Fracture Porosity Research Articles

A thermodynamics-based coupled model for multi-phase flowback in deformable dual-porosity shale gas reservoirs

Over the past decades, there has been growing interest in modelling multi-phase flowback in shale gas reservoirs during hydraulic fracturing, primarily due to the significant risk of groundwater pollution. However, the lack of fully coupled simulations and a unified theoretical model has hindered the study of coupled adsorptive hydro-mechanical behaviour and its influence on fluid flowback prediction. In this paper, we have extended the non-equilibrium thermodynamics-based Mixture Coupling Theory to derive the constitutive relationship and governing equation for adsorptive multi-phase fluid flows in deformable dual-porosity media. Helmholtz free energy density has been used to establish the relationship between solids and fluids. The interaction of adsorptive fluids in the pore and fracture space has been fully considered, leading to a new form of constitutive equation, which obtained the molecular adsorptive effect on the rock by introducing adsorption entropy production. The proposed governing equations determine the fully coupled evolution of solid deformation and multi-phase flow, with the consideration of adsorption as well as pore and fracture porosity change. Numerical simulation is then performed to study the flowback of a real shale gas well and the influence of coupled behaviour on model prediction. The simulation shows that the flowback flux is mainly by the fracture (more than 99.9 %) and the overall cumulative volume is 2427.1 m3 within 225 h which accounts for about 5.3 % of the total injected fracturing fluid, and the sensitivity analysis reveals that the manual fracture porosity is the most sensitive factor to the cumulative flowback. This work provides new insights into the flowback process of shale reservoirs in a fully coupled view and the proposed theoretical tool should contribute to the modelling of other adsorptive multi-phase flow problem in deformable dual-porosity media such as carbon storage and coalbed methane production.

A multi-level Stress–Strain relationship and hydraulic characteristics for supporting grain-porous rock fracture system

Fracturing technology, essential for enhancing permeability within oil and gas reservoirs, relies on supported particles to maintain fracture conductivity. A comprehensive understanding of the stress-strain relationships and hydraulic properties of Supported Grain-Porous Media Fracture System (SG-PMFS) is crucial for optimizing hydrocarbon extraction and advancing geomechanical modeling. In this study, we introduced a multilevel spring compression deformation model that categorized SG-PMFS deformation into an equivalent tensile component in matrix pores of the far-field region, equivalent compressive components in matrix pores of the near-field region and fractures, and multilevel compressive components in supporting grains. We derived distributed uniaxial/triaxial stress-strain relationships to quantitatively characterize these multilevel deformation behaviors across distinct characteristic regions. Digital image correlation strain measurement was employed for experimental validation, confirming the model's accuracy. The experimental results identified five distinct characteristic regions within SG-PMFS, with the distribution factors of each deformation component decreasing with distance from the fracture. Self-supported grains significantly enhanced the correlation coefficient of experimental data, reducing the fracture-related bulk modulus from 11.99 MPa to 1 MPa. The degree of grain crushing was directly proportional to the bulk modulus of the corresponding multilevel compressive component. Furthermore, we established constitutive relationships between stress and key hydraulic properties: fracture aperture, compressibility, and porosity. The model predictions and experimental results consistently characterized the relationship between hydraulic properties and stress across different regions. The hydraulic behavior of SG-PMFS under stress was segmented into three stages: porous rock fracture-dominated deformation (0–2.2 MPa), first-level particle crushing (2.2–4.6 MPa), and second-level particle crushing (4.6–25 MPa). The contribution of grain crushing to porosity changes consistently exceeded 50%, making it the dominant factor in the porosity variation of SG-PMFS.

Unconventional reservoir stimulation method based on electrohydraulic discharge shock and frequency resonance technology

