Dual-Burst Thermal Decay Time Logging Overview and Examples (1987)

Dual-Burst Thermal Decay Time Logging Overview and Examples J.-R. Olesen; J.-R. Olesen Schlumberger Well Services Search for other works by this author on: This Site Google Scholar M. Mahdavi; M. Mahdavi Schlumberger Well Services Search for other works by this author on: This Site Google Scholar D.K. Steinman; D.K. Steinman Schlumberger Well Services Search for other works by this author on: This Site Google Scholar J.P. Yver J.P. Yver Schlumberger Middle East S.A. Search for other works by this author on: This Site Google Scholar Paper presented at the Middle East Oil Show, Bahrain, March 1987. Paper Number: SPE-15716-MS https://doi.org/10.2118/15716-MS Published: March 07 1987 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation Olesen, J.-R., Mahdavi, M., Steinman, D.K., and J.P. Yver. "Dual-Burst Thermal Decay Time Logging Overview and Examples." Paper presented at the Middle East Oil Show, Bahrain, March 1987. doi: https://doi.org/10.2118/15716-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE Middle East Oil and Gas Show and Conference Search Advanced Search AbstractThe Dual-Burst TDT* (TDT-P) tool is a 1 11/16-in. diameter pulsed neutron, dual detector tool system that uses a new method of analyzing the decay of a burst of fast neutrons in the downhole environment. The method provides an accurate measurement of the neutron capture cross section of the formation by employing a physically realistic model of the decay, which accounts for neutron diffusion effects, rather than displaying the decay as a sum of exponentials from the formation and borehole. Intrinsic sigma is computed directly from the decay curve instead of by using correction charts. The model is evaluated using over 2,500 data obtained in well characterized laboratory test formations. The benefit of the method is that the formation cross section is independent of the fluid in the borehole as well as independent of the neutron diffusion characteristics of the formation, characteristics which vary with lithology.Selected results over the data base are discussed and compared to the results of a two-exponential processing over the same data. The insensitivity of the formation capture cross-section determination with the diffusion model to variations in borehole characteristics as well as tool geometry and, more importantly, timing is demonstrated.Middle-Eastern log results are presented; they include TDT-P versus TDT-M comparisons, showing the variable effect of diffusion for different cases; an example of the insensitivity of formation sigma to borehole fluid characteristics. Results from a two component (formation and borehole) diffusion processing to invert the data, featuring a level by level fit of the diffusion coefficients to the measured data, are presented and compared with the results from a fast, real-time, wellsite computation using a single component model and diffusion coefficients predetermined from external input of lithology and borehole geometry.IntroductionThe Thermal Neutron Decay Time (TDT)1 log measures the rate of capture of thermal neutrons in the formation and borehole environment surrounding the tool. The decay time, called tau, is related to the reciprocal of the neutron absorption cross section of the materials in the environment, Σa. Of the common earth-forming elements, chlorine has the largest cross section, and contrasts in tau are an excellent means of discriminating between hydrocarbons and saline water in reservoir rocks. Because it is a nuclear measurement, the TDT series of tools can measure tau behind casing, and the tools can monitor changes in water saturation in producing wells. Also, the logs can be interpreted quantitatively to determine residual oil saturations. Keywords: borehole, diffusion model, cross section, overview, detector, formation sigma, dual-burst thermal decay time, upstream oil & gas, log analysis, porosity Subjects: Formation Evaluation & Management, Open hole/cased hole log analysis This content is only available via PDF. 1987. Society of Petroleum Engineers You can access this article if you purchase or spend a download.