Natural Fractures Research Articles

CO2 Sequestration in a Carbonate Saline Aquifer: An Investigation into the Roles of Natural Fractures and Well Placement

CO2 Sequestration in a Carbonate Saline Aquifer: An Investigation into the Roles of Natural Fractures and Well Placement

A Laboratory‐Validated, Graph‐Based Flow and Transport Model for Naturally Fractured Media

AbstractFractures are a primary feature controlling flow, transport, and coupled processes in geologic systems. To date, experimental image‐based observations of these processes have been challenging. Here, we use pulse‐tracer experiments with a conservative radiotracer ([18F]‐fludeoxyglucose) spanning multiple flow rates with simultaneous positron emission tomography imaging to characterize transport in a 5.08 cm fractured Sierra granite core. A graph‐based, laboratory‐validated flow and transport model is successfully demonstrated to describe the conservative solute transport in the natural fracture. Model network complexity, determined by the number of nodes and edges, significantly impacts model fit to observed data. Large graphs over‐describe a fracture plane and act similarly to a porous medium while small graphs oversimplify the solute transport behavior. To our knowledge, this work provides the first validation of graph‐based flow and transport models across a range of experimental conditions and sets the groundwork for upscaling to more complex and computationally efficient fracture models.

The propagation laws of hydraulic fractures under the influence of natural fracture zones

This paper uses the finite-discrete element method (FDEM) to establish a fluid–solid coupling model for the propagation of multi-cluster hydraulic fractures. The validity of the proposed model is verified using the analytical solution and onsite microseismic data. The results show that with the approach angle increases, the propagation of hydraulic fracture transitions from a single capture mode to the mixed modes of capture, crossing and blocking. The total length of hydraulic fractures is negatively correlated with the approach angle and cohesion of natural fracture, and positively correlated with the distribution density and bandwidth of natural fracture. The length of shear fractures initially increases and then decreases with the increase in approach angle, showing negatively, positively, and positively correlated with cohesion, length, and bandwidth, respectively, while the trend of tensile fracture length is opposite to it. The tortuosity of hydraulic fractures (which is used to characterize the complexity of fracture morphology) initially decreases and then increases with the increase in the approach angle, negatively correlated with the cohesion of natural fracture, and positively correlated with the distribution density and bandwidth of natural fractures. Comparatively, the fracture propagation morphology is the most complex at the approach angle of 45°, which is more conducive to full reservoir enhancement. Conversely, under conditions of high cohesion, narrow width, and low-density distribution of natural fractures, the fracture morphology is relatively simple, all of which are unfavorable for reservoir enhancement in the target segment.

Subsurface Natural Fracture Identification Using an Integrated Ensemble Learning Method

Subsurface Natural Fracture Identification Using an Integrated Ensemble Learning Method

Numerical investigation of multiphase flow through self-affine rough fractures

Multiphase flow through fractures has great significance in subsurface energy recovery and gas storage applications. Different fracture and flow properties affect flow through a fracture which is difficult to control in laboratory experiments. Here, we perform lattice Boltzmann simulations in an ensemble of synthetically generated fractures. Drainage simulations are performed at different capillary numbers, wettability, and viscosity ratios. We track the invading front and quantify breakthrough saturations and show that roughness and wettability have a strong effect on fluid invasion through a complex fracture. Invading a more viscous fluid results in more stable displacement regardless of the capillary number while at very low capillary numbers, fluid migration is dependent on the inherent structure of the fracture. We develop a fluid displacement phase diagram in a single rough fracture and compare our results from that in the literature. Finally, we extend the phase diagrams across multiple fractures and demonstrate the importance of natural fracture features of roughness and wettability in identifying stable versus unstable displacement regimes during multiphase flow through rough fractures. Our work presents an end-to-end numerical pathway for testing on experimental data and expanding numerical data sets for testing combinations of different physical phenomenon and make valuable predictions on fluid flow through rough fractures.

