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Experimental Fracturing - Structural Geology - Lecture Notes, Study notes of Geology

West Bengal State UniversityGeology

In these Lecture notes, Professor has tried to illustrate the following points : Experimental Fracturing, Methodology, Experimental Work, Rock Mechanics, Triaxial Press, Strength Exceeded, Mohr Relations, Rock Failure, Permanent Deformation, Critical Stress

Typology: Study notes

2012/2013

Uploaded on 07/22/2013

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103
I. Experimental Fracturing in Rocks
A. Methodology
1. Experimental Work in Rock Mechanics
a. Rock core samples placed in triaxial press
b. Pressure applied until strength exceeded
c. Fractures examined to provide insight into Mohr relations.
B. Basic Concepts and Variables
1. Rock failure
a. critical stress relations at which sample is unable to support
further stress increase without permanent deformation
2. Strength
a. critical stress conditions at which failure of rock sample occurs
3. Brittle Failure: brittle cracking of sample
a. Brittle Fracture: surface zone across which rock sample loses
cohesion
(1) atomic bonds broken at subatomic level
4. Ductile Failure: rock material becomes permanently deformed without
losing cohesion
5. Confining Pressure: pressure applied to and surrounding the exterior of
the sample
6. Pore Fluid Pressure: pressure of fluids contained in pore spaces of rock
7. Temperature: may be controlled in experimental aparatus
8. Axial Stress: stress applied parallel to core cylinder axis
9. Radial Stress: stress applied perpendicular to core cylinder axis
a. i.e. confining pressure
C. Common experimental conditions
1. Axial compression experiments (positive stress)
a. axial stress = sigma1
b. radial stress = sigma2=sigma3
2. Axial tension experiments (negative stress)
a. axial stress = negative = pull = sigma3
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I. Experimental Fracturing in Rocks A. Methodology

  1. Experimental Work in Rock Mechanics a.b. Rock core samples placed in triaxial pressPressure applied until strength exceeded c. Fractures examined to provide insight into Mohr relations. B. Basic Concepts and Variables
  2. Rock failure a. critical stress relations at which sample is unable to supportfurther stress increase without permanent deformation
  3. Strength a. critical stress conditions at which failure of rock sample occurs
  4. Brittle Failure: brittle cracking of sample a. Brittle Fracture: surface zone across which rock sample loses cohesion (1) atomic bonds broken at subatomic level
  5. Ductile Failure: rock material becomes permanently deformed without losing cohesion
  6. (^) the sampleConfining Pressure: pressure applied to and surrounding the exterior of
  7. Pore Fluid Pressure: pressure of fluids contained in pore spaces of rock
  8. Temperature: may be controlled in experimental aparatus
  9. Axial Stress: stress applied parallel to core cylinder axis
  10. (^) a.Radial Stress: stress applied perpendicular to core cylinder axis i.e. confining pressure C. Common experimental conditions
  11. Axial compression experiments (positive stress) a.b. axial stress = sigma1radial stress = sigma2=sigma
  12. Axial tension experiments (negative stress) a. axial stress = negative = pull = sigma

D. Concepts from Rock Fracture Experiments

  1. Mode I, II and III Fractures Commonly Produced in Experiments a. Review of Terminology (1) Mode I: extension fractures, separation perpendicular tofracture surface (2) Mode II: strike-slip shear fracture (3) Mode III: dip-slip shear fracture b. Extension Fractures (Mode I) (1) From under positive, compressive stress (2) fracture plane perpendicular to minimum principal stress (3) sigma3fracture plane parallel to maximum principal stress sigma (4) displacement normal to fracture surface c. Tension Fractures (Mode I) (1) Form under negative, tensile stress (2) fracture plane perpendicular to minimum principle stress sigma d. Shear Fractures (Mode III) (1) Form under conditions of confined compression (2) Commonly form at angles < 45 degrees to maximumcompressive stress, sigma (3) Displacement, by shear, parallel to fracture surface (4) If under triaxial conditions: sigma1 > sigma2 > sigma3... (a) shear fractures form parallel to intermediateprincipal stress, sigma
  2. Conditions of Tension a. Tensile strength of rock (1) critical tensile stress (negative sigma3) at which rockundergoes brittle failure to form tension fractures

b. Fracture plane angle (1) angle between maximum principal stress (sigma1) and thefracture plane

c. Fracture angle (1) angle between maximum principal stress (sigma1) and a

(3) Angle of Internal Friction: defined by angle betweentangent lines of Mohr envelope and horizontal of mohr circle (4) Coulomb Coefficient d. Other Fracture Relations (1) Conjugate Shear Fractures (a) a set of two shear fractures that commonly developunder shear failure

