Using CNL-FDC Logging To Distinguish Oil, Water, and Gas Zones-Texas Gulf Coast
Authors: Jack Robert S. 
Published: December 2007

The use of the dual-spacing neutron log (CNL) with the compensated formation density log (FDC) in a sand-shale sequence provides valuable reservoir fluid information. As confirmed by subsequent testing, this combination has been highly reliable along the Texas Gulf Coast in differentiating the reservoir fluids.

Introduction
To reduce the cost of completing new wells and of working over existing wells, it is imperative to know the poison of oil-, gas-, and water-bearing horizons in the wellbore. For many years the logging industry could assist the operators by predicting the presence of only hydrocarbons or water in a formation. The forecasts from log analysis were not so specific as desired in distinguishing between oil and gas and often differed considerably from the results obtained when the zones were tested. In recent years, because of great advances in logging techniques, actual results have more closely approached the predictions based on log analysis. The role of one of these advances, the CNL-FDC log combination, will be discussed in further detail. The use of the dual-spacing neutron log (CNL) with the compensated formation density log (FDC) in the sand-shale sequence provides valuable information regarding the type of fluid in the zone of interest. If one has a good concept of the fluid present, he can make the best economical decision present, he can make the best economical decision regarding a completion attempt and the type of completion to be made. The CNL-FDC logging combination has been very reliable along the Texas Gulf Coast in identifying the actual reservoir fluid, with identification subsequently confirmed by actual testing. In the sections that follow are cited 12 case histories of well completions in which the CNL-FDC combination was used. The cases represent fields scattered along the entire length of the Texas Gulf Coast and constitute about 15 percent of Texaco's CNL-FDC logging runs in this area. The case histories illustrate that the CNL-FDC logging combination is valuable for identifying gas/ oil and water/oil contacts, thus insuring against perforating gas- or water-bearing zones unknowingly. perforating gas- or water-bearing zones unknowingly. In the light of experience it is evident that if they differ on a specific interval, the combination log is more reliable than the sidewall core.

Application of Combination CNL-FDC Logging
Texaco uses the CNL log in conjunction with the FDC log primarily to identify the current gas/oil contact in a newly drilled well. This has been especially successful in several salt dome fields along the Texas Gulf Coast. Such fields are characterized by steeply dipping sands divided into many fault block with varying gas/oil contacts. The wells are drilled to develop the oil column, and the exact location of the gas/oil contact is very difficult to predict before drilling the well. The CNL-FDC log has proved to be a superior means of evaluating the sand in open hole. The determination of the pressure of gas or oil by this logging technique has successfully resulted in sidetracking wells to encounter the oil column.
JPT
P. 990