So I had a request to explain what an enhanced oil recovery engineer does. I was going to write Aunty Linda an e-mail, but maybe it is worth explaining in a blog post. It is one of my favorite subjects after all.
So let's start from the beginning. This is mostly background and I might follow up, but I'll try to set the basics.
A oil reservoir is an accumulation of oil trapped underground in a porous 'reservoir' rock. Over geologic time, the oil migrated from the dense 'source' rock into the reservoir rock (which can either be sandstone or carbonate). The oil reservoir must have a 'cap' rock, a layer of non-porous rock that keeps the oil from escaping to surface. The shape of the reservoir also must contain the reservoir in a 'trap'.
Here is a link to a picture of a common type of reservoir trap, an anticline.
This only thing misleading about this picture is that it looks like the oil gas and water exist as a big swimming pool. That is not true. The liquids only exists in the pore space of the rock, too small to see with your naked eye. So when we model fluid movement in the reservoir, it's fluid flow through porous media. Darcy's Law.
Ok, so now we have oil (and potentially gas and water) trapped underground. So you can produce the oil out just by drilling a single well into it. The oil contains dissolved gas, and at that depth underground, it is pressured up enough that you can usually produce fluids to the surface just by drilling through the cap rock and letting the fluid depressurize. That is called primary depletion. It can usually recover about 20% of the oil.
This is generally not the recovery method used in oil reservoirs. It is often used in gas reservoirs, where primary depletion can recover up to 90%, but not oil reservoirs.
The next thing you can do it drill another well into the reservoir and inject water into that well. So you have one injection well and one production well. By injecting water in one area you push more oil to the producing well. You also maintain the pressure in the reservoir. This is key because you want the reservoir to remain above the 'bubble point pressure' which means the pressure at which the gas comes out of the oil. This process is called 'waterflooding' and is also known as secondary recovery. Utilizing this method can generally recover another 20% of the oil.
Ok, any recovery method past secondary recovery is enhanced oil recovery.
Sometimes called tertiary recovery. Any other thing you can do to get more oil out of the ground. At Kuparuk we have managed a large scale MWAG flood since 1996. We started to pilot the process in 1988 and the large scale implementation started in 1996. MWAG stands for miscible water alternating gas. We inject water but we also inject a gas called miscible injectant. Alternating. The miscible injectant is made of gas produced from the reservoir (mainly methane) and natural gas liquids (NGLs). They mix to make a fluid that is designed to be 'miscible' with the oil at reservoir temperature and pressure conditions. If the MI and the oil are miscible, they become one phase, The oil mixes with the lighter MI and flows easier to the producer. The water is still required for the pushing mechanism, conformance control and pressure support. This process can get another 5-10% of the oil out.
Here is a link to a paper about the MWAG flood at Kuparuk. Unfortunately, I believe you have to pay to download the whole thing.
https://www.onepetro.org/conference-paper/SPE-113933-MS
We are also always constantly looking for new EOR methods to try at Kuparuk. So I get to help design pilots of new technology. We deal with our lab in Bartlesville and our technology group in Houston. Examples of other enhanced oil recovery methods
- Low Salinity flooding
- Chemical EOR flooding, ASP (alklai surfactant polymer)
- Foam flooding
- Polymer flooding
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