Wednesday, December 19, 2012

An Introduction to Petrophysics and the Science of Finding Oil

I am starting this post while out in the oilfield. This is not as strange as it sounds; I work in the oil industry! In this post I'd like to introduce you to petrophysics, one of the scientific disciplines within the oil industry. I hope by the end of the post you will have a better understanding of some of the science that goes into finding oil.

Before I was recruited into the oil industry, I didn’t know much (anything?) about oil production, and I had no idea these types of jobs even existed. Coming into the industry, I was amazed by how many people and how much time, money, and advanced science it takes to drill a well. Even after being in the industry for three years now, I am no less amazed than I was at the beginning. I am learning new things constantly, and there is STILL so much to learn!

I will introduce you to petrophysics in the context of exploration. Many oil companies have exploration teams that try to find and develop new oil accumulations that no one else has found/produced yet. Before petrophysics even comes into play, geological work is crucial for being able to identify areas that may have potential as oilfields. (I still have a lot to learn about this part of the process, so I won’t be elaborating on it at the moment.) Once the geologists identify a potential target, the team must justify pursuing the target by gathering any available data that may support the claims that the target will produce oil economically. The company won’t want to spend money on something that has a low chance of actually making a profit, just as you wouldn’t buy a stock that was unlikely to give you a good return on your investment. Some of this available data may include well logs. The team's petrophysicist would be the one to analyze these logs.

Well logging refers to recording information about the characteristics of the rocks and fluids under the earth’s surface. After an oil well is drilled, logging tools are lowered into the well to acquire this information and send it back up to the surface. Since we can’t see down into the ground, we have to rely on this type of secondhand information to figure out where the oil is and how much there is (or to figure out that there is actually none at all). We can infer the presence of oil and the feasibility of producing it by looking at a combination of different rock and fluid properties. The basic properties I'll cover here are natural radioactivity, resistivity, porosity, density, and permeability. I’ll explain how each property contributes to our understanding of whether or not there is easily producible oil in the well or the surrounding area.

Let’s start with natural radioactivity. This data helps us determine the difference between sand and shale. For now I’ll focus on conventional sandstone reservoirs. It is easier to produce oil out of sands than out of shales; shales make up much of the new “unconventional” exploration that you may have heard of. Think of sandstone as something similar to beach sand compacted into a hard rock, and think of shale as something more like a condensed and hardened mud. So, if we are looking for a conventional sandstone reservoir, we need to be able to tell the difference between these two. This is where radioactivity comes in. The naturally radioactive elements are mostly concentrated in shale and not in sand, so if we see low radioactivity, we can infer that the tool is likely responding to a sand. We measure this with a tool that counts gamma rays, a type of energy that radioactive elements emit when they decay.

Now that we have identified a sandstone, we next try to identify if it is filled with oil or water. We do this by measuring the resistivity. Resistivity is the opposite of conductivity, and conductivity is a measure of a material’s ability to allow electricity to flow through it. The water deep underground usually has other materials dissolved in it (e.g. salt), creating ions – particles or atoms that have an electric charge. These ions are free to move around, and if they are somehow stimulated to move in one direction, an electric current is born. So, the water in the ground is able to conduct electricity and therefore has a LOW resistivity. (There are a few exceptions to this, including that some oilfields are not deep enough underground to encounter the saltier water. In shallow fields, the water may not have as many ions and may be more resistive than the deeper water. This makes analysis even harder.) On the other hand, oil does not dissolve things the way water does, so it doesn’t have any free ions floating about, and the oil molecules themselves are not charged either. There are no free charges in oil that can move around to create a current, so oil cannot conduct electricity and therefore has a HIGH resistivity. We use resistivity information to figure out whether the fluid in the ground is water or oil. If it’s water, we have to go back and figure out why there isn’t oil where we thought there would be. If it’s oil, we look at other properties to decide whether or not we should try to produce it.

