Gas Saturation Research Articles

Displacement of hydrocarbon liquids by water using the models of zonally heterogeneous deformable formations

Relevance. The need to predict the main indicators of the development of a gas condensate deposit represented by elastic zonal heterogeneous reservoirs. In this case, the real PVT properties of a two-phase hydrocarbon system and the rheology of reservoir rocks are taken into account. Aim. Based on the study of the causes of high residual gas saturation and oil saturation of hydrocarbon deposits, it is necessary to solve the problem of displacing hydrocarbon liquids with water injected into the reservoir in zonal heterogeneous reservoirs. In this case, a circular reservoir developed by a central well is represented as consisting of two zones with different reservoir-capacity and rheological properties. Objects. The processes of hydrocarbon system filtration to the central well during displacement by water in a zonally heterogeneous formation. The current position of the water front has a radius r_v. The flooding contour has a radius R_k. It is known that the motion of two-phase hydrocarbon systems in deformable reservoirs is represented by complex nonlinear partial differential equations. An analytical solution of such equations is possible only with the use of special approaches. In this paper, the averaging method and the Khristianovich function will be used to linearize the equations. To predict the well flow rate, an algorithm it is necessary to determine the noted reservoir parameters at any time. For this purpose, we will use the material balance equations for the gas and liquid phases of the hydrocarbon system and the volume of water penetrating into the reservoir. Methods. Solution of the equation of unsteady water filtration taking into account boundary and boundary conditions. Results and conclusions. The results are obtained for forecasting the development of the main indicators of displacement of hydrocarbon liquids to the well, when near the well (in the inner zone) and in the remote part of the deposit (i. e. in the outer zone) the formation has different reservoir-capacity and rheological characteristics. The above approach allows determining the main indicators of a gas condensate deposit development under various technological modes, taking into account the difference in permeability and the nature of deformations of the bottomhole zone and in the remote part of the reservoir. The obtained solution allows forecasting the main indicators of the development of a gas condensate deposit represented by elastic zonal-heterogeneous collectors. The proposed algorithm allows simulating almost any technological mode of injection and well. Thus, it is possible to reproduce the mode of a given water injection rate and a given pressure on the flooding contour.

A North Pacific Meridional Section (U.S. GEOTRACES GP15) of Helium Isotopes and Noble Gases II: Shallow Distributions

AbstractThe distribution of helium isotopes in the upper kilometer of the water column along the GP15 section in the central Pacific reflects the large‐scale patterns of upwelling hydrothermal 3He in the tropics and sub‐polar gyre, tracing two important pathways whereby bottom water exits from the deep Pacific. Heavy noble gas saturation anomalies, particularly in the upper two hundred meters of the water column, are more strongly increased by seasonal radiative heating, while lighter noble gas saturation anomalies are increased more by air injection processes. A similar, seasonally persistent radiative heating feature was observed in the Equatorial Undercurrent, and appears to be replicated in climate system model simulations. The origin of this feature, however, remains a mystery. A heuristic component model explains the noble gas saturation anomaly distributions, separating the influences of air injection, barometric pressure and radiative heating/cooling. Results show cohesive spatial patterns consistent with where water masses originate, their circulation, and gas exchange dynamics in relation to their formation regions. Using this model, we diagnose the distribution of “non‐atmospheric” 4He in shallow waters, which parallels the helium isotope anomaly and silica distributions.

Seismic and ultrasonic frequency measurements on partially saturated rocks under X-ray

