Permeability is the quality of a material that tells us the ease with which that material will allow any given fluid to pass through it. Let’s say we want water to flow through two materials, called material A and material B. Permeability is a concept we can use to measure just how easy it will be for the water to flow through each of the materials. When these two materials are rocks, permeability measures how easy it is for that rock to permit the flow of fluid through its pores.
What Makes One Rock More Permeable than Another?
Number of Pores
A porous rock is one that has pores wide enough to accept fluids, no matter how small the volume of fluids it accepts is. But, in the oil and gas industry we are not only concerned with how much fluid the rock allows into its pores, we are also concerned with how easy it will be for this rock to release these fluids from its pores when we need it to. This is where permeability comes in.
There are a number of factors that allow one rock to release the fluids in its pores more readily than another does. One of them is pore interconnectivity. After we know how many pores are available, we need to know how many of these pores are interconnected. Pores must be so interconnected with one another so that fluid flows easily throughout the rock from one end to the other. Rocks with more interconnectivity are usually more permeable than rocks with pores that are closed to one another by rock grains or any other solid precipitate.
This leads to another factor known as sorting. A poorly sorted rock will have its grains scattered all around the entire rock matrix (the entire rock volume comprising both the pores and the grains). When grains are scattered this way, some grains may find themselves in positions where they are blocking pore spaces that fluids will have naturally passed through. (For more information about reservoir rock, see Earth Matters in Oil and Gas.)
Sometimes, these poorly positioned grains may partially close the pathway, making the fluid struggle to find a way to squeeze through any space it can find or at other times, the grains may entirely block the pathway. Any fluid that encounters these grains is trapped and cannot flow through. A well sorted rock has grains of almost the same size and well positioned, so the issue of a small grain in between a bigger grain blocking a pathway is reduced to the barest. This is why a well sorted rock will be more permeable than one that is poorly sorted.
Let’s consider another factor, and then we’ll leave it at that. Since we need our fluid to flow out of the rock as easily as it flowed in, the size of the pore plays a crucial role. The bigger the size of the interconnected pore spaces, the better.
Figure 1. Comparison of loose and tight rock pore spaces.
When rocks were deposited, they were deposited in layers, with one layer lying on top of the other. Remember that we find our reservoirs in deep underground rocks, which means that over time, the rock layers on top have compressed the reservoir rock as much as possible under pressure thereby reducing the volume occupied by the rock matrix. (Learn more about this topic in the article How Hydrocarbon Reservoirs Are Formed: The Rock Cycle.) So depending on how much pressure was exerted on the reservoir rock, the permeability varies from one reservoir to another.
Tight sands and other fine-grained sedimentary rocks that have undergone tremendous pressures from overlying rocks ultimately reduce their pore volume and even grain volume. This is why tight sands have very little permeability compared to conventional reservoir rocks. So the tighter the rocks are packed, usually due to pressure, the smaller the pore throat or pore size, and the less permeable the rock will be.
What is Absolute Permeability?
We know that permeability helps us measure how easy it is for a rock to permit a fluid to flow through its pores. To fully understand this, let’s take a look at absolute permeability.
Imagine being given one rock sample and three fluid samples (oil, water and natural gas). To understand the permeability of the rock sample for each of the three fluids, we decide to pass each fluid one at a time through the rock sample, after which we clean and dry the rock sample and get it ready for another fluid.
We can start with passing water through the entire rock sample from one end to the other and measure how easy it is for the water to flow out of the rock sample. Like we agreed, we then clean and dry the rock sample and then test with oil and natural gas.
What we just did for each fluid was to determine the absolute permeability. When only one kind of fluid fully saturates a rock, then the permeability of the rock to that fluid is the absolute permeability. In other words, the pores of the rock are absolutely or 100% filled with only one fluid and hence the permeability of the rock at that time is the absolute permeability to that particular fluid.
How is Effective Permeability Different from Absolute Permeability?
Absolute permeability is great when carrying out laboratory tests on rock samples, but it is not always the case in reality. In the reservoir, we usually find two or more fluids sharing the available pore spaces in the rock. For instance, the reservoir may be an oil reservoir but with some connate water occupying some of the rock pores, so we have oil and water occupying the pores. At other times, it could be gas and water or even all three (oil, water and gas) actively present in the rock pores of the reservoir. So under typical reservoir conditions, we talk about effective permeability and relative permeability.
To understand these new concepts, let’s head back to our experiment. We’ll be using the same rock sample, but this time instead of passing one fluid at a time, we pass all three fluids (oil, water, and gas) at one end.
So we’ll end up repeating this experiment three times, but we focus on one particular fluid at a time. When we passed all three fluids into the rock sample the first time, we focused on the oil and measured the ease of flow of the oil through the rock sample in the presence of water and gas. We then repeat this same experiment and test for water, measuring the permeability of the water in the presence of oil and gas. Finally we do the same for the gas in the presence of water and oil both flowing in the rock pores. The permeability we just obtained for each fluid in the presence of other fluids is the effective permeability of the rock to that fluid.
With effective permeability, all the fluids compete for the available pore spaces, and the one that is most permeable will flow easier and faster through the rock pores than the others.
Figure 2. Comparing the permeability and drainage of gravel, fine sand and clay.
Permeability is one of the most important concepts in the oil and gas industry. The knowledge of permeability is useful from discovery all the way until well abandonment. It guides recovery strategies. Permeability will be a requisite ingredient when making depletion strategies from primary recovery to tertiary recovery (if necessary).
Absolute permeability is normally not important because most of the time we will be dealing with more than one fluid within the rock pores.
A rock with large hydrocarbon deposits and low permeability will have to be stimulated to open up the pores and artificially create pore interconnectivity.