High Fracture Density Research Articles

Developmental characteristics of vertical natural fracture in low-permeability oil sandstones and its influence on hydraulic fracture propagation

Vertical natural fractures (NFs) are prevalent in low-permeability sandstone reservoirs. Presently, the impact of NFs on the extension of hydraulic fractures (HFs) remains partially unveiled, which restricts the scientific development of strategies for low-permeability, fractured oil sandstones. In this study, taking the oil sandstone of the He-3 Member, Hetaoyuan Formation, southeastern Biyang Depression as an example, we conducted a comprehensive investigation into the factors influencing vertical fracture development and the interaction between natural and hydraulic fractures. The cohesive unit simulations indicate that geostress is the principal factor influencing HF expansion, more so than NFs, with this influence intensifying as natural fracture density increases. As natural fracture density grows, the potential for two sets of conjugate natural fractures to form short HFs arises, which are limited in expansion scope, suggesting a need to reduce well spacing accordingly. Conversely, areas with a single set of NFs are more prone to developing longer HFs, warranting an increase in well spacing to avoid water channeling. High natural fracture densities may constrain the effectiveness of HFs. In fractured reservoirs with a 10 MPa horizontal stress difference, the length of HFs is 1.52 times that of HFs with 0 MPa and 5 MPa differences. However, the hydraulic fracture effectiveness index (FE) of the latter is 1.74 times higher than the former. For fractured reservoirs, the expansion capacity of HF length within a 5 MPa horizontal stress difference remains relatively stable; beyond this threshold, the expansion capacity increases with the growing horizontal stress difference, and the fracturing effect eventually deteriorates. Furthermore, as the strength of NFs escalates, the length and modified area of HFs initially decrease significantly before stabilizing. The complexity and FE value of HFs formed under strong natural fracture conditions are heightened, indicating a more effective fracturing outcome.

Integration of 2D and 3D electrical resistivity tomography and 2D gravity modeling to evaluate groundwater investigations in the Burka Uke catchment area, western Ethiopia

The increasing demand for water resources due to the expanding infrastructure and growing population of Uke town, West Ethiopia, necessitates a comprehensive understanding of groundwater potential within the Burka Uke catchment. The aims and objectives of this study are to identify a map of the aquifer by characterizing the subsurface earth, basement rock depth, resistivity and density of geological layers, their thicknesses, and structural features. The investigation integrates electrical resistivity tomography and gravity modeling to delineate potential groundwater zones and their structural controls. Electrical resistivity tomography profiles (1, 4, and 6) revealed low resistivity zones indicative of potential aquifers. The significant resistivity contrast (378 to 1412 Ω.m) suggests a high fracture density, potentially enhancing groundwater storage. The Bouguer gravity anomaly map, separated into regional and residual components, displayed values ranging from 51.79 to 41.59 mGal, highlighting density variations in the subsurface. As a result, analysis of the residual gravity anomaly map identifies north–south striking fault elements, indicating their influence on groundwater flow.The combined 2D and 3D ERT data also revealed a 2D gravity model of the aquifer from which the concentration extended throughout the study area. Data validation with previously drilled wells has confirmed the effectiveness of the geophysical methods used in identifying potential groundwater zones. This study provided critical information for the sustainable management of existing groundwater and demonstrated the effectiveness of integrating geophysical techniques to delineate and characterize aquifers in water-stressed areas.

Structural analysis and evolution of large Venusian coronae: Insights from low-angle faults at coronae rims