Summary Low-frequency laboratory measurements provide direct access to the elastic properties of samples within the seismic frequency band, offering calibration data for seismic survey analysis. Additionally, µCT imaging can quantify actual saturations and provides insights into phase distributions at the pore scale. To conduct laboratory triaxial measurements at seismic frequencies while simultaneously imaging the rock interior, we developed an X-ray transparent low-frequency apparatus. Our apparatus determines rock mechanical properties at seismic frequencies (0.5–150 Hz) and strain amplitudes (10−7–10−5), measuring Young’s modulus, Poisson’s ratio, and attenuation. In addition P- and S-wave velocities at ultrasonic frequencies are measured. We conducted imbibition-drainage experiments to assess the effect of saturation and patch size on seismic and ultrasonic elastic properties in sandstone. Additional tests with liquid and gaseous CO2 reveal the impact of partial CO2-gas saturation. The imbibition-drainage experiment demonstrated that P-wave velocity at ultrasonic frequencies was elevated during drainage and reduced during imbibition. Drainage caused patchy saturation, while imbibition resulted in uniform saturation. This implies that ultrasonic measurements, with wavelengths comparable to the pore fluid patch size, are likely influenced by scattering. In contrast, low-frequency measurements, where the wavelength surpasses the patch size, capture effective medium properties and therefore are not affected by scattering effects. The results of the CO2 test suggest that low-frequency measurements can detect even low gas saturations (4% gaseous CO2). In contrast, ultrasonic velocity measurements primarily reflect the response of the fully saturated sample at low gas saturations and do not indicate a reduction in velocity. Identifying fluid-solid interactions and estimating saturation via µCT imaging is crucial, especially with minimal gas presence. Our combined approach allows precise determination of elastic properties at seismic frequencies and shows the importance of low-frequency over ultrasonic measurements.

Gas saturation and tuning effect investigations in high seismic amplitude anomalies: a case study of the “Taadoy” field in Nigeria

ABSTRACT The assumption that the presence of high seismic amplitude anomalies is mainly due to gas saturation has significantly contributed to drilling failure in “Taadoy” Field. To mitigate failure due to seismic amplitude anomalies, it is imperative to analyse the possible presence of tuning effect in high seismic amplitude anomaly in this field. Seismic to Well tie was used to identify reservoir “Res 500”. Presence of hydrocarbon anomaly was determined using seismic amplitudes cross-plot and attribute analyses. Rock Physics Template (RPT) helped to substantiate the presence of gas in the thin sand and acoustic impedance inversion revealed minimal impedance contrast between encasing shale and “Res. 500”. Wedge model revealed that reservoir “Res. 500” is 10 m thick while the RPT analysis estimated gas saturation of 25%. This implies that the high seismic amplitude anomaly is due to both gas presence and thickness of the reservoir. Amplitudes Variation with Angles (AVA) response indicated high seismic amplitudes at the top and base of the reservoir. The presence of gas causes high seismic amplitude and gradient trends especially at far angles but with minimal effect with increased gas volume. Superposition of seismic amplitudes due to both gas presence and tuning effect produced the high seismic anomaly observed in Res. 500 reservoir.

Construction and characteristic analysis of in-situ combustion production dynamic evaluation equation based on seepage difference of zones

The combustion-displacement relationship is complex of in-situ combustion (ISC), making it difficult to assess the combustion effect on the ground. In this work, combined with the characteristics of ISC production stage and oil-gas-water relative permeability relationship, the production dynamic evaluation equations are established, such as cumulative gas-liquid, cumulative oil-water and water cut. The determination method is obtained of oil saturation and gas saturation in the oil bank. It is found that the evaluation equation is exponential in the production rising and oil bank breakthrough stage, which is linear in the stable production stage. The obtained evaluation curves can fully reflect the stage characteristics of ISC. According to the evaluation equation, the oil saturation in the oil bank of the ISC test area in Xinjiang, China is calculated to be between 0.6 and 0.72, the temperature is roughly 110 °C, and the gas saturation is 0.09. The diagnosis believes that the test area has entered the oil bank breakthrough stage in the later stage. The established evaluation equations can directly use the actual production data to diagnose the characteristics of ISC production at different stages, and can quickly provide guidance for the adjustment of field measures.

The low acoustic impedance of fault zones in the Wisting field, Barents Sea: Gas saturation or damage zone?

The low acoustic impedance of fault zones in the Wisting field, Barents Sea: Gas saturation or damage zone?

