Shear Stresses and the Hydraulic Fracturing of Earth Dam Soils

Abstract All reservoirs are fractured to some degree. Depending on the density, dimension, orientation and the cementation of natural fractures and the location where the hydraulic fracturing is done, preexisting natural fractures can impact hydraulic fracture propagation and the associated flow capacity. Understanding the interactions between hydraulic fracture and natural fractures is crucial in estimating fracture complexity, stimulated reservoir volume, drained reservoir volume and completion efficiency. However, because of the presence of natural fractures with diffuse penetration and different orientations, the operation is complicated in naturally fractured gas reservoirs. For this purpose, two numerical methods are proposed for simulating the hydraulic fracture in a naturally fractured gas reservoir. However, what hydraulic fracture looks like in the subsurface, especially in unconventional reservoirs, remain elusive, and many times, field observations contradict our common beliefs. In this study, the hydraulic fracture model is considered in terms of the state of tensions, on the interaction between the hydraulic fracture and the natural fracture (45°), and the effect of length and height of hydraulic fracture developed and how to distribute induced stress around the well. In order to determine the direction in which the hydraulic fracture is formed strikethrough, the finite difference method and the individual element for numerical solution are used and simulated. The results indicate that the optimum hydraulic fracture time was when the hydraulic fracture is able to connect natural fractures with large streams and connected to the well, and there is a fundamental difference between the tensile and shear opening. The analysis indicates that the growing hydraulic fracture, the tensile and shear stresses applied to the natural fracture.

Download Full-text

Stress-deformation predictions for the LG 4 main dam

Canadian Geotechnical Journal ◽

10.1139/t84-025 ◽

1984 ◽

Vol 21 (2) ◽

pp. 213-222 ◽

Cited By ~ 3

Author(s):

J. J. Paré ◽

N. S. Verma ◽

H. M. S. Keira ◽

A. D. McConnell

Keyword(s):

Hydraulic Fracturing ◽

Material Properties ◽

Laboratory Testing ◽

Deformation Behaviour ◽

Earth Dam ◽

Interesting Aspect ◽

James Bay ◽

Hydroelectric Development ◽

Stress Deformation ◽

Critical Sections

The LG 4 dam, the second largest structure on the La Grande Complex of the James Bay hydroelectric development, is 125 m high, about 4 km long, and is composed of 19 × 106 m3 of fill materials.The design of the dam is characterised by (i) a zoned earth–rockfill section based on a judicious use of the limited quantities of various materials available, (ii) a 70 m high abutment in the river valley with a steep inclination of about 55°, and (iii) a 50 m high section of the dam with its axis curved in the downstream direction.Detailed stress-deformation analyses were carried out in the critical sections of the dam using finite element methods to verify any presence of arching and hydraulic fracturing potentials in the nonplastic till core. An interesting aspect of these analyses was the fact that the material properties were established based on laboratory testing as well as the observed deformation behaviour of the already completed 156 m high LG 2 main dam.The analyses have indicated that the design has adequate reserve of safety against hydraulic fracturing and arching. Nevertheless, the design sections and material placement requirements were optimised, where necessary, to ensure a satisfactory behaviour of the dam. The instrumentation design was also adapted to the findings of this study. Keywords: earth dam, steep abutment, stress-strain, hydraulic fracturing, arching.

Download Full-text

Geomechanical stress conditions to induce half-moon events during hydraulic fracturing

Geophysics ◽

10.1190/geo2019-0681.1 ◽

2021 ◽

pp. 1-39

Author(s):

Nepomuk Boitz ◽

Serge A. Shapiro

Keyword(s):

Hydraulic Fracturing ◽

Hydraulic Fracture ◽

Fault Plane ◽

Stress Conditions ◽

Shear Stresses ◽

Stress Regime ◽

Hydraulic Stimulation ◽

Fracture Pressure ◽

Vertical Fault ◽

Multiple Case

Half-moon events are a special type of microseismic source mechanism that is found at various hydraulic fracturing sites, but hardly observed in natural seismicity. This event type can be either explained by a vertical slip on a nearly vertical fault plane or by horizontal slip on a nearly horizontal fault plane. For this, special stress conditions are required, for instance nearly equal horizontal and vertical compressional stresses and significant shear stresses. Such conditions are created during hydraulic stimulation as shown by our numerical simulations. By applying fracture pressure to the surface of the hydraulic fracture, the stress field in the vicinity of the hydraulic fracture can locally rotate and horizontal or vertical faults become optimally oriented. We show that such rotations can occur in locations, where the elastic properties of rocks change (i.e., the fracture crosses a layer interface) or at the tips of the hydraulic fracture.Depending on the stress regime, our model explains half-moon events generated by slip on nearly horizontal fault planes in strike-slip environments and by slip on nearly vertical fault planes in normal faulting tectonics. Moreover, our models explain several common characteristics observed in multiple case studies. This includes the observation of high portion of half-moon events and opposed shear senses in different depths and on opposite sides of the fracture.

