Increasing Water Saturation Research Articles

Zoning Productivity Calculation Method of Fractured Horizontal Wells in High-Water-Cut Tight Sandstone Gas Reservoirs under Complex Seepage Conditions

Tight sandstone gas reservoirs generally contain water. Studying the impact of water content on the permeability mechanism of tight gas reservoirs is of positive significance for the rational development of gas reservoirs. Selected cores from a tight sandstone gas reservoir in the Ordos Basin were used to establish the variation in its seepage mechanism under different water saturations. The experimental results show that the gas slip factor in tight water-bearing gas reservoirs decreases as the water saturation increases. The stress sensitivity coefficient and the threshold pressure gradient (TPG) increase with increasing water saturation, characterizing the relationships between stress sensitivity coefficients, TPG, permeability, and water saturation. As the water saturation gradually increases, the relative gas phase permeability of tight sandstone gas reservoirs will sharply decrease. When the water saturation exceeds 80%, the gas phase permeability becomes almost zero, resulting in gas almost ceasing to flow. Through the analysis of experimental results, we defined high-water-cut tight sandstone gas reservoirs and analyzed the permeability characteristics of high-water-cut tight sandstone gas reservoirs in different regions. Combining stress sensitivity coefficients and the TPG with permeability and water saturation relationships, we established a zoning productivity calculation method of fractured horizontal wells in high-water-cut tight sandstone gas reservoirs under complex seepage conditions and validated the practicality of the model through example calculations.

Effect of microporous layer structural parameters on heat and mass transfer in proton exchange membrane fuel cells

Proton exchange membrane fuel cells offer promising clean energy solutions for various applications. However, their performance relies heavily on the properties of the microporous layer, which plays a crucial role in transporting and distributing the components in the fuel cell. To date, the potential for optimising the microporous layer material structural parameters to enhance the fuel cell performance remains largely unexplored. This study aims to fill this research gap by conducting a comprehensive investigation of the effects of different microporous layer material structural parameters on the heat and mass transfer in the membrane electrode assembly. MATLAB was used for optimising the performance of the fuel cell components. The results show that increasing the microporous layer thickness from 5 to 50 μm significantly affects the species transport, leading to a substantial reduction in the molar fraction of H2 and O2 at the electrochemical reaction sites. Furthermore, the distribution of the liquid water saturation inside the fuel cell is influenced by the porosity and permeability of the microporous layer. By increasing the porosity from 0.3 to 0.6, the liquid water saturation at the interface of the catalyst layer and microporous layer decreases by 0.52 % and 1.12 % at output voltages of 0.5 V and 0.7 V, respectively. This reduction enhances the efficiency of internal water transport. Moreover, reducing the permeability of the microporous layer from 2 × 10-12 to 1 × 10-13 at 0.5 V and 0.7 V leads to an increase in liquid water saturation at the interface of the proton exchange membrane and the catalyst layer by 1.49 % and 0.74 %, respectively, causing hindrance to the transport of internal liquid water. This study provides valuable insights into the interplay between the properties of the microporous layer material properties and heat and mass transfer characteristics in proton exchange membrane fuel cell.

Effects of Pore Water Content on Stress Sensitivity of Tight Sandstone Oil Reservoirs: A Study of the Mahu Block (Xinjiang Province, China)

Traditional stress sensitivity experiments are typically conducted under dry conditions, without considering the reservoir’s water content. In reality, the presence of water within pores significantly influences the extent of stress sensitivity damage in tight sandstone oil formations, subsequently affecting the determination of stress sensitivity coefficients during experimentation. By investigating sandstone samples from wells in the Mahu Block of China’s Xinjiang province, we observed that increasing water saturation reduces the stress sensitivity of tight sandstone. By conducting stress sensitivity experiments under varying water content conditions, we found that the stress sensitivity coefficient is not a constant value but decreases as water saturation increases. Based on experimental comparisons, an optimized power-law model for stress-sensitive damage assessment was refined. By conducting stress-sensitive damage assessment experiments under different water content conditions and integrating the concept of comprehensive compression coefficient, an improved stress-sensitive power-law model was established allowing for the influence of water content. The accuracy of this improved model was increased by 46.98% compared to the original power-law model through experimental validation. The research outcomes can enhance the accuracy of permeability and productivity evaluation, providing valuable guidance for unconventional oil and gas development.

