Hydraulic Fracturing Of Rocks Research Articles

Method for macro-microscopic damage observation of rock hydraulic fracturing based on dic and thin section techniques

This study conducted systematic studies and in-depth analyses of micromechanics of hydraulically induced crack propagation. This study introduced a method for observing macromicroscopic damage in rock hydraulic fracturing based on DIC and thin-section techniques. We developed an advanced sealed visualization system equipped with high-speed cameras and macrolenses to facilitate precise observation of the hydraulic fracturing process. Additionally, Digital Image Correlation was used to accurately track and analyze the evolution of fractures. Our methodology clearly distinguished between intergranular and transgranular cracks using polarized light microscopy, thereby offering novel insights into grain-scale damage and strain localization within crystalline rocks. This research contributes to the understanding of the mechanisms underlying hydraulic fracturing at both the macroscopic and microscopic scales.

Study on Composite Fracture Characteristics and Hydraulic Fracturing Behavior of Hard Rock

To investigate the influence of non-singular terms (T stress) in the stress field on the composite fractures of hard rock Type I–II, such as rock splitting failure and hydraulic fracture propagation, this study focused on hard rocks in metallic mines. Through splitting tests and hydraulic fracturing experiments, the impact of T stress on the characteristics of Type I–II composite fractures in hard rocks was analyzed. Utilizing the generalized maximum tangential (GMTS) stress criterion considering T stress, the stress intensity factors of hard rock Type I–II composite fractures with different pre-existing crack angles were predicted. The critical fracture pressure expression for hard rocks was derived based on the maximum tangential stress (MTS) criterion. The results indicate that the GMTS criterion, considering T stress, is more suitable for describing the characteristics of Type I–II composite fractures under rock-splitting loads. However, under hydraulic fracturing, T stress has a minor influence on the fracture characteristics of hard rock hydraulic fractures. Therefore, when predicting the critical fracture pressure of hydraulic fractures, T stress can be disregarded. This study provides a scientific basis and guidance for hard rock hydraulic fracturing engineering.

Gas-entry pressure impact on the evaluation of hydrogen migration at different scales of a deep geological disposal of radioactive waste

Although the importance of gas-entry pressure in simulating two-phase liquid–gas flow in porous media has been studied at the column and borehole scales, its impact on the simulation of transient hydraulic-gas at different scales of a deep geological repository of radioactive waste (DGR) in low permeability clay rock during the post-closure phase has not yet been studied. The purpose of this work is to show that neglecting this phenomenon can lead to underestimation of the maximum gas pressure and water–gas fluxes simulated within the host rock and backfilled drift network. This could impact the performance of the engineered barrier system of a DGR. Simulations performed for a high-level waste disposal cell and for a simplified repository composed of hundreds of disposal cells situated in a clay host rock, show that gas preferentially migrates through the DGR components with low capillary entry pressures, such as the excavation damaged zone (Refers to the zone where fractures develop due to failure of the rock mass around galleries after tunneling) (EDZ), the engineered barriers materials (backfill, bentonite-plug…) and interfaces between the EDZ and these materials. Such a result could have significant consequences on the performance of a repository, due to the accumulation of gas in the drift network and high increase of gas pressure, which could lead to the host rock hydraulic fracturing.

Extraction and identification of spectrum characteristics of coal and rock hydraulic fracturing and uniaxial compression signals

Microseismic (MS) events generated during coal and rock hydraulic fracturing (HF) include wet events caused by fracturing fluid injection, in addition to dry events caused by stress perturbations. The mixture of these two events makes effective fracturing MS events pickup difficult. This study is based on physical experiments of different coal and rock HF and uniaxial compression. The differences of waveform characteristic parameters of various coal and rock ruptures were analyzed using the Hilbert–Huang transform, leading to some useful conclusions. The phase characteristics of the acoustic emission (AE) energy differed significantly and responded well to the pumping pressure curve. The AE waveforms of HF exhibit similar energy and frequency distribution characteristics after Empirical mode decomposition. The main frequency bands for coal, sandstone, and shale samples are 100–300 kHz, while the mudstone sample is in the range of 50–150 kHz. The decay ratios for coal, sandstone, shale and mudstone samples are 0.78, 0.83, 0.67 and 0.85, respectively. When compared to the uniaxial compression test, the main frequency bands of HF were reduced for coal, sandstone and mudstone samples, whereas shale remained essentially unchanged. The duration, instantaneous energy, and total energy of the HF waveform are smaller than those of uniaxial compression, while the decay ratio is greater, especially for the mudstone samples. The waveform characteristic parameters, trained using the multilayer perceptron neural network, can effectively identify HF and uniaxial compression events with an accuracy of 96%.

