1. Introduction

Microseismic monitoring has become a valuable tool for optimizing stimulations, completions, and overall field development, particularly in unconventional reservoirs. This technology was initially rooted in geothermal energy [1,2], but subsequently was used for many years in research projects to understand fracturing in unconventional reservoirs, such as in the Multiwell Experiment [3,4], the M-Site fracture diagnostics laboratory [5–8], the Carthage Cotton Valley fracturing test [9,10], and for other processes, such as drill cuttings injection [11]. It finally reached a level of sophistication and reliability to function as a service technology in the early 21st century [12,13], and many thousands of hydraulic fractures have been monitored since that time. In addition to providing a "window" into the subsurface for fracture optimization and control, the large amount of microseismic data that has been gathered provides a significant database that can be used for environmental surety.

Microseismicity occurs because of geomechanical changes to the reservoir as a result of the fracturing process [14,15], and detection and location of these "events" provides a methodology to monitor fracture growth patterns and overall dimensions. One of the curious features of microseismic technology is that no one has ever seen the slippage plane of a microseism that was induced by a hydraulic fracture. As a result, the understanding of microseismicity has been through a down-scaling of earthquake seismology [16], examination of fracture behaviour in minebacks [17,18], comparisons with rock bursts and laboratory acoustic emissions[19,20], and geomechanics considerations of the way in which hydraulic fractures perturb a reservoir [21].

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