What do you do when you want to measure water saturation behind casing? What if you want to know the current water saturation after years of production?
The answer: You run a sigma log or a C/O log using a Pulsed Neutron Log Tool (PNL).
The PNL tool you use to run these two logs goes by many names like RST, RPM, Raptor, RAS, PNN, RMT, PNX and many more, depending on the service providers. You can even purchase your own sigma tool and give it your name.
Regardless of the names you call the tool, in general, when you run a sigma log, you run it under a pulsed neutron capture mode. For a C/O log, pulsed neutron spectroscopy mode.
Fundamentally, the RST works just like an open hole (OH) neutron tool does where we probe neutrons in the formation. However, the RST fires neutrons at a higher energy level than the open hole compensated neutron tool. Also, instead of detecting direct neutrons coming back, the RST tool detects capture gamma ray (GR).
I think it’s worth to revisit the three neutron interactions.
The first one is inelastic neutron interaction.
(Source: SLB Oilfield Glossary)
When we send fast neutrons into the formation, the neutrons interact with the nucleus of many elements in the formation. The neutron quickly bounces off the nucleus, excites the nucleus to give off inelastic gamma rays. The characters of gamma ray count depends on the element that emitted it like Ca, O, Si, Fe and so on. This is the basis of Carbon/Oxygen mode.
It’s like when we throw a billiard ball onto a bigger bowling ball or onto a smaller golf ball, the pool ball loses energy and produces different sounds as it hits these balls.
The second neutron interaction is elastic neutron interaction.
[Source: SLB oilfield glossary]
In an elastic neutron interaction, the neutron bounces off the bombarded nucleus without exciting or destabilizing it. With each elastic interaction, the neutron loses kinetic energy. The atom that slows down neutron the most is hydrogen. There’s no gamma ray emission as the kinetic energy from the neutron is transferred to the nucleus.
Picture a billiard ball (your neutron) hitting another similarly-sized billiard balls (your hydrogen atoms). The ball will get slowed down more as it continues to hit other billiard balls. Notice that they produce similar sound when it hits each other.
The third neutron interaction is neutron capture.
[Source: SLB Oilfield glossary]
After the neutron get slowed down as slow as the surrounding matter, the nucleus absorbs the neutron and becomes excited, typically emitting capture gamma rays.
This is the basis of sigma log and porosity log.
The best neutron absorber is chlorine. In saline formation water, Chlorine elements are higher. That’s why Sigma works best in saline water.
(Actually there’s a fourth neutron interaction called the fast-neutron interaction. But the possibility for it to happen is very small to be useful).
So now we know the three main neutron interactions. Let’s talk about how the neutron interactions affect the pulsed neutron mode.
Four modes of a Pulsed Neutron Log Tool.
Basically, when we run a Pulsed Neutron Log Tool, (let’s call it RST for brevity), we want to either monitor the fluid saturation inside the formation, or the fluid movement inside the borehole.
Because the objectives are different, we run RST under different modes: Sigma, C/O, WFL or TPHL. (I am not including elemental spectroscopy in this article).
We run Sigma or C/O modes to monitor the fluid saturation in the formation. If the salinity is not known, we prefer C/O than Sigma.
We run RST under Holdup mode (TPHL) to monitor the fluid occupancy inside the borehole.
If we want to monitor the water velocity inside the borehole, then we run RST under Water Flow Velocity Log (WFL) mode to supplement our production log data.I’ve talked about
Specifically for this article, let’s talk about sigma mode.
Sigma is the capture cross section. It represents how good can a given formation capture thermal neutrons.
Sigma log is effective for water saturation calculation. Sigma water is normally high in the range of 22 to 120 cu.
In comparison, sigma matrix and sigma hydrocarbon is much lower. Sigma oil is lower around 15 to 22 cu. Sigma gas is the lowest between 1 to 12 cu. Sigma matrix like dolomite, limestone and sandstone normally have sigma values of less than 10.
So the whole idea is, the higher your sigma, the higher your water saturation is.
How to calculate water saturation from sigma.
This is the formula:
In terms of time lapse, provided that the sigma water is the same, you can use this equation:
If you have different water salinities between your injected water and your formation water, then you have to use the full equation individually.
How do you detect gas from Sigma mode?
Here are the rules:
1.Gas has the lowest sigma. So whenever you see sigma log deflecting to the lower value, then you probably got gas in your well.
2. Remember that gas is neutron’s enemy number one. Thermal neutron porosity will decrease in gas.
3. TRAT will decrease in presence of gas when compared to your calculate effective porosity.
4. Inelastic count indicator will increase in gas.
4.In case of SLB’s RST, RSCF, RSCB, IRAT and WINR will increase.
When do sigma doesn’t work?
1. Sigma mode might not work if the water salinity is too low. When water salinity is low (normally below 40 kppm), sigma water becomes too low that we couldn’t distinguish between sigma water and sigma oil.
2. If we have different water salinity of injected water and formation water, you have to be really careful of the sigma water to use.
3. If we use acid (like HCl) to stimulate our reservoir, then we are creating extra porosity and extra Cl- in the formation. Porosity and sigma will increase that increases your resultant water saturation.
4. Gas in the wellbore will severely affect your sigma values.
So, that’s how Sigma works. Now, you can use it to calculate water saturation behind casing.
Even though sigma mode is quite forward, complex reservoir behaviours, changes in reservoirs conditions, and multiple completion types could make the sigma evaluation harder.
2. NExT Cased Hole Logging Course, Schlumberger.