|Title||Seismic Efficiency vs. Fracability; Effects of Mechanical Properties on Radiated Elastic Waves With Application to Hydraulic-Fracturing-Induced Microseismicity|
|Publication Type||Conference Paper|
|Year of Publication||2017|
|Authors||Sheibani, F, Hager, B|
|Conference Name||AAPG Annual Convention & Exhibition|
|Conference Location||Houston, TX|
Shear activation of pre-existing fractures through natural fracture sliding due to effective-stress perturbations during hydraulic fracturing would result in shear stress drops on the surfaces of pre-existing fractures and consequently causes the releasing of stored elastic energy in the rock. This energy would be converted to fracture surface energy, disspated heat energy and radiation of elastic waves. Only rapidly slipping (or seismic events) generate elastic waves, while the energy released during slow-slip events (aseismic events) mostly would dissipate. Seismic efficiency is the ratio of released seismic wave energy to the potential energy. The rate of stress drop and energy budget components depend on natural fracture constitutive modeling as well as fabric rock mechanical properties. The most favorable constitutive behavior for faults to cause higher seismic efficiency is “slip weakening,” where fault strength decreases during slip accumulation. Receivers used in microseismic monitoring only measure seismic events. That explains why only a small portion of energy budget during hydraulic fracturing would be estimated by information obtained from microseismic monitoring. We performed a series of numerical experiments to investigate the effects of rock mechanical properties on seismic efficiency. Using a configuration of a main opening hydraulic fracture, as well as natural fractures at field scale and applying slip-weakening constitutive modeling for natural fractures, we studied how Young's modulus affects seismic efficiency. Perhaps surprisingly, our results show that rocks with higher values of Young's modulus have lower seismic efficiency generated from sliding on pre-existing natural fractures, while lower rigidity leads to higher seismic efficiency. The results doesn't contradict general beliefs about the effect of rigidity on fracability; More rigid rocks are more favorable for hydraulic fracturing and would generate larger fracture networks; however, compared with less rigid rocks, fewer events would be detected seismically. We combined our numerical modeling with lab results using acoustic emission of saw-cut samples under triaxial loads. Using samples with different mechanical properties and the same fracture surface roughness, we measured seismic efficiency. The experiments verify our numerical results and confirm that, assuming the same conditions for fault constitutive behavior, less rigid rock results in higher seismic efficiency.