{"id":1723,"date":"2021-07-30T17:24:27","date_gmt":"2021-07-30T17:24:27","guid":{"rendered":"https:\/\/ofgeomech.com\/ofg20\/?page_id=1723"},"modified":"2021-07-30T23:20:14","modified_gmt":"2021-07-30T23:20:14","slug":"fault-stability-seismic-activity","status":"publish","type":"page","link":"https:\/\/ofgeomech.com\/ofg20\/fault-stability-seismic-activity\/","title":{"rendered":"Fault Stability \/ Seismic Activity"},"content":{"rendered":"\n
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The go-to failure criterion for most geomechanical applications, Mohr-Coloumb failure<\/em><\/strong>, shows that shear failure occurs when the normal effective stress is reduced or the shear stress increases. The reduction in normal effective stress can occur due to a decrease in the total normal stress or an increase in fluid pressure. The general assumption<\/em><\/strong> is that the total normal stress and the in-situ shear stress is constant – and therefore Mohr-Coloumb failure<\/em><\/strong> is solely driven by a pressure change.<\/p>\n\n\n\n

Fluid injection<\/em><\/strong> due to, for example, produced water disposal or hydraulic fracturing operations \u2013 resulting in an increase in pore pressure – near faults or other structural features in areas with high in-situ shear stress (e.g., a strike slip tectonic regime) may lead to induced or triggered failure<\/em><\/strong> of the weaker planes (natural fractures or faults), which are reflected as seismic events. Some of these seismic events (micro earthquakes) are of low magnitude (commonly called microseismicity<\/em><\/strong>) while other seismic events can be of a magnitude 4 or higher.<\/p>\n\n\n\n

In order to evaluate fault stability<\/em><\/strong>, it is critical to understand the in-situ stress magnitudes and orientation, pore pressure and any changes and fault geometry. In addition, proper modeling of fault stability will also allow for the assessment of fault leakage<\/em><\/strong>, which is a central concern for underground storage<\/em><\/strong> of CO2<\/sub> and hydrocarbons.<\/p>\n\n\n\n

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Earthquake data from Oklahoma. Many scientists believe that this fault-slippage (failure)-induced seismicity is due to water disposal operations. However, this has not been proven.<\/em><\/strong><\/figcaption><\/figure>\n\n\n\n
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Seismicity (microseismicity) associated with hydraulic fracturing operations. Again, investigations suggest that increasing formation pressure is the cause of shear failure reflected in the MS data.<\/em><\/strong><\/figcaption><\/figure>\n<\/div>\n\n\n\n
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Structural geometry (layers and faults) from a large-domain 3D geomechanics model. Note the well locations.<\/em><\/strong><\/figcaption><\/figure>\n\n\n\n

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Normal stress computations (warmer colors are higher stress) on deep faults from a 3D geomechanics model.<\/em><\/strong><\/figcaption><\/figure><\/div>\n\n\n\n

Induced vs. Triggered Seismicity<\/strong><\/h4>\n\n\n\n

It is critical to bear in mind that injection or depletion operations<\/em><\/strong> often do not impart significant stress changes in the formations. Simply put, these operations do not significantly alter the existing in-situ stresses, particularly the in-situ shear stresses. This implies, and is supported by field data, that cases of field micro- or macro-seismicity are most-often associated with formations under high initial shear stresses.<\/p>\n\n\n\n

Where a causative activity can account for most of the stress change or energy required to produce the seismicity, this would be an induced event; however, where the causative activity accounts for only a fraction of the stress change or energy associated with the seismicity (i.e., tectonic loading plays a primary role), the event is more properly called a triggered event \u2013 as is the case for the majority of oilfield-related seismic events. Simply put, rather than creating the driver for seismicity (shear stresses), most oilfield activities activate or accelerate failure (high shear stresses naturally exist and the resistance to the shear failure is reduced, triggering the seismic event).<\/p>\n\n\n\n

Considering the Complete Geomechanics Picture<\/h4>\n\n\n\n

As indicated above, the general assumption<\/em><\/strong> is that seismic events are solely caused by pressure changes. This is incorrect and misleading. At OFG, we evaluate the full range of possible causes for shear failure seismicity:<\/p>\n\n\n\n