Investigations on the Directional Propagation of Hydraulic Fracture in Hard Roof of Mine: Utilizing a Set of Fractures and the Stress Disturbance of Hydraulic Fracture

Abstract Directional fracturing is fundamental to weakening the hard roof in the mine. However, due to the significant stress disturbance in the mine, principal stresses present complicated and unmeasurable. Consequently, the designed hydraulic fracture (HF) extension path is always oblique to principal stresses. Then, the HF will present deflecting propagation, which will restrict the weakness of the hard roof. In this work, we proposed an approach to drive the HF to propagate directionally in the hard roof, utilizing a set of hydraulic fractures and their stress disturbance. In this approach, directional fracturing in the hard roof is conducted via the sequential fracturing of three linear distribution slots. The disturbed stresses produced by the first fracturing (in the middle) are utilized to restrict the HF deflecting extension of the subsequent fracturing. Then, the combined hydraulic fractures constitute a roughly directional fracturing trajectory in rock, i.e., the directional fracturing. To validate the directional fracturing approach, the cohesive crack (representing rock fracture process zone (FPZ)) model coupled with the extended finite element method (XFEM) was employed to simulate the 2D hydraulic fracturing process. The benchmark of the above fracturing simulation method was firstly conducted, which presents the high consistency between simulation results and the fracturing experiments. Then, the published geological data of the hard roof in Datong coal mine (in Shanxi, China) was employed in the fracturing simulation model, with various principal stress differences (2~6 MPa) and designed fracturing directions (30°~60°). The simulation results show that the disturbing stress of the first fracturing significantly inhibits the deflecting propagation of the subsequent fractures. More specifically, along the direction parallel to the initial minimum principal stress, the extension distance of the subsequent hydraulic fractures is 2~3 times higher than that of the deflecting HF in the first fracturing. The fracturing trajectory of the proposed direction fracturing method deviates from the designed fracturing path by only 2°~14°, reduced by 76%~93% compared with the traditional fracturing method utilizing a single hydraulic fracture. This newly proposed method can enhance the HF directional propagation ability more effectively and conveniently in the complex and unmeasurable stress field. Besides, this directional fracturing method can also provide references for the directional fracturing in the oil-gas and geothermal reservoir.

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Numerical simulation on the simultaneous stress interference of parallel multiple hydraulic fractures

Energy Exploration & Exploitation ◽

10.1177/01445987211001955 ◽

2021 ◽

pp. 014459872110019

Author(s):

Weiyong Lu ◽

Changchun He

Keyword(s):

Principal Stress ◽

Hydraulic Fracture ◽

Stress Ratio ◽

Fracture Network ◽

Hydraulic Fractures ◽

Fracture Spacing ◽

Induced Stress ◽

Minimum Principal Stress ◽

Fracturing Process ◽

Stress Ratios

During horizontal well staged fracturing, there is stress interference between multiple transverse fractures in the same perforation cluster. Theoretical analysis and numerical calculation methods are applied in this study. We analysed the mechanism of induced stress interference in a single fracture under different fracture spacings and principal stress ratios. We also investigated the hydraulic fracture morphology and synchronous expansion process under different fracture spacings and principal stress ratios. The results show that the essence of induced stress is the stress increment in the area around the hydraulic fracture. Induced stress had a dual role in the fracturing process. It created favourable ground stress conditions for the diversion of hydraulic fractures and the formation of complex fracture network systems, inhibited fracture expansion in local areas, stopped hydraulic fractures, and prevented the formation of effective fractures. The curves of the maximum principal stress, minimum principal stress, and induced principal stress difference with distance under different fracture lengths, different fracture spacings, and different principal stress ratios were consistent overall. With a small fracture spacing and a small principal stress ratio, intermediate hydraulic fractures were difficult to initiate or arrest soon after initiation, fractures did not expand easily, and the expansion speed of lateral hydraulic fractures was fast. Moreover, with a smaller fracture spacing and a smaller principal stress ratio, hydraulic fractures were more prone to steering, and even new fractures were produced in the minimum principal stress direction, which was beneficial to the fracture network communication in the reservoir. When the local stress and fracture spacing were appropriate, the intermediate fracture could expand normally, which could effectively increase the reservoir permeability.