We analyzed topography, fracture patterns, and faults of the asymmetric Atahensik Corona (700 × 900 km diameter), formerly known as Latona Corona, and their surrounding troughs using Magellan SAR imagery, and compare the results with the smaller, ovoid Didilia (400 × 450 km diameter) and Pavlova coronae (550 × 650 km diameter) to get insights on corona formation on Venus. Atahensik contains a high density of radial, oblique, and concentric fractures, the latter are inferred to be the youngest fractures. A high density of concentric fractures particularly occurs along the outer rise and indicates elastic downward bending of this part of the lithosphere in the later stage of corona formation. Along the steep inner slopes of Atahensik's arcuate troughs, large-scale faults are exposed that dip gently towards the corona center and crosscut all fractures. We propose that these low-angle faults were initially formed as thrust planes but subsequently became reactivated as low-angle normal faults, thereby exposing parts of their fault surfaces. Such faults have been identified not only along the arcuate troughs of Atahensik but also occur in Dali Chasma, northwest of Atahensik Corona.A phenomenological formation model of large coronae is presented: corona initiation starts with radial fracturing, which is caused by the dike emplacement and thermal uplift of the corona center due to the rise of a hot asthenospheric mantle plume. Uplift and lateral plume spreading steepen the outer rim of the uplift and cause intense radial fracturing of a central volcanic edifice and the corona's outer rim. This intermediate stage is preserved in several less-evolved coronae such as Didilia and Pavlova Coronae. The fractured ridge thrusts outward onto an intact and cooler lithosphere along strongly localized thrust planes. The overthrusted, cooler lithosphere is elastically bent downward and forms arcuate troughs and associated outer rises with numerous concentric fractures along their crest line. The fractured ridge annulus of the corona is supported by the intact and thickened lithosphere surrounding the corona. The present morphology of Atahensik Corona indicates subsequent subsidence in its central part due to declining plume activity and reduced thermal buoyancy. Reactivation of the thrusts as low-angle normal faults results from the subsidence of the corona interior, a gravitational instability of the elevated corona annulus, and a lack of shortening. The evolutionary sequence derived on the basis of structural data is in agreement with geodynamic models on corona formation involving a bending lithosphere at the plume margin.

Numerical simulation of wormhole propagation in fractured carbonate rocks during acidizing using a simplified Stokes–Brinkman model

Most numerical simulations for modeling acid reactive fluid transport and wormhole propagation during matrix acidizing, waterflooding, and CO2 sequestration in carbonate formations are computationally expensive, limiting real-time reservoir management and deep learning training datasets generation for inverse modeling research. Therefore, there is a need for less computationally expensive acid-reactive fluid flow models with adequate accuracy. This study developed and validated a simplified acid reactive-transport model by integrating a simplified Stokes–Brinkman model (as opposed to Darcy’s law), an averaged continuum model, and a pseudo-fracture model. Using FEniCS, the model effectively simulates acid-reactive fluid transport and wormhole propagation in carbonate rocks, achieving a high R-square value of about 0.97 based on a quantitative comparison of the breakthrough volume with other models. The simplified model can also simulate wormhole propagation for the reciprocal of the Damköhler number (1/Da) ranging from 0.001 to 1 with adequate accuracy. Sensitivity studies on the natural fracture parameters such as orientation, length, width, and density showed that higher fracture density, wider fracture aperture, longer fracture length, and orientation aligned with the direction of acid injection contribute to lower pore volume to breakthrough ratio but may not increase long-term acid stimulation efficiency. Also, the presence or absence of fractures in the matrix does not alter the dissolving patterns and optimum injection rate. This simple acid reactive-transport model can generate large training datasets for developing surrogate models in deep learning research. Finally, the FEniCS code in this paper is shared so future researchers can reproduce the results or extend the research work.

Three-dimensional model and environmental fragility in the Guarani Aquifer system, SE-Brazil

The Guarani Aquifer System (GAS) is a vast transboundary continental sedimentary basin in southern South America, encompassing sedimentary rocks and basalt flows across Uruguay, Paraguay, Argentina, and Brazil. It serves as a crucial water resource for urban supply and irrigation. This study aims to propose geophysical, geometric, and fragility models of the GAS in a critical exploitation area, utilizing geological data, geophysical logs, geomorphometry, and information from tubular wells. The models define distinct geophysical ranges for each formation and identify contacts between sedimentary and basaltic rocks. Integration of horizontal and vertical spatial distributions of geophysical and geological data forms a 3D model, revealing basaltic flows and intrusions distribution in fragile areas between formations, and potentially connecting aquifer layers. Natural fragility zones, characterized by high fracture densities, are observed in the central-east region, while outcrop and recharge areas of the main aquifer hydrofacies are found in the north. The 3D model highlights similarities between the topography of the Botucatu Formation (KJb – eolian sandstones) and the potentiometry of GAS at local scales, and overexploitation zones in Ribeirão Preto's city center. The assessment prioritizes areas at risk and conservation strategies for sedimentary aquifers, emphasizing interactions between surface and groundwater and addressing issues of overexploitation, identifying high-impact risks on GAS hydrodynamics and water quality.

