NEW ANALYTICAL TECHNIQUES TO HELP IMPROVE OUR UNDERSTANDING OF HYDRAULICALLY INDUCED MICROSEISMICITY AND FRACTURE PROPAGATION

A multi-stage 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 Sand 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.

Detailed analysis of the source mechanisms of various representative events reveals that more than eighty percent of the events display non-shear mechanism and are directly connected with the injected fluids. A radically new processing technique is used to locate microseismic events. While originally a couple of hundreds of events per stage were mapped, this new processing technique leads to the detection and location of several thousands of events per stage. This dramatic increase of mapped microseismic events leads to the following observations. First, initial gaps in located seismicity appear to be artifacts due to the monitoring geometry. The additional located microseismic events highlight an underestimation of the initial fracture system length; 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. Second, for the first time, 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. In average the fracture system propagates horizontally at 12 to 15 feet per minute. In addition, precise event location determination reveals a slow down in fracture propagation velocity along the vertical axis in the vicinity of shale barriers. Also the observed hydraulic fracture systems propagated faster toward the east than toward the west, resulting in a shorter western wing of the fracture. Third, analysis of the distribution of seismic events along the hydraulic fracture reveals that the seismic length based on the most distant events slightly overestimates the true length of the fracture.