If you’ve ever spent time around oil and gas operations, you’ve probably heard the term “residual oil saturation” (ROS) thrown around a lot. At its core, ROS is the amount of oil left trapped in a reservoir rock after we’ve done what we can to pull out the easy stuff—primary recovery, water flooding, maybe even some gas injection. It’s that stubborn oil that’s stuck in tiny pore spaces, clinging to grain surfaces, and it makes up a huge part of a reservoir’s total potential. But here’s the thing: if you don’t measure ROS correctly, you’re flying blind when it comes to planning enhanced oil recovery (EOR) projects, calculating how much oil you can actually get out, or even deciding if a reservoir is worth developing. Residual Oil Saturation
As someone who’s been deep in the ROS game for over a decade now (I started out in a lab running core tests, and now I’m on the supply side helping operations teams get the right tools to measure ROS reliably), I’ve seen my fair share of mistakes. Early on, I watched a team waste $2 million on an EOR project because they used a quick, flimsy ROS test that gave them numbers way too high—turns out the residual oil was actually way lower, so their injection plan was overkill. That’s why I want to break down how ROS is actually measured, what works, what doesn’t, and why the supplier side matters so much here. Let’s cut the jargon where we can, but keep it real—no textbook fluff, just the stuff that matters to folks in the field or lab.
First off, let’s get one thing straight: ROS isn’t a single number. It changes depending on the reservoir rock, the type of oil, the fluids we inject, even the temperature and pressure downhole. That’s why measuring it isn’t a “one-and-done” test. You’ve got to match the method to the specific reservoir you’re working with.
Let’s start with the most common method out there: core analysis. If you’ve ever drilled a well, you know you pull out these long cylinders of rock called cores, right? Those cores are like a time capsule of the reservoir. To measure ROS, you take a small plug of that core (an inch or two long, an inch wide) and put it in a core flood setup. That setup is basically a pressure vessel that mimics downhole conditions—we crank up the pressure, heat it to the reservoir temp, and then we flood the core with water (or whatever fluid the operation uses to push oil) until no more oil comes out. Then we measure how much oil is left in the plug.
Wait, how do we measure that residual oil, exactly? Two main ways here: retort method and Dean-Stark extraction. Let’s keep it simple. The retort method is where you heat the core plug super hot (like 600 degrees F or so) in a closed chamber. The oil and water in the rock vaporize, and we collect those vapors, cool them down, and measure the volume of oil that condenses. It’s fast—takes a few hours—and that’s why a lot of teams use it for quick checks. But here’s the catch: if the rock has any heavy oil or bitumen, some of that might not vaporize, so you get an undercount. Also, if there’s any clay in the rock, it can hold onto water that gets counted as oil sometimes.
Then there’s Dean-Stark, which is more reliable but takes longer. You crush up the core plug, mix it with a solvent like toluene, and heat it. The solvent dissolves the oil, and you separate the oil from the solvent (and water) in that Dean-Stark apparatus. This is way better for heavy oils, because you’re actually pulling the oil out with solvent instead of just vaporizing it. The downside? It takes a day or two, and you have to crush the core—once it’s crushed, you can’t test the same plug for other things like permeability later. That’s a big deal if your core samples are limited (most teams only get a few good cores from a well).
Now, if core analysis is like digging up a piece of the reservoir and testing it in a lab, there’s another method that’s 100% downhole, and that’s well logging. Well logs are the tools you lower into the wellbore to measure properties of the rock around the hole. For ROS, the big ones are nuclear logs—specifically, spectral gamma-gamma logs and nuclear magnetic resonance (NMR) logs. Let’s break that down.
Spectral gamma-gamma works by shooting gamma rays into the formation. The rock, oil, and water in the formation all absorb gamma rays differently. Oil has hydrogen, but wait—water has hydrogen too, right? So we have to adjust for that, but the key is that the amount of gamma rays that bounce back tells us how much hydrogen is there, and then we can subtract the water hydrogen to get oil hydrogen, which translates to ROS. It’s great because it’s continuous—you get ROS measurements all along the well, not just at the core plug spots. The problem? It’s less accurate than core tests, especially in messy formations with lots of different rock types or heavy oil.
NMR logs are newer, and they’re game-changing for a lot of operations. NMR measures the way hydrogen atoms spin. Water and oil hydrogen spin at different rates—water spins fast, oil spins slow. So the NMR tool can tell the difference between hydrogen in water and hydrogen in oil, and calculate how much oil is left. It’s non-invasive, gives you a profile of the entire well, and works way better for heavy oils than gamma-gamma. The downside? It’s expensive, and you have to know the right parameters for your specific reservoir (like the oil’s viscosity) to get the numbers right. I’ve worked with a team that switched from gamma-gamma to NMR for a sandstone reservoir and cut their ROS measurement error by 15%—that’s thousands of barrels of oil they can now recover, all because of a better log.
Then there are newer methods that are popping up as EOR gets more advanced, like tracer tests. You inject a small amount of a tracer (a chemical that doesn’t mix with oil, or one that binds to oil) into the reservoir, and then you track how much comes back. The amount of tracer left in the reservoir tells you how much oil is around, so you can calculate ROS. It’s super useful for whole-field scale measurements, not just single well or core plug. But tracers have their own issues—they can stick to rock, or mix with other fluids, so you have to pick the right tracer for your reservoir. A lot of teams love tracers because they can measure ROS after they’ve been injecting EOR fluids for months, which core tests can’t do (you don’t want to crush up a whole reservoir just to check ROS).
