Permian Carbonate Rocks Research Articles

On the Interrelationship of Electrical and Hydraulic Conductivities in Carbonate Rocks

Carbonate reservoir exhibits "low-resistivity and (ultra) low-permeability" anomaly due to complex pore structure. It means that hydraulic conductivity, closely related to permeability (K), is not simply equivalent to electrical conductivity in this case. As a result, the conventionally well-established permeability-resistivity relationship does not apply to carbonate reservoirs any longer, and this poses a big challenge for the interpretation and evaluation of carbonate reservoirs. In this study, we found that the relationship between permeability and formation factor is non-monotonic and scattering through direct-current (DC) resistivity and nuclear magnetic resonance (NMR) experiments, and this means that the hydraulic conductivity is not well correlated with the electrical conductivity in carbonate rocks as one expects. There are the two factors, the volume fraction ratio of macro- and micro-pores, and the number of dolomitization-formed micropores, predominantly control hydraulic conductivity and electric conductivity, respectively. Moreover, the non-uniform distribution of intercrystalline pore networks will significantly reduce hydraulic conductivity. To more accurately estimate the permeability of carbonate rocks, the new permeability model based on NMR spectra rather than formation factor has been proposed. In this model, the characteristic relaxation time () was defined, which is associated with the macropores that contribute most significantly to permeability and can be used to distinguish between macro- and micro- pores. This model demonstrates a minimal average absolute logarithmic deviation () and exhibits generalizability across various carbonate rocks. The study addresses the mechanism analysis of "low-resistivity and (ultra) low-permeability" phenomenon in carbonate reservoir, and has potential to facilitate the interpretation and evaluation of carbonate reservoir.

The Vienna Basin: Petroleum Systems, Storage and Geothermal Potential

The Vienna Basin is located in the transition zone between the Alps and the Carpathians. Its tectonostratigraphic structure is complex, and includes from base to top crystalline basement, autochthonous Mesozoic sediments, Cenozoic foreland basin deposits, the Alpine nappe system and a thick Neogene basin fill. The Vienna Basin area hosts one of the major hydrocarbon provinces of Central Europe, but also a significant geothermal potential. The petroleum system is mainly based on the Upper Jurassic Mikulov source rock in the authochthonous succession, which became mature during nappe stacking and Neogene basin subsidence. Migration along faults and via basement highs filled oil and gas reservoirs in all tectonostratigraphic units. Despite a 100-year-long exploration history, new discoveries are still being made. Two geothermal systems are known in the Vienna Basin. One system is based on hot fluids ascending along the major faults in the southern part of the basin and has been used for balenological purposes since Roman times. The other is based on deeply buried, highly permeable carbonate rocks in the Alpine wedge (e.g. Hauptdolomite Formation) and clastic reservoirs in the Neogene basin fill. Current projects aim to supply the city of Vienna with geothermal heat from Lower Miocene fluvial conglomerates.

Soil-forming accumulation of heavy metals in geological high background areas: Constraints of structure, lithology, and overlying soil geochemistry

Heavy metals (HMs) are often abnormally enriched in soils in geologically high background areas, posing a serious threat to the ecosystem and human health. However, the material origin of HMs in overlying soils is unclear. In this paper, we studied the accumulation control and formation process of HMs (Cd, Hg, As, Pb, Cr, Cu, Ni, and Zn) in soils overlying bedrock from different geological periods in a typical anticline structure area in southwest of Guizhou province. The results revealed that the content of HMs in the bedrock were ranked as Carboniferous carbonate rocks < Permian carbonate rocks < Triassic clastic rocks, while the opposite trend was observed for the HMs content in the overlying soil. The slopes of the EF-Q values of HMs in the soils overlying the Carboniferous, Permian, and Triassic rocks were 0.003, 0.007, and 0.83, respectively, which suggests that HMs enrichment or leaching was more complicated in the process of carbonate weathering soil-forming, whereas it was mainly in situ weathering during the process of clastic weathering soil-forming. The soils overlying the bedrock of the three geological periods experienced similar weathering processes and had the same source of soil-forming matrices, as it was observed that their Fe2O3-Al2O3, Hf-Zr, Nb-Ta, Th/Sc-Zr/Sc and Y/Ho-Y contents were significantly correlated (with R2 values of 0.80, 0.94, 0.72, 0.60, and 0.76, respectively) with very similar REE assignment patterns. The results corroborated the causes of the high background geological heterogeneity of HMs in the overlying soils in karst areas and supported the theory of the source of weathering soil-forming materials from carbonate rocks in karst areas.

