Fracture Growth Research Articles

Integrating microseismicity and low frequency DAS to characterize hydraulic fracturing.

Hydraulic fracturing is a widely used technique for developing unconventional reservoirs. Thus, knowledge of the fracture geometry is essential to evaluate the success of a hydraulic fracturing operation. Traditionally, the spatio-temporal distribution of the microseismicity has served as a proxy for the stimulated reservoir volume (SRV), based on the assumption that seismic events are triggered near the fractures. Recently, distributed acoustic sensors (DAS) have been utilized to measure strain changes resulting from hydraulic fracturing, allowing to measure fracture extends directly to some degree. By integrating DAS with microseismic data, we can validate parameters obtained from each dataset and improve our understanding of seismicity induced by hydraulic fracturing. We characterize hydraulic fracture growth across 26 stages of HFTS-2 in three different directions with microseismic data. For this we apply a formulation of the triggering-front based on non-linear fluid-rock interactions, assuming negligible fluid leakage and fluid conservation within the fractured volumes, which effectively describes the spatio-temporal distributions of microseismic events. Additionally, fracture extents obtained from microseismic data are compared with those measured directly with DAS. This comparison reveals that the assumption that microseismic events are triggered in the immediate vicinity of hydraulic fractures is not always accurate. Observed are both extremes: microseismic clouds being significantly larger than the fractures, or vice versa. An analysis of the spatio-temporal distributions of the microseismicity, combined with numerical modeling of the fiber responses, demonstrates that seismic events associated with the triggering-fronts are related to the destabilization of rocks surrounding fractures through elastic stress transfer. Hence, fracture growth can be tracked with varying precision using only microseismic data, with the extreme case of completely aseismic growth. The combination of microseismic monitoring with DAS yield the most valuable insights.

Non-isothermal mathematical model of blocking tecnogenic fractures

Nowadays, large oil fields have moved to the stage of declining production, to maintain reservoir pressure, it is necessary to apply flooding technologies. To maintain the previous rates of oil production, it is necessary to force selections by increasing the value of downhole pressure on the injection wells. However, the risks of exceeding the fracturing pressure are increasing, which can lead to the formation of technogenic fractures. An intensive increase in the fracture can lead to an increase in the risks of premature water reaching through it into the drainage zone of the producing wells, which will lead to an increase in the value of the water oil ratio. The analysis of current numerical mathematical models of colmatation of technogenic fracture has shown the status of determining the volume of leaks of the colmatation agent beyond the fracture, considering changes in the temperature field at the bottom of the injection well. This problem is relevant, since special research complexes have been conducted at several oil and gas fields to determine the growth of technogenic fractures that arose because of excess fracturing pressure and fell into the drainage zone of producing wells. A change in the temperature field of the reservoir will allow direct changes in the viscosity of the injected colmatation agent, as well as determine the amount of leakage of the agent beyond the limits of the technogenic fractures. The article describes the construction of a non-isothermal physico-mathematical model of injection of a suspension system (water-reagent) into the reservoir, considering changes in the temperature field of the reservoir, the volume of reagent leaks beyond the limits of the technogenic fracture, considered for the first time. The aim of the work is to establish the dependences of the leakage volume of the colmatation agent, the critical time of filling the fracture from changes in the temperature field at the bottom of the injection well. A nonisothermal reservoir simulation model has been constructed showing the stages of initiation of a technogenic fracture with its subsequent colmation. The distribution of the concentration of the colmatation reagent both in the fracture and outside it, depending on the change in the temperature field at the bottom of the well, is obtained. It is determined that the volume of reagent leaks decreases if changes in the temperature field at the bottom of the injection well are considered with identical well operation parameters and geological and physical characteristics of the formation.

