Using Microseismicity to Understand Subsurface Fracture Systems and Increase the Effectiveness of Completions: Eagle Ford Shale, TX
Detring, John1 and Williams-Stroud, Sherilyn
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 rock failure mechanisms and reactivation of existing faults and fractures. 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 without directional bias. The geometry of the trends in microseismicity related to the hydraulic fracture stimulation were representative of the true geometry of the structure. The large aperture and wide azimuth of the monitoring array facilitated determination of source mechanisms from every detected event, thereby providing full coverage of the focal sphere of each source mechanism. Two different source mechanisms were identified, with the fractures displaying a different mechanism than the reactivated faults.
Microseismicity with a NE-SW trend are interpreted to be related to induced or reactivated fractures. Microseismicity also formed contiguous trends across multiple wellbores in a ENE-WSW direction. These trends are interpreted to be the result of fault reactivation. Source mechanisms from fracturing parallel to SHmax have failure planes striking 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 parallel extensional Gulf of Mexico growth faulting. The microseismicity trends associated with fracturing are at approximately 25o to the faulting trends.
The operator identifies zones that should be avoided in subsequent treatments by combining treatment pump parameters and source mechanisms with the structural understanding gained from the analysis of fracture trends. Mapping microseismicity provides critical information for well spacing for future wells. This knowledge can be utilized to optimize the completion plan and cut costs.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013