Carbon-oxygen logging is used primarily to estimate oil saturation in cased-hole conditions when the formation water is fresh or unknown. The drawbacks of current techniques are: 1) slow logging speed, 2) large tool diameter, and 3) excessive sensitivity to borehole fluid composition. A new, slim, neutron-induced gamma ray spectroscopy logging system has been developed to overcome some of these limitations. The new logging service is called the Reservoir Saturation (RST*) Tool. Initial field tests are being carried out in the Middle East.The RST tool uses multiple detectors to separate the signal contributions from the borehole and the formation. Therefore, even when the borehole fluid composition is unknown, oil saturation can be determined in addition to the borehole oil fraction. This presents the possibility of logging flowing wells, which ensures that reinvasion and crossflow will not affect the results, and eliminates the costs of well preparation.A reduction in sonde diameter, without sacrificing logging speed, has been made possible through the use of specially developed technologies in the areas of scintillation materials; photomultiplier tubes; pulsed neutron generators; pulse acquisition circuits; and highdensity, digital, signal-processing electronics. The new tool fits through fro duct ion tubing and operates at temperatures to 150 C. Since no Dewar flasks are used, there is no limit on how long the tool can stay in the well.
Oil production from many major producing reservoirs is now maintained by water injection programs. The resulting mixed and often very low-salinity formation water presents a serious problem when monitoring water saturation with traditional pulsed neutron capture (PNC) tools. A slim carbon/oxygen (CAD) pulsed neutron tool tested recently in many regions around the world addresses the problems encountered in low- and mixed-salinity environments. The tool's dual y-ray detectors and flexible neutron burst timing allow it to be used for carbon/oxygen logging and, in a separate pass, for PNC logging. New detector and accelerator technologies as well as advanced data processing allow logging speeds that meet or exceed those of traditional, large, single-detector C/O tools.
The dual-burst thermal-neutron-decay-time (TDT™) tool brings two enhancements to pulsed-neutron capture logging. The first is a realistic physical model of pulsed neutron decay curves that accounts explicitly for the effects of neutron diffusion and decay in both the wellbore and the formation. The second is the dual-burst system itself, which permits excellent statistical precision with minimal dead-time losses. This paper discusses the physics of the model, operation of the tool, important mathematical considerations for optimum use of the tool, and a demonstration of the tool performance in a laboratory simulation of a log-inject-log (LIL) operation.
Recently, two new slim through-tubing carbon-oxygen tools were introduced and described in the literature. In addition to carbon-oxygen water-saturation capability, these new instruments can provide, during the same trip, pulsed neutron capture (PNC) measurements (formation sigma, porosity, borehole fluid salinity, etc.). The answers have improved accuracy and precision compared to prior logging instruments. This paper describes the Dual-Burst* PNC measurement scheme for the RST* Reservoir Saturation Tool, as well as the algorithm methodology for determining corrected sigma, porosity, borehole fluid salinity, log quality and other associated outputs. Also documented are database measurements on which the algorithms are based, accuracy benchmarks in industry-standard calibration facilities, and precision (repeatability) comparisons. Log examples are presented. Diffusion, borehole and lithology effects must be considered when transforming observed quantities such as decay times or near-to-far ratios to actual physical quantities. However, these effects are difficult to account for in direct analytical approaches over the entire range of oilfield conditions. Therefore, a multidimensional dynamic parameterization technique, based on an extensive set of laboratory measurements, has been developed and refined. This technique keeps the order of parameters low, resulting in a well-behaved response both inside and outside the range spanned by the database. The supporting database for each tool includes over 1000 measured points augmented with approximately 400 modeled points spanning different lithologies, porosities, borehole sizes, casing sizes and weights, formation fluid salinities, and borehole fluid salinities typically encountered in the oilfield. Also reported are the sigma and porosity accuracy benchmark measurements made in the industry-standard calibration facilities (EUROPA facility, Aberdeen, Scotland and API test pits, University of Houston, Houston, Texas, USA). Precision (repeatability) is also compared to prior PNC logging instruments, demonstrating logging speeds typically several times faster for comparable precision. Introduction Two new through-tubing carbon-oxygen (C/O) tools, the 1 11/16-in. RST tool ("RST-A") and the 2 1/2-in. RST tool ("RST-B"), were recently introduced. In addition to C/O capability, these new tools can provide, during the same trip in the hole, pulsed neutron capture (PNC) "sigma" logging with substantially improved accuracy, precision, and borehole salinity compensation compared to dedicated PNC tools presently used in the industry. PNC logging determines formation oil saturation indirectly by measuring the "absence of salinity" much in the same way an electrical log does in open hole. PNC logs measure the macroscopic thermal neutron capture cross section () of the formation, a quantity which is highly influenced by the salinity of the formation water. When the formation water is fairly saline an anomalously low sigma provides the hydrocarbon signature much like a low conductivity reading on an electrical log. Unfortunately, the ability of PNC logs to unambiguously detect liquid hydrocarbons breaks down in fresh or low-salinity environments since fresh water and hydrocarbons have nearly the same capture cross section and thus "fresh water looks like oil." Nevertheless, in most situations PNC tools can be logged at acceptable speeds with precision sufficient to detect changes in the reservoir, when run in time-lapse mode. P. 729
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