To increase the production of unconventional oil and gas, it is necessary to improve reservoir porosity through reservoir reconstruction. High voltage electro-hydraulic discharge can generate strong shock waves, which not only meets the requirements of reservoir reconstruction, but also has the advantages of high efficiency and environmental protection, and has become a research hotspot. In order to further improve the effectiveness of reservoir transformation, a reservoir stimulation method based on electrohydraulic discharge shock and frequency resonance is proposed. Firstly, an electrohydraulic discharge equipment was designed, and according to the material and structure of the designed rod plate electrode, a theoretical electrohydraulic discharge model was built based on the bubble theory. The high voltage breakdown process under the condition of applied voltage of 12 kV and electrode gap of 2 mm is simulated in detail. The simulation results show that the discharge time is within 0.3 μs, and the extremely short discharge time not only produces strong shock waves, but also forms an arc channel to cause circuit damping oscillation. Then, an electrohydraulic discharge experimental platform was built according to the developed equipment. The discharge process was recorded by a high-precision oscilloscope, and the arc formation was photographed by a high-speed camera. The experimental results not only verify the correctness of the theoretical analysis, but also confirm that the developed electro-hydraulic discharge device can generate 0-60 Hz adjustable electrical pulses and has the ability to cause reservoir resonance. Finally, a comparative experiment was carried out with shale core samples to test the influence of resonance factors on the development of internal fractures in unconventional reservoir rocks. The CT scan images show that the shale fracture porosity in the resonance group increased by 46.4 %, which is much higher than that in the non-resonance group (11.8%). To further test the frequency resonance effect on reservoir reconstruction, a field test was carried out on the coalbed methane wells in Shanxi using the developed equipment. The experimental results show that the average gas production rate after on-site construction is increased from 11.46 m3/h to 12.49 m3/h, an increase of 8.99 %, indicating that the proposed technology can significantly promote the development of rock fractures and enhance the production recovery.

Application of machine learning and fuzzy AHP for identification of suitable groundwater potential zones using field based hydrogeophysical and soil hydraulic factors in a complex hydrogeological terrain

The eastern section of West Bengal grapples with limited surface water availability in its hard rock terrain, compounded by a semi-arid climate, variable rainfall, and a plateau topography, prompting communities to adapt groundwater water-use practices, leading to unsustainable extraction and misuse. Thus, the novel objective of the present research was to produce groundwater potential maps by comparing machine learning techniques with a Fuzzy MCDM model using specific field-based conditioning factors. In the first step, 285 wells were identified, of which 70 percent were used for training and 30 percent for the validation of the models. Secondly, field-based conditioning factors including, longitudinal conductance (SC), longitudinal resistance (ρl), transverse resistance (TR), coefficient of electrical anisotropy (λ), resistivity of formation (ρm), fracture porosity (φf), reflection coefficients (r), hydraulic conductivity (K), transmissivity(Tr), bulk density, porosity, permeability, soil moisture content and water holding capacity were used to analyze the association between these conditioning factors and groundwater occurrences. In the following steps, the XGBoost, Random Forest, and Naïve Bayes models were executed using the training dataset, and factor weights were calculated using Fuzzy Analytical Hierarchy Process of Extent analysis method. To validate and compare the performance of four models, ROC curves, AUCs, MCAs, and correlation plots were used. In general, all four models were successful in evaluating the potential of groundwater occurrences. The predictive capability of the XGBoost techniques with the highest AUC values (0.79) and the highest correlation value (0.78) is superior to those of other machine learning and MCDM models. Geophysical survey revealed that transmissivity and hydraulic conductivity of the aquifer of the river basin range from 1.55 to 440.11 m/day and 10.15–2253 m2/day, indicating a moderate to good hydrodynamic potential. Planners and engineers can use such groundwater potential maps to manage water resources effectively.

Use digital rock physics to characterize velocity and attenuation signatures of deep cavity-fracture carbonate reservoirs

Abstract Conventional rock physics experiments and well logging analyses face challenges in deep carbonate formation containing meter-scale cavity-fracture system, since it is unfeasible to extract representative samples from targeted formation. In this study, differing from the conventional digital rock models at the core scale, we construct a digital rock model from realistic outcrop images with meter-scale heterogeneity of cavities and fractures. Using a dynamic stress–strain simulation, we investigate how reservoir type, fracture density and spacing, cavity porosity and size, fluid saturation, and connectivity influence wave dispersion and attenuation. By selecting the simulation frequency of 0.3 MHz approximately corresponding to the field seismic measuring frequency of 30 Hz, the simulation results can reflect the realistic seismic velocity and attenuation characteristics of deep carbonate reservoir rocks. We find that the P-wave velocity decreases linearly with increasing fracture and cavity porosity. The P-wave attenuation is sensitive to variations in fracture density but shows limited sensitivity to changes in cavity porosity and size. Fractures with narrower spacing result in higher attenuation when fracture density remains constant. Deep carbonate rocks with cavity-fracture systems saturated with two immiscible fluids display enhanced attenuation compared to fully saturated conditions. The velocity demonstrates an almost linear increase with water saturation, while attenuation initially increases before decreasing at a peak water saturation of 70%–80%. The connectivity between fractures and cavities significantly influences the attenuation behavior. Our simulation results provide valuable guidance for seismic data processing and interpretation of deep carbonate reservoir rocks with cavity-fracture systems.

