Geological Heterogeneity Research Articles

Unravelling the influence of heterogeneities and abstraction on groundwater flow and solute transport in a fractured carbonate aquifer, Sicily, Italy

The migration of a groundwater contaminant plume consisting of light nonaqueous phase liquids (LNAPLs) along the Ionian coastline of Sicily, Italy, has been reported to follow a trajectory that is inconsistent with the regional hydraulic gradient. The influence of several faults affecting the fractured carbonate bedrock aquifer, and groundwater abstraction from a well, were hypothesized to be responsible for the anomalous trajectory of the contaminant plume. A conceptual hydrogeological model was developed for the study area that incorporated structural information derived from geophysical surveys and the mapping of fractures in bedrock outcrops. This conceptual model was incorporated into numerical groundwater flow and solute transport models to simulate the groundwater transport of the light nonaqueous phase liquids. Four model scenarios representing different levels of complexity were tested to assess the relative influence of geological heterogeneity and groundwater abstraction on the migration of the contaminant plume. Results show that underground major discontinuity systems, invoking the presence of the faults in the model domain accounted for the observed migration of the contaminant plume, act as conduits for groundwater flow. Conversely, groundwater abstraction from a well was found to result in relatively minor, localized impacts to the migration of the contamination plume. This study demonstrates the importance of incorporating geological heterogeneity into groundwater modelling and environmental risk assessments associated with the storage of LNAPLs.

Optimising CO2 storage resource utilisation: insights from the Otway Stage 4 field program

The optimised utilisation of storage resources and accelerated commercial carbon dioxide (CO2) storage development is vital for carbon capture and storage to play its important role in reducing emissions at scale. CO2CRC seeks to address this challenge through an international collaboration – the Otway Stage 4 field program – that demonstrates a suite of cost-effective reservoir management techniques with the potential to improve CO2 storage capabilities, particularly in lower-quality reservoirs, maturing them for commercial readiness. Aiming to enhance injectivity, improve sweep, increase model resolution, and develop performance monitoring capabilities, Otway 4 will substantially improve CO2 storage resource usage, furthering economical viability for decreasing CO2 emissions to the atmosphere. Otway Stage 4 includes acquisition and analysis of CO2 saturation and chemical data during plume migration and trapping, combined with investigation of fine-scale geological heterogeneity’s role in CO2 flow dynamics, to refine modelling workflows and, ultimately, develop strategies for optimising commercial CO2 storage; demonstrating whether CO2 microbubbles (MBs) significantly increase storage efficiency compared to standard CO2 injection, thereby unlocking previously untenable reservoirs for CO2 storage; enhancing seismic monitoring to comprehensively assess storage performance and MB behaviour, including quantitative derivation of CO2 saturation; and demonstrating the capability of Distributed Strain Sensing to quantitatively measure geomechanical changes associated with CO2 injection. In addition to Otway 4’s forward plans, laboratory and modelling work, including core flood analysis, to understand CO2 MB and surfactant behaviour at pore scale, dynamic modelling of CO2 flow through heterogeneous reservoir, and selecting suitable injection intervals at the Otway site are discussed.

Experimental investigation to check the feasibility of an under-surface barrier for stratified layers to remediate seawater intrusion considering an inclined boundary

Passive seawater intrusion is recorded due to geological heterogeneity, such as stratification of aquifers. The purpose of this study is to check the performance of a new kind of barrier system for stratified layers with different permeabilities under the influence of an inclined ocean-aquifer interface. The value of permeability ranges from 0.165–0.457 cm/sec. A base case of homogenous aquifer was used to check the feasibility of the glass box model. Coarser material was employed to validate the results with literature by showing similar wedge formation with faster intrusion that reaches the boundary. Toe length to height ratio was in the range of 2.0 to 3.14. Longitudinal dispersion of 1 cm, transverse dispersion of 0.1 cm, and Péclet number of 1.5 were calculated, which falls under the range defined in literature. Results show that the under-surface barrier is much more successful in stopping intrusion when materials with monotonically increasing permeability are considered, as the toe length increases with increasing permeability. For perpendicular stratification, the use of conventional barrier is much less efficient as compared to an under-surface barrier as the toe length to height ratio decreases. Height of intrusion after using an under-surface barrier in different layers decreases subsequently as compared to without the barrier. Application of these results to real-life problems considering factors such as scaling, heterogeneity, applicability of boundary conditions, and temporal dynamics is suggested. Under-surface barriers have merits like ease of installation, fewer complications, etc. Results exhibit that the use of an under-surface barrier starting from the point where the equilibrium of water has reached and extending towards the impermeable strata is the most efficient approach to follow. Sensitivity analysis to check the behavior of saltwater intrusion under various height openings shows that the toe length is affected by the presence of material with higher permeability.

