Overview
Causal factors for seismicity near Azle, Texas
• Induced seismicity is a quickly moving and developing science.
-Techniques and approaches are being developed real-time

• The paper attempts to integrate the detailed assessment of seismicity and subsurface
reservoir modeling

• Many elements of a comprehensive cross-functional study were not addressed

• Natural seismicity was not strongly considered or assessed

• The analysis does not support the strong conclusion of the paper

 Elements of a Causation Stud'l
Cross-functional integration
• Precise seismic event monitoring

• Subsurface characterization

• Near surface impacts

• Geomechanical modeling

• Reservoir characterization and modeling

• Integration of the various datasets in the proper time
sequence

• Assessment of natural causal factors

 Events 11/9/13 - 11/26/13

 

Seismic event locations courtesy of SMU

Events 11/9/13 - 12/22/ 13

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 Events 11/9/13 -1/13/14

 

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 Events 11/9/13 - 2/06/14

 

Seismic event locations courtesy of SMU

Events 11/9/13 - 2/07/14

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 Misleading Sequencing of Data and Analysis
Authors make their argument focusing on the antithetic fault not the primary deep-seated fault
• Main normal fault moves first - 10,000 feet below injection interval
• The antithetic is reactionary or secondary movement, not primary
• Events are even smaller than those on the main fault
• The map gives the appearance of more events at the antithetic- stacking with deeper events

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Events 11/9/13 - 4/19/14

 Investigation of Natural Seismicit'l
Limited cursory review - The paper discounts natural seismicity as a cause / no historic events
• The paper does not address seismic event detection limits during the last 150 years

• Non-felt events have occurred historically within the greater study area
USArray measured a non-felt event below 2.5M during a two year period as it moved through the U.S.
• There is seismicity occurring below the threshold of the permanent instrumentation
• Very likely 'aftershocks' to this event that were not recorded
SMU deployed a temporary array in the basin during 2008 and 2009. Many events measured by that array
where not picked up by the permanent instrumentation
• Again, demonstrating background seismicity

• One of the authors recently presented a paper in Dallas discussing an area with a 500 year event period using
high resolution data (New Madrid fault)
-

Why do they dismiss that possibility here?

• Lack of surface expression as evidence no fault movement over the last 150 years
-

Is there a surface expression now that the fault has moved?
o •

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 Attempt to Implicate Producing Wells
No quantitative analysis
• No explanation of how withdrawal at the Barnett Shale level would
trigger events thousands of feet deeper in the basement
• The authors rely on the on references to 'demonstrate' their point
- Reference 7 - Fluid flow along potentially active faults (crystalline rocks)
- Reference 28 - Implications for fault seal in platform carbonates

- Reference 29 - Earthquakes triggered by fluid extraction (1989 paper)
• Reservoir depletion leading to significant compaction / subsidence
- Reference 31 - Stress changes - Ekofisk as an example (1998 paper)
- Reference 32 - Fault zone architecture and permeability structure

• References not relevant to this particular geologic environment
- Studies to not address unconventional reservoirs - pre 2000 papers

• No geomechanical modeling performed

 Biased Reservoir Model
•
•
•
•
•
•
•
•

Model built and by geophysicist (first model)
Wrong bottom hole pressure calculated and used for disposal wells
No hydraulic connection to the Barnett Shale
Single phase model - no gas
Paper text focuses on antithetic fault, not the main normal fault
No pressure modeled at the depth of the basement related events
Permeability reduction at faults ("high permeability Ellenberger")
Model results - A range of extremely low possible pressure changes
that do not conclusively support a tie to the observed seismicity
Supplemental Information
b

-2.5 psi
-0.017

Main Body of the Paper
+2.5 si

 Elements of a Causation Stud'l
Cross-functional integration
• Precise seismic event monitoring

• Subsurface characterization

• Near surface impacts

• Geomechanical modeling

• Reservoir characterization and modeling

• Integration of the various datasets in the proper time
sequence

• Assessment of natural causal factors