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  4. Refracking and Reservoir Types

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- [Voiceover] Upon the successful completion of this unit, the student should be able to explain when it is a good idea to refracture a well and also explain how a knowledge of geology helps optimize refracs. So we're focusing on fractures and refracing, and it's very easy to look at micro seismic and geo-mechanical information in image logs and stick with those. Those are important, but really take a focus on primarily the geomechanical elements. And take a step back and look at things from a geological perspective. So what actually creates the need for going back into a well and refracing from a geological point of view? Well, first of all, I'd like to say that many times there's a need for refracing because of incorrect assumptions about the reservoir, and generally speaking, there is an assumption that the reservoir is more homogenous than it really is. What I mean to say by that is that there has been a one-size-fits-all approach to completion, especially in horizontal wells, where there might be an attempt to rubblize the rock and maybe have thirty or fifty stages, and they're all the same. Well, that can work, but it's not going to be uniform, and so you'll have great IPs, but because of the fact that it's not a uniform frac, it's not going to be uniformly effective, and so there are going to be places that contain oil or gas, but they weren't fraced exactly in the way that they've been. And why, well, because of changes in the lithology and changes in the density of fractures, the change in fracture architecture of the natural fractures, changes in the lithology, which means that the fracability changes, so you might have areas of rock a little bit more normal than others, and it's because of more ponderance of, say, a different kind of clay mineral or different types of carbonates, so there might be unexpected permeability barriers and barren zones. Then, another reason is just simply porpoising through horizontal drilling. They drill really fast, and it's easy to drill out a zone, so there might be areas that were just missed. And then there's also the issue of formation damage, due to incorrect selection of drilling and frac fluids, so there could be a need for refracturing, especially if there is formation damage, just more or less in the annular space and not into the actual formation. So, in theory, maybe a good refrac could open that up and overcome some of the formation damage that's just right next to the borehole. So to return to my basic theme: geology matters in refracing, and you might say, well, okay, we're going to refrac a shale play or shale well. Maybe we're looking at that, but we may want to go in and refrac a carbonate, vertical well, or a sandstone reservoir that's in a mature field, or we might want to go in and refrac a shale or mudstone horizontal, and also what is near the reservoir? Has there been a lot of diagenetic alteration? And what are those clay minerals? So I'm going to just briefly go over what you need to know about sweet spots and different kinds of reservoirs. So first of all, let's look at a carbonate reservoir. Sweet spots: what you need to know. There are natural fractures, and some are closed and some are open. In a carbonate reservoir, you need to know which ones are open and which are closed and the fracture architecture of fracture networks. You also need to know diagenetic alteration, what type. Dissolution can result in vuggy porosity, so increased storage. Also, you can have dolomitic zones, so that can increase the porosity. And also diagenetic alteration can affect the fracability. Maybe have chert or sparry calcite, chert overgrowths, tripolitic chert. All those things are variant. Pore architecture. How are the pores related to each other? Also, what is the type of biological activity? Are there bioherms? Are we talking about reefs? And also, what happened in the original frac? Is there scale, paraffin? Carbonates are notorious for having a lot of scale produced very quickly, essentially because of creating in situ diagenesis, due to the mineral nature of the reservoir fluids, so do you need to add acid and a flood? What are the need-to-know elements in a sand reservoir of sweet spots? First of all, I've argued that a sweet spot is essentially a stratigraphic phenomenon, because it's contained in strata, so if you're looking at, say, a stratigraphic trap, what kind? If it's a sand lens, what kind? Beach sand, point bar, fluvial deltaic? What is the continuity and orientation of the sand body? John Holbrook at TCU has done a lot of work with fluvial delatics and looking at the different types of sand bodies. Extremely useful, and you can see in his work where you're most likely to have preferential enrichment, so if you understand your sand body, you can go back in and refrac in the right place and really get some of the oil out that's been left behind. Also, understanding the level of heterogeneity is really important. In clastics as well as carbonates, it's important to understand the degree of heterogeneity in the reservoir. One thing to do is to develop a depositional model. Here I've looked at clastics and essentially an offshore system, and you can see that it's not just a big blanket of equal, all-equivalent sand, so the idea that you could have a play the size of Delaware, that it's all going to be the same in terms of the equality of the reservoir, is not too realistic. It's really, really important to use nature as a model for the past and develop a depositional model, and it can be and should be highly heterogeneous. And sand bodies are going to be discrete, lenticular units many times, and instead of looking at heterogeneity as a frustration, production barrier, which it usually is in terms of primary production, in secondary it's an opportunity because that's where you have the left-behind oil and gas. What are the need-to-know elements in a shale reservoir? Sweet spots. So, one, there are a lot of natural fractures. Closed versus open, as we mentioned before. Also, induced fractures. This is where microseismic comes in. Where exactly are your induced fractures, and where did you establish flow between natural and induced fractures? What's the lithology? Also, where did the gas come from? We can use gas fingerprinting for that. Lots of work being done in that. And where are the water zones? Super, super critical, because fracing into a water zone usually is not a very good thing in a shale reservoir. Diagenesis can make or break your refrac. Why? Well, diagenetic sweet spots: dolomitization adds intercrystalline porosity. Diagenetic porosity and permeability can be reduced also through cement, calcite, chert. So it's interesting. Diagenesis can make your refrac by increasing the fracability and increasing the potential for induced fractures that connect with your natural fractures, but also, it's potentially dangerous, because it can be right on the edge of the water and usually is, due to hydrothermal alteration causing the diagenesis in the first place. So, really need to understand the nature of the diagenesis in your reservoir. D is correct. All of the above are true. Now, the interesting thing about A and B is that diagenesis involving secondary crystallization is usually due to vein-filling waters, but, in A, secondary crystallization often enhances porosity. But, in B, the vein-filling waters that precipitate calcite actually fill up the pores with a mineral. In the case of C, dissolution, the waters have dissolved the minerals, and so usually, there is enhanced porosity as much as even vuggy porosity. Vuggy, big pore spaces.