Experimental investigation of alterations in coal fracture network induced by thermal treatment: Implications for CO2 geo-sequestration

Hydrogen-rich syngas, produced by underground coal gasification (UCG) process, is a valuable feedstock for chemical industry. However, it can contain large quantities of carbon dioxide (CO2), which requires separation and sequestration to reduce the carbon footprint of the process. This study explores mechanisms influencing coal permeability when CO2 is injected back into the coal through existing gasification chambers. Two main mechanisms affecting permeability are thermal effects associated with underground coal gasification and CO2-related coal matrix swelling.The effects of thermal processes and CO2 flooding on the fracture network of the coal are investigated through a series of comprehensive experiments. Thermal treatment of the studied sample at a temperature of 210 °C shows a notable effect on fracture morphology, increasing width and permeability by up to a factor of 2 and 5, respectively. CO2 flooding, on the other hand, impairs fracture network due to chemisorption driven processes, leading to irreversible changes. Following CO2 flooding experiment, fracture porosity decreases by a factor of three and subsequently permeability declines by 70–80 % within the studied effective stresses range. Our characterization study indicates that thermal treatment significantly increases permeability, compensating for impairments to the fracture network associated with CO2 injection.

Multiscale discrete fracture network modelling of shallow-water carbonates: East Agri Valley Basin, Southern Italy

We investigate the multiscale geometrical and dimensional properties of fracture and fault networks crosscutting Lower Jurassic to Cretaceous shallow-water carbonate rocks. The carbonates are exposed along the flanks of the Viggiano Mt., on the eastern side of the High Agri Valley Basin of southern Italy. Furthermore, we employ the results of field and digital structural analyses to derive the input parameters for subsequent Discrete Fracture Network modelling (DFN) of geocellular volumes whose dimension and architecture resemble those of the investigated sites. DFN modelling is carried out for geocellular volumes representing the studied bed packages (5m-side volumes), outcrops (50m-side volumes), and reservoir-scale carbonate cliffs (500m-side volumes). Notably, modelling is obtained considering the minimum mechanical aperture value obtained in the field for porosity computations and theoretical values of fracture hydraulic aperture for permeability computations. This is because the mechanical aperture values and roughness profiles collected in the field along single fractures are unreliable due to weathering and localized karst-related dissolution. Our findings reveal that a scale-dependent geometry characterizes the Cretaceous carbonates over three orders of magnitude. These results support previously published data, and contrast with those obtained for the Lower Jurassic carbonates, which suffer of some bias due to the quality of the exposures. After DFN modelling, we show that the amount of computed fracture porosity is mainly due to the SB and NSB fractures, and that equivalent permeability is greater within faulted rock volumes. In terms of horizontal permeability, which is the most reliable result after DFN modelling, we document near isotropic horizontal permeability ellipses at all scales of observations. At individual outcrops, we note that the permeability ellipses are elongated parallel to the dominant fault sets that denotes how localized strain affects the directionality of fluid flow. Finally, we summarize the original data in a conceptual model illustrating the modalities of meteoric fluid infiltration through the vadose zone, and then subsequent horizontal flow in the phreatic zone present within the fracture carbonates at shallow depths.

Electro-Chemo-Mechanical Modeling of Composite Cathodes in All-Solid-State Li-Ion Batteries