Identifying fracture-controlled resonance modes for structural health monitoring: insights from Hunter Canyon Arch (Utah, USA)

Abstract. Progressive fracturing contributes to structural degradation of natural rock arches and other freestanding rock landforms. However, methods to detect structural changes arising from fracturing are limited, particularly at sites with difficult access and high cultural value, where non-invasive approaches are essential. This study aims to determine how fractures affect the dynamic properties of rock arches, focusing on resonance modes as indicators of structural health conditions. We hypothesize that damage resulting from fracture propagation may influence specific resonance modes that can be identified through ambient vibration modal analysis. We characterized the dynamic properties (i.e., resonance frequencies, damping ratios, and mode shapes) of Hunter Canyon Arch, Utah (USA), using spectral and cross-correlation analyses of data generated from an array of nodal geophones. Results revealed properties of nine resonance modes with frequencies between 1 and 12 Hz. Experimental data were then compared to numerical models with homogeneous and heterogeneous compositions, the latter implementing weak mechanical zones in areas of mapped fractures. All numerical solutions replicated the first two resonance modes of the arch, indicating these modes are insensitive to structural complexity derived from fractures. Meanwhile, heterogenous models with discrete fracture zones succeeded in matching the frequency and shape of one additional higher mode, indicating this mode is sensitive to the presence of fractures and thus most likely to respond to structural change from fracture propagation. An evolutionary crack damage model was then applied to simulate fracture propagation, confirming that only this higher mode is sensitive to structural damage resulting from fracture growth. While examination of fundamental modes is common practice in structural health monitoring studies, our results suggest that analysis of higher-order resonance modes can be more informative for characterizing fracture-driven structural damage.

Numerical Investigation into the Stress Evolution and Failure Mechanism of Deep-Buried Hard Rock Under Blasting Loads

With the production activities continue progressing into deeper underground spaces, the rising ground stress poses new challenges in the fracturing of hard rock. Previous research mostly focused on the outer actions of blasting on conventional rock mass, while research on the inner actions of blasting via discrete element method (DEM) is relatively scarce, especially for deep-buried hard rocks under high ground stress. Relying on the pre-cracking project of hard rock protective layer in the thousand-meter deep well of Pingdingshan coal mine, this paper aims to numerically investigate the fracture mechanism of deep-buried hard rock under blasting loads via DEM. To this end, the algorithm of simulating explosion load is improved. The improved algorithm ensures a more reliable correspondence between numerical results and engineering practice, significantly enhancing the accuracy and credibility of calculations. After the calibration of mesoscopic parameters on the basis of laboratory tests, a series of parametric study, including confining pressure, peak blast stress and lateral stress coefficient, have been performed to understand the effects of in-situ stress on the behaviors of rock blasting. The obtained numerical results exhibit that confining pressure inhibits the fracture growth: under low confining pressure, confining pressure mainly inhibits the development of fractures in sparsely fractured zone while the crack growth in densely fractured zone and crushed zone is also inhibited under high confining pressure. According to the stress state, hoop peak stress is more sensitive to confining pressure than radial peak stress. Rock breakage in the vicinity of blasthole is essentially controlled by the radial peak stress, while crack propagation in the far-field is mainly induced by the hoop peak stress. With different lateral stress coefficients, the failure characteristics of rock mass are principally related to the hoop stresses in the vertical direction. The obtained numerical results and mesoscopic analysis are capable of providing new insights into the fracturing mechanism of deep-buried hard rock.

Experimental Study on the Damage Evolution Mechanism of Siltstone by Water Content and Confining Pressure