Next is porosity. Unfortunately, oil doesn’t exist in large lakes within cavernous spaces underground; it would be much easier to produce it if this were the case! Instead it is trapped in the microscopic spaces, or pores, between the grains of the rock. Porosity is the fraction of the rock that these pores occupy. 

The pores are filled with fluid (oil or water) or gas. Even if this fluid is oil, there won’t be a large amount of oil to produce if there isn’t much space in the rock to begin with. It’s not only about the mere presence of oil but also about the amount of oil that can be recovered – the total amount of return on the investment. In the diagram below, the brown squares represent solid rock grains, and the blue squares represent the pores, which could be filled with oil or water. You can see that 5 squares out of 25 squares are blue, which means that the pores make up 1/5 (or 20%) of the total area (or volume, in the three dimensions of a real piece of rock). So, this rock has 20% porosity. This also means that the other 80% is solid rock, so Volume of rock = 1 – porosity.  

One of the ways to calculate porosity is by first looking at the density. This is the same “density”  you probably learned about in school at some point. Density is the weight of a material in a given volume. Imagine that you have a pair of dice. You already know that the two dice are the same size and shape. Now imagine that they are made of two different materials (with different densities). The die made of the higher density material will be the heavier one. For oilfield applications, there is a tool that measures the total density of the rock-plus-fluid underground. Water and oil have similar known densities, and different types of rock have known densities as well. So, if you have the total density measured by the tool, and you already know the density of the rock (assuming you know what kind of rock is down there) and the density of the fluid, you can figure out how much fluid there is, i.e. the porosity.

We also need to worry about the permeability, which is a measure of how connected the pores are. If the pores are all connected, the oil will be able to flow out of the reservoir easily. If the pores are not connected, the oil will be trapped in the rock and will have no path to flow out. So even if the porosity is high, there’s no guarantee that the pores are well-connected or that the oil will actually flow. Below are examples of "rock" with similar porosities but different permeabilities. Again, the brown represents rock grains and the blue represents fluid-filled space between the grains. 
This rock has no permeability - the fluid is stuck in its original location and has no path to flow.

This rock has low permeability - there are connections between the pores, but they are small. The fluid will probably flow, but it will not flow easily or efficiently.

This rock has high permeability - the connections between the pores (called the pore throats) are large and will allow the fluid to flow quickly and easily.
Permeability is one of the hardest properties to measure. There are some tools that try to estimate or calculate it, but it is better to do some tests on a sample of the actual rock (more on this below). 

So, now we know the basic petrophysical parameters of the reservoir. We know if it is a sandstone, we know if it is filled with oil or water, and we know how much fluid the rock holds. However, for all but the most established and most perfect reservoirs, things are not as straightforward as this idealistic case, and a much more detailed analysis needs to be performed. This is true of the recently targeted “unconventional” reservoirs that you may be hearing a lot about these days. As part of this necessary advanced analysis, there are tools that can tell us about the mineral composition of the rock, the amount of fluid that’s free to flow versus the amount that is “stuck” in the rock, the mechanical properties of the rock (how the rock will deform and/or break), the way the different layers of rock are arranged, and the direction of the forces and stresses at work underground.
In addition to these advanced logging techniques, it is important to core the reservoir. This means that we put a tool into the well that takes physical samples of the rock and brings them back up to the surface. Then we can do tests on the samples in a lab and verify some of the things we have inferred from the logs – or learn things that are different from what we originally inferred! In that case we have to re-interpret the data to make sense of everything.

All of these data-gathering and analysis techniques are important to finding oil and to optimizing production of the oil. We want to produce the oil in the most efficient and economic way possible, and we can do so by understanding exactly how the reservoir is arranged and how it acts.

As I mentioned earlier, there is so much science and engineering that goes into producing oil. This just barely scratches the surface! I hope it has given you an understanding of some of the science, and I hope you will be interested in learning more! Please let me know if parts of this are unclear, and I will write some follow-up explanations.