For better understanding of topic we highly recommend our readers to read introductory article of our pulsed neutron article series Cased hole formation evaluation. Kindly also go through articles on Pulsed neutron physics and pulsed neutron detectors for sound understanding of topic.
As already discussed in previous articles Pulsed neutron generators produce 14 Mev neutrons and these neutrons emitted by generators interact with formation and borehole environment to provide useful information about formation properties on the basis of hydrogen index, bulk density and lithology. As we have already discussed in article on pulsed neutron physics, Neutron goes through three major interactions after getting released from generator 1. High energy inelastic collision, 2 Elastic Collision & 3. Neutron capture decay time collision.
High energy inelastic collision is dominated by heavier elements found in rock matrix such as calcium, silicon and oxygen. Low porosity / high amount of matrix /high formation density leads to neutron losing its energy fast and coming down to elastic energy level in less distance travelled in formation matrix whereas high porosity /less amount of matrix /lower formation density leads to high energy inelastic neutron travelling more distance comparatively to lose enough energy to come down to elastic energy level. Inelastic count rate can be defined as below
NInel = AInel * e –ρµ LInel —————– Eq1
Where NInel is the inelastic count rate, AInel is total inelastic neutrons, ρ is formation density, µ is formation mass attenuation coefficient, and LInel is the attenuation distance of inelastic gamma ray to detector
Slowdown of neutron at elastic energy level is dominated by amount of hydrogen present in environment as hydrogen being single proton is having highest slowdown cross-section of neutron. Hydrogen is majorly present in fluids with water and oil having higher hydrogen index and gas having lower hydrogen index. Neutron slowdown keeps on going till it reaches thermal energy level and gets captured. Capture count rate can be defined as:-
Ncap = Acap* e –ρµ Lcap ————————Eq2
In Eq2 Ncap is the capture count rate, Acap is total neutrons reaching capture energy level, ρ is formation density, µ is formation mass attenuation coefficient, and Lcap is the attenuation distance of released gamma ray to detector as a result of neutron capture.
As we discussed above, capture count rate and inelastic count rate are complex function of hydrogen index and porosity. Green cylinder in above figure represents area of hydrogen index effect, Higher the hydrogen index smaller the green cylinder and hence larger attenuation distance to detector. This causes both inelastic and capture counts to decrease. On the other hand greater hydrogen index could be related to higher porosity or lower formation matrix which causes magenta circle to grow bigger and hence decrease in inelastic and capture counts. These two effects work against each other and hence porosity and hydrogen index needs to characterized as per inelastic and capture count rate ratio. Characterization of inelastic to capture ratio with porosity and hydrogen index leads to gas identification on the basis of hydrogen index. This technique has only has one drawback, due to lower hydrogen index of depleted reservoirs this technique shows high amount of gas in depleted reservoirs.
In three detectors pulsed neutron tool both inelastic and capture count rates are recorded from third detector placed at optimal distance from source to increase hydrogen index sensitivity and reduce matrix sensitivity.
Inelastic to capture count ratio is also influenced by formation water salinity and hence Halliburton in their characterizations uses ratio between inelastic counts to slow capture counts known as SATG. Inelastic to slow capture counts have very little affected by formation water salinity as per laboratory measurement conducted by them.
Weatherford in their Raptor tool interpretation differentiate between gas and liquid by using multi detector pulsed neutron burst (MDPN NB). Interpretation is based on characterization of fast neutron normalized burst and capture ratio acquired from nearest and farthest detector. Characterization is done by “3D neutron gamma transport modeling”. Data is characterized with formation porosity to prepare interpretation envelope in which data should respond.
Schlumberger in their multi detector pulsed neutron tool “pulsar” uses fast neutron cross-section (FNXS) to quantify gas saturation. FNXS is defined as ability of formation to interact with fast neutrons. As already discussed high energy inelastic neutron goes through three types of collisions which are high energy inelastic collision, elastic collusion and thermal capture. High energy inelastic collision is dominant in 14 to 8 mev energy level. Element having highest cross section to high energy inelastic collusion are C, O ,Ca ,Si and elastic cross section especially at lower energy level such as 100Kev is mainly dominated by hydrogen index (HI) of formation. It has been observed that at 14 Mev energy range elastic scattering cross sections of many elements merge to have close values and inelastic cross sections of different elements also merge to have close values which is around half of elastic cross section values with only exception of hydrogen which don’t have any value of inelastic cross section at 14 Mev.
Due to physics of measurement direct measurement of fast neutron is not possible and hence inelastic gamma ray produced as a result of inelastic neutron interaction with formation is recorded. As already discussed in earlier part of this article high amount of matrix with heavier elements leads to higher neutron inelastic cross section resulting into more neutrons getting attenuated and hence more inelastic gamma ray getting generated to be transported to detector. During gamma rays travel to detector gamma ray gets attenuated by pair production, comptom scattering and photo electric effect. All these attenuating mechanisms are mainly dominated by formations bulk density and effective atomic number.
Measurement of inelastic gamma ray counts at 14 Mev energy range is by itself very challenging task, to maximize the signal following things are done:
On the basis of recorded inelastic gamma ray counts a Gas ratio (GRAT) channel is created. This GRAT channel is corrected for capture background and also normalized. Further corrections for completions and hole size is applied ,data offset is also applied if needed to finally calculated Fast neutron cross section (FNSX).
Below table shows Sigma and FNSX values of different materials found in borehole. We can clearly see in below table that Gas with lower hydrogen index is having very low values of FNSX.
Multi-detector pulsed neutron tools have been developed with sole purpose of gas- liquid differentiation. Measurements provided by different service providers are slightly different but are all highly sensitive to gas.
Gas identification using multi detector pulsed neutron tool is a vast topic itself. To make this topic concise we have only kept most important parts in current article. To discuss this article with author please feel free to write to us at firstname.lastname@example.org.