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A Case Study of Vertical Hydraulic Fracture Growth, Stress Variations with Depth and Shear Stimulation in the Niobrara Shale and Codell Sand, DJ Basin, Colorado

Interpretation ◽

10.1190/int-2020-0246.1 ◽

2021 ◽

pp. 1-34

Author(s):

Kevin L. McCormack ◽

Mark D. Zoback ◽

Wenhuan Kuang

Keyword(s):

Principal Stress ◽

Hydraulic Fracture ◽

Oil And Gas ◽

Velocity Model ◽

Growth Stress ◽

Hydraulic Fractures ◽

Stress Profile ◽

Microseismic Events ◽

Stress Variations ◽

Fracture Growth

We carried out a geomechanical study of three wells, one each in the Niobrara A, Niobrara C and Codell sandstone to investigate how the state of stress and stress variations with depth affect vertical hydraulic fracture growth and shear stimulation of pre-existing fractures. We demonstrate that the higher magnitudes of measured least principal stress values in the Niobrara A and C shales are the result of viscoplastic stress relaxation. Using a density log and a VTI velocity model developed to accurately locate the microseismic events, we theoretically calculated a continuous profile of the magnitude of the least principal stress with depth. This stress profile explains the apparent vertical hydraulic fracture growth as inferred from the well-constrained depths of associated microseismic events. Finally, we demonstrate that because of the upward propagation of hydraulic fractures from the Niobrara C to the Niobrara A, the latter formation experienced considerably more shear stimulation, which may contribute to the greater production of oil and gas from that formation.

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Numerical Simulation of the Influence of Natural Fractures on Hydraulic Fracture Propagation

Geofluids ◽

10.1155/2020/8878548 ◽

2020 ◽

Vol 2020 ◽

pp. 1-12 ◽

Cited By ~ 1

Author(s):

Song Yaobin ◽

Lu Weiyong ◽

He Changchun ◽

Bai Erhu

Keyword(s):

Tensile Strength ◽

Principal Stress ◽

Hydraulic Fracture ◽

Fracture Propagation ◽

Natural Fracture ◽

Hydraulic Fractures ◽

Stress Difference ◽

Natural Fractures ◽

Approach Angle ◽

Hydraulic Fracture Propagation

According to the theory of plane mechanics involving the interaction of hydraulic and natural fractures, the law of hydraulic fracture propagation under the influence of natural fractures is verified using theoretical analysis and RFPA2D-Flow numerical simulation approaches. The shear and tensile failure mechanisms of rock are simultaneously considered. Furthermore, the effects of the approach angle, principal stress difference, tensile strength and length of the natural fracture, and elastic modulus and Poisson’s ratio of the reservoir on the propagation law of a hydraulic fracture are investigated. The following results are obtained: (1) The numerical results agree with the experimental data, indicating that the RFPA2D-Flow software can be used to examine the hydraulic fracture propagation process under the action of natural fractures. (2) In the case of a low principal stress difference and low approach angle, the hydraulic fracture likely causes shear failure along the tip of the natural fracture. However, under a high stress difference and high approach angle, the hydraulic fracture spreads directly through the natural fracture along the original direction. (3) When natural fractures with a low tensile strength encounter hydraulic fractures, the hydraulic fractures likely deviate and expand along the natural fractures. However, in the case of natural fractures with a high tensile strength, the natural fracture surface is closed, and the hydraulic fracture directly passes through the natural fracture, propagating along the direction of the maximum principal stress. (4) Under the same principal stress difference, a longer natural fracture corresponds to the easier initiation and expansion of a hydraulic fracture from the tip of the natural fracture. However, when the size of the natural fracture is small, the hydraulic fracture tends to propagate directly through the natural fracture. (5) A smaller elastic modulus and larger Poisson’s ratio of the reservoir result in a larger fracture initiation pressure. The presented findings can provide theoretical guidance regarding the hydraulic fracturing of reservoirs with natural fractures.

