Monitoring Gas, Oil, Fresh Water and Saline Water Saturations with a Single Tool
Authors: Petricola M.J.C. 
Published: December 2007

Summary
In the saline formation water environment of the Middle East, TDT* thermal decay time logging has proved very reliable for the detection of formation water level movement. This technique has also been widely used to monitor injection water front, in cases of saline injection water.
However, many injection schemes in the area are now using relatively fresh sea water, rather than saline formation water, as the displacing fluid. In these cases, thermal decay time logs are very difficult to interpret due to the lack of contrast in the capture cross section between oil and injection water.
TDT logs have also been widely used for gas detection behind casing, and in some favorable situations, a reliable gas saturation can be obtained from the TDT porosity.
A new type of slim induced gamma ray tool, which measures carbon and oxygen and can be run through completion tubing, was introduced recently. The RST* Reservoir Saturation Tool, which is run inside the casing below the wireline entry guide, does not require killing the well ad removing the completion before logging. This means that data can be acquired with the well flowing. The main measurement is the carbon/oxygen ratio (COR), which can be used to compute oil and water saturation, independent of water salinity. The tool also provides a measurement of capture cross section, and therefore waiter salinity analysis can be performed in case of complex mixing of formation and injection water. Finally, the RST tool also provides a cased-hole neutron porosity, and can therefore be used for gas evaluation.
This paper presents a new technique to simultaneously use carbon/oxygen ratio, capture cross section and TDT/RST porosity for evaluating the respective saturations of oil, gas, fresh water and saline water when they are found together in the reservoir.
An example is used to illustrate the technique and its successful application in a typical reservoir from the Middle East.

1. Introduction
TDT campaigns are conducted yearly in the Middle East, to monitor the year-by-year changes in fluid saturation in the reservoirs. More widespread use of this measurement has been hampered by difficulties in interpreting the results in some specific conditions. Well-known adverse effects are the so-called "filtrate effect" and "acid effect". Recently, however, one additional difficulty has further complicated TDT interpretation. Many water injection schemes use sea water as the displacing fluid. Sea water has a capture cross section of about 35 c.u., which is much closer to the capture cross section of oil than to that of formation water. Therefore, if oil is displaced by injection water, the change in sigma is very small. Very often, injection water displaces both oil and formation water, possibly leading to a reduction in sigma reading and an apparent decrease in water saturation.
Even when sigma increases, only a very qualitative evaluation can be performed. An algorithm that accounts for the presence of both formation water and injection water has been used with some success, but it is based on the assumption, which is not always valid, that the original irreducible formation water has not been displaced by invasion water.
The TDT tool is also used for gas monitoring. This is achieved by simultaneously using TDT porosity and sigma measurement. Although porosity measured by the TDT tool is affected by many parameters related to the surrounding environment, tubing packer, casing, cement, etc., a reliable gas detection and acceptable gas saturation computation can be performed whenever gas-free zones are available for calibration in the same environment.