Development of a convolutional neural network based geomechanical upscaling technique for heterogeneous geological reservoir

Geomechanical assessment using coupled reservoir-geomechanical simulation is becoming increasingly important for analyzing the potential geomechanical risks in subsurface geological developments. However, a robust and efficient geomechanical upscaling technique for heterogeneous geological reservoirs is lacking to advance the applications of three-dimensional (3D) reservoir-scale geomechanical simulation considering detailed geological heterogeneities. Here, we develop convolutional neural network (CNN) proxies that reproduce the anisotropic nonlinear geomechanical response caused by lithological heterogeneity, and compute upscaled geomechanical properties from CNN proxies. The CNN proxies are trained using a large dataset of randomly generated spatially correlated sand-shale realizations as inputs and simulation results of their macroscopic geomechanical response as outputs. The trained CNN models can provide the upscaled shear strength (R2 > 0.949), stress-strain behavior (R2 > 0.925), and volumetric strain changes (R2 > 0.958) that highly agree with the numerical simulation results while saving over two orders of magnitude of computational time. This is a major advantage in computing the upscaled geomechanical properties directly from geological realizations without the need to perform local numerical simulations to obtain the geomechanical response. The proposed CNN proxy-based upscaling technique has the ability to (1) bridge the gap between the fine-scale geocellular models considering geological uncertainties and computationally efficient geomechanical models used to assess the geomechanical risks of large-scale subsurface development, and (2) improve the efficiency of numerical upscaling techniques that rely on local numerical simulations, leading to significantly increased computational time for uncertainty quantification using numerous geological realizations.

Numerical modeling of temperature-reporting nanoparticle tracer for fractured geothermal reservoir characterization

Information on the temperature distribution of subsurface reservoirs is essential for geothermal energy development. One of the promising tools to detect the reservoir temperature distribution is temperature-reporting nanoparticle tracers whose functionality has been extensively investigated in both theoretical and experimental ways in the last decade. However, most related studies were limited to simplified geometries and ignored the dynamic interplays of fluid flow, heat transfer, transport and reaction of the temperature-reporting nanoparticle tracer. The response behavior and working mechanisms of such nanotracers in a realistic three-dimensional system still have not been fully revealed through a systematic study. In this work, we develop a numerical modeling approach to simulate field implementation of these nanotracers in a fractured geothermal reservoir. This study aims to evaluate whether the injection of multiple temperature-reporting nanoparticle tracers with different thresholds can be used to estimate the temperature distribution and provide information on the thermal and geological heterogeneities. Several scenarios have been investigated for the geothermal reservoir including homogeneous and non-homogeneous cases (e.g., thermal and geological heterogeneities). Our obtained results from the nanotracer breakthrough curves show that the deviation temperatures in peak concentration values provide an upper limit of the lowest temperature and precise highest temperature for the reservoir temperature range. The deviation temperature of the peak arrival time curve accurately estimates the highest temperature along the main streamlines between the wells. The proposed analysis curves based on the nanotracer breakthrough data were visibly affected by geological heterogeneities including their conductivities and orientations as well as thermal heterogeneities in the geothermal reservoir.

Geothermal harvest from a CO2 storage Reservoir: Coupled impacts of geological channel length and horizontal well configuration