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A Study of Inclined Hydraulic Fractures

Society of Petroleum Engineers Journal ◽

10.2118/4062-pa ◽

1973 ◽

Vol 13 (02) ◽

pp. 61-68 ◽

Cited By ~ 56

Author(s):

Abbas Ali Daneshy

Keyword(s):

Tensile Stress ◽

Coordinate System ◽

Hydraulic Fracture ◽

Isotropic Medium ◽

Maximum Tensile Stress ◽

Borehole Wall ◽

Hydraulic Fractures ◽

Principal Stresses ◽

The Subject

Abstract The results of a theoretical and experimental investigation of inclined hydraulic fractures, reported in this paper, indicate that such fractures do not generally initiate perpendicular to the maximum tensile stress induced on the borehole wall. Unlike axial or normal hydraulic fractures, a degree of shear failure seems to be associated with the initiation and extension of almost all inclined hydraulic fractures. These fractures often intersect the borehole along two diametrically opposite axial lines, thus giving it the appearance of an axial fracture. Inclined hydraulic fractures generally change their orientation as they extend away from the wellbore until they become perpendicular to the least compressive in-situ principal stress. Therefore, the borehole trace of such fractures cannot be used for their positive identification. Introduction The process of hydraulic fracturing of a formation essentially consists of injecting a fluid inside the borehole and pressurizing it until the induced stresses exceed the strength of the formation and cause failure. Failure is generally indicated by a sudden major drop in the variations of the borehole fluid pressure with time. In general in an isotropic medium, the over-all plane of a hydraulic fracture is either parallel, plane of a hydraulic fracture is either parallel, inclined, or perpendicular to the axis of the borehole from which it is extending. Accordingly, these fractures will be called axial, inclined or normal, respectively. This classification of hydraulic fractures refers them to the borehole where they are observed rather than the ground surface or the bedding planes. In case of vertical boreholes, axial and normal fractures become identical with vertical and horizontal fractures (which are the terms often used in petroleum industry). In the first comprehensive analysis of the mechanics of hydraulic fracturing, Hubbert and Willis proposed that axial or normal hydraulic fractures initiate when the maximum tensile stress induced around the borehole exceeds the tensile strength of the formation, and that such fractures extend in a plane perpendicular to the least compressive in - situ principal stress. The correctness of this proposal has since been verified by Haimson and Fairhurst, who conducted an extensive series of laboratory experiments on the subject. In their theoretical and experimental work, Haimson and Fairhurst assumed that one of the in-situ principal stresses is parallel to the borehole axis. Under such a condition, one can only create an axial or a normal hydraulic fracture in an isotropic medium. For the case when none of the in-situ principal stresses are parallel to the borehole, Fairhurst derived mathematical expressions for the stress components on the borehole wall, in isotropic and transversely isotropic media. Experimentally, von Schonfeldt and Daneshy independently observed that under such a condition the fracture orientation is influenced by the borehole in its vicinity. The trace of inclined hydraulic fractures at the wellbore was found to be misleading if used for the purpose of determining the over-all fracture orientation. The research reported here is an extension of a previous work on the subject of inclined hydraulic previous work on the subject of inclined hydraulic fractures. It includes the computation of the magnitude and the orientation of the maximum tensile stress induced at the borehole wall, for each experiment, and the resulting fracture shape. Such investigations can, in the course of time, provide means of determining the over-all fracture provide means of determining the over-all fracture type at great depth, which has significant importance in many fields, such as geophysics, petroleum, geological and civil engineering. STRESS DISTRIBUTION AT THE WALL OF THE BOREHOLE Let sigma1, sigma2 and sigma3 be the three in-situ total principal stresses whose values and directions are assumed to remain constant throughout the isotropic porous elastic formation under consideration. No porous elastic formation under consideration. No restriction is imposed upon the direction of any of the in-situ principal stresses, except the mathematical requirement that they should be mutually perpendicular. Consider a coordinate system ox1 x2 x3 perpendicular. Consider a coordinate system ox1 x2 x3 chosen such that ox3 is the borehole axis, and ox1, lies in the plane osigma1 sigma2, Fig. 1. SPEJ P. 61