Geochemical aspects on formation processes of vein-hosted pyrite in the Carboniferous-Permian rocks of the Karavanke/Karawanken tunnel, northwestern Slovenia

Abstract The Carboniferous-Permian organic-rich shale in the Karavanke/Karawanken tunnel exhibits extensive pyrite mineralization in the form of veins and impregnations. Significant thermal alteration, induced by the overlying Mesozoic rock sequences, led to the decomposition of organic matter into methane, which subsequently acted as a sulfate-reducing agent. The primary source of sulfate were the evaporitic layers hosted within the Paleozoic strata. Reduced sulfur species necessary for pyrite formation were produced through methane-mediated thermochemical sulfate reduction, catalyzed by specific cations (Ni2+, Fe2+) and clay minerals (montmorillonite). The mineralizing fluids originated from the expulsion of interstitial water and possibly from dehydration reactions related to the gypsum-to-anhydrite phase transition. The geochemical data indicate that the metals originated from the surrounding sedimentary rocks. These mineralizing fluids were characterized by low temperatures (&lt; 300–350 °C), moderate reducing conditions, and low chlorinity. The low permeability of Carboniferous-Permian rocks, combined with the low trace element content of the investigated pyrite, mitigates the potential risk of environmental pollution.

Metallogenic Model for Pb-Zn Deposits in Clastic Rocks of the Dahai Mining Area, Northeast Yunnan: Evidence from H-O-S-Sr-Pb Isotopes

The Dahai Pb-Zn mining area is located in the northwestern Pb-Zn district in northeastern Yunnan Province in the Sichuan-Yunnan-Guizhou Pb-Zn metallogenic triangle (SYGT), east of the Xiaojiang fault. Numerous Pb-Zn deposits (spots) occur in clastic rocks in this area. In this study, the Maliping, Laoyingqing, and Jinniuchang Pb-Zn deposits, representative clastic rocks in the Dahai mining area, were selected as research objects. The results of H-O-S-Sr-Pb isotope analyses show that the three deposits mainly formed through the mixing of a basinal brine with a hydrothermal fluid derived from deep within the underlying (deformed) basement, and brines leached organic matter from wall rocks. The δ34S values range from −2.62–30.30‰. The S isotope results show two different sources of reduced S: one in the Laoyingqing deposit derived from the S reduction generated by the pyrolysis of sulfur-bearing organic matter in the carbonaceous slate of the Kunyang Group, and the second in the Maliping and Jinniuchang deposits derived from the S reduction generated by the thermochemical sulfur reduction (TSR) of seawater sulfate in the Lower Cambrian Yuhucun Formation and Sinian Dengying Formation. The Pb isotope results show that the Pb sources of the three deposits are derived from basement rocks (Kunyang Group) with a small portion derived from Devonian–Permian carbonate rocks and Dengying Formation dolomite, both of which have undergone homogenization during mineralization. The Sr content varied from 0.09629 to 0.2523 × 10−6, and the study shows that the main source of Sr is a mixture of ore-forming fluid flowing through basement rocks (Kunyang Group) and through sedimentary cover. However, most of the Sr in the Maliping deposit is derived from marine carbonate, and in the Laoyingqing deposit, it is provided by basement rocks (Kunyang Group). Based on a comparative study of the deposits, the Pb-Zn deposits in the clastic rocks of the Dahai mining area and the SYGT belong to the same metallogenic system and were formed under the same metallogenic geological background. Finally, a unified metallogenic model of the two types of fluid migration and mixed mineralization of the Pb-Zn deposit in clastic rocks of the Dahai mining area is proposed. The metallogenic model provides a basis for the study of the Pb-Zn metallogenic system and guidance for deep and peripheral prospecting in this area.