ABSTRACTProlonged exposure of deep coal mines to erosion from groundwater results in a gradual accumulation of rock mass damage, which can lead to geological hazards such as deformation and instability. These challenges significantly impact the safe operation of deep coal mines. To understand the mechanisms behind siltstone damage progression related to water content and confining pressure, this study explores the influence of these factors on the deformation and damage evolution of siltstone, employing a combination of rock mechanics testing, numerical simulation, and CT scanning techniques. Results demonstrate that increasing water content reduces the compressive strength of rock, leading to more complex failure modes. In contrast, higher confining pressure strengthens the compressive capacity, thereby suppressing the formation and growth of transverse cracks under compression. Using Avizo software, a three‐dimensional model of siltstone was developed to visualize the distribution of fractures in a three‐dimensional field. In the MATLAB platform, a box dimension algorithm based on three‐dimensional digital volume imaging was developed, employing box dimension theory and digital image storage methods. Fractal analysis reveals that the fractal dimension of internal fractures in loaded samples increases linearly with water content, indicating more extensive fracture development and greater specimen damage. Applying the box dimension from three‐dimensional digital volume images as a metric facilitates characterizing the damage evolution in siltstone under different water content conditions. This approach provides a new means to quantitatively evaluate the growth and complexity of internal fractures in siltstone, offering insights into rock damage progression under varying moisture conditions.

Encapsulating Ultrafine In2O3 Particles in Carbon Nanofiber Framework as Superior Electrode for Lithium-Ion Batteries

Indium oxide (In2O3) is a promising anode material for next-generation lithium-ion batteries and is prized for its high electrical conductivity, environmental friendliness, and high theoretical capacity. However, its practical application is significantly limited by severe volume expansion and contraction during the lithium insertion/extraction process. This volume change disrupts the solid electrolyte interphase (SEI) and degrades contact with the current collector, undermining battery performance. Although the nano-structured design of In2O3 can mitigate the volume effect to some extent, pure In2O3 nanomaterials are prone to agglomeration during frequent charging and discharging. The pure In2O3-based electrode shows a sustained and rapid capacity degradation. In this study, we embed ultrafine In2O3 particles in a carbon nanofiber framework using electrospinning and thermal annealing. The 1D carbon nanofiber structure provides an effective electronic conductive network and reduces the length of lithium-iondiffusion, which enhances the reactivity of the nanocomposite and improves electrode kinetics. Additionally, the carbon nanofiber framework isolates ultrafine In2O3 particles, preventing their aggregation. The small volume changes due to the ultrafine size of the In2O3 are buffered by the carbon materials, allowing the overall structure of the In2O3/C composite nanofiber to remain largely intact without crushing during charging and discharging cycles. This stability helps avoid electrode fracture and excessive SEI growth, resulting in superior cycle and rate performance compared with the pure In2O3 nanofiber electrodes.

Insights into the flow characteristics during hydraulic fracturing

AbstractThis paper presents a numerical model to study fracture propagation during water-based hydraulic fracturing. To address the computational challenges associated with the numerical model, the proposed approach employs a set of overlapping spheres arranged in a monolayer to construct a porous specimen containing pre-existing cracks. The fluid-filled cracks represent various stages of initiation and propagation of fluid-driven fracture. The high-pressure fluid flow within the fractures is considered under isothermal conditions. Unlike the conventional focus on rock fracture analysis, the presented approach focuses on flow characteristics during fracture growth. The main objective of the presented study is to provide a detailed description of the computational fluid dynamics (CFD) aspects of fracture propagation during hydraulic fracturing to aid in calibration and validation of simplified discrete element method (DEM) models coupled with CFD representing this phenomenon. Experimental validations performed in previous studies support the model's reliability, making it useful in particular for calibration and validation of coupled 2D DEM-CFD models constructed from one layer of spheres. Obtaining experimental data for such cases is practically challenging, and the proposed model addresses the lack of reliable experimental data for hydraulic fracturing. To achieve this, representative specimens are designed, accurate simulations are conducted and precise assessments of the results are performed. Key variables such as density, pressure, velocity, porosity, and permeability were measured to facilitate the validation and calibration of future DEM-CFD studies.

Coupled 4D Flow-Geomechanics Simulation to Characterize Dynamic Fracture Propagation in Tight Sandstone Reservoirs.

