Cased-Hole Logging for Evaluating Bypassed Reserves
Authors: Felder R.D. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Summary.
Cased-hole formation evaluation logs are playing a major role inefforts to increase recovery from existing reservoirs. While the logs availabletoday are significant improvements over those of just a few years ago, caremust still be exercised for successful applications. Application limitationsremain, although some of these are being addressed by ongoing research.

Introduction
Current oilfield economics, while discouraging some exploration and drillingexpenditures, has also produced a growing interest in re-evaluating existingwells to locate bypassed intervals. In mature producing areas, the potentialreserve additions from bypassed reservoirs can be very large; moreover, theeconomics for recompletion can be quite favorable because of the high chance ofsuccess in adding production for moderate capital expenditures. Cased-hole logsproduction for moderate capital expenditures. Cased-hole logs are playing amajor role in locating and evaluating such bypassed hydrocarbon zones. In viewof this role, a review of the common technology, applications, limitations, andpossible future developments in cased-hole logging is appropriate.
Cased-hole logs for looking behind pipe are used in a broad spectrum ofapplications, several of which are indicated in Table 1. Although these usesrange from cement evaluation to monitoring EOR processes, for the purpose ofthis discussion our focus will be on recompletions, and specifically thelocation and evaluation of bypassed oil or gas zones.

Technology
The logs commonly used in formation evaluation behind pipe are listed inTable 2, along with the primary reservoir properties they measure and anindication of some of the other properties they measure and an indication ofsome of the other formation properties that influence their response. This listis not exhaustive, but it does provide an indication of the types of formationproperties that can be measured by common cased-hole tools. Further informationon the operating principles of these tools is readily available in theliterature.
Gamma ray logs include both conventional tools that measure total naturalgamma ray count rates and newer spectral gamma ray tools, which measure theindividual contributions of potassium, uranium, and thorium to the energyspectrum of natural gamma rays. These tools are predominantly lithologyindicators and, as such, are useful in predominantly lithology indicators and, as such, are useful in depth control, correlation, and lithology evaluation(especially shaliness). They also detect the buildup of radioactive scale, which in certain cases can indicate zones of prolonged water movement, such asperforations, watered-out zones, or channels behind pipe.
Cased-hole neutron logs are also of two primary types: the olderconventional neutron log that measures capture gamma ray count rates with asingle detector and the newer dual-detector, compensated neutron log thatpresents a log trace scaled in porosity units. These logs are good porosity, gas, and indicators, With proper calibration, quantitative estimates ofporosity and semiquantitative estimates of gas saturation can be obtained, provided that shaliness effects are -properly accounted for. Because of theirgas response. they are widely used in monitoring gas/liquid contacts. Pulsedneutron capture (PNC) logs are the primary tools Pulsed neutron capture (PNC)logs are the primary tools used in detecting oil behind pipe. Firstcommercially available in the mid 1960's, they measure the macroscopic thermalneutron capture cross section, using a pulsed neutron source. Present-daytools can be run through tubing as small as 2 in. [5.08 cm]. Because thechlorine present in salt water has a large thermal neutron capture crosssection, these logs can distinguish high-salinity formation water fromhydrocarbons, and hence can yield quantitative estimates of water saturation ifproperly calibrated. They also respond to gas saturation and properlycalibrated. They also respond to gas saturation and porosity, but theirapplications can be severely limited by low porosity, but their applicationscan be severely limited by low brine salinity, low porosity, or shaliness.
Pulsed neutron spectral (PNS) or induced gamma ray spectral logs are alsocommonly known as carbon/oxygen (C/O) or gamma ray spectroscopy logs. Theselogs, which were first commercially available in the mid-1970's, measure theenergy spectra of gamma rays from both inelastic neutron scattering and thermalneutron capture to obtain estimates of the relative concentrations ofparticular elements present in the formation. Various ratios of these elementalpresent in the formation. Various ratios of these elemental components (such asthe C/O ratio) can be used to evaluate oil saturation, lithology, and evenbrine salinity; also, count rate or component yield ratios can be used toestimate porosity.
JPT