Seasonal hydrology and gas transport in a composite cover on sulfide tailings

This study presents the field performance of a five-layer composite cover to mitigate acid mine drainage in legacy sulfide tailings in northern Ontario, Canada. Installed in 2008, this cover comprised sand, clay, geosynthetic clay liner, sand, and waste rock layers. To evaluate the effectiveness of the cover in reducing water and oxygen ingress, groundwater and vadose zone hydrological characterization, stable water isotope analysis, pore-gas measurements, oxygen flux calculations, and variably saturated flow modeling were conducted. The results indicate that the clay layer stayed nearly saturated in the spring, fall, and winter, but temporary desiccation occurred during the summer. Compared to uncovered tailings, the cover significantly lowered diffusive oxygen flux. In the summer, fall, and winter, the capillary barrier effect of the cover functioned effectively and inhibited percolation. Atmospheric pore-gas oxygen concentrations at one out of the three monitoring locations indicate potential cover imperfections that enabled oxygen transport into the tailings. In the spring and early summer, snowmelt infiltration resulted in percolation that compromised the capillary barrier effect, as well as lateral drainage. The resulting increase in water saturation in the cover limited oxygen transport. Despite potential cover imperfections, this composite cover reduced oxygen and water ingress a decade after installation.

Experimental investigation of the interfacial bonding properties between polyurethane mortar and concrete under different influencing factors

As a repair material, polyurethane mortar has excellent mechanical properties. The aim of this study is to explore the mechanical properties and microstructure of the interface between polyurethane mortar and concrete. The effects of curing temperature (−20 °C, −10 °C, 0 °C, 10 °C, 20 °C), water saturation of concrete (0 %, 30 %, 70 %, 100 %) and roughness (0, 1.5, 3.0, 4.5, 6.0 mm) on the bonding interface strength (shear strength and splitting tensile strength) were investigated through shear and splitting tests. The microstructure of the bonded interface was analysed by scanning electron microscopy at different curing temperatures and water saturation of the concrete, and an interfacial bonding mechanism was proposed. The results of the study show that the strength of the bonded interface increases gradually as the curing temperature rises. At the same curing temperature, the strength of the bonded interface decreases with increasing water saturation of the concrete and is essentially linear. The rate of strength loss of high-water-saturation specimens increases gradually as the temperature rises. When the temperature is positive, the interfacial bond strength increases and then decreases with increasing roughness, and a critical value (4.5 mm) of roughness exists that makes the bond strength reach a maximum. At negative temperatures, the interfacial bond strength increases with increasing roughness. The grey correlation analysis shows that roughness has the greatest influence on the interfacial bonding performance, followed by curing temperature and then water saturation. The interfacial shear strength and splitting tensile strength have a good linear relationship under the influence of all factors. Microscopic tests show that the polyurethane mortar body structure loosens gradually as the curing temperature decreases. As the water saturation of the concrete increases, the interface width between the polyurethane mortar and concrete shows an overall tendency to increase.

Research on Transformation of Connate Water to Movable Water in Water-Bearing Tight Gas Reservoirs

The Dongsheng gas field is a water-bearing tight gas reservoir characterized by high connate water saturation. During gas production, the transformation of connate water into movable water introduces a unique water production mode, significantly impacting gas reservoir recovery. Current experimental and theoretical methods for assessing formation water mobility are static and do not address the transformation mechanism from connate into movable water. In this study, we considered dynamic changes in formation stress and proposed the mechanism for the transformation of connate water into movable water during depressurization, involving the expansion of connate water films and the reduction of pore volume. We developed a novel methodology to calculate the dynamic changes in movable and connate water saturation in tight reservoirs due to reservoir pressure reduction. Furthermore, we quantitatively evaluated the transformation of connate water into movable water in the Dongsheng gas field through laboratory experiments (including formation water expansion tests, connate water tests, and porosity stress sensitivity tests) and theoretical calculations. Results show that under original stress, the initial connate water saturation in the Dongsheng gas field ranges from 50.09% to 58.5%. As reservoir pressure decreases, the maximum increase in movable water saturation ranges from 6.1% to 8.4% due to the transformation of connate water into movable water. This explains why formation water is produced in large quantities during gas production. Therefore, considering the transition of connate water to movable water is crucial when evaluating water production risk. These findings offer valuable guidance for selecting optimal well locations and development layers to reduce reservoir water production risks.

