Disconnected Microseisms

On occasion during a monitored hydraulic fracturing job there will appear to be some events which are disconnected from the current fracture job. Often the client wonder's how could this be? Is it a stress shadow effect? Is there fluid or proppant here?

The short answer is depends. On one project, the lateral was located in the lower formation and during completion, events were located above the upper formation but not in the middle! The velocity model and tools have been well established by this point, and though the treating pressures were in the 6500 psi range, a small temporary "bump" of ~300 psi was noticed which corresponded to the events in the upper formation. Yet there were 0 events located between the two layers. When the next stage began, events were locating both in the upper and lower formations until the ball hit and the previous stage was isolated and all new events were in the lower formation again. As before, the PSI bump occurred and events were locating in both the upper and lower formations. After the conclusion of the project, the company geologist informed us the middle layer was relatively elastic.

While we did not believe proppant was present, the immediate response to the ball seat gave credence to the idea there was fluid present and it was hydraulically connected to the wellbore.