Progressive Initiation Phenomenon of Radial and Axial Cracks of the Borehole During Rock Hydraulic Fracturing

Progressive Initiation Phenomenon of Radial and Axial Cracks of the Borehole During Rock Hydraulic Fracturing

A simplified approach to hydraulic fracturing of rocks based on Finite Fracture Mechanics

AbstractThis work presents a coupled approach, based on Finite Fracture Mechanics (FFM), to preliminarily investigate hydraulic fracturing of rocks. The novelty of the criterion relies on the assumption of a finite crack extension and on the simultaneous fulfillment of a stress requirement and the energy balance. Two material constants are involved, namely, the tensile strength and the fracture toughness. The FFM unknowns are represented by the critical crack advance, which results a structural parameter and not a simple material one, and the breakdown pressure. The study investigates the longitudinal failure behavior of both impermeable and permeable rocks by supposing that growing cracks are loaded by a pressure proportional to that acting on the borehole wall. The stability growth of hydraulic fractures is discussed case by case. The approach is validated against experimental data available in the literature by considering the effect of rock permeability, fluid viscosity and flow rate.

Experimental investigation of the fracture initiation and propagation for sandstone hydraulic fracturing under the effect of evenly distributed pore pressure

Pore pressure is disregarded in traditional laboratory rock hydraulic fracturing experiments, and the effect of pore pressure is not clear. An integrated experiment for seepage and hydrofracturing was established and used to perform sandstone hydraulic fracturing experiments under an initial evenly distributed pore pressure. The experimental results show that there is a positive correlation between the breakdown pressure and the pore pressure at the initiation stage. The data fitting results show that the breakdown pressure and pore pressure follow a linear growth trend. As the pore pressure increases, the acoustic emission energy at the moment of borehole wall fracturing correspondingly increases. After borehole wall fracturing, the reduced magnitude of the pumping pressure also increases, indicating that the initial rupture range is positively correlated with pore pressure. During fracturing propagation, the propagation range and opening of the fracture increase as the initial pore pressure increases within the same pumping time. During hydraulic fracturing, a pore pressure gradient is generated on both sides of the mineral particles. When the tensile stress or shear stress induced by the pore pressure gradient reaches the ultimate strength of the mineral particle bonding surface, the particle bonding surface breaks and opens. This experimental process is more similar to the actual hydraulic fracturing process of oil and gas reservoirs. These results provide a more comprehensive theoretical basis for resolving technical problems of unconventional oil and gas resource exploitation.

An Innovative Method to Analyze the Hydraulic Fracture Reopening Pressure of Hot Dry Rock.

This paper focuses on a new test method and theoretical model for measuring and evaluating the reopening pressure during hot dry rock hydraulic fracturing. Firstly, rock blocks of four lithologies were collected from the hot dry rock strata. Hydraulic fracturing tests at high temperatures in real-time were conducted using drilled cubic specimens and drilled cubic specimens with a pre-crack. Breakdown pressure, reopening pressure, and fracture toughness were measured, respectively. In addition, Brazilian splitting tests at high temperatures in real-time were performed using Brazilian disc specimens to measure tensile strength. Secondly, an empirical equation for evaluating the reopening pressure during hot dry rock secondary fracturing was developed based on fracture mechanics and hydraulic fracturing theory. Third, the values calculated by the new equation, considering breakdown pressure, fracture toughness, and tensile strength, were compared to the values determined by the classical equation and to measurement results. It was found that the new equation predicted closer reopening pressure to the measurement results, regardless of the lithology of the hot dry rock. Moreover, with increasing temperature in the specimens, the error between the value calculated by the new equation and the measurement value remained low. In contrast, the difference between the classical equation predictions and the measurement results was widened. In addition, the reopening pressure was positively correlated with tensile strength and fracture toughness. Variations in lithology and temperature affected tensile strength and fracture toughness, which then changed the hot dry rock reopening pressure.