Influence of Clay‐Containing Sediments on Methane Hydrate Formation: Impacts on Kinetic Behavior and Gas Storage Capacity

AbstractOn Earth, natural hydrates are mostly encountered in clay‐rich sediments. Yet their formation processes in such matrices remain poorly understood. Achieving an in‐depth understanding of how methane hydrates accumulate on continental margins is key to accurately assess (a) their role in sustaining the development of some chemosynthetic communities at cold seeps, (b) their potential in terms of energy resources and geohazards, and (c) the fate of the methane releases, a powerful greenhouse gas, in this changing climate. This study investigated the formation of methane hydrates and their gas storage capacity (GSC) in clay‐rich sediments. A set of hydrate experiments were performed in matrices composed of sand, illite‐rich clay, and montmorillonite‐rich clay at different proportions aiming to determine the role of mineralogy on hydrate formation processes. The experiments demonstrate that a clay content of 10% in a partially water saturated sand/clay mixture increases the induction time by ∼60%, irrespective of the nature of the clay used. The increase in water saturation in the two matrices promotes hydrate formation. Micro‐Raman spectroscopic analyses reveal that increasing the clay content leads to a decrease in the hydrate small‐cage occupancy, with an impact on the storage capacity. Finally, the analyses of collected natural samples from the Black Sea (off Romania) enable us to estimate the GSC of the deposit. Our estimates is different from previous ones, and supports the importance of coupling multiscale properties, from the microscale to the geological scale, to accurately assess the total amount of methane hosts in hydrate deposits worldwide.

Geochemical characteristics of the braided river reservoir in block 19 of the sulige gas field

The sand body structure and geochemical characteristics of braided river reservoirs are the key geological factors affecting gas production and development effects. The Sulige gas field in the Ordos Basin is an important large-scale gas-producing layer. Owing to the control of sedimentary facies, the geological structure of the sand body changes greatly and its connectivity is poor. The geological characteristics have not yet been elucidated, and this is an important problem restricting the development of the Sulige Gas Field. To solve this problem, this study focuses on the braided river reservoir of the Shihezi Formation in Block 19 of the Sulige Gas Field, conducts geological surveys in the study area, analyzes the geological and geochemical characteristics of the reservoir, and obtains samples through drilling. Through a thin-section test, gas-water two-phase experiment, and simulation test, the braided river reservoir configuration and pore and gas-water characteristics are obtained. The results show that the reservoir lithology in the study area is mainly composed of quartz sandstone, lithic sandstone, and quartzy lithic sandstone, with a porosity of 3%–13% and a permeability of (0.05–0.7) × 10−3 m2. The reservoir has low porosity and low permeability. After drilling samples were obtained, 32 thin-section rock samples were selected. The pore types of the block reservoir mainly (82.9%) consisted of intragranular and intergranular dissolved pores. The difference in pore structure was mainly reflected by the size and distribution of the throat. The distribution of physical properties was 6%–10%, the gas saturation was 61%, the NMR effective porosity was 7.49%, the permeability was 4.08 × 102 μm2, and the physical properties were relatively good. In terms of the study area, the average thickness of the single braided channel in the lower section of He 8 was 4.7 m, the average width of the channel was 963 m, and the composite channel was distributed in a potato shape, parallel to the direction of the main flow. The average length of the channel was 2,147 m and the average width was 844 m. As the porosity increased, the efficiency of gas-driven water also increased, and there was a linear positive correlation between porosity and gas-driven water efficiency. With the increase in movable water saturation, the water-air ratio became larger and water production was greater. In low-amplitude structures and under low-permeability background conditions, for reservoirs with good local pore structure and physical properties, the water remaining at the bottom of the reservoir or sand body was controlled by the accumulation conditions or the weak structural differentiation after accumulation. In terms of the gas and water produced simultaneously in the study area, gas production was less than 2 × 104 m3/d and water production was relatively large at more than 10 m3/d; gas and water were mainly distributed in the downdip part of the main channel structure or in the island lens-shaped permeable sand bodies trapped by the surrounding tight layers. The study results provide theoretical data support for the exploration and production of the Sulige Gas Field.

