The Use of Microseismicity To Understand Subsurface-Fracture Systems and To Increase the Effectiveness of Completions: Eagle Ford Shale, Texas
Authors: Detring John P. , Williams-Stroud Sherilyn 
Published: September 2013

Existing natural fractures often have a significant impact on both the stimulation and the production of oil and gas wells. The effective exploitation of unconventional reservoirs requires understanding of the local tectonic history and the present-day stress regime. Signal strength; high-quality reflection seismic, microseismic imaging; and the moderate structural complexity of the liquids-rich gas and tight oil Eagle Ford Shale make it an excellent place to study hydraulic fracturing in tight rocks. Microseismic-monitoring results showed clear structural trends relating to the reactivation of existing faults and fractures, and rock-failure mechanisms determined through source mechanism inversion of events. These results provided critical information to the operator for optimizing the hydraulic-fracture design.

Microseismic data collected by use of 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 was 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 failure mechanism for fractures that is different from that for reactivated faults.

Microseismicity with a northeast/southwest (NE/SW) orientation is interpreted to be related to either induced or reactivated faults or fractures. Microseismicity also formed trends that are contiguous across more than one wellbore in an east-northeast/ west-southwest (ENE/WSW) direction. These trends are interpreted to have formed as a result of fault reactivation. Source mechanisms from fracturing parallel 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 NE/SW-striking, dip-slip fractures are parallel to extensional Gulf of Mexico (GOM) growth faulting, and the ENE/WSW-striking, strike-slip faults are at an angle of approximately 25° to the dominant fracturing trends.

Microseismicity trends associated with faults are used to project where faults will intersect adjacent wells. The identification of these faults in the reservoir by means of microseismic mapping allows operators to modify their treatment parameters and stage spacing 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.