Tensile—The silent fracture opened from stage perforations.
Shear—The natural friction-filled fracture that makes a mini sound.
Microseismic, the tiny acoustic signal detected from a rock moving thousands of feet below the surface, has a new voice in the world of geomechanics, translated by Petro.ai into an applied diagnostic: Where deep learning models and minimum stress models combine to use the microseismic cloud of points as an indicator of hydrofrac growth. That’s Part 2.
First an important history lesson.
In the early years, microseismic was filled with a different projected promise for productivity. We heard feeble signals from under the earth. Being familiar with seismic analysis, where dynamite blasts ricochet off features to develop underearth geopositioning, scientists extrapolated that the cracking acoustic signal they heard was a tensile fracture locator, the sound of the rock breaking at the well bore. But the model was wrong and companies got burned.
“Scientists thought it was worth billions of dollars and was going to help us solve the frac geometry problem of frac optimization,” Dr. Troy Ruths, CEO of Petro.ai explains, “But when people started applying it, the model was very inconsistent. It didn’t jive with a lot of results. The most important result was that it didn’t jive with productivity. It didn’t finish the story. Engineers would see a cloud of points but they didn’t get a more productive well. So, operators questioned whether microseismic was useful or not.”
“There’s also a misconception about a term that became known as the SRV which is the Stimulated Reservoir Volume,” Kyle LaMotta, VP of Analytics adds, “Scientists thought this cloud of microseismic events was the stimulated reservoir volume and that if you could make some assumptions you could calculate the actual oil that was in that volume and that should correlate to production. But it wasn’t. After that people still used SRV, but the use of microseismic became less popular because of the stigma around it of not being a useful diagnostic.” In a paper by Cipola and Wallace, the researchers concluded that “SRV and similar techniques provide little insight into two critical parameters: hydraulic fracture area and conductivity.”
The first step in understanding microseismic was in figuring out the complexity of the fractures happening under the earth, and equally important, the causal order, the sequence of events.
According to the Journal of NG&E, “depending upon…the location where the hydraulic fracturing is executed, pre-existing natural fractures can impact hydraulic fracture propagation and the associated flow capacity. Understanding the interactions between hydraulic fracture and natural fractures is crucial in estimating fracture complexity, stimulated reservoir volume (SRV), drained reservoir volume (DRV) and completion efficiency.”
Linking the induced hydraulic fracture created by the injection of fluids under high pressure with the natural fractures already existing in the earth-crushed, twisted and bent shale is an essential part of understanding a well’s productivity. Microseismic provides the unconventional link.
There are two fractures at work in an unconventional well, both of which contribute to the productivity. The executed hydraulic fracture or tensile fracture, and the shear fractures or the natural fractures already in situ in the rock. Initially, scientists thought that the microseismic signals they were hearing were the tensile fracture. But the tensile fracture breaks easily along the SHmin direction and makes no noise.
“It’s a silent opening,” Ruths indicates, parting his closed hands smoothly. “And while it’s opening, it continues to break but continues to be silent. If all we were doing is tensile fractures, microseismic wouldn’t work.”
“Now, what does happen next,” Ruths continues, “is we start to hear an acoustic signal. These are pre-existing fractures in the earth that start to move. Some of those fractures are aligned in such a way that when you pressure up this tensile fracture, the pressure leaks off and that forces a natural fracture to move. That’s what we’re hearing. We’re hearing the shear fractures.
Why you hear something, is because the rocks are sliding past each other with lots of friction. In fact, that natural fracture is probably filled with another mineral and once that pressure hits, it’s a weak mineral that can’t keep the rock together, so it moves. That’s what we’re hearing in microseismic. And that’s why it doesn’t always correlate with productivity because we’re getting lots of production from the tensile fractures and then we’re also getting production from the shear fractures. Microseismic points to where the hydrofrac is growing.”
The CSEG Recorder summarizes, “Microseismic activity is generated by instantaneous geomechanical strain or slip and the microseismic source characterization provides details of the deformation or slip resulting in the microseismicity.”
Enter Dr. Mark Zoback, Petro.ai Science Advisor and SHmin and the Petro.ai platform. And microseismic becomes the diagnostic tool of the future. Check out Part 2.