Study on the Influencing Factors of CO2 Storage in Low Porosity-Low Permeability Heterogeneous Saline Aquifer

The safety and long-term storage capacity of CO2 geological storage are necessary factors for project design and engineering development. Evaluating the influencing factors of CO2 storage and quantitatively analyzing the sensitivity of each parameter have an important guiding role in the design and development of storage projects. In this paper, the Liujiagou Formation in the northeast of the Ordos Basin is taken as an example. Based on the TOUGH/Petrasim simulation tool, the RZ2D geological storage model is established. Seven influencing factors, namely salinity, temperature, horizontal and vertical permeability ratio, pore geometry factor, residual gas saturation, liquid saturation and pore compression coefficient, were compared and analyzed to control the plume migration behavior, interlayer pressure accumulation and storage capacity of low porosity and low permeability heterogeneous reservoirs, and the sensitivity of each parameter to interlayer pressure and storage capacity was quantitatively analyzed. The simulation results show that the uncertain factors affect the safety of CO2 geological storage to a certain extent by affecting the speed of the residual storage and dissolution storage mechanism. High residual gas saturation and salinity will make CO2 mostly exist in the form of free state, which will adversely affect the safety and storage capacity of CO2 saline aquifer storage. High temperature and high vertical permeability ratio will lead to higher interlayer pressure accumulation, which is not conducive to the safety of the storage project but is beneficial to the storage capacity. Temperature, transverse and longitudinal permeability ratio and pore geometry factor control the propagation velocity of plume. The larger these factors are, the faster the plume velocity is. Higher liquid phase saturation is not better; higher liquid phase saturation leads to a large build-up of pressure in the reservoir and can have an adverse effect on the storage volume. The sensitivity analysis of all factors shows that the liquid saturation and temperature have the greatest influence on CO2 geological storage, and the pore compression coefficient has the least influence. The conclusions of this paper can provide a theoretical reference for the design and development of a CO2 saline aquifer storage project in a low porosity and low permeability reservoir area.

Study on the genesis of deposits of the Productive Series of the Bibi-Heybat field by using a complex of borehole data

The majority of developed oil and gas fields in our country are associated with deposits of terrigenous origin. The results of the research carried out provide a basis to conclude that terrigenous reservoirs are not completely homogeneous. Generally, the individual layers differ in their composi’tion in terms of porosity, permeability, shape, size, etc. Therefore, the heterogeneous characteristics of reservoirs should be determined in advance. For this reason, the study of sedimentation conditions of terrigenous layers and the genesis of deposits can be useful for geologists and geophysicists when discovering new fields. This article examines the issue of studying the genesis of deposits of the Productive Series based on the results of quantitative interpretation of borehole data. Sections of wells NoNo X1, X2, X3, X4 from horizons X, XI, XII, XIII, XIV, and XV of the Productive Series (PS) were conditionally chosen as the area of research in the Bibi-Heybat field. The values of effective porosity and volumetric clay content were calculated using established methods and formulas for selected well sections, and histograms of the variation of these values with depth were plotted. The genesis of deposits was studied on the basis of histograms, and facies of stream, bar, and beach-plain origin were identified. The article also defines intervals for the identification of stream, bar, and beach-plain facies in the studied areas and presents 2D and 3D models characterizing the distribution of their thicknesses in the studied region. The research work also involved building 3D models of the distribution of determined facies thicknesses as well as effective porosity, volumetric clay content, and oil and gas saturation across the wells. According to the 3D model analyses, the permeability coefficient value for the study area ranges from 0-120 mD, the porosity value ranges from 0-20%, and the oil and gas saturation ratio value ranges from 0-100%.

A Review of the Intrinsic Parameters Affecting the Elastic Characteristics of Heterogeneous Carbonate Reservoirs: Insights from Laboratory Assessments

This research provides an in-depth analysis of how various parameters such as mineralogy, density, porosity, temperature, pressure, and structural features impact the velocities of sonic waves in carbonate rocks. Our findings reveal that the mineral composition significantly influences the elastic behavior of these rocks. The density and elastic properties of minerals, especially clay minerals, play a crucial role in affecting porosity and predominant pore types. The porosity of carbonate reservoirs impacts their elastic properties, leading to variations in sonic wave velocities depending on the different pore types present. For a given porosity, the velocities can vary considerably due to the presence of diverse pore types within the pore space. Non-interconnected porosities with spherical or near-spherical shapes, along with microporosity, alter the effective elastic properties of the rock. Additionally, temperature affects the velocity-porosity relationship in rocks, with experimental results showing a decrease in P-wave velocity as temperature increases. Under reservoir conditions, wave velocity in carbonate rocks is influenced by factors such as confining pressure, temperature, gas saturation, and effective stress. Specifically, P-wave velocity increases with confining pressure as soft pores and cracks gradually close, enhancing the dry rock bulk shear modulus. Conversely, rising temperatures cause a slight decrease in velocities and an increase in attenuation. In conclusion, this study enhances our understanding of the physical properties and behavior of carbonate rocks under reservoir conditions, thereby contributing to the exploration and production of hydrocarbon resources.