Although China's low-permeability and tight oil reservoir utilization and newly proven reserves are growing annually, the overall recovery of such reservoirs is generally low. One of the main factors influencing the low recovery is the effect of intricate dynamic fracture propagations on the remaining oil distribution. Constrained by the evolution of an in situ stress field and the accumulation of fluid injection volumes, the growth of dynamic fractures allows a production profile of water breakthrough. To reveal this phenomenon, this paper systematically summarized the workflow of geoengineering integration and took the target block of an oil reservoir in the Changqing oilfield as an example. The numerical simulation model and geomechanical model of the reservoir were established, respectively, by using core analysis and rock mechanics experiments, and the simulation of dynamic fracture propagation under the control of flow-geomechanical coupling mechanisms was carried out. It is found that injection-induced fractures open and propagate under propulsion pressure until they link with natural fractures when the injection pressure surpasses the formation rupture pressure. The natural fractures become effective after long-term water injection and propagate until they connect to hydraulic fractures near a producing well. The study results on the dynamic fracture propagation process have theoretical and practical implications for the characterization of fractures, residual oil production, and water control techniques in tight sandstone reservoirs.

Fracture and Fatigue Crack Growth Behaviour of A516 Gr 60 Steel Welded Joints

The facture and fatigue behaviour of welded joints made of A516 Gr 60 was analysed, bearing in mind their susceptibility to cracking, especially in the case of components which had been in service for a long time period. With respect to fracture, the fracture toughness was determined for all three zones of a welded joint, the base metal (BM), heat-affected zone (HAZ) and weld metal (WM), by applying a standard procedure to evaluate KIc via based on JIc values (ASTM E1820). With respect to fatigue, the fatigue crack growth rates were determined according to the Paris law by the standard procedure (ASTM E647) to evaluate the behaviour of different welded joint zones under amplitude loading. The results obtained for A516 Gr. 60 structural steel showed why it is widely used in the case of static loads, since the minimum value of fracture toughness (185 MPa√m) provides relatively large critical crack lengths, whereas its behaviour under amplitude loading indicated a need for further improvement in WM and HAZ, since the crack growth rate reached values as high as 4.58 × 10−4 mm/cycle. In addition, risk-based analysis was applied to assess the structural integrity of a pressure vessel, including comparison with the high-strength low-alloy (HSLA) steel NIOVAL 50, proving once again its superior behaviour under static loading.

Influence of Complex Lithology Distribution on Fracture Propagation Morphology in Coalbed Methane Reservoir

The mineral composition in coalbed methane (CBM) reservoirs significantly influences fracture morphology, making the description of reservoir heterogeneity challenging. This study develops a fracture propagation model for CBM reservoirs that incorporates the varying mineral properties within the reservoir’s lithology. Dynamic logging data are considered to characterize rock mechanical properties, which form the basis for in situ stress estimation. Using an adjusted critical circumferential stress calculation for coal rock, the model considers the impact of complex lithology on fracture propagation. A comprehensive fractal index is introduced to capture the influence of different minerals on fracture morphology and propagation randomness. Models representing clay, quartz, and pyrite with varied compositions were constructed to explore the effects of each mineral on fracture characteristics. In single-component models, clay-rich reservoirs exhibited the highest induced fracture density, with quartz and pyrite showing approximately 65% and 20% of the fracture density observed in clay, respectively. Fractures primarily propagated toward quartz-rich regions, while pyrite significantly inhibited fracture growth. In mixed-mineral models, increasing the quartz proportion by 40% resulted in a 20 m increase in fracture length and a 30% reduction in fracture density. Fractures predominantly propagated around pyrite boundaries, demonstrating pyrite’s resistance to fracture penetration. Clay and quartz promote fracture development, whereas pyrite hinders fracture formation. The fracture inversion model presented here effectively captures the influence of complex mineral distributions on fracture morphology, offering valuable insights for optimizing fracturing production strategies.