A model-based understanding can assist and accelerate developing all-solid-state batteries (ASSB). In addition to chemo-mechanical influences within electrode particles (e.g., NMC) [1-2], a solid electrolyte (e.g., argyrodites) introduces additional interfacial interactions between electrode and electrolyte phases [3-5]. The present research derives and implements a coupled multi-physics finite-element model that captures electrochemical, transport, and structural behaviors of composite electrode structures. The models incorporate concentration-dependent and anisotropic material properties that are based on previously published combinations of experiment and density functional theory (DFT). These include stiffness and fracture toughness, porosity and crystallographic orientation, and operating conditions such as charge/discharge rates and external pressure.Figure 1 illustrates predicted stresses developed during electrode manufacturing. The relatively complex cathode microstructure is based on replicating scanning electron microscopy (SEM) images [6]. The composite electrode consists of electrode, electrolyte, and pore phases (Fig. 1a). As illustrated in Fig. 1b, the ASSB synthesis process involves applying and removing high compressive pressure, which causes plastic deformation and introduces residual stresses. Figure 1c shows residual von Mises stresses near electrode-electrolyte interfaces that can be on the order of a gigapascal. The synthesis-generated residual stresses serve as initial conditions for modeling the chemo-mechanics during battery cycling.During cell operation, spatially varying Li concentrations cause material deformation and associated stresses. Figure 1d shows predicted crack nucleation and growth during operation. Depending on the stress levels, crack nucleation and growth leads to cell degradation and capacity fade. The models predict interfacial fracture and phase separations using phase-field fracture theory. In phase-field formulation, the sharp cracks are approximated as diffuse cracks using a process-zone. High values of the phase parameter ξ (Fig. 1d) represent cracked surfaces. The structural disintegration and loss of active surface areas increase the tortuous path for Li/Li-ion transport, which eventually manifests as capacity-fade.The simulations are validated using published experimental work. The modeling approach, which combines phase-field and finite-element algorithms, is implemented using the COMSOL Multiphysics software. The models are expected to inform microstructure/manufacturing design and optimal operating conditions that improve cycling performance and limit/prevent mechanical damage.[1] K. Taghikhani, P.J. Weddle, J.R. Berger, and R.J. Kee. Modeling coupled chemo-mechanical behavior of randomly oriented NMC811 polycrystalline Li-ion battery cathodes. J. Electrochem. Soc., 168(8):080511, 2021.[2] R. Xu, Y. Yang, F. Yin, P. Liu, P. Cloetens, Y. Liu, F. Lin, and K. Zhao, Heterogeneous damage in Li-ion batteries: experimental analysis and theoretical modeling. J. Mech. Phys. Solids, 129, 160, 2019.[3] K. Taghikhani, P.J. Weddle, R.M. Hoffman, J.R. Berger, and R.J. Kee. Electro-chemo-mechanical finite-element model of single-crystal and polycrystalline NMC cathode particles embedded in an argyrodite solid electrolyte. Electrochim. Acta, 460:142585, 2023.[4] A. Bielefeld, D.A. Weber, R. Rueß, V. Glavas, and J. Janek. Influence of lithium ion kinetics, particle morphology and voids on the electrochemical performance of composite cathodes for all-solid-state batteries. J. Electrochem. Soc., 169(2):020539, 2022.[5] P. Minnmann, F. Strauss, A. Bielefeld, R. Ruess, P. Adelhelm, S. Burkhardt, S.L. Dreyer, E. Trevisanello, H. Ehrenberg, T. Brezesinski, F.H. Richter, and J. Janek. Designing cathodes and cathode active materials for solid-state batteries. Adv. Energy Mater., 12(35):2201425, 2022.[6] C. Doerrer, I. Capone, S. Narayanan, J. Liu, C.R.M. Grovenor, M. Pasta, and P.S. Grant. High energy density single-crystal NMC/Li6PS5Cl cathodes for all-solid-state lithium-metal batteries. ACS Appl. Mater. & interfaces, 13(31):37809–37815, 2021. Figure 1

Quantitative characterization and application of deep learning carbonate reservoir

Abstract Quantitative reservoir characterization is an important research task in carbonate reservoir development. At present, the comprehensive evaluation of drilling, logging, seismic, and productivity testing data is usually used, which requires a large number of test data and tedious manual data collation. Deep learning technology can independently learn complex reservoir characteristics, has a strong ability for high-dimensional data structure mining, and has a strong advantage in solving complex nonlinear reservoir description problems closely related to various geological characteristics. Therefore, this paper applies deep learning technology to reservoir characterization. First, a deep neural network model is constructed, and then seven characteristic parameters such as fracture density, fracture opening, fracture porosity, matrix porosity, total porosity, effective thickness, and specific oil production index are taken as input variables. After model training and verification, the effective permeability distribution map of the reservoir in the target area can be automatically generated. The results show that the deep learning method has strong adaptability and practicability for the quantitative characterization of complex reservoirs, and provides a new way of thinking for the quantitative characterization of reservoirs.