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Simulation of Stress Changes of Shale Reservoirs during Hydraulic Fracturing to Create Fracture Networks

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.419.10 ◽

2013 ◽

Vol 419 ◽

pp. 10-16

Author(s):

G.M. Zhang ◽

J.D. Liu ◽

C.M. Xiong ◽

H. Liu ◽

J. Jin

Keyword(s):

Pore Pressure ◽

Hydraulic Fracture ◽

Theoretical Data ◽

Element Model ◽

Hydraulic Fractures ◽

Theoretical Studies ◽

Fracture Networks ◽

Stress Anisotropy ◽

Stress Changes ◽

Simulation Results

Theoretical studies have shown that the generation of hydraulic fractures reduces or even reverses the stress anisotropy between the fractures and results in increasing the complexity of fractures. A finite element model was established in which the pore pressure element was used to simulate the behavior of porous media and the pore pressure cohesive element was adopted to catch the characters of hydraulic fracture. A special fracturing manner was adopted to create complicated fracture networks by reducing or even reversing the stress anisotropy between fractures. The geometries of hydraulic fractures, strains, stresses, pore pressure distributions and fluid pressures within the fractures are obtained. The results of the model are fit well with the corresponding theoretical data. The simulation results show that the stress anisotropy is reduced resulting from the generation of the hydraulic fracture, multiple parallel transverse fractures of horizontal well further reduce or even reverse the stress anisotropy in some place of the reservoir. The simulation results validate the feasibility of the theoretical studies and the expected complex network fractures could be created by adopting the special fracturing manner.

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Hydraulic-Fracture Geomechanics and Microseismic-Source Mechanisms

SPE Journal ◽

10.2118/158935-pa ◽

2013 ◽

Vol 18 (04) ◽

pp. 766-780 ◽

Cited By ~ 56

Author(s):

N.R.. R. Warpinski ◽

M.J.. J. Mayerhofer ◽

K.. Agarwal ◽

J.. Du

Keyword(s):

Shear Stress ◽

Hydraulic Fracture ◽

Hydraulic Fractures ◽

Natural Fractures ◽

Total Stress ◽

Multiple Fractures ◽

Significant Information ◽

Bedding Planes ◽

Fracturing Process ◽

Source Mechanisms

Summary Interpretation of microseismic results and attempts to link microseismic-source mechanisms to fracture behavior require an understanding of the geomechanics of the fracturing process. Stress calculations around fractures show that the area normal to the fracture surface is stabilized by a pressurized fracture as a result of increased total stress and decreased shear stress. In this area, microseisms can occur only if leakoff pressurizes natural fractures, bedding planes, or other weakness features, and source mechanisms are thus likely to show a volumetric component that has either opening or closing movement in addition to shear slippage. Conversely, the tip tensile region is destabilized by a reduction in total stress and an increase in shear stress, with the likelihood that microseisms would be generated in this region because of these changes. Such microseisms would not yet be invaded by the fracturing fluid, and events that are mostly shear would be expected. Systems with multiple fractures, such as those that are potentially created in multiperforation-cluster stages, are much more complex, but similar elements can be outlined for those as well. Source mechanisms can help delineate these different types of microseismic behaviors, but the evaluation of such mechanisms reveals that they provide no significant information about the hydraulic fracture. Whereas it would be valuable if source mechanisms could provide information about the mechanics of the hydraulic fracture (e.g., opening, closing, and proppant), calculations show that both the energy and volume associated with microseismicity are an insignificant fraction of the total energy and volume input into the stimulation. Thus, hydraulic fractures are almost entirely aseismic. The analysis of source mechanisms should concentrate on what those data reveal about the reservoir (e.g., natural fractures and faults). Integrated diagnostic studies provide more value in understanding both the microseismicity and interpretation of the microseismic results.