High velocity nasal insufflation vs non-invasive positive pressure ventilation in the management of acute hypercapnic respiratory failure

ABSTRACT Introduction High-velocity nasal insufflation (HVNI) is a type of high-flow nasal cannula that depends on the dead-space clearance. HVNI utilizes small-bore nasal cannulae resulting in high velocity flow greater than the flow of the wider bore cannulae that were previously in use. Aim Assess the efficacy of high-velocity nasal insufflation in managing acute hypercapnic respiratory failure and compare between high-velocity nasal insufflations and noninvasive positive pressure ventilation (NIPPV) regarding efficacy and outcome in managing acute hypercapnic respiratory failure. Patients and Methods Over 12 months 48 patients admitted with a diagnosis of acute hypercapnic respiratory failure were enrolled in the study. Twenty-four patients were managed with HVNI, and the other 24 patients were managed with non-invasive ventilation. Patients were closely monitored in the respiratory intensive care unit (RICU). Oxygen saturation, respiratory rate, and Arterial Blood Gases (ABG) were measured before the start of treatment, then 1 hour, 4 hours, 2 days, and 3 days after treatment. Patients were allowed to crossover to the other arm of treatment or endotracheal intubation according to the clinical situation. Results pH, P Co2 levels showed similar changes in the two studied groups and improved with time. Treatment failure and mortality rates were also comparable in the two groups. Failure of treatment occurred in 16% in the group managed with HIVNI and in 33% in the group managed with NIPPV. Intubation at the end of 72 hourswas 16.7% in the group managed with HVNI and 25% in the group managed with NIPPV. Conclusion High velocity nasal insufflation is effective in managing acute hypercapnic respiratory failure. HVNI offers an effective way alternative to NIPPV in managing acute hypercapnic respiratory failure.

Data-Driven-Based Full Recovery Technology and System for Transformer Insulating Oil

This study aims to develop an efficient recovery solution for waste transformer insulating oil, addressing the challenge of incomplete separation of residual oil in existing recovery technologies. A multi-module integrated system is constructed, comprising a waste oil extraction module, a residual oil vaporization module, an exhaust gas treatment module, and an online monitoring module. By combining steps such as oil extraction, residual oil absorption, hot air circulation heating, and negative-pressure low-frequency induction heating, the complete recovery of waste oil is achieved. The recovery process incorporates oil–gas saturation monitoring and an oil–gas precipitation assessment algorithm based on neural networks to enable intelligent control, ensuring thorough recovery of residual oil from transformers. The proposed system and methods demonstrate excellent recovery efficiency and environmental protection effects during the pre-treatment of waste transformer oil. Experiments conducted on 50 discarded transformers showed an average recovery efficiency exceeding 99%, with 49 transformers exhibiting no damage to core components after the recovery process. From a theoretical perspective, this research introduces monitoring and control methods for transformer insulating oil recovery, providing significant support for the green processing and reutilization of discarded transformer insulating oil. From an application value perspective, the recovery process helps reduce environmental pollution and facilitates the disassembly of transformers. This enables better analysis of transformer operating characteristics, thereby enhancing the reliability and safety of power systems.

