The Cause and Devilish Effect of Heterogeneity
- I'd like to finish off this discussion of the causes of heterogeneity in natural fracture systems by looking at geologically induced heterogeneity. And by this, I mean four different factors: Chemical diagenesis and the process of dissolution of fractures. Chemical diagenesis again, but this time the process of mineralization of fractures. The lithologic control of fractures. And, finally, stratigraphic control of fractures. Let's begin by looking here. There's a large fracture in this cliff. You can see the scale by the two people that are standing down at the base of the cliff on just the right side near that large cavern there. There's a fracture that cross-cuts this cliff and you can see that with the red, dashed line. Along the fracture are several caverns. This is very common. Most natural cavern systems follow preexisting fractures. Now, consider what effect this might have on heterogeneity. Very small differences in one location could have a big impact on the performance of the well. Consider, for example, if we were to drill a well over in this location on the left side, hit no fractures in this interval, we might not even get any production along here depending upon what the matrix properties of the rock are. As opposed to if we had drilled just a few meters to the right of there, we would intersect an open fracture and maybe it would make a pretty nice well. And, imagine if we were to drill just a few meters further over, we would hit this large cavern. We might get grand production from this well, because the permeability along this cavern system would be so enormously high. Or, in fact, we might even risk losing our mud system and a blowout as we came through this cavern. So, extremely high heterogeneity depending on small differences in well location. And, all of this is due to the diagenetic modification of the fractures. Now, that's what happens when fractures are enlarged; which normally is occurring in carbonates. Let's look at some more modifications of fractures. Diagenesis, in addition to enlarging apertures, can also occlude apertures. And, we're seeing an example of that here. This is a sandstone layer. And, you see the photograph in the upper right is showing open fractures extending a long distance along a nearly horizontal layer of this sandstone, versus the photograph in the lower right labeled healed fractures. And, here what we're seeing is in a location that's very close; actually, a few hundred meters from the upper photograph. The fractures in this location are healed by secondary mineralization. There's ankerite cement that has formed within the fractures and it's completely closing those fractures off. So, even if the fracture system were uniformly developed, and by that I mean the basic fracture system in response to it's mechanical development, even if it was uniformly developed, the mineralization would have induced heterogeneity aerially throughout this reservoir. Now, let's move on from modification to fractures and look at the effect of different rock properties on the occurrence of fractures. Here's an example; although, this is one of many examples that show that rock properties have a strong effect or can have a strong effect on the presence of fractures. What we're seeing here is a bar chart that shows average fracture density on the Y-axis as a function of lithology on the X-axis. And, you see that the bar off to the left side shows high fracture density within clean, well-sorted siltstone. And, as we move from there to well-sorted sandstone, fracture density is lower. We start adding some clay to the rock and fracture density drops. And again, as the rock becomes less well-sorted, fracture density is lower until we get into true shales and fracture density has dropped off to zero. No fractures in the shales. Facies of rocks can also impact the presence and character of fractures. And, we can see that in this spectacular outcrop on the far side of a small river in Northwestern Australia. This is the margin of a carbonate platform. You see the platform strata are off to the right side of the picture. As we move to the left, we cross the area of the marginal reef. And then, further to the left, we come into the depositional slope. And, the basin is off to the extreme left, here. Let's just zoom in on this reef platform area and take a look at just the area within the red, dashed rectangle. Here again, we see the reef on the extreme right and the slope on the extreme left of this photograph. Let's look at the fracture character. In the platform rocks, the fractures look like joints that we saw in a previous photograph. That is, they are perpendicular to beds, they are regularly spaced and uniform in orientation, and they tend to stop at mechanical bed boundaries. Now, from there, compare that to what we see in the slope. First of all, bedding, the bedding that you see dipping about 45 degrees to the left, that's depositional dip in the slope environment in these carbonate rocks. These rocks have a high microbial component to them. Therefore, they tend to be sticky. And so, we can get grainy materials stuck to the sticky bedding planes and form very steep slopes in places. The fractures that form in this slope environment are not as uniform in their occurrence. They're not as distinctly perpendicular to bedding here as they are in the platform region. And then, when we look in the reef region, bedding itself is very hard to ascertain. In fact, in general, it's massive bedding that forms throughout this slope environment. And really, I think