Now, here’s the part that a lot of people don’t talk about: the supplier’s role in all this. I know, I know, this is a blog about ROS measurement, not a sales pitch. But let’s be real—if you’re out there buying supplies to run these tests, whether it’s core flood equipment, Dean-Stark kits, NMR logging tools, or tracers, the quality of what you get directly impacts your ROS numbers.
For example, if you buy a cheap Dean-Stark kit that’s not calibrated right, you’ll get wrong volume measurements, and your ROS will be off. Or if you buy a tracer that’s too sticky, it’ll give you false high ROS numbers because half the tracer is stuck in the rock, so you think less oil is there than there actually is. I’ve seen teams go with the cheapest core flood pumps they could find, and those pumps can’t maintain the consistent pressure needed for accurate ROS measurements—pressure fluctuations make oil move in and out of the rock, so you end up with a messed-up residual count.
That’s why when I work with ops teams, I don’t just sell them gear— I walk them through which method makes sense for their reservoir. If they’re working with a tight sandstone with light oil, I tell them core flood with Dean-Stark is the way to go, and here’s what calibration checks they need to do every time. If they’re doing a whole-field EOR project, I’ll help them pick tracers that work with their reservoir rock type and fluid, so their ROS numbers are reliable. I’ve even set up quick training sessions for field teams on how to run simple ROS checks on site, not just rely on lab results that take a week to come back.
Let me share a recent example to make this real. Last year, a mid-sized operator in the Permian reached out to me. They were planning a polymer flooding EOR project, and their previous ROS measurements from a local lab said 28%—which would mean the EOR project could recover an extra 12 million barrels. But they were skeptical, because their own quick well log readings were pointing to 22% ROS. They didn’t want to waste $1.5 million on a project that would only get 5 million barrels, so they asked for our help.
We worked with them to run a hybrid test: we used their existing core plugs, but brought in our calibrated Dean-Stark equipment (way better than the cheap stuff the local lab was using) and ran parallel tests. Turns out, the local lab had undercounted heavy components in their oil, so their ROS was actually 21%—close to their log numbers. We also ran a quick NMR log to confirm along the wellbore, and that matched the core test. So they adjusted their EOR plan, optimized the polymer injection rate, and ended up recovering 10.8 million barrels—almost exactly what the corrected ROS numbers predicted. They would have overspent and underdelivered if they’d stuck with the wrong ROS measurement from a cheap supplier.
Now, let’s get to the mistakes I see all the time that mess up ROS measurements. First, trying to use one method for everything. A lot of teams default to retort because it’s fast, but if your reservoir has heavy oil, that retort number is garbage. Second, not mimicking actual reservoir conditions during the test. If you run a core flood at atmospheric pressure and room temp instead of 3,000 psi and 180 degrees F, your oil will behave differently—you might get way less residual oil than you actually do downhole. Third, not calibrating your equipment. I can’t tell you how many times I’ve seen a team run a Dean-Stark test without calibrating the collection tube, or a core pump that’s off by 100 psi. Small errors there turn into big ROS mistakes.
So, what does this mean for you, whether you’re an ops manager, a lab tech, or someone just getting into the oil and gas space? If you need to measure ROS, don’t just grab the cheapest test or the quickest method. Ask questions: does this test mimic my reservoir’s pressure and temperature? Does it account for my oil’s viscosity? Am I using calibrated, reliable equipment? And when you’re sourcing supplies for these tests, work with a team that actually understands ROS measurement—not just someone selling gear off a catalog.
If you’re working on a project right now and your ROS numbers feel off, or you’re just starting out and need to get your first accurate ROS measurement, reach out. We’ve worked with everything from small independent operators to major oil companies, and we help pick the right method, supply the calibrated equipment, and even train your team to make sure you get numbers you can trust. No jargon, no hidden fees, just straight talk about getting your ROS right so you can make better decisions for your projects.
Tools For Geothermal Wells At the end of the day, residual oil is the biggest untapped resource in most oil reservoirs. But you can’t get it out if you don’t know how much is there, and you can’t know how much is there if you measure it wrong. That’s why ROS measurement isn’t just a technical step—it’s the foundation of every successful EOR project, every well plan, every dollar you spend. Make sure you get that foundation right.
References
- Ahmed, T. "Reservoir Engineering Handbook." Gulf Professional Publishing, 2019.
- Coates, G.R., et al. "NMR Logging in the Petroleum Industry." Society of Petroleum Engineers, 1999.
- Standnes, D.C., et al. "Methods for Measuring Residual Oil Saturation in Core Samples: A Review." Journal of Petroleum Science and Engineering, vol. 112, 2013, pp. 101-115.
- Piggott, T.M., et al. "Tracer Methods for Residual Oil Saturation Quantification in Carbonate Reservoirs." Society of Petroleum Engineers Reservoir Evaluation & Engineering, vol. 20, no. 3, 2017, pp. 567-578.
Xi’an Sitan Instruments Co., Ltd.
Xi’an Sitan Instruments Co., Ltd. is one of the most professional residual oil saturation manufacturers and suppliers in China for 27 years, mainly engaged in providing high quality products. Be free to buy discount residual oil saturation at low price here and get quotation from our factory.
Address: No.22, Keji 5th Road, High-tech Zone, Xi’an City, Shanxi, China
E-mail: sales@sitan.com.cn
WebSite: https://www.sitanpetro.com/
