I’m Being Followed by a Stress Shadow
Data Science & Analytics

I’m Being Followed by a Stress Shadow

Rosemary Jackson  •  

Stress shadow: Each use case is unique and built on your in-situ geology and anisotropic stress. Build your own model in Petro.ai with your own data leveraged with deep machine learning and geomechanical understanding.

Stress Shadow: Tight spacing, one more stage, complicated subsurface frac geometry. Visualize with tools in Petro.ai that model the complex.

The hydraulic fracturing of wells to create tight cluster spacing in low permeability shale formations develops a dynamic buildup of stress that increases as stages move along the length of the well bore. One of those stressors, the stress shadow, is a dynamic pressure that exhibits when you’re fracking the well. The frac opening is widest at that time. The effect of the rock opening pushes on the rest of the rock. The stress shadow is a mathematical representation of the increase in closure stress for a secondary parallel fracture initiated in that region.

The stress shadow comes out in a couple of different ways,” Dr. Troy Ruths, CEO of Petro.ai explains this small but consequential affect.“ The first way it’s observed is in your cluster spacing. When you’re fracking a well and the clusters are too tight, anisotropic stress and specific in situ geology is further complicated by the stress opening of several potential fractures simultaneously. If you put a cluster too close to another cluster, whatever fracture potentially opens second isn’t going to open.Who knows which one will go first, but after you get a first fracture, you won’t get a second because it’s too tight.As a result, simulating sequential fracturing scenarios is important.

"Observing your stress shadow, particularly in zipper fracking, creates a deeper subsurface predictive understanding for estimating required treatment pressures.

Which speaks to the whole issue of well spacing in the cube, multi-well drilling in the pad, and stress impact, including stress shadows, in stage development. This was recently summarized in an issue of the American O&G Reporter, “While the general trend across the industry has been toward higher stage counts and tighter stage spacing, experience shows production performance does not scale up in simple increments when fracture stages are added in closely spaced completions. In fact, instead of adding hydrocarbon production on a per-stage basis, tighter spacing can add incrementally less hydrocarbon production per stage.”

The reason why geomechanics is so important in this analysis is because when you make an hydraulic fracture it’s essentially a mechanical phenomenon,” Dr. Brendon Hall, VP of Geoscience at Petro.ai, emphasizes the importance of a model that includes all the simultaneous factors impacting a well. "A geomechanical model is a description of stress in the earth. By pumping water down, we are intentionally raising the stress at a location. Concentrating it to overcome the natural strength of the rock at that location to propagate these fractures.

“When we do that you’re changing the state of stress at that location where you’re injecting all that pressure. And that causes the state of stress around that fracture to also change. You’re opening up these corridors in the rock by actually prying the rock apart and pushing on it. The act of pushing on it increases the stress at that location.

Horizontal wells and fracturing
Horizontal wells and fracturing

“If you have a fracture that’s a little bit down the well that’s also trying to open there, it has to not only overcome the toughness of the rock but also this other frac that’s being pried open. That’s the continuing issue of stress shadow along the length of the well bore. The fractures themselves have to overcome the natural state of stress but also the additional stress that they induce on the rock formation. That changes as you continue to frac along the length of the well bore. It alters what pressure is needed to propagate these fractures. They’re not completely independent of one another.”

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