Study of petrophysical parameters of productive series by use of well data

The progress of existing geophysical techniques for the last years led to a wider scope of the problems resolved using this set of tools. The tools allow tackling pivotal tasks such as the study of lithology in a geological section, correlation, evaluation of saturation, porosity, permeability, and clay fraction, and outlining of oil-gas and water-saturated layers. Complications emerging while resolving these problems directly depend on the petrophysical parameters of reservoir layers. For instance, similar values of resistivity in oil and water layers result in high value of clay fraction and variation of porosity in a wide range and this complicates distinguishing the oil and water layers. On the other hand, it is known that an increase of cementing clay amount in reservoirs causes drop of porosity (Kpor.) and increase of residual water saturation.
 This paper is devoted to design of models defining correlation between petrophysical parameters of reservoir layers by use of integrated well data and establishing regression equations from these parameters’ distributions. Models of variation of porosity, permeability, clay fraction and oil and gas saturation through the sections of wells were also designed. They made it possible to evaluate filtration-capacity characteristics and establish correlation between petrophysical parameters of reservoir layers and gain information about this field.
 The bar graph was drawn for permeability coefficient variation through studied reservoirs in Qirmaky suite (QD-1, QD-2, QD-3, QD-4) by use of sections of wells conditionally marked as 1, 2, 3, and 4 in the Balakhany field.
 Analysis of the constructed histograms allows us to classify the studied collector layers according to their permeability. Of the 17 layers studied in section of well 1, only Layer 17 has very good permeability, the other 16 are of good permeability; all 17 layers of well 2 have good permeability characteristics; of the 17 layers of well 3, Layer 3 is of poor permeability, layers 6, 7, and 14 have average permeability, and the rest are of good permeability; all 12 layers of well 4 have good permeability.
 The study target is Qirmaky suite (QD-1, QD-2, QD-3, QD-4) of the Productive Series in Balakhany field.

Effect of water saturation on CO2 minimum miscibility pressure and oil displacement performance

The CO2 miscible flooding is an effective technology for development of low permeability reservoirs due to high oil displacement efficiency. However, CO2 miscible flooding for tertiary recovery technology typically occur after water flooding. Water hindered the contact and mass transfer of CO2 and oil, produce water resistance effect. Water can block CO2 and contact and mass transfer of crude oil (ie water blocking effect). The impact of this effect on the miscibility pressure and oil displacement effect is still unclear. In this study, homogeneous artificial cores with the same permeability as the reservoir were designed according to the miscible mechanism. Subsequently, the minimum miscibility pressure(MMP) between CO2 and crude oil at different water saturation was measured by the coreflood test. Based on the MMP, CO2 miscible flooding experiments were conducted by natural cores at different water saturation. The results showed that when the water saturation was 30%, 45%, and 60%, respectively, the corresponding MMP between CO2 and crude oil was 18.56, 19.48, and 21.76 MPa. The higher the water saturation, the longer the gas breakthrough time. But the oil recovery factor did not rise with the increase of water saturation. When the water saturation was 45%, the oil recovery factor in the miscible flooding experiment was the highest. In addition, the higher the water saturation, the weaker the CO2 extraction of various components in the crude oil. And the light components below C5 were most affected by water saturation. The mechanisms underlying these results are discussed. This study contributes to the development of CO2 miscible flooding reservoir plan.

EFFECT OF WATER SATURATION ON STABILITY OF A HILL SLOPE: MALIN CASE STUDY

The study presents a meticulous investigation into the catastrophic landslide that impacted Malin village in Pune district of Maharashtra, India. By employing a multi-faceted approach encompassing field, laboratory, and numerical analyses, in the study, stability governing parameters were thoroughly assessed with respect to varying levels of water saturation. Field investigation provided crucial insights into the geographical profile, field density, slope strata, and representative soil sample acquired from hill slopes. Furthermore, extensive laboratory investigations were conducted to gain a comprehensive understanding of the role of stability governing parameters under different water saturation levels. The limit equilibrium method was employed for numerical simulation to rigorously evaluate slope stability. The results revealed the significant influence of increased water saturation on stability governing parameters, leading to slope instability, which was confirmed by numerical simulation. The study further established that excessive rainfall triggered the landslide, saturating the soil mass and deteriorating the stability governing parameters, ultimately leading to instability. The findings of this study offer valuable insights for mitigating landslides and can be instrumental in developing effective monitoring and warning systems.

Genetic Mechanisms and Multiparameter Logging Identification of Low-Resistivity Oil Pay: A Case Study of the Triassic Chang 6 Member, Zhidan Area, Ordos Basin, China.