Reactive flow of microemulsion-based EDTA fluid for carbonate stimulation

Traditional methods for stimulating carbonate formations primarily rely on the use of highly reactive acids, such as hydrochloric acid (HCl), to create wormholes. However, the limitations of this approach in certain scenarios have prompted the exploration of alternative strategies. In this study, we investigate the synergistic potential between microemulsion and ethylenediaminetetraacetic acid (EDTA) for stimulating carbonate formations. Microemulsion systems were formulated using a nonionic surfactant, sec-butanol as a cosurfactant, xylene as the oily phase, and an aqueous solution of EDTA as the water phase. The microemulsions were characterized by phase equilibrium and rheological behavior. Coreflooding and static dissolution experiments were performed. We employ microCT imaging to evaluate the change in porosity in static dissolution experiments. The results reveal that microemulsions containing EDTA presented remarkable stability and were able to significantly enhance carbonate rock permeability up to 85%, indicating that dissolution occurred and the rock was stimulated. Analyses of microCT images on static dissolution experiments of Indiana limestones in reactive microemulsions indicate an increase in porosity, confirming the dissolution process. These findings highlight the synergistic effect of EDTA in microemulsion media for enhancing the rock permeability of carbonate formation as an alternative as low reactive system.

Neoproterozoic Zn-Pb mineralization in the world-class Sichuan-Yunnan-Guizhou Zn-Pb triangle, southwest China: Insights from apatite geochemistry and in situ sericite Rb-Sr geochronology of the Daxiao deposit

The Sichuan-Yunnan-Guizhou (SYG) zinc-lead triangle, located in southwestern China, is an important producer of zinc, lead, silver, and critical raw metals such as germanium. In this region, most Zn-Pb deposits hosted in the later Ediacaran to Permian carbonate rocks are believed to be Mississippi Valley-type (MVT) deposits, while the origin of the Zn-Pb deposits found in the Mesoproterozoic basement remains controversial. To address this gap, our study conducted in situ LA-ICP-MS/MS muscovite Rb-Sr dating and apatite major, trace and Sr isotope analysis on the Daxiao deposit hosted in the Mesoproterozoic basement. The in situ Rb-Sr dating of sericite yielded a Neoproterozoic age of 766 ± 31.3 Ma, challenging the previously proposed Indosinian orogen-related MVT model. Instead, it links the formation of this deposit to an extensional setting that coincided with the breakup of the Rodinia. We identified a specific type of apatite, recognized as ore-stage apatite based on texture and analyzed composition, within the deposit. The ore-stage apatite exhibits high 87Sr/86Sr ratios (ranging from 0.78384 to 0.81082), similar to those of the Archean basement rocks.Based on our study and previous data, we proposed a new genetic model for the Daxiao Zn-Pb deposit, and possibly other basement-hosted Zn-Pb deposits in the SYG triangle. During the Neoproterozoic (∼766 Ma), the study area experienced an extensional setting characterized by crustal thinning and magmatic activity resulting from the breakup of the Rodinia. Basinal brines infiltrated the deep Archean basement along faults and were heated to high temperatures by magma and/or high geothermal gradients. These hot brines continuously extracted Pb, Zn from the Archean basement and migrated upward into the Mesoproterozoic host rock, where they precipitated lead–zinc ores in favorable structural positions. The Daxiao Zn-Pb deposit, along with the coexisting sediment-hosted stratabound copper (SSC) deposits, basement-hosted gold and uranium deposits, and late sulfide mineralization in the iron oxide-copper–gold (IOCG) deposits, suggests a Neoproterozoic regional mineralization event associated with the breakup of Rodinia. Our study also highlights the potential of using in situ sericite Rb-Sr dating to precisely determine the age of other hydrothermal ore deposits, such as Carlin-type Au deposits, which have long been a subject of uncertainty.

