Lesson 1.01 Intro to DFITs
Lesson Contents01. DFIT - Lesson 1.01: Intro to DFITs02. Introduction03. What is a DFIT?04. Before Closure Analysis05. After Closure Analysis
01. DFIT - Lesson 1.01: Intro to DFITs
OK, so that was a standard operating procedure in the days of conventional reservoirs. And if you do happen to work offshore, for example, it's still the standard operating procedure. You generally will have a well test on everything single well. The reason why is because if we want to optimize the completion in any well, regardless of whether it's a conventional or unconventional reservoir, we need to know some basic properties.
First, we have to know the initial reservoir pressure.
And we need to know the permeability.
There's also some fracture properties that are important, especially if we're going to be doing some type of simulation. And so we need to be able to collect those fracture properties.
And then lastly, we need some other reservoir properties. For example, we may need to know the composition of the rock. Maybe it is a carbonate and we can just use a little acid on it for stimulation. If it's not, then we may have to do a hydraulic fracturing treatment for stimulation. And there are other reservoir properties, we need to know something about the fluid that's in the reservoir, so we'd have to do a PVT study for example.
That all works great in conventional reservoirs. We could do all of that required testing in a fairly short period of time and it would not be very expensive. But as we started developing lower permeability reservoirs and as their permeability went to ultra low permeability, it became more difficult to do conventional well testing.
And if you think about the classic pressure build-up test, what's required for a pressure build-up test. Well, in order to do a build-up, I have to create a drawdown. Well, in an ultra low permeability reservoir, I may not be able to establish a flow rate before I've stimulated the well. And so I can't really do a build-up if I can't get the reservoir to flow. And so you kind of see the problems that you would run into. But even if you could, let's say you could establish a flow rate in a tight reservoir and then you did the required build-up, you would have to stay shut in for a very, very long period of time in order to have any hope of making a unique interpretation of permeability, for example.
OK, so as we started to develop routinely these ultra low permeability reservoirs, what we needed was a reservoir diagnostic that could be inexpensive and it had to be expeditious. And what do I mean? We had to be able to do it quickly. It has to be cheap and we can do it relatively quickly. OK, so a DFIT is what we use to help fill the void in well test analysis in unconventional wells pre-frac. A DFIT can be relatively inexpensive and it can be done relatively quickly. Notice I use relative here because these terms are, there are still some costs associated with a DFIT and there's still some time involved with collecting the data. But if we can minimize both enough, then even in unconventional reservoirs we can do well testing. And that is what led to the development of the DFIT fracturing for well testing.
03. What is a DFIT?
And when we do this test, we're going to do a small volume injection, let's kind of go through the figure here really quickly. So we have a surface pressure on the y-axis and time in hours on the x-axis. We have another y-axis over here for injection rate. So the injection rate are the low points that are shown right here. And then this is the surface pressure that's recorded up above. So you can see there's a relatively small injection and then a long shut-in period. And during this injection, a total of about 60 barrels of fluid was injected over about 10 minutes. So a relatively small volume, not the smallest volume we'll talk about, but generally a pretty small volume and then a long falloff period.
And what happens in unconventional reservoirs is most of the time this injection rate is sufficient to create and propagate a hydraulic fracture. It doesn't all the time and it's not necessary that we create a fracture to analyze the falloff data. However, in most cases, in the vast majority of cases in tight rock, we will create a hydraulic fracture. And then we'll watch that fracture close. And then after closure diffusion during the falloff period.
04. Before Closure Analysis
Well early in the falloff period where the circle is drawn, this is the before closure region. So we created a hydraulic fracture with this injection. Hydraulic fracture opened over some frac length. And then when we shut down the pumps, the fluid that's in that hydraulic fracture will actually leak off into the formation and the fracture will close, the walls will come back together. It never closes completely. And we'll talk about this a bunch as we go through the class. It closes to a residual width, but we still refer to it as hydraulic fracture closure. We can analyze that before fracture closure data for something that we call leak-off type. And there are four leak-off types that we'll talk about as we get into lesson 2, I believe, in module 1. And then we can also estimate permeability and we can estimate reservoir pressure. Now, the issue we have with these is that this permeability and this pressure estimate have the highest uncertainty of any of the interpretations that we'll make when we analyze DFIT data. And that's primarily because the permeability requires knowing the fracture geometry and often we don't know the fracture geometry that was created. We can estimate it sometimes. But we have to recognize that that estimate may not be very good. And then also the reservoir pressure, we're actually calculating from the closer stress. And if we're not using a calibrated stress equation where we know all the input parameters very well then that pressure estimate can be wrong. Nevertheless, we can still estimate permeability and pressure from the before closure data. And when everything is calibrated and we know all the values, it actually works very well.
05. After Closure Analysis
There are other flow regimes that we do see. And as we go through the course, especially when we get to module 3 and 4 and 5, you're gonna hear a lot about flow regimes. But the other main flow regime that we see all the time is called pseudo linear flow. In pseudo linear flow we can calculate reservoir pressure and we can also calculate transmissibility or permeability, but we give up a little bit of the uniqueness when we use the linear flow regime. In most unconventionals however, that's all we have to analyze. And so we have to use that linear flow regime for the analysis.
Now, there's also a technique I'll talk about in modules 3 and 4 and 5, it's transient analysis of a DFIT test. And in that module, we'll make it clear that you don't have to have any of the flow regimes, linear or radial flow, in order to analyze the DFIT using well testing methods. But in exchange for that, we give up some uniqueness. So uniqueness is also a common issue that's going to be coming up during this class we'll talk about all the time. A lot of our interpretations can be really good, but they may not be unique. So we have to be aware that.