Existing natural fractures often have a significant impact on both stimulation and production of oil and gas wells. Effective exploitation of unconventional reservoirs requires the understanding of the local tectonic history and the present day stress regime. Signal strength, high quality reflection seismic, microseismic imaging, and moderate structural complexity of the liquids-rich gas and tight oil Eagle Ford shale makes it an ideal place to study hydraulic fracturing in tight rocks. Microseismic monitoring results showed clear structural trends relating to reactivation of existing faults and fractures, and rock failure mechanisms determined through source mechanism inversions. These results provided critical information to the operator for optimizing the hydraulic fracture design.
Microseismic data collected using a surface array allowed the full geometry of the result to be viewed with no directional bias. The geometry of the microseismicity trends related to fracturing developed during the stimulation treatment were representative of the true geometry of the structure. The large aperture and wide azimuth of the monitoring array facilitated the determination of source mechanisms from every event detected, which provided full coverage of the focal sphere of each source mechanism. The events identified two different source mechanisms, indicating a different failure mechanism for fractures than for reactivated faults.
Microseismicity with a NE-SW orientation are interpreted to be related to either induced or reactivated fractures. Microseismicity also formed trends that are contiguous across more than one wellbore in a ENE-WSW direction. These trends are interpreted to have formed as a result of fault reactivation. Source mechanisms from faulting parallal to SHmax have failure planes that strike NE-SW with normal dip-slip failure on steeply-dipping planes. Those from fault reactivation have strike-slip failure on ENE-WSW striking failure planes. The orientations of the fault-related trends are parallel to extensional Gulf of Mexico growth faulting. The microseismicity trends associated with fracturing form at an angle of approximately 25° to the faulting trends and are parallel to SHmax.
Microseismicity trends associated with faults are used to project where faults will intersect adjacent wells. Identification of these faults in the reservoir via microseismic mapping allow operators to modify their treatment parameters and stage spacing in order to avoid geologic hazards. The operator combines the treatment pump parameters for the wells with the additional structural understanding gained from the analysis of fracture trends and source mechanisms to identify zones that should be avoided in subsequent treatments. In addition, the mapped microseismicity provides critical information that was used to modify well spacing for subsequent wells, thereby optimizing the completion plan and dramatically cutting costs.