Research of influencing factors on permeability for carbonate rocks based on LBM simulation: A case study of low-permeability gas reservoir of Sinian Dengying Formation in Sichuan Basin

Instruction: Due to the declining production rate of conventional natural gas reservoir and the rising demand for natural gas resources, conventional natural gas resources cannot satisfy the needs of economic development. Unconventional gas resource is considered to be the most promising energy supplement, therefore it is an inevitable trend to explore and develop unconventional oil and gas resources such as low permeability reservoirs. At present, the carbonate gas reservoirs in the high permeability area of Sichuan Basin have been almost fully developed, while the remaining gas reservoirs in the low permeability area are not well developed. However, the key factors for effectively enhancing gas recovery rate in different types of low permeability reservoirs are not the same. Even gas reservoirs with the same macroscopic geological characteristics will show different development characteristics through the production processes.Method: In order to analyze the factors affecting the effective production rate of low-permeability gas reservoirs, it is necessary to conduct research from the microscopic perspective, to reveal the effective production conditions of natural gas in low-permeability reservoirs. In this study, low-permeability carbonate samples of Sinian Dengying Formation from Sichuan Basin were taken. Micro-CT scanning technology was used to obtain core images for the carbonate rocks, and the digital carbonate core models at microscopic scale were reconstructed. Based on the reconstructed carbonate digital core models, LBM method was applied to obtain the absolute permeability of the core models.Results: The results imply that the porosity, pore area fraction, throat area fraction, mean throat length, shape factor, coordination number and tortuosity are the factors affecting the absolute permeability of carbonate rocks.Discussion: Subsequently, the relationship between different pore structure parameters and absolute permeability was obtained by multi-parameter fitting method, which provided a new research method for directly predicting the absolute permeability of carbonate rocks by using pore structure parameters.

Simulation of flow characteristics and development of permeability model in fractured-vuggy carbonate reservoir

Fractured-vuggy carbonate reservoirs have complex pore structure, significant heterogeneity, and irregular permeability-porosity relation, which make existing evaluation methods unable to accurately calculate the permeability. To investigate the flow characteristics of carbonate rocks with triple-porosity, 3D digital rock models were constructed from X-ray computed tomography (CT) scanning images of carbonate cores collected in the field, and the physical properties of rocks were obtained by combining digital rock analysis (DRA) and nuclear magnetic resonance (NMR) experiment. The effects of fractures and vugs on the permeability of carbonate rocks was investigated using the Stokes-Brinkman method, and then a new fractured-vuggy carbonate permeability model was developed by artificially adding vugs and fractures. The results indicate that the carbonate pores can be divided into matrix pores, vugs, and fractures according to their sizes by combining digital rock analysis and T2 spectrums. The transport resistance is effectively reduced by fractures and vugs, which leads to the change of the flow path. For tight carbonate rocks, the dominant flow path formed through fractures is a crucial factor affecting permeability. On the contrary, unconnected vugs have less effect on permeability. Finally, the new permeability model was applied to the logging interpretation of field data. The new model is in better agreement with the permeability analysis of field cores, which greatly improves the permeability calculation accuracy of the triple-porosity reservoir in the study area.

Relative Permeability from Digital Analysis of a Carbonate Synthetic Core

This research discusses the digital core to understand better the petrophysics parameters, especially the relative permeability of carbonate rocks. The method used in this research is descriptive-analytic by using primary data. The primary data is sample core data (digital data) collected and processed directly to compare results from digital data to laboratory data. This research indicates a relationship between the results from digital data with the results from laboratory data such as porosity, permeability, and trendline of a curve from relative permeability. Analysis from the research results shows that the relative permeability value was caused by several factors, i.e., wettability, cementation, clay, and bound water constant.

Effects of microfractures on permeability in carbonate rocks based on digital core technology

