Abs tra c t Several schemes have been described for removal of the capture gamma-ray contribution during a neutron burst in order to obtain a clean inelastic gamma measurement. However, even a clean inelastic count-rate ( CR) measurement -while substantially dependent on the gamma transport properties of the formation -still possesses a sizeable residual neutron transport effect. M onte-Carlo modeling has been used in the proj ect reported here to better understand the physics of the neutron-gamma transport problem. M onte Carlo modeling has also been used to develop a correction technique that leads to a compensated inelastic ratio dependent almost solely on the gamma transport ( density) properties of the formation surrounding the tool. This paper will discuss how a modeling database was constructed and how it has been used to develop the correction technique. An extensive database of modeling results validates the proposed technique.I n addition, since this technique does not employ any new measurements from the PNC tool ( it uses existing near and far detector inelastic and capture CR' s) , any existing field PNC log can be used to test the process where a suitable openhole density-neutron log is available. Several log examples demonstrate a reasonable correlation between this new PNC density technique and an OH density log. Introduc tionVarious physical arguments point to the inelastic gross countrates in the detectors of PNC/ PNS tools as having significant sensitivity to formation density. Thus, some measure of formation density in cased-holes ( CH) may be obtained by proper analysis of these inelastic count rates. Several schemes have been described for removal of the capture gamma-ray contribution during the neutron burst in order to obtain a clean inelastic gamma measurement. 1-5 However, as will be shown here, even a clean inelastic count-rate measurement -while substantially dependent on the gamma transport properties of the formation -still possesses a sizeable residual neutron transport effect. M onte-Carlo modeling has been used to better understand the physics of the neutron-gamma transport problem and to develop a correction technique that leads to a compensated inelastic ratio dependent almost solely on the gamma transport ( density) properties of the formation surrounding the tool. An extensive database of modeling results validates this correction technique.This correction technique employs measurements that have been on logs for quite some time, thus allowing evaluation of the technique on previously run field logs. A couple of examples will be examined where openhole ( OH) neutrondensity logs were available for comparison. Generally, a good correlation is observed; but it must be recognized that any CH density measurement will necessarily be somewhat inferior to an OH measurement because of the complexities and uncertainties of the CH environment. Precise information on tool standoff, BH size, cement quality, and casing corrosion may not be available when the CH log is run. Variation in thes...

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The Burgos Basin, located in Northeastern Mexico, has sometimes been very difficult to evaluate with openhole wireline tools because over-pressured formations present a problem when trying to reach a balanced borehole condition. This paper will discuss three problem wells, presenting both CHI Modeling results used to solve the problems as well as an alternative method of obtaining resistivity behind casing. Case History #1 looks at the Fundador Field which has been a particular problem in this area, presenting both drilling and logging difficulties. Because of borehole problems associated with over-pressured formations encountered in this well, it was not possible to acquire openhole wireline logs across the intermediate drilling zone. After the well was completely drilled and cased, a pulsed neutron tool was run across the entire well to assist in the evaluation. By using the resistivity data acquired in the previous section of openhole along with the pulsed neutron data logged in cased hole across the same interval, the missing resistivity data was predicted over the entire intermediate zone. The pseudo resistivity log was created using only the pulsed neutron data obtained across the intermediate zone, processed with the CHI (Cased Hole Interpretation) Modeling program. The final resistivity is also compared to a second method of obtaining resistivity behind casing used by another service company. Case History #2 uses CHI Modeling to create pseudo openhole logs that were subsequently used as inputs to a volumetric evaluation. The CHI Modeling results are compared to an alternate method of obtaining resistivity behind casing. Testing results are documented to confirm the validity of both the CHI Modeling and the volumetric evaluation. Case History #3 documents where the CHI Modeling technique was successfully applied through two strings of casing where density data was not available. A comparison to the original openhole logs is presented confirming that this technique is a valid option when additional openhole information is required in a partially logged well. This paper will discuss the procedures used to evaluate three case histories using CHI Modeling, and present two comparisons with a second cased hole resistivity measurement. Production data is also presented to validate this technique. The use of pulsed neutron logs to acquire pseudo openhole data is shown to be a valid alternative when drilling conditions do not permit normal data acquisition in openhole; or when additional openhole information is required after the well has been cased. Neural Networks The oilfield has traditionally employed polynomial equations to solve for missing wireline data curves. However, this algorithmic approach is not very dependable because the calculation uncertainties are extremely large under some conditions. Because of these large uncertainties, the CHI Modeling[SM] program was written by Halliburton, using neural network software architecture, to create pseudo open hole curves using only cased hole data.