one gets a false impression looking at this photograph as to what actual bedding is in the reef setting. But, it is very clear that it's strongly fractured through the reef and the fractures are not nearly as systematic as they are in the slope and especially not as systematic as in the platform. So, what we're seeing here is facies of these rocks has a very strong control on the character and occurrence of fractures. Now let's look at stratigraphic control of fractures in a sandstone. And, what we'll be looking at here is a satellite image of the Aztec sandstone from Valley of Fire State Park in Southern Nevada. And what I'd like you to see here is that small variations in rock property can have a strong effect on fracture occurrence and character. The Aztec sandstone that we're looking at here is a Jurassic sandstone. It's a subarkose and very high-porosity overall, 15 to 25 percent; pretty uniform overall in its character through the reservoir. But, some subtle variations as we move from lower, to middle, to upper unit in the Aztec formation. And, indeed, that's why you will look at the fractures on closeup views here from the strongly jointed lower Aztec; which is in the Southwest corner of this regional satellite image. Then, we'll move on with the moderately jointed upper Aztec on the East-Central side of this regional area. And then, finally, we'll look at the sparsely jointed middle Aztec in the area of the box toward the North end of this regional view. So, here we are. The strata are nearly horizontal in the lower Aztec here. And, it's very clear that the rock is strongly jointed, strongly fractured. Those North, almost North-Northwest to South-Southeast trend that you see there are all fractures in the Aztec sandstone. We then move over to the middle Aztec; which is moderately jointed and you see here far less joint density in these rocks than what we just looked at in the lower Aztec. And now, let's look at the middle Aztec. And that shown here, it's a lighter colored rock. It's been bleached chemically. And, what we see is almost no fractures at all through this middle member of the Aztec sandstone. So, the stratigraphy and/or rock properties have had a very strong impact on the character; on really, the occurrence of fractures in this rock. In our final example of fracture system variability, I'd like to talk about the development of different fracture sets in an area and mention the impact that this can have on heterogeneity, of producibility within a reservoir. Consider the image that you're seeing here. This is a satellite image. Bedding is horizontal, the fractures are vertical. And, you can see the scale bar; 500 meters. You can well imagine we could have a well in the Northeast corner and another well in the Southwest corner and if they were an injector and producer pair, we would get a completely different recovery than we would if we had those same two wells offset from the Northwest corner connecting to a well in the Southeast corner. If you had that Northwest to Southeast alignment of wells, maybe the recovery that you would get would be something like imbibition. Whereas, if we had the alignment of wells from the Northeast to the Southwest, it would be much more likely we would see some amount of viscous displacement of oil from the matrix. So, right there, the anisotropy and orientation of the single set of fractures could have a big impact on the recovery and recovery efficiency in this formation. This rock is the Entrada sandstone and the area is in Arches National Park of Utah. So, let's compare this to another image. Here's another satellite image at the same scale. And, you see, there's one well-developed set of joints. And, a less well-developed set of joints extending from the Northeast to the Southwest. So, this could have a major impact on the way fluids would flow within the reservoir. This is also the Entrada sandstone in Arches National Park. And now, compare this image to one more satellite image again at the same scale. And here we have multiple over-printed fracture sets on this formation. And, this would have, again, a very different flow character than what we'd expect from the other two images. Fluid could flow in this reservoir in just about any direction with relative ease through a fracture network. This is also the Entrada formation in Arches National Park. So, we see a high degree of heterogeneity when we only have a single fracture set present or one well-developed fracture set present. But, we also see heterogeneity from unit-to-unit within the same formation; just due to the presence or absence of multiple fracture sets. That brings me to the end of this module in which we looked at the cause of variation in fracture system character as a consequence of well fracture intersection, or really, sampling related probabilities; fracture size variability; geologic heterogeneity caused by aperture enlargement, aperture occlusion, and rock property and stratigraphic variability; and finally, fracture connectivity variability due to variation in fracture density and the presence or absence of multiple fracture sets within a formation. So, we'll move on now into the last module in which we consider fracture system variability. Both fracture permeability and fracture system connectivity from the standpoint of models. So, it's a bit more conceptual in it's treatment of fracture system occurrence and its impact. But, it helps us to understand and better describe fluid flow within a naturally fractured reservoir. And, to understand the heterogeneity that results from fracture system variation.