Research on the Effect of Fracture Angle on Neutron Logging Results of Shale Gas Reservoirs

Fracture structures are important natural gas transport spaces in shale gas reservoirs, and their storage state in shale gas reservoirs seriously affects gas production and extraction efficiency. This work uses numerical modeling techniques to investigate the logging response law of the thermal and epithermal neutrons in the gas-containing fracture environment at various angles, applying neutron logging as a technical method. To increase the precision of the evaluation of the natural gas storage condition in shale gas reservoirs, the angle of the fractures’ neutron logging data is analyzed. It is found that even in an environment with the same porosity of the fractures, there are significant differences in the logging results due to the different angles of the fracture alignment: 1. the neutron counts in the high-angle (70–90°) fracture environment are 2.25 times higher than in the low-angle (0–20°), but the diffusion area of the neutrons is only 10.58% of that in the low-angle (0–20°); 2. in the neutron energy spectrum, neutron counts are spreading to the high-energy region (7–13 MeV) along with the increase in the angle of the fracture, and the feature is especially prominent in the approximately vertical (60–90°) fracture environment, which is an increase of 528.12% in comparison with the counts in the approximately horizontal angle (0–30°) environment. The main reason for these differences is the variation in the volume of the fracture within the source radiation. This volumetric difference results from the variation in fracture angles (even though the fracture porosity is the same). In view of the above phenomenon, this paper proposes the concept of “effective fracture volume”, which can intuitively reflect the degree of influence of fracture angle on neutron logging results. Further, based on the unique characteristics of shale gas reservoirs and neutrons, this paper provides important theoretical support for the modification of the porosity of the field operation, the evaluation of the physical characteristics of the gas endowment space, and the assessment.

Developing an inflow performance relationship for predicting hydrocarbon recovery from horizontal wells operating in partially naturally fractured reservoirs under solution-gas drive

Inflow performance relationships (IPR) have long been recognized as one of the most important diagnostic tools for predicting hydrocarbon production performance. Current IPR models associated with naturally fractured reservoirs (NFR) are only applicable to fully fractured NFR models. No previous research has investigated the IPR of partially naturally fractured reservoirs (PNFR). This work aims to develop an IPR model for horizontal wells producing from PNFRs under solution-gas drive (SGD). A 3D black oil simulator is utilized to generate the required IPR data. A total of 236 IPR plots and 26 future IPR plots are generated throughout six major cases varying in fracture intensity (FI), dimensionless well length (LD ), matrix permeability (km ), and fracture porosity (ϕf ). Percolation theory validated the existence of a critical FI threshold at which complex IPR behaviors manifest. Sensitivity analysis reveals that IPR shape becomes more complex as LD reduces. ϕf has negligible effects on IPR and future IPR behaviors. A logarithmic cycle proportionality governs the relationship between km and absolute open flow potential (qo,AOF ) in future IPR curves. A generalized IPR model applicable to horizontal wells producing from SGD PNFRs is developed through the non-linear regression of IPR candidates. The model is validated against field data and complex IPR behaviors.

Digital Fracture Porosity Measurement using Digital Outcrop Modelling (DOM)

Abstract Fracture measurements for porosity calculations often experience problems such as barely accessible locations, limited area coverage, and a long time to carry out field data measurements. Therefore, refined alternative methods are necessary to obtain the most favourable data. This study demonstrates fracture measurements through application of Digital Outcrop Modelling (DOM) using the Structure from Motion (SfM) method to utilize digital photogrammetry techniques. This method was chosen because data collection can be done remotely using Light Detection and Ranging (LiDAR) or digital photogrammetry to obtain point cloud data that is used to perform three-dimensional outcrop modeling. In addition, SfM also produces high-resolution data without the need for experts, is cost-efficient and safer than other methods. Hand-held platform Canon 700D digital single lens reflex (DSLR) camera was used in this study to capture data from an area of a square meter located on the riverbed that requires detailed modeling. DOM using SfM is applied to conduct aperture measurements in fractures inside the scan window, later used as data for calculating basement fracture porosity predictions. Field data collection is also carried out to validate data generated by DOM. Calculations achieved from field data and DOM data will be compared to determine differences of the porosity prediction values. Field data collection was carried out on 3 scan windows, each one is an area of 1 m2 and in totals obtained 117 fracture data, consist of 44 extension joints, 34 release joints, 7 shear joints, and 32 non-systematic joints. In this study, 115 photos were taken from scanwindow 1, 162 photos from scanwindow 2, and 158 photos from scanwindow 3 to construct the DOM using agisoft metashape software. The difference in these values can be used as a correction formula or evaluation for further research. The results of measuring basement fracture porosity using data generated by DOM will remain in the range of less than 1%.

