Using Intermediate-Field Terms in Locating Microseismic Events

Wednesday, 17 December 2014
Juan M Lorenzo, Louisiana State University, Department of Geology and Geophysics, Baton Rouge, LA, United States, Arash Dahi Taleghani, Louisiana State University, Baton Rouge, LA, United States and Joel LeCalvez, Schlumberger Oilfield Services, Sugar Land, TX, United States
Microseismic mapping is a passive seismic technique used extensively for assessment of hydraulic fracturing treatments during the last two decades. Basically, microseisms are microearthquakes induced by the changes in stress and pore-fluid pressure associated with the hydraulic fracturing treatment. Current practice to locate events and determine the source mechanism of microseismic events associated with hydraulic fracture treatments only includes far-field terms for the moment tensor inversion. The intermediate-field terms and near-field term are normally ignored, perhaps simply following the tradition in locating distant earthquakes. However, source-receiver distances in hydraulic fracturing are usually 1000 ft (~300m), which is much less than the typical distances in earthquakes; therefore the effect of near and intermediate-field effects are not yet known.

We try to include these near-field effects to improve the precision of locating the events and consequently determining the source mechanism. We find that the intermediate-field term may contribute up to 1/3 of the signal amplitude when the source-receiver distance is about 300 m. The intermediate-field term contributes ~1/20 of the signal amplitude when the source-receiver distance is ~ 2000 m . When the source-receiver distance exceeds ~ 2000 m, the intermediate-field terms can be ignored in our inversion. In our case, we also confirm that the near-field term can be ignored in microseismic analysis. Our results indicate that the intermediate-field terms can improve moment tensor inversion between 2% to 40% at source-receiver ranges less than 300 m. However for the case of large distances, the improvement using this technique is limited to 1%. In the presence of strong environmental noise, intermediate-field terms help to effectively improve the moment tensor inversion: i.e., 15% improvement with noise present vs 3% improvement without noise.