Low-resistivity pay has been found throughout the world. The causes and logging responses of low-resistivity reservoirs are complex and variable. The weak variations in resistivity between the oil pay and the adjacent water pay makes it difficult to identify fluids by resistivity log analysis, which reduces the overall exploration benefit of the oil field. Therefore, it is very important to study the genesis and logging identification technology of the low-resistivity oil pay. In this paper, we first analyzed the core results such as X-ray diffraction, scanning electron microscopy, mercury intrusion, phase permeability, nuclear magnetic resonance, physical properties, electric petrophysical experiment, micro-CT, rock wettability, etc. The results show that: ① the development of low-resistivity oil pays in the studied area is mainly controlled by irreducible water saturation. The complicated pore structure, high gamma ray sandstone, and rock hydrophilicity are the factors that lead to the increase of irreducible water saturation. ② The salinity of formation water and the invasion of drilling fluid also have a certain influence on the variation of reservoir resistivity. ③ In order to magnify the difference between oil and water, sensitive parameters of logging response are extracted according to the controlling factors of low- resistivity reservoir. Then, AC-RILD, SP-PSP, GR*GR*ΔSP-RILD, and (RILM-RILD)/RILD-RILD cross-plots, some overlap method, and movable water analysis are used to identify low-resistivity oil pays synthetically. In the case study, the comprehensive application of the above identification method can effectively improve the accuracy of fluid recognition step by step. It provides reference for identifying more low-resistivity reservoirs with similar geological conditions.

Influence of matrix physical properties on flow characteristics in dual network model

The tight oil formation develops with microfractures and matrix pores, it is important to study the influence of matrix physical properties on flow characteristics. At first, the representative fracture and matrix samples are selected respectively in the dual media, the fracture and matrix digital rocks are constructed with micro-CT scanning at different resolutions, and the corresponding fracture and matrix pore networks are extracted, respectively. Then, the modified integration method is proposed to build the dual network model containing both fracture and matrix pore-throat elements, while the geometric-topological structure equivalent matrix pores are generated to fill in the skeleton domain of fracture network, the constructed dual network could describe the geometric-topological structure characteristics of fracture and matrix pore-throat simultaneously. At last, by adjusting the matrix pore density and the matrix filling domain factor, a series of dual network models are obtained to analyze the influence of matrix physical properties on flow characteristics in dual-media. It can be seen that the matrix system contributes more to the porosity of the dual media and less to the permeability. With the decrease in matrix pore density, the porosity/permeability contributions of matrix system to dual media keep decreasing, but the decrease is not significant, the oil–water co-flow zone decreases and the irreducible water saturation increases, and the saturation interval dominated by the fluid flow in the fracture keeps increasing. With the decrease in matrix filling domain factor, the porosity/permeability contributions of matrix system to dual media decreases, the oil–water co-flow zone increases and the irreducible water saturation decreases, and the saturation interval dominated by the fluid flow in the fracture keeps increasing. The results can be used to explain the dual-media flow pattern under different matrix types and different fracture control volumes during tight oil production.

Productivity Analysis of Fractured Horizontal Wells in Tight Gas Reservoirs Using a Gas–Water Two-Phase Flow Model with Consideration of a Threshold Pressure Gradient

Tight gas reservoirs are valuable yet challenging unconventional resources as a result of their complex flow mechanisms. Typical challenges are characterized by gas–water two-phase flows with a threshold pressure gradient (TPG). In this work, experimental studies are initially conducted to investigate and quantify the influence of water saturation on the TPG. A gas–water two-phase flow model is then established for a fractured horizontal well considering the influence of water saturation on gas–water permeability and TPG, along with the dynamic changes in the water–gas ratio. Sensitive studies are performed to investigate the impact of various parameters on the productivity of a horizontal well. The results demonstrate that, as water saturation increases, the TPG increases and the relative permeability of the gas phase decreases, resulting in the gas well productivity decrease. As the permeability decreases, the changes in the TPG become more prominent, leading to a greater impact on productivity. In comparison to a constant TPG, the changes with water saturation in the TPG have a more significant effect on productivity. Hence, it is beneficial to regulate the rate of the water cut increase to enhance production in tight gas reservoirs.