Carbonate reservoirs develop many different types of microfractures that play an important role in increasing the effective reservoir space and permeability. Thus, the qualitative and quantitative characterisation of the effect of microfractures on permeability in rocks is essential. In this study, a quantitative method for evaluating the impact of different microfracture parameters on carbonate rock permeability was proposed. Lattice Boltzmann simulations were carried on two carbonate digital cores with different types of artificially added microfractures. Based on the simulation results, a partial least squares regression analysis was used to investigate the impact of microfractures on the permeability of the cores. Increases in the fracture length, aperture, and density were found to linearly increase the permeability of the carbonate rocks, and as the fracture length increased to penetrate the whole core, an exponential increase in permeability was observed. Additionally, the effect of microfractures on the digital core permeability was more significant in cores with high permeability compared to that in low-permeability cores. Although both fractures and matrix permeability contribute to the permeability of the digital cores, the former were found to have a greater effect on the permeability. Cited as: Liu, C., Zhang, L., Li, Y., Liu, F., Martyushev, D. A., Yang, Y. Effects of microfractures on permeability in carbonate rocks based on digital core technology. Advances in Geo-Energy Research, 2022, 6(1): 86-90. https://doi.org/10.46690/ager.2022.01.07

On the evaluation of permeability of heterogeneous carbonate reservoirs using rigorous data-driven techniques

This study probes the application of Cascade Forward Neural Network (CFNN), Least Square Support Vector Machine (LSSVM), Multilayer Perceptron (MLP), and Generalized Regression Neural Network (GRNN) techniques for modeling the absolute permeability of carbonate rocks in terms of pore specific surface area, porosity, and irreducible water saturation. The control parameters of the MLP and CFNN models were tuned through Levenberg Marquardt Algorithm (LMA) and Bayesian Regularization (BR) optimizers, and the LSSVM paradigm was optimized using Gravitational Search Algorithm (GSA). Accordingly, six intelligent schemes, viz. MLP-BR, MLP-LMA, LSSVM-GSA, CFNN-BR, CFNN-LMA, and GRNN were trained by utilizing 80% of a valuable set of core data compiled from reliable literature and were tested through the rest of the data points (20%). The accuracy of the proposed paradigms was evaluated using several statistical and graphical assessments. The overall results were fulfilling and fair enough for the scope of this study. The proposed MLP-BR, MLP-LMA, LSSVM-GSA, CFNN-BR, CFNN-LMA, and GRNN models were associated with the Root Mean Square Errors of 6.8019, 5.6225, 165.8852, 6.6841, 5.2136, and 11.1799, respectively. The results were endorsed through 3-fold cross-validation. Furthermore, outlier detection was carried out by means of the plot of standardized residuals versus Leverage values. For all models, the majority of the points were valid values distributing in the applicability domain of the models. In the end, the developed models were compared against two literature smart models and a traditional correlation. The results demonstrated that the recently generated models offer remarkably higher accuracy than other alternatives followed by the Gene Expression Programming (GEP) modeling approach.

A hydrogeochemical approach to the characterization of low-enthalpy geothermal systems: the Scordia – Lentini graben (Sicily, Italy)

We describe the geochemical characteristics of groundwater samples collected in 23 water wells located on the northern margin of the Hyblean plateau (East Sicily). This area, mostly made of highly permeable carbonate rocks, is rich in low temperature (T &lt; 50° C) hydrothermal groundwaters, distributed in an active sismogenetic zone, with several ENE-WSW-directed tectonic structures that drove magma to the surface during Upper Pliocene and Pleistocene. The chemical features suggest complex mixing between rainwater, CO2-rich groundwater, steam-heated groundwater and geothermal brines, as highlighted by Principal Component Analysis (PCA). Some parameters, however, indicate widespread pollution of the aquifers from human activities. Stable isotopes analysis confirms the meteoric origin of groundwater and supports the origin of dissolved CO2 mostly from mantle degassing through deep tectonic faults. Geothermometric estimates, mostly based on quartz and Saturation Indexes geothermometers, suggest minimum reservoir temperature between 100 and 120° C.