Abu Roash F reservoir hydrocarbon potential, Western Desert, Egypt

The Western Desert of Egypt is recognized for its significant potential in oil and gas reserves, making it a prominent hydrocarbon-producing area with conventional reservoirs. Recent findings from various wells suggest that unconventional reservoirs in the Western Desert may hold considerable promise. The primary objective of this study was to explore this potential. Accordingly, a comprehensive and multidisciplinary approach has been employed to assess the Abu Roash/F (AR/F) reservoir. The methodology involved meticulously examining formation characteristics and integrating borehole images with existing core data and thin sections. This integration aimed to analyse and interpret the lithofacies of the reservoir, with a specific focus on identifying fractures and decoding potential diagenesis effects. As a result, we successfully identified wells exhibiting optimal reservoir quality, unveiling the presence of fractured dolomite or vuggy dolomite in wells containing promising oil and gas pay intervals. Notably, some wells displayed low porosity (3%), yet seismic attribute maps in their vicinity indicated a high density of fractures. This underscores the importance of combining borehole images with seismic attributes to evaluate carbonate formations effectively. The fractures, predominantly oriented NW–SE, were found to be associated with deformation zones. Our findings emphasize the critical role of this integrated approach in unlocking the full potential of hydrocarbon reservoirs in the Western Desert.

Natural fractures and their attributes in organic-rich shales: Insights from the Paleozoic Wufeng-Longmaxi formation, southeastern Sichuan Basin

Fractures in organic-rich shale affect the evolution of permeability and control shale gas preservation. We characterize fracture attributes in the Qiyue-Huaying Fold-Thrust belt in the southeastern Sichuan Basin, revealing the distribution, origin and factors controlling fracture localization through investigation of cores, image logs, and thin section petrography. We found that the deformation intensity, organic matter content and lithology are the major factors for controlling fracture occurrence and location in the Wufeng-Longmaxi deep shale. The major fracture pattern in the Fuling Block is characterized by abundant inclined shear fractures, bed-parallel shear fractures, and bed-normal extension fractures, while bed-parallel veins prevail in the Luzhou Block. In general, fracture density and size in the Fuling Block are larger than those in the Luzhou Block. The competent layers (siliceous shale with high TOC) have the highest fracture density, and noticeably, organic matter content controls bed-parallel vein localization. Based on the distribution of fractures in two blocks, we suggest that the dominant origin of fractures in organic-rich shale gradually changes from tectonic events to fluid pressure changes due to organic maturation (organic events), from the Fuling Block to the Luzhou Block.

Integrated Insights into Lithofacies, Fracture Networks, and Reservoir Productivity in Basement Reservoirs: A Comprehensive Case Study from the Cauvery Basin, India, Using Well Logs, Petrographic Studies, and Production Logs

Summary The architecture and heterogeneity of basement rocks can play a crucial role in reservoir evaluation and hydrocarbon exploration. This study focuses on the comprehensive characterization of basement reservoir heterogeneities and fracture networks in the Cauvery Basin using a combination of petrophysical and geochemical analyses. The intricate interplay of lithological variations and chemical alteration processes in basement rocks significantly influences their complex nature. In this study, openhole log data acquired in the basement section from wells within the Cauvery Basin were analyzed. These log data were correlated with core studies to understand the lithofacies heterogeneities present within the basement reservoir. Furthermore, fracture data obtained from resistivity image log data were examined to understand the distribution and properties of fractures in relation to lithofacies heterogeneities. Notably, a higher fracture density was observed in the mafic-rich sections compared to the felsic-rich sections. The integration of resistivity image log data with lithofacies contributed to the understanding of the reservoir architecture. Additionally, elemental spectroscopy logging facilitated geochemical analysis, providing measurements of the elemental composition of basement, which were incorporated into the study. Production logging (PL) was performed in the study wells to ascertain the production from the basement reservoir. This analytical approach augmented the characterization of the basement reservoir and contributed to a holistic understanding of its properties. This study emphasizes the role of conducting a thorough analysis of petrophysical log responses in conjunction with detailed core studies to characterize basement reservoirs. The study successfully differentiated two distinct lithofacies units within the Cauvery Basin’s basement reservoirs: mafic-mineral-dominated and felsic-mineral-dominated and their impact on reservoir heterogeneities. Production log data validated hydrocarbon contributions primarily from felsic rocks, underscoring the importance of lithological variations.

