When fractures are open and propped as volume is stimulated, the build-up of stress shadow along the well bore may increase treatment pressures on the order of 1000 psi. If stages are developed too quickly, you can elevate the stress shadow to the point of increasing geomechanical barriers. Your first fracs may not be affected, but your last fracs probably will, because you’re going as fast as possible on the completion. And you may blow out a zone.
Let’s start at the toe. Picking ISIPs, initial shut-in pressures, here at the first stages of the well, are more straight forward. You’re fracking for the first time; the geology is virgin and predictable. Measurements for the initial shut-in pressure and the vertical stress profile are less complex.
Now, the well starts porpoising in the zone next to other wells porpoising in the zone. Multiple cluster stimulation stresses add to in-situ stresses, fluid properties complicate geology changes and the vertical stress profile needs more characterization detail.
Dr. Troy Ruths, CEO of Petro.ai, explains, “You may have changes in the ISIPs that are actually based on geology changes and not on stress shadow. The first thing that’s important is being able to pull those effects apart. Am I getting changes in pressure because of geology or because of the stress shadow? You’re getting both. These wells aren’t staying in the same lithology or the same geology and they’re also building up a stress shadow at the same time.
“As we move down the well bore, we’re continuing to receive mountains of diagnostic information. Using that we can build a vertical stress profile.Why is a vertical stress profile so important? Fracs don’t care about oil saturation. Fracs don’t care about pore pressure. Fracs don’t care about all the things you care about in a producing reservoir. Fracs only care about the state of stress.
“And the stress shadow is the frac creating pressure. The fractures are very small, just big enough to get the sand particles in. These are narrow fractures that go very far. The proppant stays close and so does the stress shadow. Even a small fracture increases the pressure a lot, very close to the well bore. That’s what we’re measuring with the stress shadow.
“Once we know what the stress shadow is, we can subtract that and get to the SHmin which is typically the minimum ISIP within the lithology."
“The last step is to assume a slight shift. Dr. Mark Zoback, our science advisor, feels from laboratory studies that ISIPs are typically 100 to 200 psi higher than what SHmin actually is. What confuses people is that the measurement needs to reflect the stress shadow that gets built up as you develop the well.”
As the stress shadow grows, it can be difficult to estimate the treating pressure without ISIPs and Shmin. The initiation pressure of the frac is the energy in that frac.If that frac is going through a vertical stress profile with lower stress, the frac will blow right through it. And it will continue to grow because of that. If the stress shadow is increased to too high a level, you can increase your geomechanical barriers and blow out a zone.
“That’s where the stress shadow makes a big impact right now,” Ruths adds.“ Engineers think they’re making a pressure boundary. An artificial geomechanics boundary, but it’s the opposite. If you don’t do it right and you try to make a pressure boundary, what you end up doing is elevating the stress which blows through all of the natural pressure boundaries that you have. Fundamentally, we need to understand and characterize the vertical stress profile with a lot of detail. And the stress shadow is a subtle but important part of that characterization.”