Partial stored CO2 in a suitable geological reservoir could be used as the heat-bearing media to harvest geothermal energy, as a CO2 utilization method integrated with sequestration. Impacts of geological heterogeneity on CO2 sequestration or water-based geothermal production have been widely recognized and extensively studied. However, very limited studies have been conducted to investigate its impact on this emerging CO2-circulated geothermal harvest. The purpose of this study is to evaluate the impacts of a particular heterogeneity type, channel structures in parallel with a horizontal well couplet, on the performance of CO2 circulation and geothermal extraction from thin-layered reservoir blocks. Particularly, the coupled impacts of channel length, horizontal well length, well space, and reservoir extension on the system performance are quantitatively evaluated by response surface models and global sensitivity analysis. The results demonstrate that channels with lengths of 250–500 m and 2000 m lead to the worst and best system performance, respectively. Injection-production well space is identified as the most sensitive parameter, and 700 m is found optimal for system cost-efficiency. Well lengths show moderate impacts on the performance metrics. Lateral boundary cell volume magnifier affects CO2 storage considerably, and 60 is identified as a cost-efficient value for the license area of the integrated system. It is also found that both channel and well lengths are negatively correlated to the CO2 storage/injection ratio, an indicator of the revenue ratio between CO2 storage credit and geothermal energy sales. The findings could be a useful guidance for implementation of CO2-circulated geothermal development in the similar reservoirs of this study.

Petrographical and petrophysical characterization of pre-salt Aptian carbonate reservoirs from the Santos Basin, Brazil

Reservoir quality in carbonates is influenced by various factors such as the depositional environment, burial history and diagenesis processes. Understanding these geological heterogeneities is essential for successful petroleum exploration. This study characterizes Brazilian pre-salt reservoirs and aims to understand how their heterogeneity impacts reservoir quality. We analysed carbonate samples from the Barra Velha Formation (Santos Basin) through an integration of petrographical and core plug descriptions, petrographical facies characterization, porosity and permeability measurements, and image analysis to identify the principal controls on porosity and permeability, pore-size distribution, and groups with similar petrophysical properties using the hydraulic flow unit (HFU) concept. Five facies groups were recognized: Spherulitestone (F1); Shrubstone (F2); Intraclastic Grainstone (F3); Intraclastic Packstone, Spherulitestone with mud and Shrubstone with mud (F4); and Shrub–Spherulite Intercalations and Bioclastic Grainstone (F5). The analysis of porosity and permeability showed that their variations are associated with pore type and cementation rate. Greater contributions of inter-aggregate, interparticle and vugular porosity, combined with a reduced amount of cement, results in higher porosity and permeability but an increase in cement tends to reduce the porosity and permeability. Among the facies groups, F2 and F3 exhibited the best porosities and permeabilities, followed by F1, F4 and F5. From image analysis, small pores (1.5 × 10 −5 –0.01 mm 2 ) are the most common in all rocks. However, these small pores contributed significantly to total porosity only in F4 and some samples of F3. For F2 and F3, the large pores (from 0.01 mm 2 to a maximum of 19.62 mm 2 ) are the main contributors, while F5 has a homogeneous contribution. Finally, the data were grouped into five HFUs: HFU1 and HFU2 represent the zones with the best reservoir quality, primarily composed of F2 and F3.

Integrating multivariate hydrogeochemical analysis with human health risk assessment: An inverse geochemical and statistical modeling approach

Chandigarh, an urban center meticulously planned in India, partially relies on groundwater for its water supply. However, rapid urbanization, geological heterogeneity, and anthropogenic actions have elevated the significance of ensuring water quality. This research employs advanced methodologies including inverse geochemical modeling through PHREEQC, multivariate statistics, and the Nemerow Pollution Index (NPI) to discern the governing factors influencing water quality. Furthermore, a human health risk assessment evaluates the potential non-carcinogenic impacts of NO3− and F− on human health. Results indicate during PRM (pre-monsoon), 90 % of shallow aquifer samples exceed an NPI value of 1.0, denoting water impurities. In POM (post-monsoon), 70 % of both shallow and deep groundwater samples surpass this threshold. Approximately 70 % of PRM deep groundwater samples and 75 % in POM exhibit “hard water” characteristics. Hydrogeochemistry is primarily influenced by rock-water interactions, corroborated by Gibb's diagram and negative values of chloro-alkaline indices (CAI-I and CAI-II) indicating cation exchange reactions. Piper's plot illustrates a prevailing Ca-Mg-HCO3 hydrochemical facies, with oversaturation of calcite, dolomite, aragonite, and fluorite at varying depths. Saturation indices (SI) affirm calcite oversaturation, implying CaCO3 precipitation and CaF2− dissolution. Additionally, 33 % of PRM and 52.4 % of POM shallow groundwater samples indicate carbonate weathering's influence on mineral content. Principal Component Analysis (PCA) and Pearson's correlation demonstrate contributions from irrigation return flows and anthropogenic activities alongside geogenic factors, enriching Na+, SO42−, NO3−, and Cl− concentrations. The combined statistical and geochemical models offer a comprehensive insight into groundwater geochemistry which will help the policy makers and municipality to manage groundwater resources effectively in the area.

Modelling Geological Heterogeneities in a Deep Saline Aquifer for Co2 Storage: Insights from the Paris Basin (France)

Modelling Geological Heterogeneities in a Deep Saline Aquifer for Co2 Storage: Insights from the Paris Basin (France)

Analysis of the causes of flooding of long-term deposits under conditions of high geological heterogeneity

In this work, the object of research was the long-term deposits of liquid hydrocarbons located within the boundaries of fields with high geological heterogeneity, the proportion of which is actively increasing every year due to the selection of a significant part of the “light” oil reserves. Based on the generalization of the results of direct and indirect research on the topic of the work, the key causes of well flooding were identified and their primary sources were identified, which made it possible to successfully describe the mechanisms of formation of the transition zone of the oil-water system, depending on the stage of development. The conclusion is made about the importance of applying relevant and reliable scientific and methodological approaches in interpreting the results of geophysical research to solve various problems of oil field development.

The current chemical composition of inland water bodies of the Volga-Akhtuba floodplain (Russia)

The problem of clean water is one of the most important environmental problems in the world. It is impossible to prevent the occurrence of adverse environmental situations without careful monitoring of the aquatic ecosystems state. The assessment of the current chemical composition of the Volga-Akhtuba floodplain shallow channels (Peschanyj, Dudak, and Dudachenok) in the water-bottom sediments system was carried out. Studied shallow channels dried out during the summer-autumn lowwater period for the last few decades. Studies were carried out after the clearance and ecological rehabilitation of the shallow channels. It was found that almost all the studied indicators in water samples correspond to the established quality standards. There is uneven distribution of heavy metals in the studied shallow channels. It can be caused by the influence of hydrodynamic conditions, changes in water and sediment flow, as well as local factors, including the catchment heterogeneous geology. The analysis of the metals content in bottom sediments with permissible ones made it possible to detect excess for Hg (0.18?0.75 mg/kg), Cd (1?2.12 mg/kg), Mn (370.8?493.3 mg/kg), Ni (2.6?67.9 mg/kg), Pb (14.3?22 mg/kg), Zn (75.2?147 mg/kg). The content of As (1?1.4 mg/kg) and Cu (8.54?28.7 mg/kg) in bottom sediments does not exceed the permissible concentrations. The obtained results will form the basis for a general comprehensive assessment of the clearance and ecological rehabilitation of the Volga-Akhtuba floodplain watercourses. It is necessary to continue monitoring the watercourses state and strengthen efforts to preserve the resilient ecosystem of the Volga-Akhtuba floodplain.

Computer Modeling of the Electrical Conductivity of the Geological Environment in Problems of Monitoring the Reliability of the Foundation of Roads, Buildings and Engineering Structures

Main problem: Monitoring the reliability of the foundation of roads, buildings and engineering structures is an important aspect of geophysical research. Modeling of geological heterogeneities is relevant. Moreover, the use of computer modeling in this problem is associated with computational costs. Purpose: Analysis of the vertical electrical sounding (VES) method for modeling the physical properties of soil and their changes when exposed to elastic vibrations. Methods: To achieve the goal, the vertical electrical sounding (VES) method was used, using a one-dimensional model of the geological environment. The direct VES problem was solved by calculating the Hankel integral. Results and their significance: The study made it possible to model a large number of soil layers using the VES method. The results obtained make it possible to determine the parameters of detected objects and changes in the characteristics of the controlled environment when exposed to elastic vibrations. The significance of the results lies in the possibility of improving the method for modeling the physical properties of soil and taking into account their changes under the influence of elastic vibrations, which helps to increase the efficiency and accuracy of monitoring the reliability of the foundation.