Integration of airborne magnetic and geological data for mineral prospectivity mapping in the Ambam-Amvom rainforest area, Archean Ntem Complex, southern Cameroon

The accurate mapping of mineral resources in rainforest areas is challenging due to dense vegetation and thick lateritic cover obscuring rock outcrops. This study utilized airborne magnetic survey data to map geological features and target mineral resources in the Ambam-Amvom area at the NW margin of the Congo craton. Linear and non-linear filtering algorithms were used, including reduction to the equator (RTE), upward continuation (UC) at 1000 m and 2000 m, first vertical derivative (FVD), and analytic signal (AS). Field surveys were carried out for sampling and petrographic investigations. The TMI map showed magnetic signals varying from -4489 nT to 2317 nT, while the RTE map displayed magnetic signals varying from -2984 nT to 3528 nT. The RTE-Reg map exhibited anomaly signal variations ranging between -2704 nT and +3637 nT and between -2829 and +3864 nT, respectively for UC at 1000 m and 2000 m. Geological structures were interpreted from the FVD, AS, and TMI-RTE images, and linear structures of regional extend mainly trend NW-SE, E-W, and NE-SW. Field investigations revealed that the study area comprises iron formations, amphibolite, gneiss, gabbro, biotite granite, and charnockite. The bulk chemical composition of iron formations showed a high content of TFe, averaging 39.80 wt.% for itabirite, 53.79 wt.% for hematite-geothite ore and 59.63 wt.% for quartz-hematite ore. The zones of peak magnetic anomalies coincided with the occurrence of the greenstone belt, and the integration of aeromagnetic and geological data led to the identification of three main target zones for iron ore minimization. High fracture density zones within the research were identified as high prospective targets for gold mineralization. The study's findings highlight the high potential of the Ntem Complex for mineral resources and provide valuable information for future research and development.

Comparative lithology and alteration mineral chemistry of host rocks at the Seabee Gold Operation, northern Saskatchewan, Canada: Implications for a connected orogenic gold-mineralizing system

The Seabee Gold Operation, located in northern Saskatchewan, Canada, hosts the Santoy and Seabee gold deposits, as well as the Fisher property; the latter is an important exploration target. This gold mineralization is hosted in rocks of the Pine Lake greenstone belt that is within the Glennie domain of the larger Paleoproterozoic Trans-Hudson orogen. Mineralization is focused within shear zones, including the Santoy shear system that extends south onto the Fisher property. The role of ore-adjacent intrusions, systematics of gold mineralization, and relationship between the mineralization of the properties are largely undefined. In this study, hydrothermally-altered samples from locations along the Santoy shear system, namely the Santoy 7, Santoy Gap, and the Fisher property, were analyzed to identify and compare the mineralogical and geochemical characteristics related to sulfide-gold mineralization and associated alteration.The altered host rocks primarily comprise upper greenschist to lower amphibolite-grade sheared metabasalt to chlorite-hornblende-biotite ± garnet schist, and intrusive rocks dominated by granodiorite and tonalite. These lithologies are cut by similar quartz vein generations and are associated with similar alteration assemblages and mineralization, including (i) syn-late-D3 “distal” (∼0.4 to 3 m wide) sericite-biotite-chlorite-clinozoisite ± albite, titanite to vein-adjacent coarse-grained diopside-K-feldspar-albite-actinolite ± apatite, titanite calc-silicate alteration, associated with main-stage Py2a, 2b-Cpy2-Po1-Au (syn-late-D3) mineralization; (ii) remobilized, fracture-filling Py3-Po2-Cpy3-Au-BiTel (late-D3) mineralization within the calc-silicate assemblage; and (iii) early colloform Py1a (pre-syn-D3) and intrusion-hosted, inclusion-rich Py1b (pre-D3). Microprobe analyses reveal similar mineralogical trends between Fisher and Santoy alteration minerals typical of orogenic gold systems. Empirical geothermobarometric calculations of gold-associated vein-adjacent and vein-hosted chlorite and biotite exhibit orogenic-style formation conditions (∼320–330 °C, 3 kbar, ∼9 km depth). As such, these analyses suggest that the Fisher property and Santoy Mine Complex underwent inter-connected, multi-stage hydrothermal processes. These stages comprise (i) a potential early introduction of gold into the system closer to the timing of plutonism (Py1b magmatic-hydrothermal; pre-D1) or during the onset of gold mineralization (Py1a; orogenic; late-D2 to early-syn-D3) during the evolution of the Reindeer zone; (ii) orogenic main-stage mineralization (syn-late-D3) concurrent with late- to post- collisional deformation throughout the Reindeer zone; and (iii) remobilization during continuous deformation (late-D3) where elevated cyclical fluid fluxes and higher fracture density at Santoy produced higher-grade mineralization.

A semi-theoretical method for determining the permeability tensor of fractured rock masses in three-dimensional space

The permeability tensor is a critical parameter for analyzing the hydraulic behavior of anisotropic permeability in fractured rock masses. However, determining this tensor for three-dimensional (3D) fractured rock masses has proven to be challenging and resource-intensive. Both field tests, requiring numerous costly in situ tests, and numerical experiments, hindered by complex discrete fracture networks with a high fracture density, present difficulties in obtaining accurate results. In response, this study proposes a semi-theoretical method for determining the permeability tensor of 3D fractured rock masses, significantly reducing labor and economic costs. The proposed method focuses on establishing the theoretical relationship of directional permeabilities in a 3D space, with emphasis on the properties of the permeability tensor and the influence of fractures' geometry on the flow rate. To facilitate the construction of the method, anisotropic ellipse and ellipsoid are introduced, providing a description of permeability anisotropy. With this innovative approach, engineers can calculate the permeability tensor even when only one value of permeability is available along any flow direction. The utilization of the anisotropic ellipse and ellipsoid concepts helps simplify the determination process. Through numerical experiments, the method is validated and its accuracy demonstrated, making it a valuable tool for analyzing the hydraulic behavior of 3D fractured rock masses.

Petrogenetic of Igneous Complex of Ilota Nanasi Gold Deposit, Gunung Pani, Gorontalo, Indonesia

Ilota Nanasi Au-Ag deposit located in the western flank of the Gn. Baganite, a part rhyodacite dome in Gn. Pani district, Gorontalo Province, where the extensive exploration programs were carried out by PT Gorontalo Sejahtera Mining (a subsidiary of PT J Resources Nusantara) from 2012 – 2020. As the result, a total of 72.7 Mt @ 0.98 g/t Au and 0.85 g/t Ag (2.3 Moz Au and 2.6 Moz Ag) mineral resource was delineated in 2019. This paper mainly aimed to the petrogenesis association of granitoid basement and rhyodacite unit as the host rock in the Ilota Nanasi gold deposit. The geology of Ilota Nanasi is overlain by Late Miocene granodiorite, andesite and diorite basement, Pliocene volcanic complex predominantly dacite – rhyodacite, tuff, breccia, and quaternary deposit as alluvial and surface breccia. Hydrothermal alteration and gold – silver mineralization centered in the porphyritic rhyodacite host rocks where the intense silicification forming a zone around the hydrothermal crackle breccia unit and/or high-density quartz vein, veinlets and stockworks. Mineralization in Ilota Nanasi is interpreted as a low sulfidation epithermal system dominated by a large volume of hydrothermal crackle breccia, intense quartz veining, veinlets, and high-density fracturing. A combined analytical result of petrography and whole-rock geochemistry has been used to assess the petrogenetic association of the rhyodacite and granitoid basement in the Ilota Nanasi. The result of the AFM diagram plot shows sample trends in calc-alkaline magma field. In contrast, the SiO2 – K2O plot distributed in two trends: the older unit was associated with high-K calc-alkaline and the shoshonite series for the younger unit. The changing of magma series is considered as product of crystallization differentiation of evolved parental magma or derived from the melting of mantle material after the mantle or lower crustal was metasomatized during a former episode of subduction.