Microemulsion for stimulating wells with high gas and water cut

This article is the first to suggest that microemulsions can also be used to reduce gas and water saturation in the reservoir in the process of well stimulation. This article presents the results of an experimental study on the possibility of using a multifunctional microemulsion for well stimulation at a bottomhole pressure below the saturation pressure and high water cut. It has been found that the treatment of the bottomhole zone with the developed microemulsion leads to a significant reduction in the size of gas bubbles due to a decrease in interfacial tension, which facilitates their removal from the bottomhole zone and increases hydraulic diffusivity of the porous medium for oil approximately five times. In addition, adsorption of the composition on the pore walls, changes the wettability of the walls from oil-wet to the water-wet providing a sliding surface for the oil. The increase in phase permeability for oil, which is proportional to the hydraulic diffusivity and the wettability alteration enhance the oil flow rate by 13.5% and consequently oil recovery by 21%. In order to visualize the effect of microemulsion the simulation of oil flow through porous media was performed using COMSOL Multiphysics.

A novel method for estimating gas and liquid saturation according to the change of temperature field

Nitrogen foam treatment is widely used to control water production in oil wells by selectively reducing water phase relative permeability in oil-water-transition zone. A novel method is proposed to analyze nitrogen gas saturation distribution under varying foam generation pressure drops, which the heat conduction coefficients and heat capacities of nitrogen gas and water are considered. A one-dimensional sand pack model was used to evaluate the impact of gravity on gas saturation distribution, tested in both horizontal and vertical orientations. The gas and water saturation varying along the flow direction of the tube can be calculated according to temperature change. The results indicate that nitrogen injection with pressure drops of 0.9 and 1.2 MPa can generate more evenly gas and water distribution as well as longer foam flow distance comparing to the higher injection pressure drops of 1.5 MPa. Average gas saturation generated in the vertical manner is found to be lower than that of horizontal one due to the higher gas flow resistance in the vertical direction. The simulation result of gas saturation distribution along the tube using Fluent software matches the experimental results well.

THE ORIGIN OF ICE MOUNDS NEAR THE ERKATA-YAKHA RIVER, YAMAL PENINSULA, FROM THE RELATIONSHIP BETWEEN δD AND δ18О ISOTOPE RATIOS

Ice mounds are widespread cryogenic landforms that occur during freezing of water-saturated sediments and in ice segregation or ice injection with the formation of ice cores. Complex mechanisms of occurrence of these landforms have their own characteristics depending on the type of enclosing sediments, water and gas saturation, freezing rate, and other factors. Ice formation processes are often accompanied by explosions of the central part with the occurrence of negative landforms. In response to the widespread occurrence of perennially frozen gas-saturated rocks, these processes are accompanied by gas emissions, gas inflammation. and other, less intense gas shows. It is quite difficult in these conditions to determine the causes of the catastrophic phenomenon, to reconstruct the dynamics of the process, and to understand the role of gas-saturated fluids. In this paper, an attempt has been made to determine the ice formation conditions using isotopic techniques. The isotopic composition of ice can reflect the conditions of ice formation at the time of occurrence of ice mounds and its related possibility of participation of gas-saturated fluids from deep-lying gas-bearing horizons [Buddo et al., 2023, 2024]. The composition of stable isotopes δD and δ18O was determined for three ice mounds in the south of the Yamal Peninsula, where there were catastrophic explosions of ice mounds with the formation of large craters. The results of the study made it possible to reconstruct the conditions of ice mound occurrence and to determine different ice formation modes.

Acoustic wave propagation in a porous medium saturated with two fluids and a solid

Abstract Wave propagation in multi-phase porous media is a significant research topic. There are a series of studies about porous media saturated with a single fluid, a solid and a fluid, two fluids, and three fluids. Some gas hydrate-bearing sediments are typical multiphase porous media saturated with a solid (gas hydrates) and two fluids (water and gas). Based on existing theories of porous media, we develop a theory and give a comprehensive analysis of wave propagation in a poroelastic medium saturated with two fluids and a solid. Initially, we establish the constitutive relations and equations of motion. Based on Biot’s approach for describing the equations of motion in fluid-saturated porous media at the macroscale, the kinetic energy density, potential energy density, and dissipative energy density are derived. After deriving the equations of motion, a plane wave analysis predicts the existence of four compressional waves, denoted P1, P2, P3, and P4 waves, and two shear waves, denoted S1 and S2 waves. Numerical examples are presented to demonstrate how velocities and attenuations of various waves behave with gas saturation, gas hydrate saturation, and frequency. A model degradation to porous media saturated with a single fluid supports the validity of the theory, which enriches the theory of multiphase porous media and provides a foundation for the evaluation of gas hydrate-bearing sediments.