Fracturing criterion of rock hydrofracturing considering pore pressure effect

Traditional hydraulic fracturing theory believes that as the initial pore pressure increases, the breakdown pressure of the rock will decrease. Previous experimental studies have shown that the breakdown pressure of rock hydraulic fracturing may increase with the increase of initial pore pressure (gradient), which cannot be explained by traditional theory. The current understanding of the effect of pore pressure and its gradient during hydraulic fracturing is still unclear. In this study, the pore pressure effect of rock hydraulic fracturing is analyzed based on a large number of macroscopic and microscopic phenomena of rock hydraulic fracturing in the previous study. Then, a new fracture criterion of rock fracturing is built considering the pressure-gradient effect. This new fracture criterion can reflect the main influence factors, including the rock particle size, porosity, pumping flow, inner diameter of open hole section, length of main fracture, height of main fracture (or length of open hole section), fluid viscosity, pore pressure, minimum initial in situ stress and rock tensile strength. The new fracture criterion is examined by the rock fracturing experiment which considering the pressure-gradient effect. The results show that the proposed fracture criterion considering the pore pressure effect can well predict the breakdown pressure of rock, and the prediction trend is consistent with the experimental results. The average error is less than 1% when adopting the present fracture criterion. The parameter sensitivity of the fracture criterion is analyzed. Results show that the fracture pressure increases with the rock porosity and this trend becomes more apparent with a larger initial pore pressure. It shows that the fracture pressure increases with the pumped flow rate. Besides, it shows that the fracture pressure decreases when increasing the particle size of the rock, but the decreasing trend gradually slows down. The research results can provide a theoretical basis for the mechanism of rock hydraulic fracturing and the structural modification effect of fluids in rock engineering.

Technology Of Drilling Of Slant Well On The Field Of Western Siberia

The technology of drilling slant exploitation well in the oilfield of Western Siberia has been studied. Development of well construction has taken into account the next geological features of the section: permafrost rocks occur to the depth of 260 m, and iciness also takes place. Well construction is selected on the base of intervals of incompatible drilling conditions. The designed profile includes several intervals: one vertical interval, one zenith angle buildup section, two hold sections and one zenith angle drop section. A check calculation of the setting depth of the intermediate casing for the condition of preventing hydraulic fracturing of rocks near the shoe was carried out, and the safety factor of rocks for hydraulic fracturing was found. Drill pipes calculation includes determination of their length and diameter as well as allowable setting depth for drill pipes. This article demonstrates drilling methods, borehole under-reaming methods and the main parameters for the performance of technological operations of directional drilling. A drilling rig has been selected according to the allowable hook load. The density of the used drilling muds has been substantiated for the intervals of compatible drilling conditions, types and parameters of muds.

New method of in situ high-resolution experiments and analysis of fracture networks formed by hydraulic fracturing

Hydraulic fracturing plays a key role in the oil and gas development of unconventional tight reservoirs. Therefore, it is important to study the fracturing mechanism of rocks. Thus far, the exploration of this problem through laboratory experiments has been regarded as the best method. However, the current laboratory experiments cannot achieve in situ high-resolution observation and analysis. Accordingly, an online scanning platform composed of hydraulic fracturing and micro-computed tomography (HFCP) was developed in this study. The in situ measurements obtained using the HFCP include three parts: a computed tomography (CT) system with a maximum imaging resolution of 7 μm, a power system with a continuous changeable fluid pressure, and a post-processing system with rock reconstruction, grayscale calibration, threshold segmentation and efficient extraction of the fracture network. Furthermore, we carried out hydraulic fracturing experiments on tight sandstone samples using the HFCP, and the entire experimental process was conducted in situ. The statistics showed that under different confining pressures, the breakdown pressure and average fracture aperture increased with increasing injection rate, but the branch fracture volume fraction exhibited diverse characteristics. The HFCP provides a new method of studying the mechanism of the hydraulic fracturing of rocks, and it has the potential to reveal the in situ differences in fracture networks in tight reservoirs.

A finite-discrete element based appoach for modelling the hydraulic fracturing of rocks with irregular inclusions

A systematic hybrid approach for modelling the hydraulic fracturing process of heterogeneous rocks with irregular inclusions is developed. This approach is based on a series of computational algorithms, including Fourier series transformation, level-set-based overlapping detection, and the finite-discrete element method. Three major steps are included: (1) circular parameterization and Fourier transformation are employed to reproduce realistic inclusion contours with arbitrary irregular shapes; (2) a novel overlapping detection method based on a level-set function is employed to allocate irregular inclusions effectively and efficiently; and (3) the finite-discrete element model is established by integrating cohesive elements with pore pressure nodes into the solid mesh to simulate the progressive hydraulic fracture and interface crack of heterogeneous rocks. To validate the proposed hybrid approach, modelling results by the established model are compared with numerical simulations in the literature. In addition, the influences of injection speed and interface strength on the mechanical and fracturing responses of heterogeneous rocks are discussed. The results demonstrate that the proposed hybrid approach is capable of simulating the hydraulic fracturing process of heterogeneous rocks.