БИОПОЛИМЕРЛІК ЖҮЙЕГЕ НЕГІЗДЕЛГЕН АҒЫНДЫ АУЫТҚУ ТЕХНОЛОГИЯСЫН ҚОЛДАНУДЫ ТАЛДАУ

Annotation. At present, many oil fields are on the third and fourth stages of development, which are characterized by significant volumes of associated water production and low degree of depletion of oil reserves from individual sections of the deposit. The volume of oil produced from the deposits is constantly decreased. This is due to the influence on the operation of wells of complex geological structures of productive formations, and fluid compositions that manifest themselves in production processes. In dense reservoirs with high geological heterogeneity, separate undeveloped areas and areas that are not covered by flooding remain in the development process. In these conditions, in order to increase the efficiency of flooding systems, the task is to maximize the coverage of reservoirs by exposure, which can be achieved by introducing water into low-permeable oil-saturated intervals. In this case, the elastic-capillary-cyclic method and the method of changing the direction of filtration fluid flows in the reservoir have significant possibilities, both in area and in the section. In this case, the elastic-capillary-cyclic method and the method of changing the direction of filtration fluid flows in the reservoir have significant possibilities, both in area and in section. One of the most effective and promising methods of stabilizing oil production is physics-chemical technology based on the application of injection of polymer compositions. This paper provides an analysis of the application of the technology of redistribution of filtration flows in a multilayer field. This technology used a biopolymer-based chemical composition. To study the effect of the biopolymer system on the oil production process, an analysis of changes in the technological parameters of the reacting production wells was carried out. The article provides an assessment of the technological effectiveness of methods for changing the direction of filtration flows in the reservoir based on the analysis of the results of the use of biopolymer compositions at one of Mangystau deposits. Keywords: Well, oil deposit, influence on the formation, flow-deflecting technology, filtration flow, polymer composition, biopolymer system, efficiency

The influence of bedrock faulting and fracturing on sediment availability and Quaternary slope systems, Talla, Southern Uplands, Scotland, UK

In bedrock-dominated upland terrains, local heterogeneity in the erodibility of rock masses is a critical but under-explored factor constraining sediment erosion, mobilisation and transport. Here we examine how fault-related fracturing controls variations in the erodibility and grain-size of bedrock source material at the hillslope-scale. We then assess how this influences the evolution of slope sediment systems using a case-study from the Southern Uplands, Scotland, UK. Faults are associated with fracture densities that are an order of magnitude greater than background joint- and bedding-related fractures in weakly metamorphosed sedimentary rocks. Thus, fault zones are enhanced source areas yielding more abundant, smaller clasts. They are associated with enhanced erosion, gullying and debris flows, and the development of blanket colluvium on steep open hillsides. The orientation at which faults intersect the hillslope constrains the evolution of the sediment system. Faults with trends closely aligned to the direction of slope are associated with higher erosion via confined-channel debris flow activity in strongly coupled gullies. Faults that are oblique to slope direction disrupt and segment gully systems developed on minor transfer faults. Overall, faults that are oblique to slope direction are associated with lower erosion and give rise to decoupling within debris flow systems. Inclusion of geological weighting parameters in the formulation of a sediment connectivity index to characterise the effect of faulting on the erodibility and mobility of source material improves correspondence of the model with observations and provides a simple approach that could be adapted for other sources of geological heterogeneity.

Effect of reservoir heterogeneity on well placement prediction in CO2-EOR projects using machine learning surrogate models: Benchmarking of boosting-based algorithms

Rising Carbon Dioxide (CO2) levels from human activities are driving climate change. Carbon capture and storage (CCS) during enhanced oil recovery (EOR) in underground reservoirs offer both environmental and economic benefits. This method boosts oil production, cuts greenhouse gas emissions, and supports sustainable energy. Precise well placement in CO2-EOR is a crucial task for effective oil displacement, but traditional reservoir simulators are costly. This study explores and compares boosting algorithms, as fast surrogate models, to achieve accurate well placement during CO2-EOR in light oil carbonate reservoirs. The research considers various reservoir scenarios with different geological heterogeneity levels (i.e., homogeneous, moderately heterogeneous, and highly heterogeneous reservoirs). Various parameters, such as injection and production well locations, the distance between production and injection wells in an inverted five-spot pattern, pattern angle, and injection and production rates are explored using a compositional reservoir simulator to assess their impact on the well placement problem. A comprehensive analysis of various boosting algorithms, including AdaBoost, CatBoost, Gradient Boosting, LightGBM, and XGBoost is performed using the simulated dataset to assess their efficacy. The results demonstrate that LightGBM outperformed the other algorithms with the lowest Mean Absolute Error and Root Mean Square Error of 115.3 × 106 $ and 188.2 × 106 $, respectively. Additionally, it demonstrates exceptional speed, averaging 3 to 8 times faster than other boosting algorithms in the three reservoir scenarios. This superior performance coupled with its efficient runtime makes LightGBM the ideal choice for the study objectives. Moreover, the mass balance approach highlights the significant CO2 storage efficiency, emphasizing the effectiveness of CO2-EOR in storing CO2 in underground heterogeneous reservoirs.