A robust 3D finite element framework for monolithically coupled thermo-hydro-mechanical analysis of fracture growth with frictional contact in porous media

This paper presents the formulation of a robust integrated framework for the coupled multiphysics and multiple fracture growth analysis in porous media. The finite element-based thermo-hydro-mechanical method for fracture growth with frictional contact (THMf-g) simultaneously solves monolithically coupled equations, incorporating contact and frictional constraints from fracture sliding. It also implements an adaptive process to update fields and geometries during fracture growth, effectively modeling emerging new surfaces. The thermo-hydro-mechanical fields are derived from fundamental principles of mass, momentum, and energy conservation and discretized numerically using the finite element method. Fractures are represented as sub-dimensional surfaces embedded within the volume of the porous medium. The growth of multiple fracture is modeled based on stress intensity factors at fracture tips, with fracture aperture and permeability emerging as dynamic properties of the system. The main novelty of this work lies in extending the implicitly solved monolithic coupling to include frictional and growth modeling for multiple non-planar fractures of emerging geometry in three dimensions. This includes the direct incorporation of cubic terms in the fracture flow equations and convection terms in the heat transfer equations, adopting an incremental method to solve these coupled, nonlinear equations. To ensure energy conservation, the heat equations are resolved using an implicit scheme, establishing a velocity dependency on pressure fields and introducing quadratic terms into the heat equation. Furthermore, the heat transfer equation has been revised to account for the work done on the fluid, enhancing the accuracy of thermal modeling. A contact mechanics leader–follower strategy tracks a conformal mesh split at each fracture, accounting explicitly for permeability changes during deformation and growth, effectively reducing computational complexity and cost. The iterative process for applying these constraints and the solution of the coupled THMf-g with friction and growth is described. The method does not require calibrated material properties or artificial length scale parameters, and relies on laboratory-measured properties with direct physical interpretation. A detailed algorithm is presented, including the updating of apertures and handling of new surfaces during the simulation, accounting for the variability of fracture permeability and its interplay with contact and friction during the non-linear solution. Several validating numerical experiments are benchmarked against analytical solutions, demonstrating the accuracy and reliability of the proposed framework in capturing complex fracture behavior and multiphysics interactions in porous media.

Microstructure-Based Modeling of Inner Oxygen Pressure in Solid Oxide Electrolysis Cells: Analysis of Electrode Delamination and Mitigation

One major degradation mechanism during long-term operation of solid oxide electrolysis cells (SOECs) is delamination of oxygen electrodes. The driving force for the electrode delamination could be the generated inner oxygen pressure near the electrode-electrolyte interface. Here a microstructure-based electrochemical model, which includes the conduction of electrons and oxygen ions coupled with Butler-Volmer-type chemical reactions at triple-phase-boundaries (TPBs), is employed to investigate the oxygen pressure in lanthanum strontium manganate (LSM)-based SOECs. The model is applied to both two-dimensional (2D) preset microstructures and three-dimensional (3D) realistic microstructures, and the oxygen pressure is analyzed as a function of transport properties and electrode thickness under both potentiostatic operation and galvanostatic operation. The results based on a 2D preset microstructure are shown in the figure. The simulation results suggest the following conditions to suppress electrode delamination under potentiostatic operation: (1) the ionic conductivity of the electrolyte either smaller than a certain threshold or higher than another value; (2) larger ionic conductivities of LSM and yttria stabilized zirconia (YSZ) in the oxygen electrode; (3) smaller ionic conductivity of YSZ in the hydrogen electrode; (4) thicker electrolyte; and (5) thicker oxygen electrode. On the other hand, under galvanostatic operation, the following conditions are favored: (1) larger ionic conductivity of YSZ in the electrolyte and oxygen electrode; (2) larger ionic conductivity of LSM; (3) thinner electrolyte; and (4) thicker oxygen electrode. The simulation results are compared to the analytical solutions of an electrochemical model based on equivalent circuits. With the calculated oxygen pressure as input to a phase-field fracture model, crack initiation and growth at the electrode-electrolyte interface are demonstrated in the simulation results. Figure 1

Exploring the role of sample size on fracture growth mechanisms in intact rock: insights from 3D DEM-DFN analysis