Experimental study on the role of clay mineral and water saturation in ultrasonic P-wave behaviours across individual filled rock joints

Understanding wave propagation across rock joints is of great importance in rock engineering, petroleum engineering, and other geophysical applications. Previous studies have shown that wave behaviours across filled rock joints are highly affected by the properties of filling materials. However, how clay minerals and water saturation of the infilling affect wave attributes across filled rock joints is still poorly understood. In this study, ultrasonic tests were conducted on kaolinite- and bentonite-filled rock joints to investigate the roles of clay minerals and the water saturation of infillings on elastic P-wave behaviours across individual filled rock joints. The test results show that the infilling's water saturation and mineralogical compositions have combined effects on P-wave signatures of single rock joints filled with clayey soils. Particularly, for the kaolinite-filled rock joint, P-wave velocity and attenuation first increase and then slightly decline with the increasing water saturation. By contrast, for the bentonite-filled rock joint, an increase in water saturation causes gradual decreases in velocity and attenuation. The integrated interpretation of the experimental results indicates that the difference in heterogeneity saturation due to clay hydration is the first-order reason for the discrepancies in saturation-dependent P-wave attributes between kaolinite- and bentonite-filled rock joints. The findings of this study can help further understand wave propagation and attenuation across rock joints filled with fluid-saturated clayey soils.

Research on the seepage mechanism and influencing law of shale oil fracturing

In order to clarify the characteristics of oil and water seepage during shale oil fracture, the tight reservoir in Jimsar Basin, Xinjiang Province was an example to get the characteristics of two-phase seepage between fracturing fluid and shale oil during fracture through the core experiment during injection. Effects of initial water saturation, viscosity ratio of oil to water, injection pressure, core permeability about imbibition displacement efficiency of fracturing fluid were systematically studied by using an improved one-dimensional core displacement device from Xinjiang tight sandstone artificial core under indoor conditions. The research results show that under constant pressure of 1 MPa, initial water saturation of the core is 43%, permeability is 0.12 md, and oil-water viscosity ratio between shale oil and fracturing fluid is 3:1, drive efficiency about fracturing fluid was about 23.5%. At the same time, results indicated at certain injection conditions, when initial water saturation increases, the permeability gradually decreases, and the viscosity ratio of oil to water gradually increases, the lower drive efficiency of fracturing fluid was, the worse its effect is. This conclusion could be used in the process of shale oil tight reservoirs.

Two-phase displacement dynamics in fractured porous media of varying wettability states: A micromodel experimental investigation of matrix/fracture interactions

Matrix-fracture interactions are significant in determining multiphase flow behavior in fractured porous media. We investigated the impact of these interactions on dynamic displacement events and flood-front patterns in a fractured micromodel, with two matrix regions having different permeabilities, of varying wettability states. The effects of injection rate and viscosity were studied during the injection of blank brine and polymer solutions, respectively, under oil-wet (hydrophobic) and water-wet (hydrophilic) conditions. The influence of reducing interfacial tension was examined by injecting a surfactant solution into the oil-wet micromodel. Under oil-wet conditions, brine invaded the high-permeability matrix producing a stepwise increase in water saturation during the low and intermediate rate experiments. The high rate induced fluid displacements within both matrices, but trapped more oil in the high-permeability zone due to what we labeled as the high-rate interruption effect. The stepwise invasion was also observed in the two matrices during the polymer solution injection, contrary to the smoother invasion of the surfactant solution. In the water-wet model, the high-permeability matrix exhibited final fluid saturations resembling those in the oil-wet model after brine injection. However, unlike in the oil-wet model, brine invaded the low-permeability matrix. The polymer solution suppressed trapping and improved the sweep efficiency across the two matrix regions. Our results provide new insights into the pore-scale evolution of fluid saturation during two-phase flow in fractured porous systems under varying wettability conditions.

Георадарные обследования горнотехнических сооружений западной части Арктики

The application of georadar surveys is described as a non-destructive, giving a detailed continuous (profile) picture, method for studying the internal structure and water saturation of mining structures. Examples of surveys of specific objects of large mining enterprises in the western part of the Arctic are given. It is shown that the electromagnetic field induced by the georadar is distorted in areas where the physical properties of rocks and soils differ (heterogeneities, increased fracturing, tectonic disturbances, faults, zones of increased water saturation, etc.). The key parameter of georadar sounding is determined, which is the intensity of wave field distortion, determined by the contrast (ratio) of the physical properties of rocks, spatial orientation and depth of heterogeneity. Based on this, structural features of the rocks of the Zhelezny quarry JSC “Kovdorsky GOK”, the mine Vostochny KF of JSC «Apatit», etc. were revealed. Hidden tectonic faults and heterogeneities of the massif, which pose a potential danger to mining operations and industrial facilities, were identified and traced. On the basis of long-term monitoring georadar surveys of the enclosing dams of the tailings of Kovdorsky GOK JSC, Kola Mining and Metallurgical Plant JSC, Oleniy Ruchey Mining and Processing Complex of NWPC, OLCON JSC, patterns of formation of filtration-deformation processes were established. For the Oleniy Ruchey mine, quality assessments were made for the construction of tailings dams, concrete foundations for mills, and railway loading and unloading terminals. On the island of Svalbard, georadar surveys have clarified the upper boundary of frozen rocks under the site of the thermal power plant of Arktikugol JSC.