Dual length scale non-local model to represent damage and transport in porous media

Recent experiments and physical evidence show that fractured porous media feature cracks and fluid capillary networks at various scales. We present a multi-physics macro-scale model that can distinguish between the mechanics and transport interactions. The porous media is represented by a poroelastic domain incorporating non-local damage and non-local transport. The evolution of each of these processes is governed by a unique length scale and driving force, which allows for better flexibility in modeling hydraulic-deformation network systems. For consistency the governing equations of the non-local multi-physics problem are derived from thermodynamics principles. Hence, a four-field (u,P,σ̃eq,D,κ̃) mixed finite element formulation is developed. The non-linear system of equations is linearized and solved using Newton’s method and a backward Euler scheme is used to evolve the system in time, for which a consistent Jacobian matrix and residual vector are derived analytically. Two benchmark examples are investigated: hydraulic fracturing of rocks and soil consolidation. The numerical examples show the viability of this model, and how the variation of the two length scales and damage parameters can be used to describe different physical phenomena.

An Improved Smoothed Particle Hydrodynamics Method and Its Application in Rock Hydraulic Fracture Modelling

An Improved Smoothed Particle Hydrodynamics Method and Its Application in Rock Hydraulic Fracture Modelling

Prediction of hydrogeomechanical processes in undermining of water bodies

Rivers, arable land, engineering structures, urban buildings, and communications often fall into the zone of influence of mining operations. Insufficiently justified mining development often leads to negative environmental and social consequences of the violation or destruction of these objects. On the other hand, surface and underground waters often complicate the development of minerals, negatively affecting the safety of production. The paper deals with the interaction of hydrogeological and geomechanical processes near water bodies in complex mining and hydrogeological conditions. One field of research is based on the development of an engineering approach to the analysis of the interaction of hydrogeological and deformation processes. In this case, the results of instrumental field observations and experience of mining operations near water bodies are used. In this approach, it is assumed that six zones can be distinguished in the under-worked rock mass: collapsed rock; through cracks; partially through cracks; individual cracks that do not form a single system; the zone without discontinuities of continuity; the zone of high stresses and compression deformations. As an example of the application of this approach, the development of a mineral deposit under a river is considered and recommendations are made for the preservation of the earth’s surface from waterlogging. The second field of research is related to mathematical modeling and the use of crack theory. The results of the simulation showed that a spontaneous hydraulic fracturing crack may develop in the underworked rock mass, which can serve as a main channel for the penetration of underground or surface water into the mine workings. The nature of spontaneous hydraulic fracturing of rocks is determined by the fact that when a man-made impact on the rock mass, one of the main stresses of the rock mass can become less than the hydrostatic pressure of water. In this case, a hydraulic fracturing crack develops in the area where this condition is met. As an example, the breakthrough of water from a surface reservoir into an underground mine is considered. It is shown that at a low crack resistance of rocks, it is possible for a crack to grow into the mine, which may result in a sudden water breakthrough. The acquired knowledge expands the scientific basis for the development of appropriate supplements to the instructions for mining operations under water bodies.

Acoustic Emission Source Location Monitoring of Laboratory-Scale Hydraulic Fracturing of Coal Under True Triaxial Stress

Crack monitoring is an important issue in coal and rock hydraulic fracturing (HF). In this study, we conducted experimental coal HF acoustic emission (AE) monitoring tests under true triaxial stress by using a self-developed experiment system. The results showed that the AE signal statistical parameters had a good positive correlation with the water pressure curve. Based on the STA/LTA, improved AIC and simplex algorithm, we developed a new method for AE event automatic identification and source location for coal HF. By using this new method, we were able to detect and locate more accurately the AE events induced by HF. Hydraulic crack propagation and temporal-spatial AE event distribution were located with high accuracy. According to the AE event temporal-spatial distribution of AE events, coal hydraulic crack propagation was not uniform. In the middle and late HF stages, especially when hydraulic cracks approach the coal sample boundary, the hydraulic crack propagation speed was accelerated significantly. The important factors that affected hydraulic crack initiation, propagation, steering, and acceleration included the triaxial stress state, original joints and cracks, and local stress disturbance caused by hydraulic cracks. These results offer a reference for designing optimized, safe and effective HF programs, which are important in preventing coal/rock dynamic disasters and ensuring safe and efficient mining.

Study on Breakdown Pressure in Hydraulic Fracturing Process of Hard Rock Based on Numerical Simulation

Pre-conditioning by hydraulic fracturing is a vital work in block caving mining of hard-rock metal mine, and breakdown pressure is one of the important parameters in rock hydraulic fracturing process, which determines the equipment selection and job-parameter control. Based on numerical simulation method, a pre-holed cylindrical hard rock sample was built for simulation of hydraulic fracturing process to investigate the effect of stress condition, rock properties and water injection rate on breakdown pressure. The results show that the breakdown pressure is increased with the confining stress, and the breakdown pressure is 2.28 times confining stress based on rock tension strength. The variation of rock tension strength affects the breakdown pressure and a positive linear relation exists. Pre-formed notch could obviously reduce the breakdown pressure and control the orientation of hydraulic fracture. And the borehole diameter has a reverse influence on rock breakdown pressure.