MODELING OF WATER-OIL FLOW IN SHALE POROUS STRUCTURES: EFFECT OF WETTABILITY AND CAPILLARY NUMBERS

Shale oil reservoirs are characterized by dense, extremely low permeability, and poorly developed natural fractures. Hydraulic fracturing technology is often used in extraction to improve recovery. It is important to clarify the mechanism and influence mechanism of displacement in complex porous media coupled with fractures and matrix to enhance oil recovery. In this study, based on the lattice Boltzmann method (LBM) utilizing the fracture-matrix pore coupling model, the authors carried out a study of displacement in organic and inorganic pore space. They systematically investigated the influence mechanisms of wettability and capillary numbers on the oil recovery rate. It was found that the stronger the wettability of the water phase, the higher the oil recovery rate, the lower the residual oil in the form of adsorbed oil film, the larger the capillary numbers, and the higher the oil recovery rate. Oil in organic pore space is more difficult to discharge compared with that in inorganic pore space, and the recovery rate of oil in organic pore space can be effectively improved by increasing the driving pressure and enhancing the properties of the water phase (fracturing fluid).

Three-Dimensional Lattice Modeling of Interaction Behavior Between Hydraulic Fractures and Natural Fractures with Varied Morphologies in Hot Dry Rock

Three-Dimensional Lattice Modeling of Interaction Behavior Between Hydraulic Fractures and Natural Fractures with Varied Morphologies in Hot Dry Rock

Integrated approach to determining the effectiveness of the reservoir pressure maintenance system in carbonate reservoirs

Relevance. The need to improve the oil recovery factor from carbonate reservoirs by increasing sweep/displacement coefficients, as well as by searching for ways to improve the efficiency of the reservoir pressure maintenance system. Aim. To increase the efficiency of developing carbonate reservoirs using a reservoir pressure maintenance system through an integrated approach to studying the impact of injected fluid on reservoir systems with natural and man-made fracturing. Objects. Bashkir (C2bsh) and Vereisky (C2vr) Middle Carboniferous objects of the Dachnoe structure of the Dachnoe deposit of the Republic of Tatarstan, tectonically confined to the Ulyanovsk structure of the South Tatar arch. Methods. Seismic, acoustic, radioactive, hydrodynamic methods, as well as methods of ground microseismic monitoring to identify the nature of the activation of natural and man-made fracturing during hydraulic fracturing and cyclic injection into injection wells. Results. The authors have carried out an analysis of the hydrodynamic system of carbonate reservoirs of the Vereiskian-Bashkirian deposits; identified filtration channels; assessed the direction of injected fluid movement; carried out the analysis of the impact of the formation pressure maintenance system on producing wells at various injection modes and compensation levels in carbonate reservoirs. When carrying out hydraulic fracturing in the reservoirs under consideration, it was established that the crack propagates to both objects, thereby forming a single hydrodynamic system. The conclusions obtained from the work indicate the effectiveness of the system for maintaining reservoir pressure in these carbonate reservoirs. Clarification of data on the filtration channels of both the working agent and the oil displaced by it, in combination with knowledge of the rates of movement of the injected liquid and its distribution profiles, makes it possible to effectively manage the oil field development.

ASSESSMENT OF CRITICALLY STRESSED FRACTURES DUE TO WATER INJECTION — A CASE STUDY FROM THE DEVONIAN CLASTIC RESERVOIR OF THE BERKAOUI FIELD, ALGERIA

A well-constrained geomechanical model of the Devonian clastic is presented to assess the fracture slip potential due to water injection in the Berkaoui field, Algeria. The studied interval indicates a strike-slip tectonic regime with a mean maximum horizontal stress azimuth of 98°N. A total of 131 natural fractures were interpreted from the acoustic image logs. These fractures have dominantly E-W strike, with true dips ranging between 51°-80° towards north and south. Critically stressed fracture analysis exhibits the onset of shear displacement of optimally oriented steeply dipping fractures occurring at 2.7 MPa of injection-induced pressure build-up. The practical injection threshold was inferred as 7 MPa based on caprock integrity assessment. A total of 10 out of 131 fractures (with dips = 70°) can experience injection-caused slip within the maximum allowable injection limit. The E-W oriented fractures, subparallel to the maximum horizontal stress azimuth, have a higher likelihood of being critically stressed during injection and therefore can contribute to permeability enhancement.

