A Technique for Locating Injected Gas Behind Casing in Oil-Bearing Formations
Authors: Wallace J.P. 
Published: December 2007

Summary
Movement of injected gas can be monitored behind pipe by using count rate data from a compensated neutron log (CNL). The method uses normalized count rates to delineate water, gas, and shale zones.
When the rescaling procedure of the near and far count rates is used, an analysis of the separation of these curves provides a qualitative determination of the distance from provides a qualitative determination of the distance from the point of injection to the top of the displaced gas in the formation, Example analyses for both gas and liquid filled cased boreholes are shown. This technique will decrease both time and expense in monitoring gas movement.

Introduction
It is desirable, when injecting gas within an oil-bearing formation, to monitor the movement of gas with time. Ideally, this should be accomplished with minimal cost and shutdown time. Modem methods include evaluating porosity computed from the CNL and/or water saturation porosity computed from the CNL and/or water saturation computed from a pulsed-neutron log (PNL). Both these methods are effective; however, each requires a neutron moderating fluid in the borehole. The expense of filling the borehole with fluid coupled with extracting the fluid after the logs have been run, and returning the well to production, is often exorbitant. Consequently, many gas-injector fields go unmonitored.
However, a standard CNL instrument can be run in a gas-filled borehole to monitor the gas movement and thus moderate many of the costs involved.
PNL PNL The neutron source for this instrument is an electronic neutron generator. This source emits packets of high-energy neutrons into the formation. These neutrons then are decelerated, by the process discussed above, and captured, releasing gamma radiation. These captured gamma rays are the input used for computing porosity and water saturation. An estimate of water salinity is necessary for the computation of water saturation.
With close inspection of the calculated water saturation and a visual inspection of the count rates, a detailed analysis can be offered and movement of gas behind pipe can be monitored properly. However, there are limitations. Low water salinities (i.e., where sodium chloride ions decrease and/or heavy mineral content increases) affect the PNL instrument significantly. Shale or fluids containing minerals with large capture cross sections (boron, etc.) tend to reduce thermal neutron concentrations. This reduction leads to inaccurate computation of porosities and water saturation. porosities and water saturation. CNL Tools
The CNL is the most widely used neutron device today. High count-rate efficiency, resulting from the detection of thermal neutrons, improves the porosity resolution. The measurement, however, is affected by lithology and borehole effects. Again, elements with large cross sections, such as chlorine and boron, will cause a decrease in neutron count rates. Since these decreases are not porosity related, thermal neutron data are often difficult porosity related, thermal neutron data are often difficult to interpret.
The near and far count rates can be used to indicate gas in a formation for both liquid- and gas-filled boreholes; however, an estimate of porosity is not possible for the gas-filled case.

Openhole Count Rate Analysis
It is apparent, under favorable conditions, that the near and far count rates of the CNL can be used to locate light hydrocarbons in much the same manner as in PNL interpretation. Most of the logging companies now provide count rate data on magnetic tape recorded in the field. The openhole neutron count rates serve as the base log for a gas monitoring program.
The openhole method begins by zonation of the formation under investigation for lithology and porosity. It is essential that lithology and porosity be consistent throughout each zone (Fig. 1).
The near and far count rate curves then are scaled so that they overlie in a clean water zone. If there is no clean zone, a 3:1 ratio of near to far should be used. This ratio works in the examples (Fig. 2). Once the scaling has been determined and the count rates have been plotted increasing to the right, gas, water, and/or shale zones plotted increasing to the right, gas, water, and/or shale zones show distinctive patterns (Fig. 3).
In gas zones, the far curve will plot to the right of the near. In clean oil zones, the far curve will plot slightly to the right of the near. In clean water the far and near counts overlie, and in shales the far plots to the left of the near. The amount of separation is indicative of the relative volumes of each.
JPT
P. 1375