The effect of sample size on the deformation characteristic and fracture growth mechanism of intact rock subjected to unconfined compressive stress state is examined in this study via discrete element method (DEM) coupled with discrete fracture network (DFN). Three-dimensional (3D) numerical models are first developed based on reported laboratory experiments and verified to realistically replicate the effect of sample size on the macro-mechanical response of the intact rock. This includes strength behavior, fracturing activities, and energy budgets. Micromechanical analyses are then performed to understand the role of sample size on the deformational behavior and damage progression in intact rock. Emphasis is placed on the distributions of coordination number, evolutions of crack density and the degree of crack anisotropy with regard to invariants of crack tensors. In addition, the 3D distribution of microstructure and contact networks are also presented. Results reveal that deformability and strength of intact rock are vastly reliant on the sample size. Increasing sample size facilitate the accumulation of induced microcracks due to the increased probability of interparticle bond failure. It is found that the increase in sample size can result in more tensile openings with strong dilatancy, as evidenced by local-scale porosity based on the implementation of Voronoi cells. Finally, the monotonical increase in seismic b-values with the increase in sample size suggests an acceleration in the number of small-magnitude acoustic emission (AE) events.

Exploring the fracture toughness and fatigue crack growth behavior of MoRe alloys

The fracture and fatigue crack growth behavior of two molybdenum alloys, containing 41 wt-% and 47.5 wt-% rhenium, respectively, are investigated. These alloys were provided in form of cold-wrought rods of 6 mm diameter and exhibit a refined microstructure with highly elongated grains and a strong fiber texture. Similarly processed pure molybdenum was used as reference material and exhibits a significantly coarser microstructure. SEN(T) and C(T) specimens were tested with R-L and L-R orientation. In both, quasi-static and fatigue crack growth experiments, L-R oriented cracks immediately kinked by 90° into the direction of grain elongation. This yields fracture toughness values and effective and long crack threshold values about twice as high as for R-L oriented cracks, which is in good agreement with calculations of a reduced local crack driving force. In both MoRe alloys a cyclic R-curve behavior was captured in fatigue crack growth tests at a load ratio of R = 0.1, while for MoRe47.5 it was still present at R = 0.7. This is attributed mainly to the coarser microstructure. The effective thresholds ∆Kth,eff of both MoRe alloys are remarkably low and deviate from a commonly used estimation, especially for the MoRe47.5 material. It is proposed that plasticity in these materials is facilitated by twinning, leading to the emission of partial dislocations from the crack tip. Although no clear microstructural or fractographic evidence was found, a recalculation of ∆Kth,eff considering partial dislocations indicates a good correlation with experimental values.

Modeling Hydraulic Fracture Entering Stress Barrier: Theory and Practical Recommendations

Numerical modeling of hydraulic fracturing is complicated when a fracture reaches a stress barrier. For high barriers, it may require changing the computational scheme. In view of the strong influence of stress barriers on the final footprint and opening of a hydraulic fracture, for decades, their modeling has been the subject of special investigations. Actually, classical models of propagation within a pay layer with impenetrable boundaries referred to the case of extremely high-stress contrast in neighbor layers. Further improvements tended to account for the fracture growth in these layers by including stress contrasts as input parameters of a model. This tendency resulted in the suggestion and successive enhancements of pseudo-three-dimensional models. All of them have used stress intensity factors (SIFs) to characterize the combined resistance caused by stress contrasts, material strength, and fluid viscosity. Specifically, the SIFs served to formulate the conditions that control the front penetration into a neighbor layer. This key concept presents the background of our research. Despite examples of modeling propagation through barriers, there is no general theory clarifying when and why conventional schemes may become inefficient and how to overcome computational difficulties. This paper presents the theory and practical recommendations following it. We start with the definition of the barrier intensity, which exposes that the barrier strength may change from zero for contrast-free propagation to infinity for channelized propagation. The analysis reveals two types of computational difficulties caused by spatial discretization as follows: (i) general, arising for fine grids and aggravated by a barrier, and (ii) specific, caused entirely by a strong barrier. The asymptotic approach, which avoids spatial discretization, is suggested. It is illustrated by solving benchmark problems for barriers of arbitrary intensity. The analysis distinguishes three typical stages of the fracture penetration into a barrier and provides theoretical values of the Nolte–Smith slope parameter and arrest time as functions of the barrier intensity. Special analysis establishes the accuracy and bounds of the asymptotic approach. It appears that the approach provides physically significant and accurate results for fracture penetration into high, intermediate, and even weak stress barriers. On this basis, simple practical recommendations are given for modeling hydraulic fractures in rocks with stress barriers. The recommendations may be promptly implemented in any program using spatial discretization to model fracture propagation.

