U‐Th‐Pb dating. However, our results, interpreted in the regional context of the São Francisco Craton, suggest that the Cambrian tectono‐thermal events could have been amongst the possible drivers for this hypogene process in the basin.

Effect of long-term infiltration on porosity-permeability evolution in carbonate rocks: An online NMR coupling penetration test

Investigating the porosity–permeability evolution in carbonate rocks during long-term infiltration is a challenging hydrodynamic problem in rock engineering (hydroelectric engineering, CO2 geological storage, and oil/gas development). In general, we consider the evolution of porosity and permeability to be positively correlated. However, the complex pore structures in carbonate rocks include multiscale pore-fracture structures, such as intergranular pores, dissolution pores, and even fractures. Although the permeability levels of carbonate rocks are similar, the evolution of the pore structure is highly variable. Here, we use the “rock seepage coupled nuclear magnetic resonance (NMR) online analysis and imaging system” platform to conduct a long-term penetration test in carbonate rocks under the coupling of stress and reactive fluid. The variation in the multiscale pore volume was quantitatively characterized by the inversion of the T2 spectrum. Subsequently, in combination with permeability measurement, the permeability-porosity evolution during long-term infiltration can be roughly divided into three stages. The experimental results show that the evolution of permeability exhibits noticeable stress sensitivity at the initial stage of the test (0–156 h). Since the compression of the pore volume is the dominant mechanism, the permeability decreases with porosity at this stage. A negative correlation between the evolution of porosity and permeability was observed during the subsequent stage of the test (156–400 h). This phenomenon may be attributed to mineral dissolution at this stage, causing the cementation to be exfoliated from the pore surface and to re-precipitate at the pore throat, locally blocking the seepage channels. As the test continued (400–720 h), mineral dissolution transitioned from micropores to mesopores and macropores, with a synergistic increase in porosity and permeability. These results revealed the competing mechanism of stress compression and mineral dissolution during long-term infiltration, providing an accurate prediction of the seepage properties in carbonate rocks.