Mature fields account for more than 70% of production today. In addition, waterflooding and EOR, including CO2 and steamflooding, have become more prevalent to improve recovery of assets. It is estimated that more than 75% of the reservoirs being flooded have heterogeneities that affect recovery efforts. Monitoring is crucial for optimizing recovery efforts; however, these secondary and enhanced recovery methods change the formation water salinity and introduce new fluids into
the pore spaces.

Pulsed neutron offers measurements to overcome these challenges to obtain corrected oil saturations. It can provide three independent measurements, which can be used to determine three saturations. These measurements for sigma, carbon-oxygen and saturation gate (SATG) saturations can be used to calculate water, oil and gas saturation.

Halliburton’s Reservoir Monitor Tool 3-Detector (RMT-3D) pulsed-neutron tool can do this in one trip in the hole. This provides the ability to uniquely solve simple or complex saturation profiles in reservoirs while reducing phase-saturation interdependency. This information can be used to monitor reservoirs, increase production, improve recovery and find bypassed pay.

Sigma-derived saturations
Saturation analysis from sigma is the oldest and most traditional of the three saturation methods and can be used for oil vs. water or gas vs. water saturation in known salinity formation waters. Sigma has a large dependency on salinity and has a reduced dependency on lithology. As salinity and volume of the formation waters increase, so does sigma (Figure 1).

In essence, sigma is a measurement of the rate of neutron decay in the formation, observed by monitoring associated gamma ray activity. Salinity is one of the largest contributors to neutron capture, increasing sigma. Hydrocarbons have lower neutron capture characteristics, indicative of a lower sigma. With representative sigma, porosity and shale volumes, a material balance equation can be used to accurately calculate water and hydrocarbon saturations.

Carbon-oxygen-derived oil saturations
Carbon-oxygen saturation, unlike sigma saturation, has no dependency on the formation water salinity, which is ideal for low, unknown or mixed formation waters, but it only works for oil vs. water saturation. Carbon-oxygen saturation measures the elemental yield ratio of carbon to oxygen. Oil has a lot of carbon and thus a high carbon-to-oxygen (carbon-oxygen) ratio, while water has a lot of oxygen and a low carbon-oxygen ratio (Figure 2).

Carbon and oxygen also are found in the lithology, and this formation component has to be accounted for to obtain accurate oil
saturations. Calcium and silicon elemental yields are processed to create a Ca/Si lithology ratio (LIRI). The LIRI value distinguishes sandstone from carbonate, which has high inorganic carbon content, and must be corrected for a proper hydrocarbon calculation.

SATG-derived gas saturations
SATG gas saturation utilizes a ratio of inelastic-to-slow capture counts using the RMT-3D tool’s long 1-m (3-ft) detector to increase the sensitivity to gas while at the same time reducing both salinity and lithology dependency. In addition, due to its convex fan saturation chart, SATG gas saturation works well at low-to-medium porosities. SATG gas saturation is sensitive to gas vs. fluid saturation where oil, low-salinity water and high-salinity water all appear similar (Figure 3).

Three-phase saturation
For high-salinity formations where formation water salinity is both high and known accurately, sigma saturation and carbon-oxygen saturation can be combined for water, oil and gas saturations. This has been available for many years. Fields with low, unknown or mixed salinity could be monitored using the carbon-oxygen-derived oil saturations.

As fields mature and EOR projects are used to increase production, they should be monitored to maximize recovery. IOR projects typically involve waterflooding, CO2 flooding, steamflooding, nitrogen and/or acidization. Formation water salinity quickly becomes unknown with any certainty, and with that sigma saturation becomes unreliable.

The carbon-oxygen service works well in flooded reservoirs for determining oil saturations; however, in the presence of gas, it requires correction to account for the pore volume displacement. Previously, in one logging trip in the hole, it was difficult to do threephase saturation in a cased-hole environment for unknown formation salinity. The SATG gas saturation fills this need for accurate gas vs. fluid saturations.

SATG and carbon-oxygen are the solution for threephase saturation in low, mixed or unknown salinity environments. SATG gas saturation is used to solve the gas volume fraction or components that mimic gas properties that have a low hydrogen index. Carbon-oxygen can be used to solve for oil vs. water but must be corrected for the volume of gas. By combining SATG gas vs. fluid and carbon-oxygen oil vs. water, there is now a solution for the three phases.

Permian case study: CO2 flood
In a Permian Basin well in West Texas, the formations are carbonate with porosity ranging from 10% to 20%. The well has been repeatedly acidized and is currently in an active CO2 fl ood. There is openhole logging data and newer cased-hole pulsed-neutron logs consisting of sigma, carbon-oxygen and SATG measurements.

Pulsed neutron can provide a good porosity measurement if openhole porosities are not available or have changed. For neutron-type porosity, a ratio of near-to-far capture counts was used, which is sensitive to the hydrogen index. For density-type porosity, a ratio of near-to-far inelastic counts was used, which is sensitive to the atomic density of the formation and fl uids. These were calibrated above where the well was acidized. Figure 4, Track 2 shows the openhole total porosity in blue compared with cased-hole total porosity in red. The yellow shading highlights the increasing porosity due to the acidization.

Traditional sigma decay saturation depends on high and known formation water salinity to work accurately. Years of waterflooding, acidization and CO2 flooding have left the original water saturation unknown.

A CO2 flood introduces challenges since additional components of carbon and oxygen enter into the analysis. For carbon-oxygen saturation to work accurately, it is important to quantify and remove the CO2 volume from the carbon-oxygen measurement.

SATG gas saturation will compute the CO2 saturation and volumes with respect to fl uid.

The SATG volume of CO2 is then subtracted from the carbon-oxygen equation to solve the correct amount of oil vs. water, providing both the volumes and saturations for water, oil and CO2.

Log Tracks from Acidized Well

FIGURE 4. A triple saturation evaluation was performed using SATG and carbon-oxygen: Track 1, lithology; track 2, openhole and cased-hole total porosity, with the difference due to acidization highlighted in yellow; track 3, gas and fl uid saturations; track 4, uncorrected oil and water saturations; track 5, corrected volumes; and track 6, corrected saturations. (Source: Halliburton)

Acknowledgment
This article is based on SPE 165230.

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