GEOTHERMAL SYSTEMS IN THE NORTHERN APENNINES, ITALY: MODERN ANALOGUES OF CARLIN-STYLE GOLD DEPOSITS

Abstract Carlin-type gold deposits in northern Nevada are inferred to overlie concealed late Eocene plutons, which are increasingly thought to have provided magmatic input to the meteoric water-dominated fluids from which the gold was precipitated. The Larderello, Monte Amiata, and Latera geothermal systems in the Northern Apennines of southern Tuscany and northern Latium, central Italy, may represent Pliocene to present-day analogues because of their demonstrated association with subsurface plutons and jasperoid-hosted antimony-gold mineralization. The plutons, which at depths of &amp;gt;5–7 km remain at least partially molten, continue to supply heat and magmatic fluids to the meteoric water-dominated geothermal systems. Formerly mined antimony deposits of Pliocene or younger age are exposed on the peripheries of the CO2 ± H2S-emitting geothermal systems, and antimony sulfides are still actively precipitating. Stibnite and submicroscopic gold in disseminated pyrite, along with Au/Ag of &amp;lt;0.5 and anomalous As, Hg, Tl, and Ba values, accompanied jasperoid formation in the Northern Apennines systems. Carlin-type mineralization in northern Nevada and the antimony-gold mineralization in the Northern Apennines are hosted by permeable carbonate rocks, particularly stratabound breccias, where they are intersected by steep normal or oblique-slip faults and confined beneath tectonically emplaced hydrologic seals. The Northern Apennines antimony-gold mineralization formed at shallow, epithermal depths, like that recently recognized in the southern Carlin trend of northern Nevada. Although underexplored, the Northern Apennines gold prospects are unlikely to ever attain the giant status of the Carlin-type deposits in northern Nevada, probably because of lower magmatic fertility (ilmenite-series rather than magnetite-series magmatism) and host-rock receptivity (less reactive iron). Nevertheless, shallow carbonate-rock aquifers within high-temperature, intrusion-related geothermal systems, be they extinct or still active, may be prospective for Carlin-style gold deposits.

Vertical evolution of Ag-Pb-Zn-(Cu)-Mo in porphyry system: A case study from the Laochang deposit, SW China

Distal massive sulfide Ag-Pb-Zn mineralization is commonly associated with proximal porphyry Mo-(Cu) systems, but the vertical evolution of Ag-Pb-Zn-(Cu)-Mo distribution within porphyry systems remains poorly understood and documented. The Laochang deposit located in the southeastern Sanjiang metallogenic belt, SW China contains both the massive sulfide Ag-Pb-Zn-(Cu) and porphyry-skarn Mo-(Cu) mineralization (1737 t Ag@222 g/t, 0.51 Mt Pb@5.1 wt%, 0.34 Mt Zn@3.1 wt%, 117, 800 t Mo@0.15 wt%, and 0.1 Mt Cu@0.2 wt%), thus representing an excellent case study. The Laochang deposit is mainly hosted in early Carboniferous volcanic rocks, late Carboniferous to early Permian carbonate rocks, and links with Eocene granite porphyry. Here, we report Rb-Sr age of sphalerite and pyrite from the massive sulfide Ag-Pb-Zn-(Cu) ores, zircon U-Pb age for the granite porphyry, C-O isotopes of hydrothermal calcite, and S-Zn isotopes of sulfide minerals to reveal the vertical distribution of Ag-Pb-Zn-(Cu)-Mo mineralization in porphyry system. The sulfides Rb-Sr age (45.1 ± 3.6 Ma) is similar to the zircon U-Pb age (44.6 ± 0.2 Ma), indicating the Ag-Pb-Zn-(Cu) mineralization has a genetic link with the granite porphyry and formed in the same system as the porphyry-skarn Mo-(Cu) mineralization. Furthermore, the C-O isotopic compositions of the hydrothermal calcite suggest a diminishing contribution of the host rocks and an increasing input of the meteoric water in the hydrothermal fluids from deep to shallow. The S-Zn isotopic compositions of the sulfide minerals indicate a magmatic source of the ore-forming elements. The δ34S values decrease, and the δ66Zn values increase with elevated levels of sampling depth, implying a Rayleigh fractionation process. This means fluids and late-stage sulfides evolved toward isotopically lighter sulfur and heavier zinc isotopes. Hence, we propose that the Laochang deposit is a porphyry system comprising distal Ag-Pb-Zn-(Cu) and proximal Mo-(Cu) mineralization. In addition, the sulfur and zinc isotopic systematics observed in this study provides a new insight into the mineralization processes of the porphyry-distal systems.

