Term 2: Faults, Fractures and fractured crustal reservoirs

Question 1

What is a fracture?

Answer 1

any sort of discontinuity in rock due to stress, pressure etc.

Question 2

Characteristics of a Single Fracture

Answer 2

• General fracture geometries relative to principle stress axes

Tensile fractures run parallel and in line to σ1
Shear fractures - Conjugate, slip failures converge at σ2, creating X’s
Dissolution seam or styolites run in line to σ3

Question 3

Fracture type

Answer 3

Jointing to faults:

• σ3 extends outwards in joints
• σ3 inwards in faults

Joint to vein: increased opening/tensile
Joint to fault: increased slip/shear

Question 4

How do shear fractures form:

Answer 4

 Mechanical strength overcome…
• A new fracture initiates when a rock is loaded by a force (stress) and the force builds up to the point the mechanical strength of the host rock is reached.
• Sources of stress can be far-field or local

 Sub-critical crack growth
• Once a fracture has formed it is a weakness in the rock and can re-activate at lower differential stress than the original fracture toughness of the rock.

Whenever you break a rock it is easier to break again

Question 5

Primary/Secondary shear fractures

Answer 5

A) Pure shear

B) Reidal model of simple shear (R’)

Question 6

Fracture change with depth

Answer 6

As you move closer to surface, normal fractures steepen and become vertical (tensile)

Question 7

Fault rock type changes with depth

Answer 7

• Shallow crustal rocks are ground against each other - gouges till 12km
• Cataclasites
• 15 km and below ductile rocks with mylonites beginning to form
• Deeper levels: mylonites in Gneiss

Question 8

Lithology controls fracturing

Answer 8

• Variability is high, even within rock types
• Lithology controls fracture density
• Subjective relationship between density of rock and the how hard it is to fracture
Relationship roughly follows rock strength, coal most densely fractured, granite least

Other rock properties have some control
Carbonates: increasing fracturing with decreasing grain size and porosity
Composition: Fracturing increases with increasing %dolomite

Question 9

Beddding controls fracture height

Answer 9

• Lithology can exert an important control on fracture frequency:
− Mechanical properties of units can lead to variations in fracture frequency (due to stress causing failure in some rocks and not in others)
− Mechanical contrast of units also impacts propagation of fracture growth
• Lithology does not always play a role in determining fracture frequency

Question 10

Fracture density in relation to bed thickness

Answer 10

As bed thickness increases:
• Fracture density decreases
• Fracture spacing increases

Question 11

Veins – mineral filled fractures

Answer 11

• Vein material fractures more easily that matrix producing narrower, open fracture;
• High flow parallel to fracture (flow µ aperture3);
• flow normal to fracture may be reduced due to mineralization;
• Quartz bridge
• Fracture from 3009m depth in Cretaceous Travis Peak Formation, East Texas.
• Fracture porosity (filled with blue epoxy)

Question 12

Fracture Aperture

Answer 12

• Aperture & fill
− Wall-to-wall separation
− Average separation
− Hydraulic aperture

• Mineral fill
− Fibers/Euhedral
− Partial/Complete
− Single or multiple stages

• Limitations
− Effect of stress
− Sampling method
− Outcrop data – not reflective of sub-surface

Question 13

Fracture Attributes for Networks (important for describing fluid flow properties of fractures)

Answer 13

• A fracture network is a group of one or more fracture sets developed in the same rock volume.
• Sets may be connected to one another directly or through other fractures.
• Characterization of fractures should include measurement of both the individual fractures sets and fracture networks – how fractures sets are linked
• We use fracture attributes to try to quantify these properties

Question 14

Fracture Density and Spacing

Answer 14

Fracture Intensity (density)
	a measure of fracture intensity,
e.g. #/L

Fracture spacing
 a measure of the distance between fractures, e.g. L/#
 only has much meaning for individual fracture sets (based on orientation)
 Spacing = 1/density

Question 15

Fracture Frequency

Answer 15

How many fractures are in this area?

Fracture frequency (P20)
- number of fractures (N) divided by the area (A)

P20 = N / A
 Main problem is in arriving at an unbiased estimate of N
 Simple counting tends to over-estimate frequency.
 Counting tips better.
 What happens when fracture length&raquo_space; sample area?

Question 16

Geometry and Topology

Answer 16

Geometry
• measurable elements (“shapes”)
• orientation and size of lines, planar surfaces etc.
• units and dimensions
Topology
• relationships between elements
• invariant with strain
• relates to “connectivity”
• dimensionless
 A trace or branch has a node at each end
This provides an unbiased estimate of line and branch frequency using nodes rather than lines

Question 17

Topology importance

Answer 17

Shows connectivity of rocks

IYX plots

Question 18

Fractured Reservoirs and mineral systems

Answer 18

 A large proportion of the world’s proven oil reserves have been found in reservoir rocks that are naturally fractured (Fractured reservoirs)
 Nelson (2001) listed some 370 fields where natural fractures are important for production, a significant proportion being in basement settings,
 Examples: Carbonates, ‘basement’, volcanic rocks [Tight reservoirs such as sands and shales contain few natural fractures, and cannot be produced economically without hydraulic fracturing]
 Many mineral deposits and aquifers are fracture controlled
 Fracture attribute analysis – a key tool to quantify natural fracture systems – Practical exercises

Question 19

Basement reservoirs: issues

Answer 19

• Globally, fractured basement reservoirs widely known but underexploited
• Oil stored within fracture systems as crystalline basement rocks have low porosity + permeability
• Oil has migrated in from adjacent sedimentary source rock Seismic imaging issues
• Limited core & industry knowledge
• How does the oil get in?
• Passive fracture conduits or fault driven migration (or both)?