Towards an Improved Understanding of and Response to Large Microseismic Events Associated with Petroleum Operations
A number of recent seismic events felt on surface near petroleum field operations have led to growing concern over seismic hazard associated with hydraulic fracture stimulation and injection programs. In response, several jurisdictions have enacted regulations requiring modification or temporary shut-down of operations in injecting wells near earthquakes which exceed certain magnitude thresholds. These recently proposed magnitude-based “traffic light” systems break from existing standards and regulations related to seismic hazard, which are typically based on measured ground shaking so that ground motion can be directly related to building codes and structural design specifications. Observed shaking, and the associated risk of damage or injury, is determined by several factors including both earthquake source characteristics and site- and raypath-specific conditions. Injection-induced seismicity tends to produce significantly less shaking than tectonic events of the same magnitude due to differences in stress release behavior. To achieve a more consistent and reliable regulatory standard, we suggest that regulations be based on ground motion, rather than magnitude, for the evaluation of seismic hazard associated with injection-induced seismicity. A recent example of a MW4.5 earthquake is used to illustrate some of the issues that can arise in induced seismicity monitoring, as well as a realistic assessment of the hazard posed to society by such events. While magnitude is calculated from recorded signals, and is subject to inherent uncertainties and biases, ground motion is directly measured by seismic instruments. Ground motion offers a more reliable alternative for assessing the potential damage due to fluid-induced earthquakes than magnitude alone. Proper calibration of seismic sensors is key to obtaining good quality data for hazard assessment. Knowledge of induced seismicity generation processes is relatively limited at present. Surface stations provide good basic characterization of M>0 events; monitoring such events with sensors downhole allows observation of high-frequency dynamic behavior which is attenuated in the sub-surface. Integrated surface and downhole monitoring shows great potential to detail the generation of large (M>0) events by giving a complete view of the rupture process. Recent studies utilizing such acquisition geometries have shown evidence of complex rupture behavior influenced by the presence of injected fluids.
AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016