Controlling Fracture Propagation Research Articles

Laboratory investigation on fracture initiation and propagation behaviors of hot dry rock by radial borehole fracturing

Creating complex and interconnected fracture networks between injection and production wells is crucial for exploiting hot dry rock (HDR) geothermal energy. However, the simple planar fractures created by conventional hydraulic fracturing, primarily controlled by in situ stress, fail to connect directionally with the target well. This study proposes a novel stimulation method, i.e. radial borehole fracturing, which shows great potential for guiding the directional propagation of fractures. The fracture initiation and propagation behaviors of high-temperature granite under radial borehole fracturing are investigated and compared with those of conventional fracturing. Three-dimensional morphological scanning and reinjection tests are used to quantitatively evaluate fracturing performance. Additionally, the influences of key parameters, including rock temperature, in situ stress, injection rate, fluid viscosity, azimuth of the radial borehole, and the number of radial boreholes on the fracture morphology and breakdown pressure are investigated. The results show that radial borehole fracturing can effectively guide the initiation and propagation of fractures along the radial borehole. The breakdown pressure of radial borehole fracturing can be reduced by 14.1%-43.7% compared to conventional fracturing. A higher fluid-rock temperature difference reduces the directional propagation range of fractures guided by the radial borehole. Increases in the vertical density of radial boreholes, injection rate, and fluid viscosity enhance the guiding ability of radial boreholes. Furthermore, there is a competitive relationship between in situ stress and the azimuth of radial boreholes in controlling fracture propagation. This research provides a viable alternative for the directional connection of injection-production wells in HDR reservoirs.

Brittle fracture studied by ultra-high-speed synchrotron X-ray diffraction imaging.

In situ investigations of cracks propagating at up to 2.5 km s-1 along an (001) plane of a silicon single crystal are reported, using X-ray diffraction megahertz imaging with intense and time-structured synchrotron radiation. The studied system is based on the Smart Cut process, where a buried layer in a material (typically Si) is weakened by microcracks and then used to drive a macroscopic crack (10-1 m) in a plane parallel to the surface with minimal deviation (10-9 m). A direct confirmation that the shape of the crack front is not affected by the distribution of the microcracks is provided. Instantaneous crack velocities over the centimetre-wide field of view were measured and showed an effect of local heating by the X-ray beam. The post-crack movements of the separated wafer parts could also be observed and explained using pneumatics and elasticity. A comprehensive view of controlled fracture propagation in a crystalline material is provided, paving the way for the in situ measurement of ultra-fast strain field propagation.

Optimization method of fracture parameters in multistage fracturing of horizontal wells

ABSTRACT Analyzing the simultaneous propagation of multiple fractures in horizontal wells is a challenging task. Although many fracture propagation models exist, the primary controlling factors are unclear. A fast and relatively accurate optimization method is urgently needed. In this study, a semi-analytical model of fracture propagation is established that couples stress interaction with fracture propagation using the energy balance principle, which can run a complete simulation process within 10 seconds. The simulation results show that the pumping rate, perforation cluster number, and fracturing fluid viscosity are the primary factors controlling fracture propagation. For two areas with different elastic modulus, the process of parameter optimization is analyzed. The reservoir with an elastic modulus of 10 MPa has an optimal perforation cluster number of 5, while the reservoir with an elastic modulus of 35 MPa has an optimal perforation cluster number of 8. The results show that the area with larger elastic modulus need to set a larger number of clusters, while the area with smaller elastic modulus have a smaller number of perforation clusters. This work aims to provide a practical and simple workflow for optimizing the fracture design of fractured horizontal wells.

Application of Koopman operator for model-based control of fracture propagation and proppant transport in hydraulic fracturing operation

This work explores the application of the recently developed Koopman operator approach for model identification and feedback control of a hydraulic fracturing process. Controlling fracture propagation and proppant transport with precision is a challenge due in large part to the difficulty of constructing approximate models that accurately capture the characteristic moving boundary and highly-coupled dynamics exhibited by the process. Koopman operator theory is particularly attractive here as it offers a way to explicitly construct linear representations for even highly nonlinear dynamics. The method is data-driven and relies on lifting the states to an infinite-dimensional space of functions called observables where the dynamics are governed by a linear Koopman operator. This work considers two problems: (a) fracture geometry control, and (b) proppant concentration control. In both cases, an approximate linear model of the corresponding dynamics is constructed and used to design a model predictive controller (MPC). The manuscript shows that in the case of highly nonlinear dynamics, as observed in the proppant concentration, use of canonical functions in the observable basis fails. In such cases, a priori system knowledge can be leveraged to choose the required basis. The numerical experiments demonstrate that the Koopman linear model shows excellent agreement with the real system and successfully achieves the desired target values maximizing the oil and gas productivity. Additionally, due to its linear structure, the Koopman models allow convex MPC formulations that avoid any issues associated with nonlinear optimization.