Seismic wave attenuation (1/Qp) in the crust underneath the Eastern and eastern Southern Alps (Europe): imaging effects of faults, fractures, and fluids

We present a novel three-dimensional model of compressional wave attenuation (1/QP) for the Eastern and eastern Southern Alps in Europe that includes the eastern part of the Adriatic indenter, termed here the Dolomites Sub-Indenter. Our approach employed waveform data from the SWATH-D network, a dense temporary network operational between 2017 and 2019, as well as selected stations of the larger AlpArray Seismic Network. A spectral inversion method using frequency-independent quality factor QP, was applied to derive 3578 path-averaged attenuation values (t*) from 126 local earthquakes. These were then inverted using the damped least square inversion (local earthquake tomography) for the attenuation structure. The resulting QP model, which builds on and complements a previously calculated 3-D velocity model (VP and VP/VS), exhibits good resolution down to ~ 20 km depth. Several anomalies can be correlated with the distribution of other physical parameters (VP and VP/VS) and regional tectonic features. Notably, the Friuli-Venetian region exhibits the highest attenuation (lowest QP) anomaly, coinciding with low VP values and increased VP/VS. This anomaly is likely associated with a high density of faults and fractures, as well as the presence of fluid-filled sediments along the active thrust front in the eastern segment of the Southern Alps. Another intriguing observation is the low attenuation (high QP) anomaly along the northwestern edge of the Dolomites Sub-Indenter (NWDI), located south of the Periadriatic fault and east of the Giudicarie fault, where seismicity is notably absent. This anomaly coincides with Permian magmatic rocks at the surface and may be a measure of their strength at depth.Graphical

Identification of Mud Volcano’s Structure using Gravity Satellite and Fault Fracture Density Analysis: Case Study Ciuyah Mud Volcano, Kuningan, West Java

Mud volcanoes, known as mud extrusion phenomenon, is a geological feature that expels suspended fine-grained sedimentary materials and fluids to the surface due to buoyancy and pressure difference. This geological feature is found almost all over the world and formed in compressional tectonic environments, one of which is the Ciuyah Mud Volcano, Kuningan, Indonesia. Previous studies have shown that the appearance of the mud volcano was influenced by tectonic activity that formed a ‘hypothetical’ fault structure as a pathway for mud migration and extrusion to the surface. Integration of geophysical studies using satellite gravity and geology using fault fracture density analysis was conducted to prove the existence of the ‘hypothetical’ fault structure. The results show that the mud volcano site is located in a low to high gravity anomaly pattern associated with significant density contrast differentiation followed by the maximum value of FHD and low-high SVD pattern oriented west southwest - east northeast (WSW - ENE). The gravity anomaly pattern can be associated with the presence of faults. This is reinforced by the fault fracture density map which shows that the mud volcano site is located in a medium to high fracture density zone (weak zone) associated with good permeability conditions below the surface. Thus, the research results have proven the existence of a ‘hypothetical’ fault as the migration and extrusion pathway of Ciuyah Mud Volcano mud that has been studied previously.

