Q: What is fracture containment?
A: Dr. Brendon Hall, VP of Geoscience, tackles this important question, “Fracture containment is all about getting fracs to go where you want them to, so you can stimulate as much good pay rock as possible, the rock that you’re accessing with hydrocarbons in it.
“The concept of fracture containment starts with understanding the importance of stress in the subsurface. One of the main reasons we care about the stress profile, specifically the minimum horizontal stress, is because that’s the pressure that the fluid in the fracture has to overcome in order to propagate.
“On average the minimum horizontal stress increases as you move downwards. If there are no variations in the minimum horizontal stress, hydraulic fractures will propagate mostly upwards because the stress is lower upwards. Fractures like to go into regions where the stress is lower because it’s easier for them to open up, to overcome the minimum horizontal stress. The lower that is, the lower the fracture pressure has to be.
“Part of the challenge in designing fracs is understanding how the stress changes so you know where fracs might be contained, where stress is a barrier.
“Where containment is an important question, Petro.ai can perform a what-if scenario. We can show what the stress looks like with containment or without containment to see what the effect is on the NPV result. If there’s a high-risk stress barrier and you’re not sure if it’s containing the fracs or not, Petro.ai can run the analysis with and without it there to see what the difference is. How important is it to get this right, what’s the importance to NPV?
“The minimum stress will vary for other reasons as you move through the subsurface. You might have an area in the subsurface that has a reasonably low minimum horizontal stress. This is the case, for example in the middle Bakken. It has a relatively low minimum horizontal stress and the layers above and below it have higher minimum horizontal stresses. This is for various reasons. In this case it’s largely compositional. The lower and the upper Bakken have a higher stress. This has been confirmed with measurements and logging.
“If you start a fracture in the middle Bakken with lower stress, fluid pressure has to be lower for that fracture to propagate. As that fracture propagates, it might meet the boundary or go to the interface between the middle Bakken and the upper Bakken or the middle Bakken and the lower Bakken. Remember, the upper and lower Bakken are in a higher minimum stress state. But the fluid in the fracture from the middle Bakken isn’t higher than the upper and lower boundary layers. This means that the fracture might not be able to break through to those layers. It might be contained in the middle Bakken and propagate outward.
“Stuck in the middle Bakken, it won’t go up or down but outward and will remain in the same layer. That’s really an ideal situation. The middle Bakken is where you want to contain that fracture. It’s where the oil is. It’s where you want to create the most fracture surface area.
“Besides natural stress differences, you can artificially manipulate the stress in the earth in different ways that can also alter fracture containment. Depletion is one of those ways. If a parent well has been there for a while and has been producing, it will tend to decrease stress as it reduces the pore pressure around where the well has been producing from, as it’s drawing fluids out. This depletion reduces the minimum horizontal stress overall and affects fracture containment.
“There is also something called pressure banking. By artificially creating stress shadows you can potentially create a higher-pressure region and prevent fracs from going in that direction.
“Some things that are important to note is that containment isn’t just stress. You can have high stress regions if the area is highly fractured, pre-existing fractures might be there. There might be some structural aspects that prevent fracs from being contained in an area that you thought had a stress barrier.
“It’s great to look at as much data as you can to see what direction your fracs are going. Microseismic, for example, helps us to see how fracs are being contained, what zones the rock is breaking up in. Knowledge of the geology helps, but the most important factor controlling fracture height and direction are the stress contrast between the pay zone and a bounding layer.”