Microfacies and Depositional Environments Analyses of the Hartha Formation at Balad Oil Field, Central Iraq

The Hartha Formation (Late Campanian-Maastrichtian) has been recognized as a potential reservoir for hydrocarbons in various oilfields within the Mesopotamian basin in central Iraq. The formation has conformable upper contact with the overlying Shiranish Formation but unconformable lower contact with the underlying Mushorah Formation. This formation has been stratigraphically divided into two main parts. The lower part is particularly significant as it is recognized as an important stratigraphic unit due to the presence of oil shows. Five subsurface sections and many thin sections of the Hartha Formation (Late Campanian-Maastrichtian) were studied to unravel the depositional facies and environments. The sedimentary microfacies of the Hartha Formation in the Balad oil field include mudstone, wackestone, wackestone to packstone, packstone, Packstone to Grainstone, and Rudstone. These depositional microfacies have been subdivided according to their primary and diagenetic constituents into six sub-microfacies. The microfacies have been deposited in deep shelf margin, open shelf, and foreslope of varying salinities and energy levels. Cementation, dolomitization, compaction and pressure solution (stylolitization ), and dissolution are observed affecting variably both ground mass and particles. These diagenetic processes have both positive and negative effects on porosity types, causing fluctuations in porosity levels. The primary pore types identified within the study wells include interparticle porosity, intercrystal porosity, moldic porosity, vuggy porosity, channel porosity, and fracture porosity.

A heat mining strategy for the potential Enhanced Geothermal System of the Acoculco Caldera Complex, Puebla (Mexico): A numerical approach based on Multiple Interacting Continua

As the world faces a transition to low-carbon electricity, energy markets have witnessed a fast expansion of Variable Renewable Energy, such as solar and wind power. In contrast, the installed capacity of geothermal electricity has increased with at a much lower rate. Continuous innovation is required to maintain geothermal electricity as a cost-competitive technology that helps building flexible power generation systems. Here we propose a heat mining setup for the promissory Enhanced Geothermal System (EGS) of the Acoculco caldera complex, which hosts a heat source in low permeability rocks. The conceptual model of the EGS reservoir consists of a fractured-porous layer between impermeable confining beds. An injection-production triplet is located in line with a fault plane that intersects the model area. Under the assumption of hydraulic stimulation to the fault zone, flow of the injection-production system restricts to a fracture network with secondary and variable permeability. Mass and heat transport is modeled following the Multiple Interacting Continua method with the software TOUGH2. We evaluate the performance of the heat mining system considering water and CO2 as working fluids for a given flow rate. We also evaluate the impact of fracture spacing in the reservoir performance. Results show that for a reservoir temperature of 300 °C, the CO2-based system experiences considerable adverse flow conditions that demands a higher pressure differential between the producer and injector (about 60 bar higher than the water-based reservoir). Likewise, the low heat capacity of CO2 imply a considerable limitation for heat mining. While for the water-based reservoir the cooling front extends throughout the entire reservoir at 30 yr of operation, for CO2 this happens in only about 60%. Additionally, the energy extracted per unit time in the water-based reservoir results more than twice the energy extracted with CO2. Finally, our numerical results show that fracture spacings in the range of 10 to 75 m behave similarly in terms of heat mining performance; decreasing the heat exchange area by increasing the fracture spacing from 10 to 75 m produces a minor impact under the assumption of high permeability fractures. Additional constrains for the fracture network, such as geometry, effective porosity and permeability of both fractures and rock matrix, have to be investigated in future research to better understand the possible performance of the production system.

Pore structure characterization and reservoir quality prediction in deep and ultra-deep tight sandstones by integrating image and NMR logs