Using fracture-based continuum modeling of coupled geomechanical-hydrological processes for numerical simulation of hydraulic fracturing

This paper describes numerical simulation of hydraulic fracturing using fracture-based continuum modeling (FBCM) of coupled geomechanical-hydrological processes to evaluate a technique for high-density fracturing and fracture caging. The simulations are innovative because of modeling discrete fractures explicitly in continuum analysis. A key advantage of FBCM is that fracture initiation and propagation are modeled explicitly without changing the domain grid (i.e. no re-meshing). Further, multiple realizations of a preexisting fracture distribution can be analyzed using the same domain grid. The simulated hydraulic fracturing technique consists of pressurizing multiple wells simultaneously: initially without permeating fluids into the rock, to seed fractures uniformly and at high density in the wall rock of the wells; followed by fluid injection to propagate the seeded fracture density hydraulically. FBCM combines the ease of continuum modeling with the potential accuracy of modeling discrete fractures and fracturing explicitly. Fractures are modeled as piecewise planar based on intersections with domain elements; fracture geometry stored as continuum properties is used to calculate parameters needed to model individual fractures; and rock behavior is modeled through tensorial aggregation of the behavior of discrete fractures and unfractured rock. Simulations are presented for previously unfractured rock and for rock with preexisting fractures of horizontal, shallow-dipping, steeply dipping, or vertical orientation. Simulations of a single-well model are used to determine the pattern and spacing for a multiple-well design. The results illustrate high-density fracturing and fracture caging through simultaneous fluid injection in multiple wells: for previously unfractured rock or rock with preexisting shallow-dipping or horizontal fractures, and in situ vertical compressive stress greater than horizontal. If preexisting fractures are steeply dipping or vertical, and considering the same in situ stress condition, well pressurization without fluid permeation appears to be the only practical way to induce new fractures and contain fracturing within the target domain.

Characteristics and controlling factors of natural fractures in deep lacustrine shale oil reservoirs of the Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China

Maximum burial depth of the Permian Fengcheng Formation, which contains significant oil resources in the Mahu Sag, exceeds 4500 m constituting a deep reservoir. Fractures are widespread in the deep lacustrine shale, providing important storage space and fluid conduits. Based on cores, borehole image logs, and petrographic analysis, we identified and analyzed the types and characteristics of fractures and conducted X-ray diffraction (XRD) and Cathodoluminescence (CL) tests to investigate the factors influencing fracture development. We found faults and three types of opening-mode fractures: bed-bounded fractures, unbounded fractures and bed-parallel fractures. Mechanical stratigraphy (depositional rock types modified by diagenesis) and proximity to faults, are main controls of opening-mode fracture occurrence and attributes. Specifically, Dolomitic mudstone with a high carbonate content shows the highest fracture density of 5.24 m-1. Bed-bounded fractures are mainly developed within brittle layers, with fracture density lower in thicker beds. On the other hand, unbounded (tall) fractures primarily occur near reverse faults (0–1500 m), with orientations parallel to extension directions implied by the kinematics of nearby faults. Additionally, the effectiveness of early-formed fractures as fluid conduits is likely modified (reduced) by cementation and enhanced by dissolution. Fractures have a positive influence on oil production, corresponding to high fracture density and aperture in the vertical direction.

A Hybrid Approach to Rock Pre-conditioning Using Non-explosive Demolition Agents and Hydraulic Stimulation

This study presents a novel approach to rock pre-conditioning to promote the sustainability of low-grade ore mining applications such as in-situ recovery and cave mining. The proposed method involves a two-stage hybrid approach, utilizing soundless cracking demolition agents (SCDAs) to initiate radial fractures in a predrilled host rock, followed by hydraulic stimulation to extend the fractures. SCDA injection in the first stage creates multiple radial fractures around the injection well. However, the extent of fractures is limited to the near vicinity of the injection well. To overcome this limitation, the second stage involves the application of hydraulic stimulation to extend the initiated fractures, which produces a greater fracture density compared to pure hydraulic stimulation. The concept was assessed using a fully coupled hydro-mechanical discrete element model that simulated the hybrid fracturing method on crystalline rock at the grain scale. The results indicate that the proposed method can create a high density of fractures around the injection well. Additionally, we identify and evaluate the key factors affecting the performance of the proposed method, including rock mass heterogeneity, stress anisotropy, and pre-existing defects, providing valuable insights for further experimental design and execution. Overall, the study offers promising results for a potential solution to enhance the efficiency of low-grade ore mining through the hybrid rock pre-conditioning method.