(b) angle between two conjugate shear fracturesapproximately 60 degrees (c) each shear at 30 degree angle between sigma and conjugate each shear plane (d) under triaxial conditions: sigma1>sigma2>sigma3, conjugate shears will form parallel to sigma i) most common stress condition in nature ii) however, usually one dominant sheardirection will prevail. (e) under confining conditions of sigma1>sigma2=sigma3, conjugate shears may form in infinite number of orientations (2) Reidel (Secondary) Shear Fractures (a) determined from clay-shear experiments (b) R shears = synthetic secondary shears that form within 15 degrees of primary conjugate set, samesense of shear motion

(c) R' shears = antithetic secondary shears that form at75-80 degrees of primary conjugate set, with opposite sense of shear motion E. Controlling Factors of Fracturing

  1. Confining Pressure a. As confining pressure increases, Mohr circle shifts to the right of the mohr diagram (1) under very high confining pressure, rocks commonly undergo ductile deformation

b. Frictional Sliding

(1) At low confining pressures, Mode I fractures commonlydevelop (2) Mode I fractures commonly reactivated as Mode II or III fractures at higher stress states (a) At lower confining pressures: shear motion = (b) continuous slidingAt higher confining pressures: reactivated shear motion = stick-slip i) "stick" interval = > internal shear stress ii) "slip" interval = rapid sliding and release ofinternal shear stress

  1. Pore Fluid Pressure a. Effects of internal fluid pressure in rock (1) internal fluid pressure effectively reduces confining pressure in straight arithmetic relationship (2) Effect: internal fluid pressure shifts mohr cirlce to the left (a) stress conditions that are stable at 0 pore pressure, may become unstable at > pore pressure (3) Thought to be primary mechanism for creating extensionfractures at great depths, under great confining pressures.

(4) Pore pressure along fault planes, reduces effective normalstress, < friction, triggers fault motion (a) "hydroplaning" along fracture plane (b) proposed as a mechanism to artificially relieve stress along known fault zonesi) e.g. San Andreas

  1. Mechanical Anisotropy a. Terms (1) mechanical isotropy: rocks have same rheology andfracture mechanics in all directions

d. earth quake prediction

  1. Borehole Analysis (Strain Gauging) a.b. drill hole in rock, set up strain gages in holemeasure strain of bore hole in response to rock removal
  2. Hydrofracturing a. use of hydrofrac studies to delineate stress conditions
  3. Earthquake First Motion Studies a. using seismic analysis of earthquakes to determine sense of b. shear on faultshelps delineate stress field C. Stress in the Earth
  4. Types of Stress a. Vertical Normal Stress (1) downward compressive stress, perpendicular to horizontal (2) generally equal to overburden stress, related to weight ofrock parallel to vector of gravitational force b. Nontectonic Horizontal Stress (1) burial of sediments in sedimentary basin (a)(b) (^) horizontal stress: horizontal component related tovertical stress = overburden compression overburden confining pressure and basin subsidence c. Tectonic Horizontal Stress (1) rift tectonics = tensile horizontal stress (2) convergent zones = compressive horizontal stress (a)(b) (^) ductile deformation at depths > 15-20 km due tobrittle rheology in upper 15-20 km of crust temp. and pressure increase
  5. Driving Mechanisms of Stress a. Overburden Pressure (1) stress due to weight of overlying column of rock (a) common average density of rocks: 2.7 g/cu. cm.

(2) Thickening of overburden pressure (a) sediment loading (b)(c) tectonic / thrust sheet loadingice loading during glacialtion b. Tectonics (1) stress associated with plate motion (a)(b) subduction pullspreading center push (c) mantle convection c. Vertical Motions

(1) Isostacy (a)(b) volcanic loadingice loading (c) erosion and isostatic uplift (2) igneous intrusion (a) e.g. laccoliths d. Effects of Temperature and Pressure (1) stresses due to thermal expansion and contraction of rocks (2) magmatic heating and cooling e. Pore Fluid Pressure (1) connate pore fluids create internal pressure (a) (^) pore pressuresediment compaction of impermeable seds., > (b) (^) i)prograde metamorphism dewatering and release of carbon dioxide, > pore pressures (c) magmatic melts exerting pore pressure, vein formation

  1. Joint Formation and Timing of Stress a. General (1) Joints: essentially mode I fractures, extension in nature (2) Problem: all stresses that have been measured in theearth are compressive in nature, tensile stresses are rare