Gas relative permeability evaluation in tight rocks using rate-transient analysis (RTA) techniques

The evaluation of effective and relative gas permeability is critical for modeling hydrocarbon recovery from low-permeability (unconventional) reservoirs, and for evaluating the potential for clean energy pathways such as carbon capture and sequestration and hydrogen storage in subsurface. Nevertheless, due to the low flow rates and long experimental times associated with low-permeability (nanodarcy/microdarcy range) sample testing, measuring relative permeability through conventional techniques is technically challenging, time-consuming, and expensive. Additionally, and importantly, none of the routine laboratory techniques for measuring relative permeability can mimic the boundary conditions under which wells are produced from, or injected into, in the field.For the first time, effective and relative gas permeability of partially-saturated low-permeability sedimentary rocks were investigated using state-of-the-art core analysis techniques based on rate-transient analysis (RTA). Conventional and new straight-line analysis methods, commonly applied to production data in the field, were used to estimate effective gas permeability of core plugs under variable stress conditions and water saturations. Five low-permeability siltstone and organic/clay-rich samples, differing in mineralogy, porosity, and pore size distribution, were analyzed for proof-of-concept testing.The observed flow regimes consist of transient linear flow followed by boundary-dominated flow regimes, as previously reported for field data. Satisfactory correlation (average coefficient of variation less than 7%) between permeability estimates using different RTA techniques demonstrates the applicability of this method for effective and relative permeability assessment of partially-saturated ultra-tight reservoirs (down to less than 1.0∙10−5 md). Importantly, test times after the initiation of the production phase, were on the order of minutes, which is significantly shorter than those commonly achievable using conventional techniques (e.g., hours using pulse-decay gas permeability testing). Effective gas permeability (from 1.5∙10−3 to 7.5∙10−6 md), and slip factor (from 715 to −286 psia−1), decreased with increasing water saturation (0–53%), reflecting the lower contribution of slip flow to gas transport. Effective gas permeability decreased with increasing effective stress (500–4000 psia), likely due to reduced effective (transport) pore throat size and increased tortuosity. The reduction of slip factor and the observed stress dependency with water saturation is attributed to the channel flow mechanism.

Study on mechanical properties and energy evolution of coal under liquid nitrogen freezing

To investigate coal’s mechanical properties and energy evolution laws under different liquid nitrogen (LN2) freezing conditions, uniaxial compression experiments were carried out on frozen coal samples, with water saturation and LN2 freezing time as variables. Based on the experimental results, the mechanical properties, acoustic emission characteristics, and energy release laws of coal were analyzed, and the mechanism of mechanical strength enhancement of coal frozen by LN2 was also discussed. The results prove the positive role of LN2 freezing in enhancing coal’s mechanical strength, that is, under the action of LN2 freezing, both the uniaxial compressive strength and elastic modulus of coal increase with the increase in water saturation and LN2 freezing time. Besides, the longer the LN2 freezing time, the longer the time the loaded coal maintains the elastic stage. At the compaction stage, acoustic emission events with large amplitude frequently occur due to the sliding friction between ice particles and the coal matrix. Under the action of LN2 freezing, the water in coal pores turns into the ice with an expanded volume and then fill the pores, which weakens the stress concentration effect at the crack tip and further prevents the crack extension of the loaded coal. With the increase in water saturation and LN2 freezing time, two peak points gradually appear on the curve of elastic energy variation of frozen coal with strain, indicating that LN2 freezing enables water-containing coal to withstand deformation even after the initial load damage. The formation of pore ice has three positive effects on coal’s mechanical strength enhancement, i.e., pore ice bears a certain mechanical strength, plays a cementing role, and reduces the stress concentration at the crack tip.