Опасные газонасыщенные объекты на арктическом шельфе Восточной Сибири, Дальнего Востока (Россия) и Аляски (США)

For the first time, a comprehensive megaregional analysis of features of potential gas saturation of shallow deposits of the Arctic seas of Eastern Siberia, Chukotka and Alaska was carried out for 2086 objects revealed in CDP seismic sections with a total length of 40 thousand km. It was established that the gas saturation of shallow deposits in the studied areas depended on their tectonic activity on late stages of sedimentation. The highest density of distribution of potentially gas-saturated objects was revealed in the Laptev, Chukchi and Bering seas; in the East Siberian Sea it was 5-6 times lower. Also, in this sea, a minimal share of objects near the bottom (up to 100 m) was observed. The results as a whole confirm low tectonic activity (dislocation) at the late stages of sedimentation in the East Siberian Sea, which concurs with the pattern of the CDP seismic sections. The results can serve as important information for increasing the efficiency and safety of oil and gas exploration in the studied offshore areas.

Prediction of coal structures and its gas-bearing properties based on geophysical logging parameters: A case study in Anze block, China

Coal structures are widely regarded as a critical influencing factor for the dynamic behaviors of CH4 migration in coalbed methane (CBM) reservoir. In this paper, geophysical logging data were analyzed to explore the logging response characteristics of coal structures, and their application on identification of coal structures by using the machine learning methods. Meanwhile, the correlations between coal structures and gas-bearing properties were revealed. The results show that with the increase in coal deformation intensities, acoustic transit time, caliper logging, compensated neutron, and natural gamma values positively increase and that for density logging and lateral resistivity show a negative correlation. The multi-logging parameter identification models of coal structures were constructed by using random forest algorithm, radial basis function neural network, and long short-term memory neural network, with their accuracy reaching to 96.67%, 93.33%, and 91.67%, respectively. Based on the identification results of RFA model, the highest distribution percentages of cataclastic coal are 50.2%, which is controlled by tectonic activities and buried depth. The origins of gases are mainly thermogenic gases whose average value of δ13C(CH4) is −37.51‰. The gas content in granulated coal is smaller than 12 cm3/g, but it is higher than 15 cm3/g in cataclastic coal, resulting the higher gas saturation of cataclastic coal. The average extension length of artificial fractures in cataclastic coals is nearly two times as long as in granulated coals. It is suggested that cataclastic coal zone is the favorable area for CBM development.

Soft interface instability and gas flow channeling in low-permeability deformable media

Understanding gas percolation through a clay layer or a shale formation is of great importance for the development of a geologic repository for nuclear waste disposal, a subsurface system for gas storage, and an engineering approach for hydrocarbon extraction from unconventional reservoirs. Gas injection experiments have revealed complex dynamic behaviours of gas percolation through water saturated compacted bentonite, characterized by a high breakthrough pressure, rapid breakthrough, a pressure/stress decay after the breakthrough, a relatively high migration rate, high-frequency periodic/nonperiodic variations in flow rate, stepwise rate reductions during relaxation, and low gas saturation over the whole process, all indicating channelling nature of the processes. Using linear stability analyses, we show that this channelling can autonomously emerge from the instability of the deformable interface between the injected gas and the compacted bentonite matrix driven by local stress concentration, pore dilation, and hydrologic gradient. Channel patterns formed would possess a fractal geometry. We further show that, once a percolating channel is established, the gas injected would percolate through the channel in a chain of gas bubbles, also due to the interface instability, resulting in periodic/chaotic variations in gas flow rate. Our work provides a unified explanation for key features observed for gas percolation in low-permeability deformable media. The work also suggests a possibility of designing an engineered barrier system for a nuclear waste repository that can have controllable gas release while limit water transport.

