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Is That Frac Job Really Breaking New Rock?

Mike J. Mullen1 and Milt Enderlin2
1Halliburton, Denver, CO
2Gearhart Company, Ft Worth, TX

With all that horse power sitting on location shaking the ground while pumping a frac job it is really hard to imagine what the fracturing treatment is doing down hole. Is it breaking into new rock in a linear elastic mode or is it just opening pre existing planes of weakness in the reservoir? Let's consider the implications of both scenarios.

If the rock is failing down pre existing planes of weakness in the rock, the frac job will tend to stay in the weakened zone without growing through weak shale barriers. This could explain why most of the frac models show poor containment in tight sands yet tracer logs show good frac containment. How about Microseismic surveys during frac treatments? They tend to see a rather large stimulated reservoir volume and shorter lengths than is modeled. If the fracture treatment is breaking new rock the created fracture will tend to be more like what is modeled in current frac simulation models.

So how do you tell if you are opening a pre-existing weakness in the rock or breaking new rock? A novel concept to do this is to integrate the geomechanically determined stress state of the rock with hydraulic fracture diagnostics. By comparing the estimates of minimum horizontal stress from both disciplines one can determine which type of fracturing one is doing. This leads to an improved post stimulation diagnostic analysis and trouble job analysis.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado