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• W142945517 abstract "Summary. The thermal multigate decay (TMD) logging system, which uses a two-exponential model to isolate borehole and formation effects, can be effective device for the high-accuracy environment of monitor logging. The minimized influence of borehole parameters on the formation sigma (CORR FM) results in residual oil saturation (ROS) calculations that compare favorably with expected values. The measurement of borehole parameters enables the user to determine whether proper formation flushing has been achieved and whether borehole fluid contamination of perforated intervals presents a potential interpretation problem. Test pit data and field results indicate that the use of flushing salanities less than approximately 70,000 ppm NaCl can result in significant borehole effects to the observed formation decay rates. The necessary cross section may induce systematic errors into the ROS calculations that are generally larger than any increased statistical uncertainties associated with using a smaller water salinity contrast. Introduction The TMD logging tool is designed to Measure FM, the macroscopic cross section for thermal neutron absorption in downhole formations. For many years, instruments that measure FM have been used to differentiate between water and hydrocarbons in salt water reservoirs. The ability to measure and quantify hydrocarbon saturation behind casing has allowed pulsed neutron capture logs to be used for a wide variety purposes, including locating bypassed hydrocarbons, observing the movement of hydrocarbon/ water contacts, monitoring reservoir depletion, determining ROS in a log-inject-log process, and measuring the efficiency of secondary and tertiary recovery projects, such as CO2 floods. Other pulsed neutron instruments capable of measuring FM have been described previously. While the primary objective of all these instruments is to determine FM, significant variations in the values measured have been caused by the methods used for data analysis. These FM variations have been often been induced solely by changes in borehole conditions between log runs. Borehole effects especially need to be identified and/or minimized in monitor logging applications where borehole fluids are often intentionally or unavoidably varied between logging runs. This paper will review the principles of operation of the TMD logging system, the details of which have been discussed is previous publications. The method of data analysis will be described, in particularly how the technique permits more borehole-insensitive, and yet statistically highly reproducible, FM values. The technique also allows generation of additional logging curves that are useful in measuring log quality, providing information concerning borehole contents and are indicative of water flow in and around the borehole. In addition, examples will be presented that demonstrate the use of the TMD for reservoir monitoring purposes, including applications to log-inject-log and CO2 floods. Principles of Operation A simplified diagram of the TMD system is shown in Fig. 1. As in previous pulsed neutron capture systems, the TMD uses a 14-MeV accelerator to create a time-dependent thermal neutron distribution in the vicinity of the borehole. By measuring the decay rate of the gamma rays generated from the capture of the thermal neutrons, the macroscopic cross section FM can be obtained. The TMD differs significantly from other systems in the method used to determine the value of FM. A schematic of pulse-and time-gating parameters of the system is shown in Fig. 2. The neutron source is pulsed with a fixed repetition frequency of 1,250 bursts per second with a burst width of 60 microseconds. Gamma rays are measured in the time between each neutron burst in six separate gates that collectively span almost the entire decay from the end of one neutron source burst until the beginning of the next. As shown in Fig. 2, the first two gates are opened shortly after the end of the neutron source burst and are positioned to detect gamma rays originating from the capture of neutrons in both the formation and the borehole. The next four gates are each progressively wider and at longer delay times from the source burst. These gates detect primarily formation events, with the latest gate (Gate 6) being sufficiently delayed that 3% or less of the counts measured in the gate are from the borehole region (unless the borehole is badly washed out and contains neat cement in the annulus outside the casing. At the end of 1 second of operation, the neutron generator is turned off for 60 milliseconds to measure the background count rate in each detector. The opening of the 55-millisecond background gate is delayed 5 milliseconds to avoid measuring residual gamma rays from thermal neutron capture. The background is filtered and subtracted from the dead-time corrected count rates in the six primary gates for each detector. The resultant count rates represent points on the composite borehole plus formation decay curves. These count rates are filtered over a short, vertical interval of the borehole (one to several feet, depending on filter parameters) and entered into the field computer system, which uses the six points along the decay curve in an iterative least-squares technique to separate the composite curve into its borehole and formation decay components. Fig. 3 shows the results of a calculation of the borehole plus formation components of a decay curve. The points on the curve are experimentally measured count rates, while the solid line represents the sum of the calculated borehole and formation components (the two dashed lines). Fig. 4 depicts the results of a similar calculation for different borehole and formation decay rates. The field computer calculates a formation-diffusion-corrected capture cross section CORR FM. Detectors near any pulsed source measure an artificially high apparent FM caused by a spreading of the thermal neutron cloud with time because of neutron diffusion away from the source. At long detector spacings, diffusion effects are much smaller, yielding a more accurate but more statistical FM. The CORR FM curve is computed in most formations from FM-SS by FM applying a FM-based diffusion correction derived from test formation data. FM-LS is also incorporated into the computation of CORR FM in low RN/F (i.e. low 0) formations, where diffusion effects are largest. The overall diffusion corrections used to obtain CORR FM are therefore based on FM itself, and also on RN/F (and hence formation porosity). Note that basic neutron transport theory indicates that FM and 0 are the only two variables needed to compute formation neutron diffusion length. SPEFE P. 401^" @default.
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• W142945517 date "1987-12-01" @default.
• W142945517 modified "2023-09-25" @default.
• W142945517 title "Reservoir Monitoring With the Thermal Multigate Decay Log" @default.
• W142945517 doi "https://doi.org/10.2118/14137-pa" @default.
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