Fault relay zones evolving through distributed longitudinal fractures: The case of the Teruel graben system (Iberian Chain, Spain)

A new type of fault relay zone in extensional contexts, dominated by distributed along-strike or longitudinal fractures, is defined. It contrasts with the classical models reported in the literature, in which transverse connecting faults controlled by the own relay kinematics prevail. The new model is based on structural features of the Teruel graben system, as well as on analogue modelling. Relay zones between the NW-SE to NNW-SSE striking faults that delimit the eastern boundary of the Jiloca Graben (Calamocha, Sierra Palomera and Concud faults), together with the Teruel Fault, have been studied. All of these relay faults show recent (Neogene-Quaternary) ruptures at different scales, mostly parallel to the macrostructural trend and to the maximum horizontal stress (SHmax) trajectories (i.e., orthogonal to the ENE-WSW regional extension direction that characterises the nearly biaxial or radial stress regime active during Upper Pliocene-Quaternary times). Transverse ruptures are almost absent, with the exception of the northern relay zone (Calamocha-Sierra Palomera), where an incipient NE-SW striking connecting fault does exist. Analogue models have been run under a biaxial extension regime similar to the regional one. They allowed analysing the main factors controlling fracture propagation, depending on the ratio of extension velocities and the orientation of the master faults relative to extension directions. Laboratory fracture patterns, as in the natural studied examples, are mostly controlled by the inherited anisotropies and, in a greater extent, by the imposed extension trajectories, which results in a clear prevalence of longitudinal fractures. Such external controls, usually disregarded in numerical and analogue modelling, tend to induce fault coalescence through along-strike (parallel or at very-low-angle) propagation resulting in a final braided fault pattern.

Artificial Fracture Stimulation of Rock Subjected to Large Isotropic Confining Stresses in Saline Environments: Application in Deep-Sea Gas Hydrate Recovery

The low permeability of gas hydrate deposits leads to poor extraction rates. Artificial fracture stimulation could significantly improve the recovery rate of an estimated 300 trillion m3 of this untapped future energy source, which form in seabed sedimentary deposits. Because conventional methods of rock fragmentation are inapplicable in such deposits due to sudden release of energy that may impose the risk of methane release to the atmosphere, alternative rock fragmentation technologies are necessary for artificial fracture stimulation of gas hydrate deposits. We checked the effectiveness of a new hydrophobic non-explosive demolition agent as a rock fracturing technique, which could potentially be used as a third-generation disruptive technology for mining (3G-DTM). Laboratory experiments performed by mimicking deep-sea environments suggest that the density of the gradually generated rock mass fractures increases with confining pressure and pore fluid salinity. Importantly, due to the fracturing nature of 3G-DTM, the fracture density can be significantly improved (by 116%) with increasing the confining pressure (from 70 kPa to 20 MPa). Increased salinity of the rock pore fluid also improved the fracture density by 38% at 20 MPa confining pressure when the salinity increased from 0% to 20%. Furthermore, the rock is subjected to a gradual fracturing process in the 3G-DTM fracturing (10–15 h in the laboratory experiments) allowing for a safer, more controlled fracture propagation, making 3G-DTM a substitute for conventional rock fragmentation in marine environments.

Fracture Property Experimentation On Hadrosaurid Dinosaur Wavy Enamel Reveals Energy‐Robbing Crack Deflection and Channeling To Localize Damage: A Rare Case of Mammalian‐Like Dental Sophistication In Reptiles