A Study on the Feasibility of Natural Frequency-Based Crack Detection

Owing to the long-standing statement that natural frequency-based crack detection is not sensitive enough to localised damage to identify small cracks, many natural frequency-based crack detection methods are validated by detecting cracks of moderate size. However, a direct comparison between the crack severity causing a measurable natural frequency change and the crack severity reaching the initiation of crack propagation or leading to brittle fracture is constantly ignored. Without this understanding, it is debatable whether the presented crack detection methods are feasible in practical situations. Through natural frequency calculation and linear elastic fracture mechanics, this study is dedicated to filling the above gap in knowledge. To directly utilize the solution of stress intensity factor, common fracture toughness test specimens featuring a single-edge crack are used. These specimens are essentially cracked rectangular plates under uniform uniaxial tension. Considering the stress resultants obtained via the extended finite element method, the natural frequency of the loaded cracked plates is calculated using the Rayleigh–Ritz method incorporating corner functions. In addition, assuming the specimens as structural components under remote uniform tension, the development of critical load versus crack length is derived based on the solution of the stress intensity factor. Thus, critical crack lengths corresponding to a series of safety factors are obtained by equating service load with critical load. After obtaining natural frequencies of the cracked plates with critical crack lengths, the natural frequency drop caused by a critical crack can be computed. Hence, the critical crack length can be compared with the crack length when the frequency drop is measurable. It is found that the brittleness of the employed metals plays a vital role in the feasibility of natural frequency-based crack detection.

A Study on the Influence of Natural Fractures in Tight Sandstone Reservoirs on Hydraulic Fracture Propagation Behavior and Post-Fracture Productivity

Tight sandstone gas reservoirs are rich in reserves and are an important part of unconventional oil and gas resources. However, natural fractures’ impact on hydraulic fracture propagation behavior and network formation mechanisms remain unclear. Exploring how to optimize fracturing parameters to maximize post-fracturing productivity requires further investigation. Therefore, this study focused on the characteristics of tight sandstone gas reservoirs and established a three-dimensional numerical simulation model for hydraulic fracture propagation and post-fracturing productivity using production history matching to validate the reliability of the model. Based on this model, this study investigated the influence mechanisms of natural fracture angles, density, and lengths on hydraulic fracture propagation behavior and network formation. The spatial distribution of hydraulic fracture widths in three dimensions is also explored. When natural fracture angles are lower, a greater number of natural fractures are activated, leading to more developed secondary hydraulic fractures and the formation of complex fracture networks. Hydraulic fractures tend to penetrate directly through high-angle natural fractures. Single-well cumulative gas production increases initially with increasing natural fracture angles, then decreases, but increases with higher natural fracture density and length. Optimal fracturing in areas with longer natural fractures, lower angles, and higher density distribution enhances single-well productivity effectively.

GeoCrack: A High-Resolution Dataset For Segmentation of Fracture Edges in Geological Outcrops

GeoCrack is the first large-scale open source annotated dataset of fracture traces from geological outcrops, enabling deep learning-based fracture segmentation, setting a new standard for natural fracture characterization datasets. GeoCrack contains images from photogrammetric surveys of fractured rock exposures across 11 sites in Europe and the Middle East, capturing diverse lithologies and tectonic settings. Each image was cleaned, normalized, and manually segmented, followed by a recursive annotation vetting process to ensure the quality and accuracy of the digitized fracture edges. The processed images and corresponding binary masks were divided into 224 × 224 patches, yielding 12,158 pairs. GeoCrack captures representive real-world challenges in fracture edge annotation, such as contrast variations between fracture traces and the host medium due to geological and geomorphological factors like aperture dilation, host rock composition, outcrop weathering, and groundwater staining. Physical occlusions like shadows and vegetation are also considered to minimize false positives. GeoCrack was validated using a U-Net implementation for fracture segmentation, achieving satisfactory IoU of 85%. GeoCrack holds strong potential to advance deep fracture segmentation in geological applications, effectively tackling the diverse challenges of real-world fracture identification.

