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Hydraulic fracture growth in naturally fractured rock: mine through mapping and analysis

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Renshaw and Pollard's study of frictional. orthogonal interface crossing ... crossing criteria of Renshaw and Pollard suggests the in-situ stress may ... – PowerPoint PPT presentation

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Title: Hydraulic fracture growth in naturally fractured rock: mine through mapping and analysis


1
Hydraulic fracture growth in naturally fractured
rock mine through mapping and analysis
  • Andre van As, Northparkes Mines
  • and
  • Rob Jeffrey, CSIRO Petroleum

Paper presented at the NARMS-TAC conference,
Toronto July 7-10, 2002.
2
The mine through experiment
  • Create, mine and map a hydraulic fracture placed
    into naturally fractured rock.
  • Characterise fracture growth as function of
    stress field and natural fractures present.
  • Measure size and orientation of fracture
  • Measure amount of fresh rock broken versus growth
    along pre-existing natural fractures.
  • Characterise crossing interactions

3
Purpose
  • To design hydraulic fracture treatments applied
    to preconditioning of ore for caving.

4
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5
The mine through site is located about 450 m
below the surface Ahead of the E26 Lift 2
decline.
6
The fracturing pump provides high pressure fluid
to the injection rod string deployed by the
drill rig. Inflatable straddle packers are used
to isolate a 0.5 m section of the hole.
drill rig
rod string
pump
7
The treatment consisted of water and red plastic
chips used To mark the hydraulic fracture formed
by the treatment.
8
Fracture closure and minimum principal stress
estimate 11.5 MPa bottom-hole.
9
Measured Stress at 450 m
Overcore measurement located in extraction
area below lift 1
10
The critical pressure is the pressure that causes
slip on a ubiquitous joint set oriented at angle
theta to sigma 1.
11
Hydraulic fracture
Natural fracture
Part of the hydraulic fracture exposed
during mining. Note offset as fracture interacts
with natural fracture.
12
The propped trace of the fracture extended for
about 18 m along the west side of the access
decline.
Shear zone
13
Natural Fracture Sets
14
Renshaw and Pollards study of frictional
orthogonal interface crossing
15
Combining these two conditions and writing them
in terms of crack and far-field stresses leads to
the criterion
For crossing to occur
16
Renshaw and Pollards study of frictional
orthogonal interface crossing
Stress and joint conditions at mine through site.
NPM
17
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18
At location P1
19
The propped trace of the fracture extended for
about 18 m along the west side of the access
decline.
Shear zone
20
Near location P3
21
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22
In face of decline
23
Conclusions
  • The hydraulic fracture formed in the naturally
    fractured rock at Northparkes was branched and
    develop offsets in the main fracture path.
  • Natural fracture crossings occurred with and
    without offsets developing.
  • The interface crossing criteria of Renshaw and
    Pollard suggests the in-situ stress may have
    interacted with the natural fractures to result
    in a non-circular hydraulic fracture shape which
    will need to be accounted for in design.
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