Reptiles rarely approached the biomechanical sophistication of feeding or dietary diversity seen in mammals. Their teeth are typically non‐occluding, semi‐conical structures with simplistic parallel‐crystalite enamel surrounding an orthodentine core. Conversely, most mammals possess multi‐cusped teeth that are drawn across one another during mastication and self‐wear to their functional morphology. Mammalian enamel is complex ‐‐ a fiberglass‐like composite composed of bundles of hydroxyapatite crystals known as prims, surrounded by compliant proteinacous sheaths. Among the most sophisticated prism architectures is the modified radial enamel (MRE) of grazing ungulates whose coarse tooth surfaces enable grinding of tough, abrasive‐laden plant matter. MRE conveys exceptional fracture toughness and controlled fracture propagation minimizing damage to the brittle enamel crests. Hadrosaurid (duck‐billed dinosaurs) are notable in independently evolving self‐wearing grinding dentitions and complex wavy enamel (WE) composed of folded layers of hydroxyapatite crystals whose biomechanical import is unknown. We tested the hypothesis that WE served an analogous role to ungulate MRE by: 1) tribologically modeling the effects on hadrosaurid occlusal surfaces should a section of enamel become fractured; 2) introducing enamel fractures to the teeth of hadrosaurids, outgroups lacking WE, and horses and bison using Vickers‐tipped indenters; 3) testing for controlled‐crack propagation using Watson's Two‐Sample Test of Homogeneity; and 4) contrasting the results in comparative ecological contexts. Removal of enamel sections leads to aberrant self‐wear to hadrosaurid chewing pavements (= “wave mouth” in equine veterinary science) inhibiting functionality. Non‐WE enamels show isotropic fracturing and catastrophic damage to the enamel shells. WE and MRE three‐dimensionally limit damage in enamel crests through energy‐robbing crack deflection at kinks in the enamel fabrics or prism boundaries and channeling perpendicular to the enamel‐dentine junction. Hadrosaurid WE represents an alternative reptilian solution for damage resistance in grazing taxa, one that evolved millions of years earlier than in mammalian ungulates. Our findings add to a growing body of evidence that material properties are preserved in fossil teeth and can be used to explore dental form and function throughout the fossil record and to develop biologically‐inspired industrial materials.Support or Funding InformationNSF EAR 0959029 awarded to GME

Ultrashort Pulse Laser Cutting of Glass by Controlled Fracture Propagation

We demonstrate that the use of low energy but high repetition ultrashort laser systems can be advantageous for glass laser cutting using method of controlled fracture propagation. In specific regime using low numerical aperture objective, elongated zone of energy deposition induces high mechanical stress allowing single-pass full cutting of 1mm-thick soda-lime glass. The crack can follow straight as well as curve trajectories with a radius down to 1 mm with a 1mm/s feedrate. In order to achieve high quality precise control on the fracture propagation we have studied the effects of experimental parameters such as numerical aperture, repetition rate, pulse energy, pulse duration, but also regimes of irradiations by single pulses or bursts.

Fracture mechanics of impacted laminated glass subjected to various fatigue stressing conditions

This paper investigates the fracture mechanics of micro-cracks in the laminated glass plates of car windshields. Foreign objects such small pieces of gravel impacting a car windshield can induce micro-cracks, which may propagate and lead to dangerous glass fracture events. A major issue is to identify the parameters controlling fracture propagation from the initial micro-crack to catastrophic macroscopic failure. The main conclusion of this study is that cyclic fatigue is not a controlling factor in crack propagation in impacted glass. A subcritical propagation effect, related to chemical reaction between SiO2 glass and humidity, appears to be the main mechanism of crack propagation. An equivalent “stress intensity factor – time” power formalism is introduced to rationalize crack propagation endurance, for constant and variable cyclic stress conditions.

2. Size effect in the fracture toughness of Ogino tuff

Splitting tests were conducted using large specimens of Ogino tuff in order to investigate the process of fracture propagation and also to clarify the size effect in fracture toughness of the rock-including specifically the effects of thickness, width and height of specimen as well as that of crack length. Two types of stiff loading apparatus, a wedge type and a screw type, were used for controlling fracture propagation; and fracture toughness during the process of fracture propagation was determined with compliance calibration. From the results on size effects, a standard size of the specimen was proposed to obtain a specimen size-independent fracture toughness value in the splitting test.

Carbonyl iron powders

This paper investigates the fracture mechanics of micro-cracks in the laminated glass plates of car windshields. Foreign objects such small pieces of gravel impacting a car windshield can induce micro-cracks, which may propagate and lead to dangerous glass fracture events. A major issue is to identify the parameters controlling fracture propagation from the initial micro-crack to catastrophic macroscopic failure. The main conclusion of this study is that cyclic fatigue is not a controlling factor in crack propagation in impacted glass. A subcritical propagation effect, related to chemical reaction between SiO2 glass and humidity, appears to be the main mechanism of crack propagation. An equivalent “stress intensity factor – time” power formalism is introduced to rationalize crack propagation endurance, for constant and variable cyclic stress conditions.