Modeling the propagation of hydraulic fractures in reservoirs with natural fracture networks using high aspect ratio interface elements

The application of hydraulic fracturing in unconventional reservoirs is a technique widely used to overcome the problem of low permeability in porous media. However, there are complex factors involved in understanding this technique, since the propagation of hydraulic fractures can be impacted by factors such as the state of stress in situ and the distribution of natural fractures, which may have different lengths, inclination angles and aperture values. Thus, based on the continuum mechanics and using the finite element method, this paper seeks to simulate the effect of hydraulic fracturing in porous media with a complex natural fractures network under the influence of different stress states. The modeling of the problem considers a fully coupled approach for solving the hydro-mechanical problem, with the Darcy’s law governing the fluid flow in the porous media and by the classical cubic law inside the fractures. For the representation of natural and hydraulic fractures, High Aspect Ratio Interface Finite Elements (HAR-IEs) associated with a suitable tensile damage model are used and inserted into the regular mesh via the Mesh Fragmentation Technique (MFT). The results obtained are validated with the literature and show that the model is capable of reproducing the complex scenarios of propagation and interaction between multiple fractures.

Phase-field modeling of hydraulic fracture interaction with natural fractures

The problem of hydraulic fracturing has been the subject of several studies in recent years, including different proposals for analytical, experimental, and numerical models. This interest is justified by the complexity of the problem and its great relevance, with applications in various areas including the industrial, energy and engineering sectors. Several applications deal with natural reservoirs which are usually characterized by the presence of inclusions, heterogeneities and natural fractures, the latter being the subject of this study. These pre-existing fractures affect the circulation of the pressurized fluid inserted into the reservoir, influencing the path of the hydraulic fracture. The interaction between hydraulic and natural fractures generates complex fracture propagation patterns involving arrest, cross and branch phenomena between the cracks. In this sense, an interesting approach to modeling hydraulic fracturing is the use of phase-field models (PFM). Through its variational approach, the PFM is able to automatically deal with any number of cracks without restricting their shapes or trajectories and the crack path is obtained directly as part of the solution to the energy minimization problem. Therefore, this paper proposes a study of the interaction between hydraulic fractures and different networks of pre-existing natural fractures using a PFM that considers the pressure load on the surface of the hydraulic fractures. Numerical examples are presented to illustrate different possible interactions between the cracks and to explore different initial crack scenarios. All simulations were performed using the INSANE (INteractive Structural ANalysis Environment) software.

Simulating Water Invasion Dynamics in Fractured Gas Reservoirs

The Longwangmiao Formation gas reservoir in the Moxi block of the Sichuan Basin is a complex carbonate reservoir characterized by a low porosity and permeability, strong heterogeneity, developed natural fractures, and active water bodies. The existence of natural fractures allows water bodies to easily channel along these fractures, resulting in a more complicated mechanism and dynamic law of gas-well water production, which seriously impacts reservoir development. Therefore, a core-based simulation experiment was designed for oil–water two-phase flow. Three main factors influencing the water production of the gas reservoir, namely fracture permeability, fracture penetration, and water volume multiple, were analyzed using the orthogonal test method. The experimental results showed that the influences of the experimental parameters on the recovery factor and average water production can be ranked as water volume multiple > fracture penetration > fracture permeability, with the influence of the water volume multiple being slightly greater than that of the other two parameters. It provides a certain theoretical basis for water control of the gas reservoir.

