Q: Why are parent/child interactions such an issue now in unconventional drilling?
A: Dr. Brendon Hall, VP of Geoscience explains, “When shale was first developed, there weren’t a lot of existing wells in the field. You drilled a well and produced from it. You didn’t have to worry about the effects of existing wells. Over time as more and more wells were drilled, these wells started to interact with one another.
“When you drill these wells, you complete them. The idea is you create a hydraulic fracture that breaks up the rock. The fluid activates natural fractures that are already there. You create this permeability network, an area around where the completion happens. The extent of that permeability network is somewhat uncertain and differs depending upon what kind of rock you’re in and what the state of the stress is there, what the permeability of the actual rock is, and how much proppant is used.
“That well will drain hydrocarbons from the area around it. Until now, exactly how much it drains can be an unknown. Certainly, right around or near the well, you’ll drain a lot of those hydrocarbons and as you go further and further out, it drains less and less. It takes more time for those hydrocarbons to reach that well bore.
“Later, you come in and drill a new well near there, because the first well doesn’t get everything. You have good acreage and you’re draining good hydrocarbons, so you want to come in and place another well nearby to make sure you’ve drained as many hydrocarbons as you can.
“Those new wells that are drilled are often called child wells with the existing well being a parent. That really establishes the relationship of which wells were drilled first. And Petro.ai has found that frequently child wells underperform because they’re trying to access the same area that the parent is already draining.
“When that child well is drilled and completed, there are a number of effects that need to be accounted for. One is the production that has already been produced by the parent because the areas that you’re stimulating might overlap somewhat. We account for that by subtracting the parent drainage that we’ve calculated in the overlap region leaving what’s available to the child to be produced.
“The other thing that the parent can do when it drains these hydrocarbons is it can reduce the pore pressure in that area. So, you imagine you let these hydrocarbons flow back, the pressure can decrease. And changing the stress state locally like that can change the stress state experienced by the child well. When hydraulic fractures are created because they respond to stress, the stress basically decides where they go. The perturbation of the stress field from the parent well production can affect where the fracs propagate.
“For example, the lower stress can be an attractor for the fracs from the child well. They can change their course and tend towards the parent well which is what you don’t want to have happen. You want to avoid that, so you don’t take production from somewhere else.
“When we work up the parent/child interaction for a client, we try to characterize the baseline stress state around the parent. First, we want to understand what are the natural stress barriers that exist between it and any child wells that are going to be drilled. If there’s a big stress barrier between a parent well and a child well that you’re going to drill, then maybe it doesn’t matter that much because that stress barrier is going to block any interactions.
“When deriving the parent/child interactions, Petro.ai asks for any testing the client might have done such as pressure testing. We’ll look at ISIPs as part of our completion analysis routine. We can look at ISIPs along the lateral and we can analyze those to see if there’s been any changes due to the stress state which could be due to parent depletion.
“When we work this up, we’ll get all the data from the parent wells, such as the amount of production so we can incorporate that into the model.
“We’ll show the drainage of the parent wells overlapped with the child wells. That highlighted overlap area is visualized in the gun barrel. How much parent production that we’re subtracting leaves what’s available for the child wells.
“One of the differentiating factors for Petro.ai is that this conserves inventory. You can’t place an infinite number of wells in between two parents, for example. So how you space them determines how much each gets because it’s sharing from the same pie. It’s not going to be able to arbitrarily increase how much oil is there to be produced. That’s a fixed amount.”