High-strength polyvinyl alcohol/gelatin/LiCl dual-network conductive hydrogel for multifunctional sensors and supercapacitors

The synthesis of conductive hydrogels with high mechanical strength, toughness, optimal fracture growth rate and the capability to detect diverse human body movements poses a significant challenge in the realm of flexible electronics. In this study, a one-pot technique utilized effectively to fabricate conductive materials by doping LiCl into a mixture of polyvinyl alcohol (PVA) and gelatin. The PVA/gelatin/LiCl0.3(PGL) conductive hydrogel demonstrates exceptional robustness, flexibility, and resistance to deformation, enabling the monitoring of various physiological signals such as temperature and humidity. Additionally, the PGL demonstrates exceptional elongation properties (up to 1111.32 %), high lifting capacity (up to 25 kg), resistance to deformation, and sustained stability of peak signals even after 300 cycles at 50 % strain. The hydrogel electrolyte exhibits a conductivity of 2.114 S/m at 25 °C and a specific capacitance of up to 48.75 F/g, along with favorable mechanical and electrochemical characteristics. These findings suggest that the PVA/gelatin/LiCl0.3 hydrogel supercapacitor (PGLSC) conductive hydrogel shows significant potential for integration into flexible electronics and wearable technology devices.

Time-lapse 3D imaging and modelling of acid-rock interactions within a mudstone: Implications for cap-rock behaviour during carbon storage

Assessing caprock integrity is crucial for safe geological carbon storage. This study addresses this by utilising time-lapse micro-CT imaging to capture real-time, dynamic changes in mudstone during acid injection, providing a detailed analysis of the 3D geometry and evolution of minerals and fractures. Additionally, the integration of reactive transport modelling offers insights into the potential changes occurring immediately after acid injection, enhancing our understanding of fluid-rock interactions and their impact on caprock stability. Different acid concentrations were used to mimic a range of possible acid concentrations during CO2 injection and storage. Changes in mudstones due to acid interaction include initial pre-existing fracture closure, followed by fracture growth and sample swelling. Acid predominantly followed pathways like fractures or laminations. Reactive transport models showed most dissolution occurring near the inlet, driven by a decrease in H+ concentration along fluid pathways. Carbonate distribution controlled dissolved areas, with calcite dissolution and acid migration rates decreasing over time. Simulation indicated reduced shear stress after acid injection, especially with low-pH acids. This correlates to decreased fluid velocity, resulting in slower fluid flow. Slower acid movement and prolonged presence in immobile fluid flow zones led to more extensive calcite dissolution compared to mobile zones. This study provides valuable insights into microstructural changes in caprock following CO2 injection, emphasizing the potential risk of fracture development and leakage.

Experimental and numerical studies on propagation behavior between hydraulic fractures and pre-existing fractures under prepulse combined hydraulic fracturing

