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Hydrocarbon saturation determination in cased hole plays an important role in reservoir management. Hydrocarbon saturation estimation over the period of production time helps in making plan for workover operation, understanding reservoir depletion, diagnosing production problems and even making strategies for enhanced recovery plans.
Pulsed neutron tools are used for hydrocarbon saturation evaluation in cased hole, these tools works mainly in three modes for hydrocarbon saturation estimation as given below:
1. Thermal decay time capture mode: Based on neutron induced capture gamma decay time measurement. Primary measurements in capture mode are capture cross-section of formation or Sigma and capture count rate from near and far detector. Other measurements are near to far capture count rate ratio and various capture yields. Data from this mode is used for Water saturation evaluation in saline formation water. As a thumb rule, this method works well in formations with water salinity higher than 35Kppm. Depth of investigation in this mode is 12 to 15 inches.
2. Carbon-Oxygen inelastic mode: Primary measurements in inelastic mode are Carbon-Oxygen ratio, Calcium-Silicon ratio, inelastic count rate and various inelastic elemental yields from both near and far detector. Other measurements are near to far inelastic count ratio, inelastic to capture ratio. Data from this mode is used for oil Saturation evaluation independent of formation water salinity. Depth of investigation in inelastic C/O mode is 6 to 8 inches. To get data with high certainty it is recommended to do 3 to 5 passes at logging speed of 3 ft/min or less if possible in this mode.
3. Gas saturation mode using third detector: In this mode Gas saturation evaluation is done using inelastic to slow capture ratio /fast neutron from third detector. Acquired data from third detector differentiates between gas and liquid. Conversion of inelastic to slow capture ratio /fast neutron into gas saturation is done through characterization of data. Depth of investigation in gas saturation mode is 12 to 14 inches.
In a freshly drilled well, openhole logs such as Gamma Ray, Neutron, Density, Sonic and Resistivity are run and these open hole logs are used for hydrocarbon saturation estimation. Saturation evaluation using mentioned openhole tools also act as base case for estimation of sweep efficiency/depletion.
Once hole is cased, lots of open hole tools most importantly resistivity does not work and hence we have to rely on pulsed neutron tools which can see through casing. As mentioned above there are three modes available in pulsed neutron tools for saturation estimation as discussed below:
The thermal decay capture mode of pulsed neutron tool looks at neutron absorption by the formation also known as capture cross section or sigma of formation. Chlorine has high capture cross-section and hence higher salinity of formation water results into higher capture cross-section. Capture cross-section has lower value of around 22Cu in case of fresh water. Oil also has same capture cross-section as fresh water in the range of 18-22capture units depending upon gas oil ratio, where as gas has very low capture cross-section depending upon reservoir pressure. Due to the above mentioned reasons decay time measurements provide reliable water saturation estimation in saline formation water, higher porosity further increases confidence in saturation estimation. As a thumb rule we can take porosity in Pu multiply it by salinity in Kppm and if answer is higher than 1000, Saturation estimation using sigma will be reliable. Value between 500 to 1000 is a grey area and sigma will not work if value is less than 500.
If formation water salinity is low or unknown, Carbon-oxygen ratio method can be used for saturation evaluation. Only issue with Carbon-Oxygen method is that it can provide oil saturation but it is blind to gas. Due to low carbon density in gas, Carbon-Oxygen ratio method is not able to see gas. Carbon-Oxygen ratio is recorded in inelastic energy range which is higher than 8 million electron volts (Mev).Recording data at such a high energy level is itself a big challenge hence detector needs to be dense, with high effective atomic number, high energy resolution and having faster decay time. Apart from having good detector pulsed neutron source also should be capable of producing higher number of neutron per second. We will discuss pulsed neutron source and detector in a separate article. For confident oil saturation estimation in carbon-oxygen mode porosity has to be higher than 12pu.The reason for need of high porosity in this mode is dynamic range of chart used for interpretation is small at low porosity, this leads to high uncertainty in oil saturation estimation.
Third mode for saturation evaluation in pulsed neutron tool is gas saturation evaluation using inelastic to capture ratio recorded by third detector of pulsed neutron tool. This mode is not available in normal two detector pulsed neutron tools. This mode is only available in three detectors pulsed neutron tools offered by different vendors. Sensor which records the gas data in these three detector pulsed neutron tool is strategically placed at gas sensitive distance from detector to record data highly affected by hydrogen index of formation, some tools record inelastic to slow capture ratio and some record fast neutrons but basically it’s a hydrogen index estimation.
As we can see data in all three modes are recorded in different ways and are having different limitation and benefits. Infact all three modes estimate fluid saturation in different ways. Carbon-Oxygen method only looks at oil, Third detector inelastic to capture ratio only looks at gas and capture sigma looks at water saturation irrespective of hydrocarbon type in saline formation water. Data from all three modes or from two modes can be combined in a software workflow to do a three phase fluid saturation interpretation.
In the next article of pulsed neutron tool series we will discuss in details Physics of pulsed neutron measurement, detector crystals and pulsed neutron source offered by major vendors and their advantages/disadvantages. Finally we will conclude this series with Pulsed neutron interpretation.