Traditional seismic exploration has it that you make a bang or vibration on the surface and interpret the nature of sound waves that bounce back from beneath. A leap forward was to be making use of three-axis vibrators to vastly improve interpretation. It was something the industry experimented with but generally abandoned as unreliable due to wave-coupling problems with the heavy vibrator machines.
But thanks to the booming trend in sourcing gas and oil from shales, the industry is taking new looks. Working in shale formations demands what’s known as horizontal multi-fracturing—drilling a horizontal well and fracturing the rock in numerous places along the wellbore to allow the hydrocarbon to flow. Tracking these ‘fracs’ increasingly employs microseismic technology. Smaller vibrators can easily generate shear waves that don’t couple. Moreover, the underground fracing activity itself generates its own shear waves which can help locate fracturing events in real time.
Microseismic doesn’t generate energy in the traditional sense except for the initial setup for orientation. It’s just listening to the frac, listening for the breaks as they occur. Researchers say they are close to being able to identify where each frac is in three dimensions, its magnitude and whether it has stayed open or resealed. That’s very important as an indicator of how much gas a well is going to produce. Industry leader CGGVeritas has trials underway this year.
Perfection of the method will give an operator real time processing, but because it takes a lot of computing power not usually available on-site, they’ll achieve it through event detection. To detect an event they say they’ll be buffering seismic data. Using the software, they slip back five milliseconds and process only a very tiny slice so they can quickly see where it is in the queue. “Our target is to be able to communicate closely with the fracing operation and let them know what they’re doing so they can decide whether to increase flows, increase pressure, stop, or whatever,” explains Darin Silvernagle of CGGVeritas. Much more in-depth analysis is then done in follow-up processing.