Natural fractures in a metamorphic buried hill reservoir, Bozhong 19–6 area, Bohai Bay Basin, China

The Bozhong (BZ) 19-6 gas field in the Bohai Bay Basin is a fractured metamorphic reservoir with proven reserves of over 1000 × 108 m3. However, the fracture development in the Archaean metamorphic buried hill reservoir in the BZ19-6 area is still not clear. In this study, cores, thin sections, imaging logs and laboratory measurements were used to elucidate the influences of lithology, tectonics and weathering on the formation and development of natural fractures. Both tectonic fractures and dissolution fractures are well developed, and 96.8% of these are interconnected. Lithology exerts the primary control on fracture formation, while stress and weathering influence the development of the fractures. Leucogneiss, cataclasite and gneiss with high contents of brittle minerals are favorable for fracture formation. Buried hill reservoirs near faults commonly exhibit a high linear density of fractures due to greater stress perturbation. Based on the degree of weathering in the study area, a metamorphic buried hill reservoir may be divided vertically into three zones: a weathered fracture zone, an inner fracture zone, and the basement. Fracture linear density and fracture aperture decrease significantly with a decrease in the intensity of weathering, while paleo-topographic highs with more intense weathering have a higher fracture density. A reservoir evolution pattern is proposed that can be used for targeting the favorable reservoirs in the study area.

The correlation between blockiness and the existence of a hydraulic conductivity representative elementary volume of fractured rocks

Whether the hydraulic conductivity representative elementary volume (KREV) exists is a fundamental question for understanding the hydraulic behavior of fractured rock masses. The fracture density and size of rocks vary greatly, and are the two most important parameters affecting the existence of a KREV. The International Society for Rock Mechanics (ISRM) presented quantitative descriptions of fracture persistence and density, dividing the persistence into 5 rates and the density into 7 rates, defining 35 basic types of isotropic fractured rock masses with different persistence and densities. Based on ISRM classification suggestions, we construct an additional 40 anisotropic fractured rocks and conduct a systematic investigation of the KREVs of all 75 rock types. The 3D DFNs of the 75 rocks are established with the Monte Carlo method; the water flow in the DFNs is simulated with the finite difference method; the equivalent conductivities of each rock are calculated in 10 domain sizes and 21 different flow directions; and the optimum conductivity ellipsoid and the fitting errors of the equivalent directional conductivity vectors are analyzed to determine the existence and size of the KREVs. A KREV is more likely to exist in rock masses with a high fracture density and persistence. The effect of fracture density and persistence can be comprehensively represented by blockiness, defined as the ratio of the volume of isolated blocks formed by fractures to the total rock volume. A strong correlation is found between the blockiness and the existence of a KREV. A KREV always exists when the blockiness is higher than 0.5%, and its size is between 2 and 18 times the fracture spacing. A KREV may or may not exist in the range of 0.1% < B < 0.5%. When B < 0.1%, a KREV does not exist.

How high can fracture porosity become in the ultra-deep subsurface?

Knowledge of how high can fracture porosity become in the ultra-deep burial conditions is important but remains problematic. Fracture aperture and porosity are measured using X-ray computed tomography (CT) at atmospheric pressure and then calculated by image logs. Special attention is paid to how high fracture porosity can become in ultra-deep (>6000 m) settings, and which situations will result in high fracture porosities. In situ stress magnitudes, which can be calculated using well logs, control fracture performances, and dissolution along fracture improve fracture porosity at ultra-deep burial depths. Low horizontal stress difference (Δσ < 25 MPa), very high fracture density will result in a high fracture porosity. Fracture porosity can keep as high as 2.0% in relatively low in situ stress conditions even at ultra-deep burial depths. In intense in situ stress conditions (Δσ > 45 MPa), a high degree of dissolution along the fracture dramatically increases fracture porosity. Dissolution will result in the vuggy fracture planes and improve fracture porosity up to 2.0%. The results provide insights into the detection, characterization, and modeling of subsurface fractures.