TDT(TM) Log Applications in California (includes associated paper 12555)
Authors: Wiese Harry C. 
Published: December 2007

The Thermal Neutron Decay Time TM (TDT) log has had wide applications throughout the world for many years. Recent improvements in equipment design, combined with the effect of the presence of trace elements in the low-salinity formation waters of California, have extended the TDT's use to these reservoirs.

Interpretation of TDT logs employs the dual water model concept. The dual water model concept is explained and an example of its use is shown in this paper.

The TDT logs run in California are processed through the Schlumberger Computing Center using the Cased Reservoir Analysis TM (CRA) program. The CRA program uses the dual water model. An example is shown including (1) pass 1 EDIT output. (2) parameter selection, and (3) CRA output.

Example of California applications are presented, including (1) selection of zones for recompletion, (2) location of zones of high GOR, (3) evaluation of diatomite, (4) monitoring of reservoir performance and production, and (5) logging of TDT through drillpipe.

With the introduction of the Schlumberger CyberService Units TM(CSU's) to the field. the scope of wellsiteQuicklook TM computations has increased greatly.

Recently introduced to the field is Cyberscan, TM aQuicklook wellsite computation. The process isdescribed. and an example is presented. Also shown is acomparison of the Cyberscan example and the CRAcomputed on the same well.

Introduction

The TDT log has had wide applications throughout the world for many years. Its measurement of the time rateof decay of thermal neutrons is affected primarily by chlorine in the high-salinity formation waters found in many parts of the world. Combining this effect with its ability to be run through casing has ted to its use in determining water saturation and porosity in cased holes. The TDT log's ability to detect hydrocarbons, to monitor changes in hydrocarbon saturation, and to identify hydrocarbon types in high-salinity water formations is well documented. The purpose of this paper is to demonstrate that its use can be extended into some areas of very low salinity water.

The extension of the TDT log's use into low salinitywaters (less than 50.000 ppm) is a result of severalfactors. The dual spacing tool in current use, TDT-K, hasmany features that are not available in previous tools. The use of dual detectors, "complete scale factor,"detection techniques. and improvements in neutrongenerator design have led to significantly improved measurements. Logs being run currently are repeated three times and then processed through a computer program that produces a weighted average log used in the computer analysis.

All the previously mentioned factors help extend the TDT use into the lower salinity range, but an equallyiniportant factor is the increased percentage of traceelements in the low-salinity formation waters of California. TDT-K logs run in California consistently show values of apparent sigma water higher than those predicted by sodium chloride content alone.

The fresh waters of California usually contain significant amounts of boron, lithium, and other traceelements. Fig. 1 shows a plot of formation-water salinityvs. sigma water calculated from the TDT logs run on 23wells. In the common California salinity range of4,000 to 35,000-ppm NaCl salinity, we observe sigma waters from a low of 45 capture units (c. u.) to a high of 115 c.u. While all the ramifications of this apparent anomaly are not understood fully, its practical effect is to extend the use of the TDT log to most California reservoirs. TDT log use in other areas of low-formation water salinity will depend on the presence of significant amounts of the high-capture-cross-section elements such as boron. lithium, etc.

JPT

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