Dual-Spacing TDT Applications in Marginal Conditions (includes associated papers 6992 and 6989 )
- J.B. Jameson (Schlumberger Well Services) | B.F. McGhee (Schlumberger Well Services) | J.S. Blackburn (Schlumberger Well Services) | B.C. Leach (Schlumberger Offshore Services)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- September 1977
- Document Type
- Journal Paper
- 1,067 - 1,077
- 1977. Society of Petroleum Engineers
- 2.2.2 Perforating, 3 Production and Well Operations, 4.5 Offshore Facilities and Subsea Systems, 2.4.3 Sand/Solids Control, 5.2 Reservoir Fluid Dynamics, 4.6 Natural Gas, 5.8.7 Carbonate Reservoir, 1.14 Casing and Cementing, 5.6.1 Open hole/cased hole log analysis, 4.1.5 Processing Equipment
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The Dual-Spacing TDT log has broadened the application of pulsed neutron-capture logs to include shaly and fresh-water formations that formerly were considered marginal for cased-hole analytical methods. Accurate saturation determinations are thus made possible in many additional reservoirs. An auxiliary technique using count rates of the two detectors has proven very sensitive to the presence of gas.
The Dual-Spacing Thermal Neutron Decay Time (TDT*) tool is an important device for cased-hole formation evaluation logging. It uses two radiation detectors and a simultaneous gamma ray log to provide in one logging run the data for determining formation porosity, saturation, fluid identification, and correlation.
The technical details of the measurements have been discussed previously. Curves recorded include macroscopic capture cross-section (Sigma), a ratio curve from near- and far-detector count rates, and a display of the actual count rates from near and far detectors. A curve showing the level of background radioactivity is also recorded.
The simultaneous recording of all this information, along with the gamma ray log, provides data for a complete reservoir evaluation in cased hole. Details of the interpretation techniques have been discussed. However, until the introduction of the Dual-Spacing TDT in 1973, such cased-hole evaluation logs were restricted in practice to clean formations of medium to high porosity containing relatively saline waters. The dual-spacing tool, embodying features that considerably broaden its range of application, permits reliable cased-hole evaluations even in formations formerly considered marginal foe pulsed-neutron logging. Moreover, a new fluid identification technique permits the qualitative location of gas under a wide variety of conditions. Several examples from the Rocky Mountain area Illustrate the usefulness of the Dual-Spacing TDT in tight, low-salinity formations.
The Dual-Spacing TDT Tool
The Dual-Spacing TDT tool has a diameter of 1-11/16 in. With the normally combined gamma ray tool, it is 32 ft long. Its pressure and temperature ratings are 16,500 psi and 300 deg. F (or higher with special equipment). The TDT measurements are made at a point 82 in. above the bottom of the tool.
The small diameter of the logging tool allows it to be lowered through 2-in. tubing to log the cased hole below the packer. Good quality logs usually can be obtained through both tubing and casing above the packer, or even in concentric strings of casing (except in the presence of a gas-filled annulus - see Example 3).
The basic measurement of the tool is the thermal neutron decay time, tau. This is a measurement of the time required for a certain percentage of the thermal neutron population to be captured in the formation. This decay time is related to the neutron capture cross-sections of the elements in the formation. To make the measurements, bursts of high-energy neutrons are emitted periodically from a pulsed-neutron source. After each burst, the neutrons scatter throughout the region around the borehole. By successive collisions with the nuclei of elements in the surrounding media, the neutrons are slowed to thermal velocities. The thermal neutrons then disappear progressively as a result of capture, mainly by elements in the formation.
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