03 July 2017
The microseismic response to refracturing of horizontal wells often shows a significant delay after pumping starts. In many cases it takes several pumping stages, equivalent to several thousand barrels of injected fluid, before any major microseismic activity is observed. Given microseismic is the acoustic representation of rock failure, no microseismic activity is potentially an indication of no rock failure.
In most refracturing jobs new perforations are added between old ones in an attempt to break fresh rock, and thereby to reach bypassed reserves. From a geomechanical perspective, it takes less pressure to re-open pre-existing fractures, from the old perforations, than to break fresh rock through the new perforations. The lack of microseismicity during the early pumping stages is evidence indicating that no rock failure occurs during this period, but the injected fluid finds its way through the pre-existing fractures which are connected to the old perforations along the well. The lower ISIPs recorded during the same stages are further evidence supporting this argument. Given that this condition almost persists throughout all stages of a refracturing treatment, the question to be answered is whether the new perforations are effective at all? If not, why not skip them to reduce cost and save time?