The impact of pore-throat shape evolution during dissolution on carbonate rock permeability: Pore network modeling and experiments

Pore network model simulation (PNM) is an important method to simulate reactive transport processes in porous media and to investigate constitutive relationships between permeability and porosity that can be implemented in continuum-scale reactive-transport modeling. The existing reactive transport pore network models (rtPNMs) assume that the initially cylindrical pore throats maintain their shape and pore throat conductance is updated using a form of Hagen-Poiseuille relation. However, in the context of calcite dissolution, earlier studies have shown that during dissolution, pore throats can attain a spectrum of shapes, depending upon the imposed reactive-flow conditions (Agrawal et al., 2020). In the current study, we derived new constitutive relations for the calculation of conductance as a function of pore throat volume and shape evolution for a range of imposed flow and reaction conditions. These relations were used to build animproved new reactive pore network model (nrtPNM). Using the new model, the porosity-permeability changes were simulated and compared against the existing pore network models.In order to validate the reactive transport pore network model, we conducted two sets of flow-through experiments on two Ketton limestone samples. Acidic solutions (pH 3.0) were injected at two Darcy velocities i.e., 7.3 × 10−4 and 1.5 × 10−4 m.s − 1 while performing X-ray micro-CT scanning. Experimental values of the changes in sample permeability were estimated in two independent ways: through PNM flow simulation and through Direct Numerical Simulation. Both approaches used images of the samples from the beginning and the end of experiments. Extracted pore networks, obtained from the micro-CT images of the sample from the beginning of the experiment, were used for reactive transport PNMs (rtPNM and nrtPNM).We observed that for the experimental conditions, most of the pore throats maintained the initially prescribed cylindrical shape such that both rtPNM and nrtPNM showed a similar evolution of porosity and permeability. This was found to be in reasonable agreement with the porosity and permeability changes observed in the experiment. Next, we have applied a range of flow and reaction regimes to compare permeability evolutions between rtPNM and nrtPNM. We found that for certain dissolution regimes, neglecting the evolution of the pore throat shape in the pore network can lead to an overestimation of up to 27% in the predicted permeability values and an overestimation of over 50% in the fitted exponent for the porosity-permeability relations. In summary, this study showed that while under high flow rate conditions the rtPNM model is accurate enough, it overestimates permeability under lower flow rates.

Increasing the production of gas condensate by using ammonium carbonate salts

The work is devoted to the problem of increasing gas condensate production in gas condensate fields. It was found that ammonium carbonate salts, in the absence of calcium chloride type waters, interact with carbonate rocks, increase the permeability of reservoirs. Solutions of ammonium carbonate salts when interacting with formation water of the calcium chloride type form chemically precipitated chalk in the pore space, while the permeability of carbonate rocks decreases. A set of experimental studies was carried out to study the displacing and washing properties of ammonium carbonate salts. It was found that ammonium carbonate salts have high displacing properties, the displacement ratio of kerosene by NH4HCO3 solution is 0.75-0.80, while reservoir water – 0.55-0.58. According to the results of laboratory studies of the displacing and washing characteristics of ammonium carbonate salts, conclusions were made about the effect of bicarbonate solution (ammonium carbonate salts) on the production characteristics of a well in reservoir conditions at temperatures of 80-100 °C and above. Industrial tests of ammonium carbonate salts showed an increase in gas flow by 30-50% at wells № 23 of Opishnia, № 115 of Mashivka, № 3 of Tymofiivka gas condensate fields. The effect of formation treatment with ammonium carbonate salts is achieved due to clearing of well bottom zone and increasing the formation permeability. At wells № 56, 108 of Yablunivka and № 58 of Tymofiivka gas condensate fields, an increase in the condensate ratio was observed by 22-35%. The effectiveness of this treatment is associated with the simultaneous bottomhole zone cleaning from asphalt-resinous contaminants and permeability increase, as well as with the hydrophilization of the pore space and mobility increase of condensate precipitated as a result of carbon dioxide effect, which was rejected as a result of decomposition of ammonium carbonate. Thus, experimental and industrial tests in Opishnia, Mashivka, Tymofiivka, Yablunivka gas condensate fields of Poltava region confirmed the effectiveness of using ammonium carbonate to increase hydrocarbon production. The prospect of further research is aimed at developing a technology for increasing the production of liquid hydrocarbons by using ammonium carbonate salts.

Geological structure of the northern part of the Kara Shelf near the Severnaya Zemlya archipelago according to recent studies

Until recently, the North of the Kara Shelf was completely unexplored by seismic methods. Seismic and seismo-acoustic data that have appeared in recent years have made it possible to decipher features of the regional geological structure. This study solves the urgent problem of determining the prospects for the oil and gas potential of the North Kara sedimentary basin. The relevance of the research is associated with determining the prospects of the oil and gas potential of the North Kara sedimentary basin. The aim of the study is to clarify the age of the reflecting horizons using data on the geology of the island, as well as to determine the tectonic position of the sedimentary cover and basement structures in the north of the Kara shelf. The sedimentary cover is divided into three structural levels: Cambrian-Devonian, Middle Carboniferous-Cretaceous, Miocene-Quarter. The Cambrian-Devonian complex fills the deep troughs of the North Kara shelf. The most noticeable discontinuity is the base of Carboniferous-Permian rocks, lying on the eroded surface of folded Silurian-Devonian seismic complexes. The blanket-like plate part of the cover is composed of thin Carbon-Quarter complexes. The authors came to the conclusion that the fold structures of the Taimyr-Severozemelskiy fold belt gradually degenerate towards the Kara sedimentary basin and towards the continental slope of the Eurasian basin of the Arctic Ocean. A chain of narrow uplifts within the seabed relief, which correspond to narrow anticlines is traced to the West of the Severnaya Zemlya islands. Paleozoic rocks have subhorizontal bedding further to the West, within the Kara shelf. Mesozoic folding in the North of the Kara Sea is expressed exclusively in a weak activation of movements along faults. At the neotectonic stage, the shelf near Severnaya Zemlya was raised and the Mesozoic complexes were eroded. The modern seismic activity of the North Zemlya shelf is associated with the ongoing formation of the continental margin.

Impact of Land Use on Karst Water Resources—A Case Study of the Kupa (Kolpa) Transboundary River Catchment

This paper presents a qualitative approach for assessing land-use pressures on the water resources of a transboundary Dinaric karst catchment of the Kupa River in Southeast Europe. Spatial analyses of the water quality indices for surface water and groundwater were carried out in a GIS environment, as well as a detailed assessment of man-made hazards based on recommendations of COST Action 620. The produced maps provide an insight into the qualitative status of water resources at a regional scale by indicating areas of potential negative impacts of land use through the identification of point and diffuse sources of pollution. Higher values of the water quality indices for surface water and groundwater are observed in lowland areas, karst plateaus and poljes, where the impacts of anthropogenic activities such as agriculture and quarries take place on karstified permeable carbonate rocks. Hazard assessment showed how transport infrastructure induces a low hazard level. Settlement areas without proper sewerage systems impose moderate hazard levels, while direct wastewater discharges into groundwater and waste illegally disposed in karst swallow holes and caves located near settlements were classified as having high hazard levels. The applied methods proved to be suitable even in challenging karst environments where the complex properties and structure make the exploration and monitoring of groundwater resources difficult and scarce.

A new model to estimate permeability using mercury injection capillary pressure data: Application to carbonate and shale samples

Estimating permeability of carbonate rocks using mercury injection capillary pressure (MICP) data has been carried out by many researchers in the past few decades. However, a major issue with almost all of the existing models is that they focus on a single aperture value from the capillary pressure curve. This study builds a new model to extract permeability from the entire pore throat sizes. Fermic-Dirac function was applied to fit the MICP curve to obtain some critical parameters such as R1 (the large curvature value) and R2 (the small curvature value). Afterwards, the partial least squares regression method was employed to develop a new permeability model. To verify the new model and check other models, we studied ten carbonate rock samples from an Iranian oil reservoir. The results showed that the R1 values vary from 1.00 to 2.73 while R2 values are found between 0.23 and 1.00. The new model performed better than the published models. The idea of building the model for the carbonates can be used in developing the permeability estimating model for shale samples, which could be a new model for the shale permeability estimation.