An NMM-based fluid-solid coupling model for simulating rock hydraulic fracturing process

Simulation of hydraulic fracturing process based on numerical manifold method (NMM) is a valuable research direction considering its advantages in dealing with continuous-discontinuous deformation problems. In this paper, an NMM-based fluid-solid coupling model is proposed, including numerical solution of unsteady fracture flow, hydraulic pressure-load conversion algorithm, rectification of hydraulic aperture and programming realization of arbitrary crack growth increment etc. In terms of the fluid phase, the fracture flow network is constructed based on the closed loops formed by cutting the mathematical mesh with fractures; in order to eliminate the sensitivity of explicit algorithm to time-step size, an unconditionally stable numerical solution scheme for unsteady flow is established. In terms of the solid phase, for the sake of realizing coupling process of hydraulic pressure to solid deformation, hydraulic pressure is converted into load vector by hydraulic pressure-load conversion algorithm to calculate the deformation of fractured surrounding rock; and Delaunay triangulation algorithm is introduced to subdivide manifold elements with multiple kinks, which effectively solves the problem that the crack tip cannot stop or kink in the same manifold element in the subsequent growth steps, realizes arbitrary crack growth increment. Several benchmark examples are utilized to demonstrate the accuracy of the proposed hydraulic fracturing model in simulating fracture fluid flow and solid deformation. Finally, the developed fluid-solid coupling model is used to simulate rock hydraulic fracturing processes under different conditions. The ability and reliability of the proposed model to describe the hydraulic fracture propagation laws are verified by comparing with previous experimental and numerical results.

ЧИСЛЕННОЕ РЕШЕНИЕ ЗАДАЧ ГЕОМЕХАНИКИ (НА ПРИМЕРЕ МЕСТОРОЖДЕНИЯ ШЕЛЬФА БАЛТИЙСКОГО МОРЯ)

Geomechanical modeling aims at solving problems associated with ensuring accident-free well drilling. The paper deals with building a numerical 3D geomechanical model for a studied field with a further production well stability computation. The region of operations is located in the Baltic Sea shelf. Apart from the field summary, the study contains results of acquisition and audit of initial data for modeling. The method is discussed for unidimensional geomechanical modeling on key wells, including determination of dynamic and static elasticity and strength of the rock varieties, computations of pore pressure, vertical and horizontal stresses. Well stability computations have been obtained and analyzed on the basis of the findings of 1D geomechanical modeling. A further analysis deals with the results of 3D geomechanical modeling, i.e. determination of boundaries and building a structural framework of the model, geometry testing, filling the grid with mechanical properties, and computation of complete stress tensor using the finite element method (FEM). Results of the 1D- and 3D-modeling have been compared. Thus, a numerical 3D geomechanical model has been built for the field under study. The following stage of works was focused on the wellbore stability computation for the planned wells. Additionally, computation was performed for drilling mud absorption pressure gradient cubes, caving pressure, and rock hydraulic fracturing pressure at different inclination angles and drilling azimuths. Recommendations were developed for accident-free construction of wells in the field under study, including upkeep and update of the geomechanical model in real time during drilling of wells. The obtained results and techniques can be used in design and construction of wells in other fields though taking into account regional specifics.

Stratified rock hydraulic fracturing for enhanced geothermal system and fracture geometry evaluation via effective length

Rock stratification and anisotropy extensively exist in geothermal reservoir, and have great effects on the results of hot dry rock hydraulic fracturing. Therefore, the understanding of fracturing mechanism is critical to successfully building an engineered reservoir with high permeability for enhanced geothermal system. Some previous researchers have attempted to reveal hydraulic fracturing process of stratified rock, but the rock hydraulic fracturing propagation and fracture quality evaluation are still not clear enough. We established a numerical model to study the propagation of non-planar fractures in stratified rock by using extended finite element method. For fracture quality evaluation, we proposed fracture effective length as an index, which is the length of the segment where fracture width greater than 1 mm, considering the volume of sand within injected proppant. It is found that fractures generated from stratified rock is more conducive to heat extraction process. A large in-situ stress difference is more conducive to an increase in the fracture effective length. The methodology and results would guide the perforation location selection for stractified rock hydraulic fracturing process to achieve a reasonable fracture morphology.