Pore structure and fracture determine reservoir quality in deep and ultra-deep tight sandstones, however, the limited availability of core data makes it challenging to describe the complexity of pore-throat structure and characterize the wide range of fractures. To fill this gap, a comprehensive analysis was conducted to evaluate pore structure and fractures from multiple perspectives. Integration of core, thin sections, scanning electron microscopy (SEM), high pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) tests are used to evaluate pore structure and characterize fracture of deep and ultra-deep tight sandstones in the Cretaceous Bashijiqike Formation in the Kuqa Depression. Pore structure and reservoir quality types can be divided into dominantly three types according to characteristics of NMR transverse relaxation time (T2) distribution and HPMI curves. Correlation analysis among HPMI and NMR parameters with petrophysical parameters indicate that maximum pore throat size (rmax), logarithmic mean of T2 (T2gm), irreducible water saturation (Swi), and reservoir quality index (RQI) are the most sensitive parameters for pore structure characterization. The Type Ⅰ pore structure is composed of intergranular and intragranular pores, and NMR T2 spectrum shows the highest magnitude, with the calculated mobile fluid saturation content also being the highest. The Type II and III pore structures are composed of intergranular pores, intragranular dissolved pores and intercrystalline micropores in clay minerals. T2gm of Type II and III are smaller, with the calculated mobile fluid saturation content being the lower.The fracture features and parameters including fracture density, porosity, aperture, and length are analyzed using image logs and core. The pore structure characteristics are quantificationally and qualitatively analyzed by NMR logs. Integration of pore structure and fractures by image logs and NMR logs are used for prediction of high quality reservoir and hydrocarbon productivity. High hydrocarbon productivity is obtained in well intervals with abundant natural fractures, and favorable pore structure will also contribute to high matrix porosity. Therefore, NMR logs and image logs are crucial for evaluating reservoir quality and predicting productivity when calibrated with core and analysis data. The comprehensive analysis integrating core data with geophysical well logs provide important insights for reservoir quality prediction of deep and ultra-deep tight sandstones.

Fracture evaluation of the plutonic basement in the Upper Magdalena Basin: Implications for the development of naturally fractured reservoirs in the Northern Andes

Plutonic rocks typically have negligible matrix porosity and permeability. However, fractures and mineral alterations create storage space and flow pathways that turn plutonic rocks into fluid reservoirs. Despite significant hydrocarbon discoveries, naturally fractured reservoirs in plutonic rocks have been poorly studied. In most Colombian basins, the crystalline basement has undergone multiple deformational events and is thrust over the Cretaceous to Cenozoic source and reservoir rocks of the conventional petroleum system. This structural configuration is ideal for the migration of oil into a fractured basement. A multiscale fracture analysis, including field, petrographical and petrophysical techniques was conducted on the Permian and Jurassic plutonic basement of Upper Magdalena Basin in order to understand the controls on brittle deformation, the development of fracture networks and their potential to form hydrocarbon reservoirs. The results indicate that protolith textures and structures, including magmatic and mylonitic foliation, favours fracturing. Dykes exhibit higher fracture density (7–48 fractures/m), porosity (mean = 0.4%) and permeability (mean = 125,818.75 mD) than the host rock (2–25 fractures/m; 0.23%; 12,066.09 mD). Intersection zones from regional faults, are characterized by the highest fracture and lineament intensity. Our results suggest that dyke swarms and interacting damage zones can significantly enhance the reservoir quality of plutonic rocks by providing storage in fractures and fluid pathways to the host rock.

Thermo-Hydro-mechanical coupling analysis of geothermal reservoirs: optimizing extraction capacities by revealing influential factors

Abstract Extraction of high-temperature geothermal resources from reservoirs is a challenge due to the complex interactions between temperature, permeability, and stress fields. Variations in physical fields are critical to thermal reservoir engineering. In this study, the coupling mechanism between temperature, permeability, and stress fields is systematically explored using theoretical analyses and numerical simulations, by utilizing the three-field coupling model. This study models the changes in porosity and permeability evolution during geothermal extraction, emphasizing the importance of the coupling term. The effects of pressure differences between injection and production wells, reservoir temperature variations, and fluid property changes on the temperature distribution and output thermal power within the geothermal reservoir were modeled and analyzed. The results reveal the significant effect of pressure differences between wells, and show that the geothermal extraction capacity of supercritical carbon dioxide (SC-CO2) is 1.83 times higher than that of water. Based on the statistical characteristics of the distribution pattern of natural fractures within the geothermal reservoir, the study simulated the distribution of natural fractures and developed a coupled THM model containing natural fractures. The results show that increasing the porosity of hydraulic and natural fractures can effectively increase the geothermal production capacity, especially the increase in the porosity of natural fractures is significant. These findings provide a key theoretical basis for understanding the THM coupling mechanism during geothermal extraction, and provide substantial support for optimizing the development and utilization of geothermal resources.