Assessing pore network heterogeneity across multiple scales to inform CO2 injection models

Geologic heterogeneity is a key feature that must be considered when translations of scaled data are performed. This paper presents the assessment of geologic heterogeneity using a multiscale workflow that includes image analysis-based methods coupled with well log analysis to provide data in which fractals and machine learning methods estimate the carbon dioxide (CO2) storage resource potential of a reservoir. The heterogeneity of rock properties of the complex Bell Creek reservoir in Montana, USA, was explored at the pore scale (∼nm to mm), core scale (∼mm to m), and well scale (∼cm to m). The data used in this study included advanced image analysis of micro-CT (computed tomography) images (pore scale), thin sections (pore scale), plugs and core images (core scale) and well logs (well scale). The micro-CT images were segmented using a U-net segmentation approach into objects of pores and grains. The segmented images were reconstructed into subvolumes of different sizes. Physical properties (porosity and permeability) and fractal dimensions were calculated for the various subvolumes, and Lorenz coefficient (Lc) values, a single parameter to describe the degree of heterogeneity within a pay zone section, were calculated from thin-section images and well logs. Porosity and fractal dimension values were used to estimate the 188-µm threshold of representative elementary volume (REV) in this study. Both the Lc and fractal dimension values were found to be negatively correlated. When these two parameters are combined, it is possible to discern differences in the complex porous networks of the samples analyzed in this study.

Development of a risk mapping along a railway

The Ramal Trem Turístico (RTT), a 17 km long railway between Ouro Preto and Mariana, a centenary line, currently in operation as a tourist attraction with historical importance. The railroad was built along a steep region, with hundreds of slopes, and eventually slope instabilities compromised the tour. In linear infrastructure projects, a risk assessment is an important tool for risk management, mapping it is the first step. After slope instabilities in 2019, a risk map was elaborated early 2021 for RTT. Risk is obtained by multiplying the probability of a certain event to occur by its consequences. Define its value in a quantitative way demands geotechnical investigation, for precise failure probability. Regardless of its importance, its determination needs high resources, specially for linear infrastructure in a region with heterogeneous geology and pedology. Therefore, for the risk mapping of the RTT, a semi-empirical methodology was adopted, based on the Rockfall Hazard Rating System (RHRS), originally developed for rocky slopes, and adapted for the registration of slopes in soil and in landfill. The aim of the method is to register a series of slope characteristics to be assigned a specific score, framing the slopes within pre-defined risk classes. This methodology was applied to this 17 km railway and identified 286 slopes. In January 2022 extreme rainfall triggered slopes instabilities at RTT. A critical analysis of this event shows a satisfactory result for the applied methodology. Risk mapping is an important tool for risk management, helping to prioritize investments in mitigation measures.

Mechanisms and Operational Strategies of Multi-Lateral Steam-Assisted Gravity Drainage (SAGD) for Heterogeneous Reservoirs

As the SAGD steam chamber and production performance in heavy oil reservoirs under fluvial sedimentation environment are heavily impacted by reservoir heterogeneity, an innovative strategy was proposed in this study coupling rock dilation and multi-lateral wells in SAGD projects to break the mud barriers and achieve uniform steam chamber growth. True tri-axial experiments and numerical simulation were designed to validate the feasibility of this strategy, based on which the branches of the SAGD well pairs were designed and the operational parameters were optimized for different geologic heterogeneity conditions. The tri-axial experiment results indicate that the rock formations in the heavy oil reservoirs of the F oilfield exhibit significant shear dilation effects under low confinement pressure conditions, with a volumetric dilation capacity of up to 7%. The branches should be deployed in an interleaved manner, with a horizontal displacement of 20 m and a vertical displacement of 6 m. The optimal results are achieved when the branches intersect the interbeds, allowing for enhanced steam chamber conformance and enlarged volume. Dilation zones of 3–8 m can be created above the steam-injection horizontal wells and around the branches in the reservoir during the dilation of SAGD steam chambers. The maximum volume of dilation fluid used for hydraulic dilation is suggested to be less than 2000 m3. This strategy has been validated as being successful in the pilot SAGD well pair in the F oilfield, China, with the SAGD preheating time reduced by 50% and an incremental oil rate of 4.5 tones/day, indicating encouraging potentials in similar heavy oil reservoirs.