Impact of cold fluid injection on caprock integrity

In Brazilian pre-salt fields, the extracted carbon dioxide (CO2) is reinjected with lower temperatures into the reservoirs to maintain reservoir pressure, improve oil recovery and reduce greenhouse gas emissions. The cold CO2 injection can alter the mechanical and hydraulic properties of the subsurface and induce variations in formation stresses. Stress changes can be sufficiently high to generate undesirable fracture propagation, damage the caprock, and activate natural fractures or geological faults. Consequently, they can induce seismicity, leaks, and contamination of shallower aquifers and CO2 release into the atmosphere. This work investigates the thermo-hydro-mechanical effect of cold fluid injection on caprock integrity. To this end, a fully coupled thermo-hydro-mechanical (THM) finite element model governs the rock formation behavior. The proposed model considers poroelasticity, fluid flow, and convection/diffusion heat transfer within the permeable rock formation under single-phase fluid flow conditions. The temperature diffusion, pore-pressure buildup, and stress variation under isothermal or non-isothermal injection scenarios are investigated. The results show reservoir expansion in the isothermal scenario due to fluid injection, which is expected in the pressurization process. In the non-isothermal scenario, the thermally disturbed region throughout the reservoir resulted in compaction rather than expansion, even though it was subjected to injection. The instantaneous drop in temperature induces a hard drop in pore pressure and plastic deformations in the reservoir. The lower interval of the caprock presents critical horizontal tension stresses compromising its integrity. Finally, the study adds valuable insight to better understand the complex cold fluid injection process, considering hydraulic, mechanical, and thermal coupling.

Impact of natural fractures on hydraulic fracture propagation in Denver-Julesburg Basin: Insights from a decade of research

The Denver-Julesburg (DJ) Basin is a key region for unconventional oil and gas extraction, particularly from the Niobrara and Codell formations. This study synthesizes findings from a decade of research conducted in three different geographic areas of the basin as part of the industry consortium Reservoir Characterization Project at Colorado School of Mines. The research explores how natural fractures affect hydraulic fracture propagation, leveraging geophysical data sets including 3D seismic imaging, image logs, microseismic monitoring, and crosswell distributed strain sensing. Key observations reveal significant variations in hydraulic fracture orientations and propagation speeds between the Niobrara and Codell formations, which are the main targets for horizontal drilling and stimulation. High natural fracture densities and orientations notably influence the propagation of hydraulic fractures in the Niobrara Formation, deviating them from the regional maximum horizontal stress (SHmax) directions, while hydraulic fractures in Codell consistently align with SHmax where lower natural fracture densities are observed. The study also highlights the critical role of stress anisotropy and zipper fracturing sequences in dictating hydraulic fracture behavior. Additionally, a marked negative correlation between natural fracture density and hydraulic fracture propagation speed underlines the fracture network complexity added by natural fractures. These findings advocate for a comprehensive natural fracture characterization and tailored fracturing strategies to optimize well placement and completions. The implications for DJ Basin development from this study are profound, suggesting that understanding local geomechanics and fracture networks can significantly enhance hydrocarbon recovery. Future work should focus on more precise assessments of hydraulic fracture interactions with natural fracture systems to refine operational strategies further.

Numerical Analysis of the Dynamic Mechanisms in Hydraulic Fracturing With a Focus on Natural Fractures