Download Full-text

Quasi-Static Crack Growth Under Symmetrical Loads in Hydraulic Fracturing

Journal of Applied Mechanics ◽

10.1115/1.4036988 ◽

2017 ◽

Vol 84 (8) ◽

Cited By ~ 11

Author(s):

Wenhao Shen ◽

Ya-Pu Zhao

Keyword(s):

Shear Stress ◽

Crack Initiation ◽

Hydraulic Fracturing ◽

Integral Equations ◽

Integral Transform ◽

Boundary Integral Equations ◽

Boundary Integral ◽

Stress Fields ◽

Shear Stresses ◽

Normal And Shear Stresses

Symmetrical load on the crack surfaces is found in many fluid–solid problems. The combined effect of symmetrical normal and shear stresses is investigated, which impacts on the displacement and stress fields and the predictions of crack initiation and deflection. The boundary integral equations of displacement and stress fields are formulated using the integral-transform method. The equations of the displacement and stress are reduced using the Abel integral equations. The analytical solution of the full space for uniform normal and shear stresses is obtained. The asymptotic solution of the displacement of the crack surface is obtained near the crack tip under specific normal and shear stresses. Results show that shear stress tends to inhibit the crack, and the predictions of crack initiation and deflection could be inappropriate for a slit crack under a singular shear stress. This study may be useful for future investigations of the fluid–solid problems and help to understand the hydraulic fracturing.

Download Full-text

Consequences of rock layers and interfaces on hydraulic fracturing and well production of unconventional reservoirs

Geophysics ◽

10.1190/geo2017-0089.1 ◽

2018 ◽

Vol 83 (6) ◽

pp. MR333-MR344

Author(s):

Seounghyun Rho ◽

Roberto Suarez-Rivera ◽

Samuel Noynaert

Keyword(s):

Hydraulic Fracturing ◽

Surface Area ◽

Homogeneous Model ◽

In Situ Stress ◽

Unconventional Reservoirs ◽

Rock Properties ◽

Hydraulic Fractures ◽

Shear Stresses ◽

Principal Stresses ◽

Well Production

Hydraulic fracturing is a fundamental condition for economic production of hydrocarbons from unconventional reservoirs. Hydrocarbon production is proportional to the propped surface area that is in contact with the reservoir and remains connected to the wellbore. Yet, the propped surface area controlling production appears to be considerably smaller than the surface area created during pumping. Somehow hydraulic fractures are disconnected, truncated, and reduced during production. One important mechanism causing this segmentation is the shear displacement of weak interfaces between rock layers. Shear stresses are generated in response to abrupt changes in material properties and changes in bed orientation, in relation to the orientation of the existing principal stresses. If the layered rocks are strongly laterally heterogeneous, they provide a high potential for shear failures and fracture segmentation along the interfaces between layers. The induced shear stress and shear slip also depend on the current geologic structure and following in situ stress loading, the stress alteration and fluid leakoff during hydraulic fracturing, and the existence of wells. We conducted numerical simulations using the finite-element method on layered and discontinuous rocks, and specifically in organic-rich mudstones and carbonate sequences. Our work was part of a field study. Three different layered rock models were simulated and compared: laterally homogeneous, laterally heterogeneous, and strongly laterally heterogeneous. For the latter, the heterogeneity was introduced by randomly varying the elastic rock properties of each layer. Our results indicate that localized shear stresses develop along interfaces between materials with contrasting properties and along the wellbore walls. This includes the generation of localized shear in planes that were principal in the homogeneous model. It was also seen that rock shear and slip, along interfaces between layers, may occur when the planes of weakness are pressurized (e.g., during hydraulic fracturing).

Download Full-text

DYNAMIC CALCULATION OF THE PLANE ELASTIC "DAM-FOUNDATION" SYSTEM

Construction Materials and Products ◽

10.34031/2618-7183-2021-4-5-16-23 ◽

2021 ◽

Vol 4 (5) ◽

pp. 16-23

Author(s):

K. Salyamova

Keyword(s):

Strain State ◽

Mathematical Formulation ◽

Earth Dam ◽

Shear Stresses ◽

Earth Dams ◽

Safe Operation ◽

Stress Strain ◽

Foundation Soil ◽

Dam Foundation ◽

Stress Strain State

design, construction, and reliable and safe operation of earth dams (more than 60 of them are in operation in the Republic of Uzbekistan located in seismic region) put forward requirements for the continuous improvement of the calculation methods for loads; as required by regulatory methods for fundamental (static) and special (dynamic) load combinations. These regulatory methods do not take into account the nonhomogeneous nature of the behavior and piecewise heterogeneity of the characteristics of foundation, and the stress-strain state (SSS) of an earth dam under constant or temporary loads, which is necessary for reliable and safe operation, especially in seismic regions. A general mathematical formulation of problems for earth dams in a plane elastic formulation is given. Dynamic calculations were conducted to determine the stress-strain state of an earth dam, taking into account the design features and real piecewise-nonhomogeneous physical and mechanical characteristics of soil of the structure body and base (these characteristics were provided by the design organization). The problem was solved by the numerical finite element method. The eigenfrequencies and modes of vibrations of the plane "structure-foundation" system are determined, considering the homogeneous and piecewise-nonhomogeneous characteristics of the foundation soil; the corresponding analysis of the behavior of the system was made. The stress-strain state of the “dam-foundation” system was investigated using calculated frequencies. The calculation results were lines of equal displacements (horizontal, vertical), normal and shear stresses in the “dam-foundation” system.

Download Full-text