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Semianalytical Model for Fault Slippage Resulting from Partial Pressurization

SPE Journal ◽

10.2118/199348-pa ◽

2019 ◽

Vol 25 (03) ◽

pp. 1489-1502 ◽

Cited By ~ 2

Author(s):

Kui Liu ◽

Arash Dahi Taleghani ◽

Deli Gao

Keyword(s):

Shale Gas ◽

Hydraulic Fracture ◽

Fault Reactivation ◽

Hydraulic Fractures ◽

Gas Wells ◽

Principal Stresses ◽

Casing Deformation ◽

Spacing Selection ◽

Casing Failure

Summary Casing failure in shale gas wells has seriously impacted production from Weiyuan and Changning fields in Sichuan Province, China. Linearly distributed microseismic data and the corresponding casing shear deformation close to these microseismic signals indicate fault reactivation in these areas during hydraulic-fracturing treatments. Apparently, interaction of hydraulic fractures with nearby faults causes fault slippage, which in some situations has led to well shearing. Hence, we propose a semianalytical model in this paper to estimate the length of slippage along the fault that is caused by pressurization of a fault intercepted by the hydraulic fracture. These calculations have been performed for different configurations of the fault with respect to the hydraulic fracture and principal stresses. Using the semianalytical model provided in this paper, two fault slippage cases are calculated to assess the casing failure in nearby wells. In one case study, the calculated results of the fault slippage are consistent with the scale of casing deformation in that well and a microseismic magnitude caused by fault slippage is calculated that is larger than the detected events. The presented model will provide a tool for a quick estimation of the magnitude of fault slippage upon intersection with a hydraulic fracture, to avoid potential casing failures and obtain a more reliable spacing selection in the wells intersecting faults.

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Numerical Investigation of Fracture Network Formation under Multiple Wells

Mathematical Problems in Engineering ◽

10.1155/2020/1763713 ◽

2020 ◽

Vol 2020 ◽

pp. 1-11

Author(s):

Xiaoxi Men ◽

Jiren Li

Keyword(s):

Pore Pressure ◽

Hydraulic Fracture ◽

Network Formation ◽

Process Analysis ◽

Flow Direction ◽

Failure Process ◽

Fracture Network ◽

Hydraulic Fractures ◽

Fracture Evolution ◽

Simulation Results

A two-step fracturing method is proposed to investigate the hydraulic fracture evolution behavior and the process of complex fracture network formation under multiple wells. Simulations are conducted with Rock Failure Process Analysis code. Heterogeneity and permeability of the rocks are considered in this study. In Step 1, the influence of an asymmetric pressure gradient on the fracture evolution is simulated, and an artificial structural plane is formed. The simulation results reflect the macroscopic fracture evolution induced by mesoscopic failure; these results agree well with the characteristics of the experiments. Step 2, which is based on the first step, investigates the influence of preexisting fractures (i.e., artificial structural planes) on the subsequent fracturing behavior. The simulation results are supported by mechanics analysis. Results indicated that the fracture evolution is influenced by pressure magnitude on a local scale around the fracture tip and by the orientation and distribution of pore pressure on a global scale. The constant pressure in wellbore H2 can affect fracture propagation by changing the water flow direction, and the hydraulic fractures will propagate to the direction of higher local pore pressure. Furthermore, the artificial structural planes influence the stress distribution surrounding the wellbores and the hydraulic fracture evolution by altering the induced stresses around the preexisting fractures. Finally, fracture network is formed among the artificial structural planes and hydraulic fractures when multiple wells are fractured successively. This study provides valuable guidance to unconventional reservoir reconstruction designs.

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CO2-Driven Hydraulic Fracturing Trajectories across a Preexisting Fracture

Geofluids ◽

10.1155/2021/5533945 ◽

2021 ◽

Vol 2021 ◽

pp. 1-12

Author(s):

Qi Zhang ◽

Jiehao Wang ◽

Yufeng Gao ◽

Shengfei Cao ◽

Jingli Xie ◽

...