AbstractThe hydraulic fracturing process is a prominent example of fracture network evolution under stress. However, the interactions between hydraulic fractures and natural fracture networks, along with the connectivity evolution of the resultant fracture networks, require more research. This research incorporates discrete fracture networks to characterize subsurface structures and employs the Discrete Element ‐ Lattice Boltzmann Method to simulate the hydraulic fracturing process. The dynamic evolution of subsurface structures, including the initiation of hydraulic fractures and their interaction with natural fractures, is systematically investigated. Results indicate that natural fractures significantly impact fracture initiation, propagation, and connectivity evolution, which in turn affects fluid production. Fracture strength is key for the interaction, and hydraulic fractures tend to propagate along weak natural fractures with low resistance. Fracture strength variability determines the final fracture networks, with low‐strength fractures breaking due to the altered in‐situ stress and forming local clusters. High injection rates and fluid viscosity result in a large pressure buildup and exaggerate the influential region. A multi‐cluster system is thus formed during the hydraulic fracturing process, and its connectivity can be well quantified with a novel connectivity metric. In low‐permeable reservoirs, fracture clusters connected to production wells can contribute instantly, while local clusters may contribute to production from a long‐term perspective. Injection rate, fluid viscosity, fracture orientation, and clustering effects have consistent positive correlations with the total connectivity and production. Heterogeneity has a weak positive correlation with fluid production, while a moderate negative correlation with total connectivity.

A hydraulic-mechanical coupling model of dynamic waterflood-induced fractures in fractured tight reservoirs

The propagation of waterflood-induced fractures (WIFs) occurs during prolonged water injection and is influenced by the distribution and properties of natural fractures (NFs). Available numerical models rarely consider fracture activation and rupture in an integrated manner, which makes it difficult to reflect complex fracture morphology. In this paper, we propose a hydraulic-mechanical model with strain-dependent damage variables to describe the dynamic expansion characteristics of WIFs. There are discrete filled NFs in the matrix with non-equal-thickness joint elements, for which we derive the constitutive equations to calculate fracture widths during water injection and production. Damage variables for the matrix and fractures are calculated according to the maximum tensile stress criterion and the Mohr–Coulomb criterion. A comparison between the coupled model and experimental results is conducted to demonstrate its validity. Finally, we simulated and analyzed four influencing factors of the pressure response and fracture evolution. The study demonstrates that fracture behavior and damage area evolution are highly sensitive to injection rate, communication sequence, NF density, and orientation. The activation, cross, and capture interactions between NFs and WIFs complicate the fracture-damage network and enhance seepage efficiency. High injection rates promote crack tip propagation, while lower rates facilitate the evolution of secondary fractures at low pressure. For high NF density reservoirs, low-pressure injection fully activates NFs, aiding damage evolution. In low NF density reservoirs, excessive pressure induces simpler fracture morphologies, making unstable water injection more effective than continuous injection. This work guides appropriately induced fractures to improve water absorption in tight reservoirs.

Investigation of geometric morphology and fluid flow characteristics of natural fracture in tight sandstone under non-loading and true triaxial cyclic loading

Understanding the flow characteristics of rock fractures under stress is critical for many geological engineering applications. In this study, flow experiments are conducted on tight sandstone samples with a single natural fracture under true triaxial cyclic loading using the geotechnical consulting and testing systems. The geometric morphology of the fracture is scanned before and after loading using a profilometer. An improved cubic law is developed by including correction factors for stationary roughness, surface tortuosity, and hydraulic tortuosity. The evolution of fracture permeability during cyclic loading of each principal stress is measured using the steady-state method. The results show that (1) the surface tortuosity of the natural fracture correlates as a binary quadratic function with its fractal dimension and joint roughness coefficient. (2) The improved cubic law model has higher accuracy in predicting the permeability of the opening natural fracture than other commonly used modified cubic law models. (3) The principal stresses exhibit an anisotropic influence on fracture permeability. During cyclic loading of principal stress parallel to the fracture, the changes in fracture permeability are neglectable. (4) During cyclic loading of principal stress perpendicular to the fracture, the fracture permeability decreases significantly in the first loading cycle, exhibiting a hysteresis effect. In subsequent cycles, the changes in fracture permeability are nearly reversible, indicating the stress-memory effect of the natural fracture. This study provides direct evidence for the hysteresis and stress-memory effects in the permeability evolution of fractured rock during true triaxial cyclic loading.