Prepulse combined hydraulic fracturing facilitates the development of fracture networks by integrating prepulse hydraulic loading with conventional hydraulic fracturing. The formation mechanisms of fracture networks between hydraulic and pre-existing fractures under different prepulse loading parameters remain unclear. This research investigates the impact of prepulse loading parameters, including the prepulse loading number ratio (C), prepulse loading stress ratio (S), and prepulse loading frequency (f), on the formation of fracture networks between hydraulic and pre-existing fractures, using both experimental and numerical methods. The results suggest that low prepulse loading stress ratios and high prepulse loading number ratios are advantageous loading modes. Multiple hydraulic fractures are generated in the specimen under the advantageous loading modes, facilitating the development of a complex fracture network. Fatigue damage occurs in the specimen at the prepulse loading stage. The high water pressure at the secondary conventional hydraulic fracturing promotes the growth of hydraulic fractures along the damage zones. This allows the hydraulic fractures to propagate deeply and interact with pre-existing fractures. Under advantageous loading conditions, multiple hydraulic fractures can extend to pre-existing fractures, and these hydraulic fractures penetrate or propagate along pre-existing fractures. Especially when the approach angle is large, the damage range in the specimen during the pre-pulse loading stage increases, resulting in the formation of more hydraulic fractures.

Propagation of Hydraulic Fractures and Natural Fractures: The Bypassing Behavior in 3D Space

Summary The interaction between natural fractures (NFs) and hydraulic fractures (HFs) in 3D space poses significant challenges to numerical simulation. The interaction behaviors between HFs and NFs in 3D space include crossing, offset, stopping, and bypassing. Many existing numerical simulators are 2D, which inherently limits their ability to account for the vertical growth of fractures. Consequently, they are unable to model the bypassing behavior effectively. In this paper, a mathematical model for the propagation of HFs and NFs in a naturally fractured reservoir is established. The displacement discontinuity method (DDM) is used to solve the rock deformation, while the finite difference method (FDM) is utilized to solve the fluid flow within fractures. The undirected graph structure is used to represent the complex fracture network, and a dynamic adjustment of grid connectivity method is implemented to describe the process by which HFs bypass NFs. By integrating with graphics processing unit (GPU) computing, an efficient 3D simulator for HFs and NFs propagation is developed. The accuracy is verified against analytical solutions and reference solutions. Subsequently, a series of numerical studies on the bypassing behavior are conducted. The primary findings are as follows: (1) The simulator can accurately capture the interactions between HFs and NFs in 3D space, including crossing, arresting, and bypassing behaviors. (2) The bypassing behavior is characterized by three distinct processes—intersection, height growth of HF, and bypassing. (3) During the intersection stage, both the injection pressure and maximum NF width increase. During the height growth stage, the pressure is relatively stable, while the maximum NF width continues to increase. These two stages together result in the “storage” phenomenon. Once the bypassing behavior occurs, both the pressure and the maximum NF width decrease sharply, leading to a “release” phenomenon. Additionally, the “storage” and “release” phenomena may impact proppant transport. (4) Given the presence of bypassing behavior, it is essential to consider the NFs in 3D fracture simulators. The simulator and its findings can provide valuable insights for field design.

Visualization experiments and numerical simulation on the growth of fracture height in bedding-rich reservoirs

The development of shale bedding and differences in properties have significant effects on the growth of the fracture height. To investigate the impact mechanism of bedding on the fracture growth height, the visualization fracturing experiments based on polymethylmethacrylate (PMMA) samples were performed to investigate the impact of the injection rates, viscosity and temporary plugging parameters on the growth pattern of hydraulic fractures. Furthermore, the solid–fluid-damage fracturing coupled model considering the vertical distribution of bedding was constructed by the continuous-discontinuous element method, and the impact of key formation parameters and treatment parameters were investigated. The results show that the height growth pattern of PMMA samples was affected by the flow rate and fluid viscosity. The fracture can cross the bedding at high-viscosity fluid. But in low-viscosity fluid the fracture tends to be arrested by the bedding. And the fracture cannot cross the bedding again after In-fracture temporary plugging. The fractures vertical growth pattern mainly includes three types at various stratigraphic parameters and treatment parameters, “工” type fracture, “丰” type fracture, and “I” type fracture, respectively. For vertical stress differentials below 3 MPa or Young's modulus below 20 GPa or injection rates below 1.8 m3/min or the fluid viscosity below 5 mPa·s, the fracture will be limited within the bedding.