Keyword(s):

Hydraulic Fracture ◽

Damage Evolution ◽

Stress Ratio ◽

Bedding Plane ◽

Hydraulic Fractures ◽

The Finite Element Method ◽

Bedding Planes ◽

Initiation And Propagation ◽

Gas Seepage ◽

Simulation Results

Defining the trajectory of hydraulic fractures crossing bedding planes and other fractures is a significant issue in determining the effectiveness of the stimulation. In this work, a damage evolution law is used to describe the initiation and propagation of the fracture. The model couples rock deformation and gas seepage using the finite element method and is validated against classical theoretical analysis. The simulation results define four basic intersection scenarios between the fluid-driven and preexisting fractures: (a) inserting—the hydraulic fracture inserts into a bedding plane and continues to propagate along it; (b) L-shaped crossing—the hydraulic fracture approaches the fracture/bedding plane then branches into the plane without crossing it; (c) T-shaped crossing—the hydraulic fracture approaches the fracture/bedding plane, branches into it, and crosses through it; (d) direct crossing—the hydraulic fracture crosses one or more bedding planes without branching into them. The intersection scenario changes from (a) → (b) → (c) → (d) in specimens with horizontal bedding planes when the stress ratio β ( β = σ y / σ x ) increases from 0.2 to 5. Similarly, the intersection type changes from (d) → (c) → (a) with an increase in the bedding plane angle α (0° → 90°). Stiffness of the bedding planes also exerts a significant influence on the propagation of hydraulic fractures. As the stiffness ratio E 1 ¯ / E 2 ¯ increases from 0.1 to 0.4 and 0.8, the seepage area decreases from 22.2% to 41.8%, and the intersection type changes from a T-shaped crossing to a direct crossing.

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Modelling Rock Fracture Induced By Hydraulic Pulses

Rock Mechanics and Rock Engineering ◽

10.1007/s00603-021-02477-0 ◽

2021 ◽

Author(s):

Xun Xi ◽

Shangtong Yang ◽

Christopher I. McDermott ◽

Zoe K. Shipton ◽

Andrew Fraser-Harris ◽

...

Keyword(s):

Hydraulic Fracturing ◽

Rock Fracture ◽

Cohesive Crack ◽

Constitutive Relationship ◽

Crack Model ◽

Hydraulic Pressure ◽

Breakdown Pressure ◽

Hydraulic Flow ◽

Discrete Crack ◽

Fracturing Process

AbstractSoft cyclic hydraulic fracturing has become an effective technology used in subsurface energy extraction which utilises cyclic hydraulic flow pressure to fracture rock. This new technique induces fatigue of rock to reduce the breakdown pressure and potentially the associated risk of seismicity. To control the fracturing process and achieve desirable fracture networks for enhanced permeability, the rock response under cyclic hydraulic stimulation needs to be understood. However, the mechanism for cyclic stimulation-induced fatigue of rock is rather unclear and to date there is no implementation of fatigue degradation in modelling the rock response under hydraulic cyclic loading. This makes accurate prediction of rock fracture under cyclic hydraulic pressure impossible. This paper develops a numerical method to model rock fracture induced by hydraulic pulses with consideration of rock fatigue. The fatigue degradation is based on S–N curves (S for cyclic stress and N for cycles to failure) and implemented into the constitutive relationship for fracture of rock using in-house FORTRAN scripts and ABAQUS solver. The cohesive crack model is used to simulate discrete crack propagation in the rock which is coupled with hydraulic flow and pore pressure capability. The developed numerical model is validated via experimental results of pulsating hydraulic fracturing of the rock. The effects of flow rate and frequency of cyclic injection on borehole pressure development are investigated. A new loading strategy for pulsating hydraulic fracturing is proposed. It has been found that hydraulic pulses can reduce the breakdown pressure of rock by 10–18% upon 10–4000 cycles. Using the new loading strategy, a slow and steady rock fracture process is obtained while the failure pressure is reduced.

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