Title: Hydraulic fracture growth in naturally fractured rock: mine through mapping and analysis
1Hydraulic 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.
2The 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
3Purpose
- To design hydraulic fracture treatments applied
to preconditioning of ore for caving.
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5The mine through site is located about 450 m
below the surface Ahead of the E26 Lift 2
decline.
6The 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
7The treatment consisted of water and red plastic
chips used To mark the hydraulic fracture formed
by the treatment.
8Fracture closure and minimum principal stress
estimate 11.5 MPa bottom-hole.
9Measured Stress at 450 m
Overcore measurement located in extraction
area below lift 1
10The critical pressure is the pressure that causes
slip on a ubiquitous joint set oriented at angle
theta to sigma 1.
11Hydraulic fracture
Natural fracture
Part of the hydraulic fracture exposed
during mining. Note offset as fracture interacts
with natural fracture.
12The propped trace of the fracture extended for
about 18 m along the west side of the access
decline.
Shear zone
13Natural Fracture Sets
14Renshaw and Pollards study of frictional
orthogonal interface crossing
15Combining these two conditions and writing them
in terms of crack and far-field stresses leads to
the criterion
For crossing to occur
16Renshaw and Pollards study of frictional
orthogonal interface crossing
Stress and joint conditions at mine through site.
NPM
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18At location P1
19The propped trace of the fracture extended for
about 18 m along the west side of the access
decline.
Shear zone
20Near location P3
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22In face of decline
23Conclusions
- 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.