New Analytical Techniques To Help Improve Our Understanding of Hydraulically Induced Microseismicity and Fracture Propagation

摘要

A multistage hydraulic fracture treatment was performed on a producing well in a mature tight gas field in West Texas and induced microseismic activity was monitored from a nearby observation well. The objective of this microseismic monitoring campaign was to determine the overall geometry of the hydraulically induced fractures in the Canyon sandstone formation. Information and results initially derived from the microseismic interpretation were used to provide the operator with recommendations for reservoir management such as drilling patterns, new well placement, and completion practices. Microseismic events were located with a newly developed location technique based on S-wave back-azimuth. While originally a couple of hundreds of induced events per stage were mapped, this new processing technique leads to the detection and location of several thousands of events per stage. This increase of mapped microseismic events provides following insights into reservoir management. First, initial gaps in located seismicity appear to be artifacts owing to the monitoring geometry, not shear shadow as commonly interpreted. The additional located microseismic events show greater fracture system length and height, thus confirming the effectiveness of the treatment. We also show that the upward component of the vertical propagation is more developed than the downward component, resulting in vertical connection of the first three stages of the stimulation. Second, the high density of located microseismic events allows us to define the velocity of the fracture system propagation along both the horizontal and the vertical directions. On average, the fracture system propagates horizontally at 12–15 ft/min eastward. The observed hydraulic fracture systems propagated slower toward the west, resulting in an asymmetric fracture with twice shorter western wing. Vertical propagation speed of fracture propagation is similar in sandstone layers, however it slowdown in the vicinity of shale barriers as expected. Analysis of the source mechanisms of induced seismic events reveals that more than 80% of the representative events have a nonshear component of source mechanisms. This observation implies that the induced microseismic events are directly connected with the created fracture and represent movement of the injected fluids.