Observation and Modeling of Fault Damage Zones at Reservoir Depths
Abstract
We report a study of sub-surface fault damage zones adjacent to the San Andreas Fault in central California and series of first- and second-order faults in a gas field in Southeast Asia. We compare the observations with theoretically-predicted damage zones from dynamic rupture propagation modeling. The importance of characterizing damage zones arises from the important role that damage zones and natural fractures play in governing fluid flow in low permeability rocks. While there are many published studies of exposed damage zones, there is an absence of studies utilizing subsurface data that characterizes damage zones at depth. Damage zones associated with second-order faults adjacent to the San Andreas Fault are studied in well-cemented arkosic sandstones immediately southwest of the fault at the SAFOD site using electrical image logs and physical property measurements. The peak fracture intensity is between three and six fractures per meter in thee damage zones which persist about 50-80 meters from the second-order faults. Fracture intensity in these damage zones in both the regions of study decreases according to a power law where the rate of decrease is approximately -0.8 . The gas reservoir in Southeast Asia is associated with a large, basement master fault and twenty-seven seismically resolvable second-order faults. Four to seven fractures per meter are observed in electrical image logs from five wells in the 50-80m wide damage zones of the second-order faults. The second part of this work involves predicting damage zone widths utilizing two-dimensional plane-strain dynamic rupture models with strong rate-weakening fault friction and off-fault Drucker-Prager plasticity. The number of induced third-order faults and fractures are calculated by assuming that the dilatational plastic strain is manifested in the form of discrete fault planes. The theoretical results suggest that the damage zones are approximately 60-100 meters wide and the fracture intensity decreases with distance from the fault according to a power law with the rate of decrease approximately -0.8, both predictions in very good agreement with our observations. However, the damage zone width and the rate of decrease of fracture intensity are not constant all along the fault and depend on fault geometry (roughness), depth and the distance from points of nucleation. We hypothesize that the superposition of numerous earthquakes on the faults in situ may tend to smooth out such variations.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T12C..05J
- Keywords:
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- 8004 STRUCTURAL GEOLOGY / Dynamics and mechanics of faulting;
- 8010 STRUCTURAL GEOLOGY / Fractures and faults;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting;
- 8163 TECTONOPHYSICS / Rheology and friction of fault zones