Effects of geological heterogeneity on gas mixing during underground hydrogen storage (UHS) in braided-fluvial reservoirs

Hydrogen, as an energy carrier, is at the centre of research attention for its potential advantages over electricity to transport and store excessive renewable energy at the GW scale as part of the energy transition. To store energy at such a large scale and in a seasonal manner, energy storage technologies such as compressed air storage and high-temperature aquifer thermal storage are proposed, where Underground Hydrogen Storage (UHS) in porous reservoirs may be an important technology for hydrogen economy. Research studies suggest the necessity of using alternative gas to hydrogen as cushion gas due to the very low density at reservoir conditions and the high production cost of hydrogen. In addition, the potentially lower development cost of UHS in existing depleted natural gas reservoirs or former sites for underground gas storage compared to that of saline aquifers makes gas mixing a real possibility for future UHS operations. However, this topic is rarely studied, let alone its geological governing factors. In this study, we focus on the most likely geological settings for early UHS projects (Depleted gas fields in high permeability braided fluvial reservoirs) to understand the potential impacts of geological heterogeneity on project economics.To quantify the possible gas mixing induced by macro-scale heterogeneity and find the dominant factors that affect storage performance, in this study, we start by building synthetic high permeability water-gas reservoir model with geological characteristics of braided-fluvial systems often encountered in the oil and gas industry. Then we examine the effects of structural and litho-facies heterogeneity on gas mixing processes during typical UHS projects (10 mol% hydrogen-methane mixture as stored gas) via compositional numerical simulation. Homogeneous cases with different injection/production rates are also part of the sensitivity analysis. The cumulative days hydrogen fraction in produced stream is used as the metric for quantifying gas mixing during this process. Our results show that, compared to the homogeneous cases, macro-scale geological heterogeneity will intensify gas mixing and degrade the hydrogen fraction in the produced stream, affecting up to 15.8% of the recovery in 10 years (6% more than the homogeneous cases). Geological structure (reservoir dip angle and closure area) is a first-order determining factor above facies heterogeneity (braided channel dimensions). It determines the level of methane breakthrough during UHS projects in all the test cases, leading to contrasting gas mixing behaviors. Our study hereby provides a systematic method for evaluating gas mixing in UHS projects and facilitates future UHS techno-economic analysis.

Meter-scale geologic heterogeneity in the near surface explains seismic speckle scattering noise

Complex scattering in the near surface can introduce significant distortions in deep reflection data. To model and explain these effects, a multiplicative random noise model based on the speckle mechanism of small-scale scattering has been proposed. While this model effectively captures the observed phenomena in field data, it has been considered rather abstract as it relies on random mathematical clutter to replicate the distortions. This study goes beyond by delving into the analysis of the actual meter-scale geologic heterogeneity found in carbonate formations from desert environments. By employing elastic wave propagation simulations, we show that geologic heterogeneity is equally capable of generating the observed speckle noise in field data when compared to idealized mathematical clutter. Our simulations reveal that the phase perturbations exhibit a quasi-random nature and follow a symmetric near-normal distribution, thereby supporting the validity of the multiplicative noise model and aligning with field observations. Furthermore, we discover that the spread or standard deviation of phase perturbations increases with frequency. This finding provides a plausible explanation for the loss of higher frequencies commonly seen in our data. By considering the complex waveform distortions induced by near-surface heterogeneity, our new noise model represents a significant advancement over residual statics that only account for the kinematic aspect. In summary, our study shows that geologic heterogeneity can easily generate the speckle noise observed in field data. The complex waveform distortions can be captured using quasi-random phase perturbations, as the multiplicative noise model outlines. This advancement leads to a more comprehensive understanding of the influence of near-surface heterogeneity on seismic data. Consequently, this understanding serves as a foundation for despeckling deep reflection data and enhancing the resolution of seismic imaging. These findings have significant implications for improving the quality and accuracy of seismic imaging in areas where speckle noise dominates.