STUDY OF MICROSCALE PORE STRUCTURE AND FRACTURING ON THE EXAMPLE OF CHINA SHALE FIELD

Accurate characterization of pores and fractures in shale reservoirs is the theoretical basis for effective exploration and development of shale oil and gas. Currently, the scientific community has developed methods for determining the characteristics of the pore and fracture structure of shales with different resolutions, and also discussed the mechanisms of the influence of the characteristics of pores and cracks on the filtration of shale oil and gas. In this work, firstly, a three-dimensional model of a variable-scale pore-fracture structure was created. Secondly, on the basis of data obtained using computed tomography, the characteristics of the development of pores and cracks at different geometric sizes (25 and 2 mm) were statistically analyzed. Third, pore and fracture structures of mixed, felsitic and laminated felsitic shale have been relatively studied. The results show that: (a) the fractured porosity of group I felsite shale is 0.36–0.89 %; mixed shale of group I is 0.34–0.47 %; mixed shale of group III is 0.12–0.86 %; mixed shale of group III is 0.13–0.15 %. (b) Compared to mixed shale of group I, fracturing in felsite shale of the same group is more developed. In addition, the average fractured porosity of felsite shale is 0.28 % greater than that of mixed shale. After analyzing group III, it was noticed that the cracks of the mixed dolomite shale are less developed compared to the mixed shale of the same group, while the average porosity of the cracked shale is 0.33 % less than that of the cracks of the mixed shale (c). Various components of shale samples are distinguished on a gray scale. High density components are bright white (e.g. pyrite); medium density components — light gray (for example, quartz, carbonate and clay minerals); low-density components — dark gray (for example, organic matter); cracks — black (d). The volume of matrix components is relatively high and constitutes the bulk of the total volume of shale samples. The distribution of high-density minerals has a certain randomness. High-density ellipsoidal or irregular minerals (mainly pyrite) range in diameter from a few microns to hundreds of microns, and their volume is relatively small (e). Three types of microscale fractures are mainly developed in shale: microscale fractures within mineral particles (intragranular fractures), microscale fractures between mineral particles (intergranular fractures), and shrinkage fractures. Intragranular cracks are mainly microscale cracks formed by rigid particles that collapse in a certain direction under stress.

An anisotropic rock physics modeling for the coalbed methane reservoirs and its applications in anisotropy parameter prediction

The elastic properties and anisotropy of coalbed methane reservoirs are still poorly understood due to the complicated pore structure and the existing state of the coalbed methane. Therefore, an anisotropic rock physics model for the coalbed methane reservoirs is proposed based on the related effective medium theories, which focuses on the equivalent calculation of the elastic properties for adsorbed coalbed methane and the characterization of the multiple pore structure. Many factors related to the elastic properties of coal are considered, including the composition, pore structure, adsorbed gas content and pore fluid. The measured ultrasonic and well-logging data demonstrate that the proposed physics model is effective in predicting the elastic constants and velocity anisotropy parameters. The results suggest that the elastic constants increase with the increase of adsorbed gas content to some extent, and the effect of porosity on the elastic constants is much more obvious than that of adsorbed gas content. The velocity anisotropy parameters increase with the aligned fracture porosity and the fracture aspect ratio, and the P-wave anisotropy is more sensitive to these two factors. With increasing organic matter and clay content and porosity, Young's modulus and brittleness index decrease while the Poisson's ratio increases, and the impact of the porosity on the brittleness index is more significant than that of organic matter and clay content. The results are helpful in establishing the relationship between reservoir physics parameters and seismic response and can provide a basis for coalbed methane reservoir prediction and hydraulic fracturing.

Understanding poromechanical response of a biogenic coalbed methane reservoir

Biogenic coalbed methane (BCBM) reservoirs aim to produce methane from in situ coal deposits following microbial conversion of coal. Success of BCBM reservoirs requires economic methane production within an acceptable timeframe. The work reported here quantifies the findings of previously published qualitative work, where it was found that bioconversion induces strains in the pore, matrix and bulk scales. Using imaging and dynamic strain monitoring techniques, the bioconversion induced strain is quantified here. To understand the effect of these strains from a reservoir geomechanics perspective, a corresponding poromechanical model is developed. Furthermore, findings of imaging experiments are validated using core-flooding flow experiments. Finally, expected field-scale behavior of the permeability response of a BCBM operation is modeled and analyzed. The results of the study indicated that, for Illinois coals, bioconversion induced strains result in a decrease in fracture porosity, resulting in a detrimental permeability drop in excess of 60% during bioconversion, which festers itself exponentially throughout its producing life. Results indicate that reservoirs with high initial permeability that will support higher Darcian flowrates, would be better suited for coal bioconversion, thereby providing a site-selection criteria for BCBM operations.