The Veracruz basin, located in eastern Mexico, has sometimes been very difficult to evaluate with openhole wireline tools because over-pressured gas formations present a problem when trying to reach a balanced borehole condition. One of the fields was planned for development using nearly horizontal wells to maximize production, but acquiring openhole logs in these wells has proven to be difficult. Well A-21 is the first highly horizontal well to be completed using only LWD resistivity data in the nearly horizontal section. To assist in the evaluation after the well was completely drilled and cased, a pulsed neutron tool was logged across the entire well using a tractor device to reach the total depth of the well. An evaluation model for the A field was then developed using the CHI (cased hole interpretation)modeling program, by using the openhole resistivity and porosity data acquired in the vertical A-1 well, combined with the cased hole pulsed neutron data acquired across the same interval. Pseudo resistivity and porosity logs were then created for the A-21 well using only the pulsed neutron data acquired across the nearly horizontal section of the well, based on the Chi Model developed for the field. The pseudo resistivity was then compared to the LWD resistivity data acquired in the A-21 well. Next, the interpretation was completed by defining the optimum perforating interval for the reservoir conditions and the mechanical condition of the well. After evaluating the interpretation, the On-Balance perforating technique using coiled tubing was decided upon as the optimum technique to perforate the A-21 well, to minimize reservoir damage. This paper will present the procedures used to evaluate and complete the A-21 and the A-31 wells, as well as a comparison of the CHI Modeling pseudo resistivity with the LWD resistivity measurement. The use of pulsed neutron logs to acquire pseudo openhole data is shown to be a valid alternative when drilling conditions do not permit normal data acquisition in openhole. The integration of the data obtained, along with applied reservoir geomechanics, for perforating design and production planning is shown to be a valid alternative to maximize production, and to prevent sanding and completion problems while reducing costs. Introduction Highly deviated wells have traditionally required the use of either logging-while-drilling (LWD) or tool pusher techniques to acquire the information needed to perform formation volumetric evaluation. This evaluation is used to determine the best intervals to be completed. A new technique is currently available that can be used in development fields to optimize drilling and completion cost while minimizing the risk of getting stuck with a set of LWD tools. Sticking LWD tools downhole can lead to a complicated recovery of the drilling string. In addition, because they contain chemical radioactive sources, LWD porosity tools left in the well present further complications. The innovative technique presented in this paper includes an application of pulsed neutron log data to acquire pseudo openhole data after the completion of the well. This acquisition can even be accomplished without a drilling rig, which further minimizes both the risk and the cost of logging the openhole section. This paper presents the theory behind the technique and the process required to reach a result. It also includes techniques currently used to acquire the information in cased hole, such as wireline logging and perforating using either coiled tubing or the wireline tractor tool.

Traditionally, all major service companies have had an openhole spectral gamma tool in their arsenal that will measure the naturally occurring potassium (K), uranium (U), and thorium (Th) radioactivity surrounding a borehole. KUTh data are used in various ways, such as subtracting out high API uranium indications from total API Gamma Ray readings in radioactive formations and using the relative percentages of these elements as input to proprietary mineral calculation programs. Recently, a standard C/O logging tool with the neutron generator turned-off was used for cased-hole KUTh logging with good results. Although this pulsed neutron spectrometry (PNS) tool was a slim tool (2 1/8-in. diameter), the log quality was comparable to that achieved by earlier, large diameter gamma spectrometry tools because this particular PNS tool utilized a high density scintillator. This experience clearly demonstrated that reliable spectral gamma measurements can be made in cased hole. Calibration with a standard thorium calibration sleeve allows the operator to set the spectral gain of the system (using the Th peak); and this value of gain is used during the logging operation. After logging, the data is reprocessed with software that tracks and corrects any gain drift due to temperature producing the final corrected log. Concentrations of K, U, and Th are extracted using a weighted-least-squares (WLS) fit of standard elemental (basis) spectra to the log data during the re-log phase. Measurements made in this way can readily precede the standard formation evaluation run in a well and use essentially the same instrumentation and thus replace traditional openhole KUTh measurements. The enhanced design and the software associated with it provide high quality formation evaluation information; and this information can be made available with no associated drilling rig cost if the tool is run going in the hole. This paper will show field examples of KUTh data. The paper will also discuss procedures necessary to obtain quality measurements, will point out various savings that can be realized by an operating oil company using the new design and the information it gathers, and will present the interpretation procedures necessary to obtain a final product. Introduction Openhole spectral gamma-ray (SGR) tools have been available for several decades and measure the naturally occurring potassium (K), uranium (U), and thorium (Th) radioactivity surrounding a borehole. KUTh measurements made with a pulsed neutron spectrometry (PNS) tool can readily precede the standard PNS formation evaluation run in a well while using essentially the same instrumentation. The PNS tool can thus replace traditional openhole KUTh measurements. Knowledge of these elements allows the analyst a more detailed stratigraphic analysis, and depth correlation, than can be obtained with conventional through-tubing gamma ray tools. By using the KUTh analysis, the analyst can also better differentiate lithology, as well as identify the type and volume of clay contained in the logged interval.[W3] Technical innovation has always been critical to the optimization of mature, sometimes declining fields. Recent high oil prices, coupled with possible supply shortages, are forcing operators to continually look to these existing fields as a potential source of increased production. As production water flow increases in the permeable zones of these wells, it tends to deposit uranium salts around the perforations, on the casing, inside of the production tubing, and also in the formation itself. Such deposition can make formation evaluation difficult for even the most experienced petrophysicist. Uranium can also indicate a fractured interval when combined with a low potassium and thorium content. Recognizing clay types to optimize infield drilling procedures -and drilling mud types - is also very helpful. All of these interpretation problems are now solved much easier by using the new through-tubing pulsed neutron KUTh log and its interpretation procedures. KUTh information can also be combined with a photoelectric measurement, as an additional indicator, to aid in identifying lithology and clay typing.