GEOLOGICAL CONDITIONS OF FORMATION AND DISTRIBUTION OF GAS-GEOCHEMICAL FIELDS IN BOTTOM SEDIMENTS OF THE LAPTEV-SIBERIAN SEA ZONE OF THE EAST ARCTIC SHELF

Gas composition of bottom sediments of the Laptev-Siberian Sea geostructural zone (LSZ) was determined to include: helium and hydrogen in concentrations of 0.0028–0.2092 and 0.0012–0.8727 cm3/kg, hydrogen sulfide – 0.0006–0.0072, carbon monoxide – 0.0016–0.0577, carbon dioxide – 0.1482–21.1602, methane – 0.0017–8.3047 and its homologues (in total) – 0.00001–0.0355 cm3/kg. It was found that the values of average concentrations of gases in bottom sediments increase with the depth of their occurrence and exceed the anomaly criteria identified for the East Arctic shelf by 2.6–27.7 times, including: CO2, CO and H2S – by 2.6, 3.0 and 3.4 times, ΣC2–C5 and He – by 9.3 and 10.8 times, CH4 – by 27.7 times; this actually indicates a high degree of gas saturation of bottom sediments of the region. The formation of CO2, CO, H2S, H2, CH4 and ΣC2–C5 anomalies in sediments of the upper and the base of the middle sampling horizon is determined by relatively high contents of sapropelic and humic organic matter and gas influx from the lower horizon. Distribution of anomalies with maximum concentrations of natural gases and He in sediments of the lower horizon, with minimum contents of Corg, is associated with zones of faults, large dislocations, tectono-magmatic and seismic activity, which are the main routes of diffusion-migration of gas transfer to bottom sediments. Formation of concentrations of gases and their anomalies in sediments of the LSZ follows the rules of additivity, i.e. successive accumulation of migratory natural gases of different genesis with prevalence of gas phase and isotopic indicators of more gas-saturated parental source, epigenetic and syngenetic to sediments. It was found that maximum values of average gas saturation with CH4, ΣС2–С5, He and H2 in LSZ are typical for bottom sediments of the flanks of troughs, CO and CO2 – for the arch parts of uplifts. In sediments of monoclinal folds (structural terraces), minimal values of average gas saturation are observed, with the exception of the South Anyui Fault Zone. In general, gas saturation of LSZ sediments is determined by the complex impact of geological factors, with the major ones being: gas content of rock complexes and gas saturation of underlying sediments, fault and fold tectonics, a high degree of tectono-magmatic and seismic activity, a geostructural position, coal content, oil and gas content, as well as the depth of occurrence, organic saturation and material composition of OM in bottom sediments. The influence of the latter predetermined the formation and distribution of five geochemical fields of CH4, three of CO, ΣС2–С5, He and one of CO2, and one of H2 in LSZ.

Numerical investigation on heat extraction performance of supercritical CO2 in depleted oil and gas reservoirs

Using supercritical CO2 (sCO2) for geothermal exploitation not only improves the heat extraction rate and saves injection-production energy consumption, but also gains environmental benefit of carbon sequestration. Compared to hot dry rock, depleted oil and gas reservoirs are nature porous reservoirs harboring abundant geothermal resources, in which artificial reservoir fracturing is unnecessary prior to the geothermal extraction. Besides, a lot of pre-existing oil-gas well networks along with ground facilities can be reutilized, significantly reducing geothermal drilling costs. This paper conducts a numerical investigation on using sCO2 for heat extraction in depleted oil and gas reservoirs. Firstly, A coupled wellbore-reservoir model is established to analyze the flow and heat transfer characteristics of sCO2 in the reservoir and injection-production well. Secondly, the variation of sCO2 gas saturation, production temperature, production rate and heat extraction rate are studied. Finally, the influence of different factors on sCO2 heat extraction performance is examined. The results indicate that the maximum sCO2 heat extraction rate is 18.45 MW, and the temperature rise of sCO2 within the reservoir is still higher than 24 °C after 30 years. To enhance sCO2 heat extraction performance, reducing injection temperature, increasing production pressure and well spacing are advisable. In the case of multi-well scheme, a higher ratio of production wells to injection wells and a decentralized arrangement pattern are encouraged. The findings of this paper are anticipated to provide theoretical basis and technical support for efficient geothermal harvest in depleted oil and gas reservoirs and potential appropriate option in building heating.