A New Technique to Correct Slim Hole Through-Tubing Carbon-Oxygen Logs for Re-Invasion Effects, for Re-Invasion Effects
Authors: Al-Behair A. , Al-Hajari A. , Zainalabedin K. , Minh C. Cao 
Published: December 2007

Abstract
The slim hole, 1 11/16 in OD, Carbon-Oxygen tool (RST-ATM) has been widely used in Saudi Arabia for the last 4 years for reservoir monitoring purposes. Because of the detectors size and configuration, the tool has to be run in shut-in conditions for the obtention of C/O logs. To verify whether the tool readings are affected by re-invasion effects, shut-in and flowing Sigma logs are recorded after, and overlaid. Usually, the Sigma difference is small (less than half of a capture unit). This suggests that there is little re-invasion and the C/O logs can be interpreted with confidence. However, there are instances where the Sigma differences are large (in the order of one capture unit or more), suggesting that re-invasion effects are not negligible. Yet, the C/O logs are not corrected for this effect. The problem has existed for some time but remains unanswered because of the complexity of the subject. Considering the time and effort needed to characterize the interpretation database for re-invasion cases, or at least, running numerous Monte Carlo simulations in order to provide correction charts for re-invasion, then it is easy to understand why there is not yet a solution.
The originality of our approach is to think beyond the tool environmental correction problem (a tough nuclear physics problem) and reason from the reservoir viewpoint (a simple material balance calculation). The problem indeed has a simple elegant solution. Consider the knowns and unknowns. The knowns are C/O, Sigma shut-in, and Sigma flowing logs. The unknowns are Sw, water salinity and the saturation of the re-invading fluid, oil or water. Therefore, it is possible to solve for the 3 unknowns. Note here, we don't correct the logs for re-invasion before the interpretation (as usually done with Resistivity logs), but instead correct the result.
We have successfully applied the technique to several wells in Saudi Arabia and found that neglecting re-invasion effects can lead to saturation errors up to 20 saturation units. Also, we believe that neglecting re-invasion is one of the causes of the C/O shift that has been applied frequently in the past to get an acceptable interpretation. The technique is fast, easy to implement and should be of interest to other users of RST-A logs.

Introduction
Numerous papers about RST have been published before. Four years ago, Saudi Aramco evaluated both the slim hole, 1 11/16 in OD tool (RST-A), and the small hole, 21/2 in OD tool (RST-B). The conclusion of the evaluation was that, although the B tool can be run under flowing condition, the A tool is preferred because of its smaller size, which allows it to be run in all tubing sizes in Saudi Aramco fields. Moreover, the A tool has better statistics than the B tool, which also means that less logging time is spent for acquisition with the A tool than with the B tool. It was recognised quickly that one of the main drawbacks of the A tool is the possibility of re-invasion that might affect the reading of the tool. However, local experience with time-lapse pulsed neutron capture (PNL) logs over 30 years indicates that often, the re-invasion effect is minimal, and the A tool is used exclusively. To verify whether the C/O readings are affected by re-invasion effects, shut-in and flowing Sigma logs are recorded with the same tool 5, and overlaid. The problem is that, when the Sigma difference is significant (1 cu or more), nothing was done to account for re-invasion effect in the interpretation of the C/O data. At best, if the analyst is skillful, he might apply a shift to the C/O data to try to compensate for it. Needless to say, the procedure is subjective, and assumes that re-invasion is uniform across the zone where data is shifted. P. 171^