The how and why of building geological models

Originally posted June 2015 here

I am a geologist who builds geological models for the oil and gas industry, and this is what I do

I’ve been building geological models for more than 6 years now, and I like it because it involves integrating all the data you have into a useful model of what might be down there. You need to be reasonably fluent in every discipline which has useful data on the formation you are trying to model. I thought I might take you on a quick tour of how these models are built, (at least in the sedimentary environments of the oil industry) and what they are used for.

Firstly you need to build a stratigraphic framework of the area of interest, i.e. where are the major boundaries between rock formations. Check out figure 1 for an outcrop example. When our rocks are hundreds of meters underground we must examine well data. Companies drill and log deep thin holes, called wells, into the earth to figure out what is down there, i.e. they drag tools along the hole measuring properties like porosity, resistivity, gamma ray response. There are subtle variations in how different rock types responds to these tools, and we use these to correlate between wells, providing information on how the geology is connected in space.

Figure 1 An outcrop from Dinosaur Provincial Park, AB with major stratigraphic surfaces annotated in red. This is what you need to do for the subsurface in 3D

Figure 1 An outcrop from Dinosaur Provincial Park, AB with major stratigraphic surfaces annotated in red. This is what you need to do for the subsurface in 3D

Once collected, these logs are loaded into a computer, examinied and used to correlate between wells. Looking at them in 3D gives a feel for their distribution, and you see that you have one point per well, per surface. It’s time to get geophysical. Seismic data is collected by shooting energy into the ground (dynamite onshore, and air guns over water) and listening for echoes returning to surface. Using many listening and shooting locations, and some unbelievable mathematics, it is possible to build a 3D image of the subsurface. Combine this with the stratigraphic picks you have made and you can intelligently join the dots between wells, as in figure 2.

Figure 2 A seismic interpretation of the stratigraphic framework of a reservoir from Findlay et al. 2014

Figure 2 A seismic interpretation of the stratigraphic framework of a reservoir from Findlay et al. 2014

Now we need to take these surfaces we have defined using well and seismic data and build a model. The first step is to use geostatistical tools to build a facies model. This means examining the data for trends and combining them with what you know about the sedimentary environment (outcrop analogues like figure 1, examining similar modern environments etc.) to build a realistic representation of the reservoir while honouring the data. Do this right, and you might end up with something like figure 3.

Figure 3 Facies model (bottom) vs outcrop (top) of an ancient river system. Given the angles we see in the outcrop, It would be a mistake to model this flat wouldn’t it?

Figure 3 Facies model (bottom) vs outcrop (top) of an ancient river system. Given the angles we see in the outcrop, It would be a mistake to model this flat wouldn’t it?

 

The facies model is used to construct other models like porosity or permeability, anything that might be useful. If you are interested in calculating how much oil you have, then you need to know the oil saturation and porosity. If you want to know how the rock might respond to hydraulic fracturing, you might need porosity, density and brittleness. In my case, I want to know how steam might travel through the reservoir, so I need porosity, permeability, and oil saturation. Let’s have a quick look at a simulation.

Figure 4 Results of a simulation based on a model similar to figure 3. From Findlay et al 2014

Figure 4 Results of a simulation based on a model similar to figure 3. From Findlay et al 2014

In this case, the oil in the reservoir is very thick, and must be melted with steam in order to extract it. We can see in figure 4 that areas of low permeability can be an impediment to steam rise, but steam will likely get around over time. By simulating how steam behaves in the reservoir, it is possible to optimise how it is operated, and save lots of money.

This is one example of how a model might be used, but I have personally used a model to inform well placement, predict what a new well might encounter, what kind of equipment to put down the well and many other things. I’m already over my word budget, but I hope this gives you some idea of what a geological model is, and why they are useful. You can find a more in depth article here, but feel free to drop me a line at duncan@realisticgeoscience.com if you want to know more.

Duncan

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