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  1. Introduction of Naturally Fractured Reservoirs (NFRs)

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- Hi. I'm Wayne Narr. I'm a geologist, and I've spent much of my career working on the characterization and modeling of naturally fractured reservoirs. I've put together a series of modules that focus on heterogeneity in naturally fractured reservoir performance. Heterogeneity is a key characteristic of naturally fractured reservoirs. We see it in all naturally fractured reservoirs. The title of my talk is guilt and absolution in naturally fractured reservoir characterization, the cause and devilish effects of heterogeneity. So, I'll talk about evidence for heterogeneity in naturally fractured reservoirs. And then I'll talk about the causes of that heterogeneity. But first, what am I talking about guilt and absolution? Production performance is extremely difficult to predict in most naturally fractured reservoirs. We, as reservoir geologists, reservoir engineers, are expected to be able to predict the performance of our reservoirs. When we can't, we fail, and that leads to guilt. However, we can use knowledge of natural fracture systems to better understand naturally fractured reservoirs, to take action based on our understanding, and finally, to accept that there are limits to what we can predict. And it's by these steps that we acquire absolution. Let's begin by defining a naturally fractured reservoir or N-F-R, as it's commonly referred to. An NFR is a reservoir in which fractures enhance the permeability field, significantly impacting well productivity and recovery efficiency. The fractures in the reservoir do this by enhancing the permeability. They generally do not enhance the reserves very much. That is, fracture porosity is very small in general and can typically be ignored. The fractures do effect the recovery mechanism in the reservoir. That is, they effect the way that fluids move from the matrix to the wells. The other term I'd like to define is heterogeneous. And by that I simply mean consisting of dissimilar ingredients or constituents. So when I say that the reservoir has production heterogeneity, what I'm really saying is, as we look from well to well, we see very strong differences in the behavior of the production from those different wells. The outline of these modules is shown here. I'll begin talking about heterogeneous production behavior of NFRs. I'll then talk about the basis for that heterogeneity, which is both geological and as a function of how we drill wells into the reservoir. I'll give some perspectives on fracture-induced heterogeneity from a model, and then we'll wrap up and come back to the point of how we find absolution.