Production can be divided into major three stages. As production from each stage declines, the operator assesses the well and decides whether it would be economical to pursue the next stage of recovery. Each stage is more expensive to implement than the preceding stage. Let’s learn a bit more about these stages.
Transcript
Stages of Extraction – Matt Balhoff – The University of Texas at Austin
I’m going to explain three stages of extraction that are used in the oil and gas industry. These stages get progressively more difficult, expensive and time consuming as we work to remove a higher percentage of the original oil or gas in place.
I’ll use oil throughout this example, but please keep in mind that natural gas recovery works the same way.
It starts with the first well. When we think we know there is oil in a reservoir, the first well we drill is called a production well. It’s an optimistic term because it hasn’t produced anything yet.
At the time of drilling, we are hopeful we can produce oil or gas out of that well. Sometimes we’re wrong and the well hits a “dry hole”, which may not really be “dry,” but, unfortunately, contains only water.
If the reservoir does contain oil and gas, its high natural pressure allows for oil to flow through the well to the surface on its own. The reservoir is a lot like a shaken-up bottle of carbonated beverage. When we open the bottle, fluid gushes out quickly.
Likewise, when we drill the well, oil initially gushes up very quickly, much like you have probably seen in TV and movies.
As time goes on the pressure in the reservoir (and beverage bottle) equilibrates and the flow rate declines and eventually ceases altogether.
Usually, this kind of flow continues for a few months or maybe a couple years and we call this stage “primary production”. Depending on the reservoir, we only produce approximately 5-20% of the original oil in place during primary production.
At the end of primary production, we could cease production altogether (and 100 years ago, that is exactly what we did), but we would be leaving a lot of oil behind.
Instead we normally proceed with secondary production which involves drilling another well (or several wells) called “injection wells”. In these wells we inject fluids, often water, to build up pressure again in the reservoir, push the oil and gas towards the producer well or wells.
Water does not mix with oil, so the water ideally pushes the oil like a piston and sweeps through the reservoir in an attempt to produce the remaining oil (Secondary production is often called “waterflooding”). During this stage, the production well produces not only oil and gas, but also some water.
On the surface, we separate the water from the oil and send the water back down the injection wells to maintain pressure in the reservoir. Unfortunately, at some point during this stage of production, water begins to bypass the oil, leaving behind and trapping a lot of it.
Eventually the production well produces almost all water and at that time the process is no longer economical.
Secondary recovery can last for decades and produce an additional 15-20% of the original oil in place. But even after secondary recovery, we still leave behind a lot of oil and many times we really do give up on the reservoir at that time.
Sometimes, however, we proceed with tertiary or “enhanced” oil recovery. Here we inject fluids other than water to produce some more of the remaining oil.
For example, steam can be injected and since it’s hot, it reduces the viscosity of the oil, making it easier to flow (like the way cooking oil becomes runny on a hot griddle).
Surfactants (better known as detergents or soaps) are sometimes injected to reduce the surface tension between oil and water and remove the trapped oil, just like soaps are used to clean trapped grease in clothes.
Polymers are thick, viscous fluids (similar to hair gel) that are efficient at pushing the oil to the producer wells.
Finally, Carbon Dioxide may be injected. CO2 is liquid-like at reservoir pressures and temperatures, and it mixes with the oil, and flows more readily through the formation, making the oil easier to produce.
Even with enhanced oil recovery techniques, we still leave a lot of oil behind. Consequently, we are always looking for safe, new, affordable ways to get a few more drops of oil out of every reservoir we drill.
Let’s review what we saw in the video:
Primary recovery refers to the first stage of recovery. During this phase natural reservoir pressure, (sometimes assisted by surface pumping) causes hydrocarbons to flow towards into the wellbore at an economical rate. As the pressure within the reservoir declines, so does the rate of primary recovery. Eventually, even surface pumping isn’t enough to draw hydrocarbons into the wellbore economically.
When primary recovery begins to decline, operators may consider implementing secondary recovery methods. Secondary recovery methods encourage the flow of hydrocarbons to the producing well by artificially raising pressures underground. For example, in the waterflood technique, water is pumped into an injection well located near the producing well. The water serves to push oil and gas towards the producing well. Similarly, natural gas and carbon dioxide are also sometimes used to provide the artificial reservoir pressure needed to push the in-place hydrocarbons in the reservoir toward the producing well.
Tertiary recovery techniques are sometimes applied after secondary recovery becomes uneconomical. These techniques encourage oil to flow by changing the physical characteristics of the oil. Most commonly, heat and chemicals are used to make the oil less viscous, encouraging flow.
We’ll come back to these in more detail in a bit.
Images: “Stages” by Jim Ladlee for Top Energy Training