ABSTRACT

The ever-growing demand for fossil fuels makes it imperative that the oil industry find and efficiently produce additional reserves. An important source of new reserves are the wells of older fields--wells drilled a number of years ago, and already cased. The Dual-Spacing Thermal Neutron Decay Time Log (Dual-Spacing TDT (FOOTNOTE *)) provides the data necessary to find oil and gas when they are present in these older cased wells.

This paper briefly discusses the Dual-Spacing TDT equipment, the log presentation, and its interpretation for porosity and water saturation. Examples are presented to illustrate a few of the numerous applications of the measurements made by the tool.

Examples shown include:

1. Illustration of gas-oil and oil-water contacts shown by the Dual-Spacing TDT (Fig. 5).
2. Use of the Dual-Spacing TDT to select a zone for a gas or oil recompletion from a series of productive sands (Fig. 6).
3. Use of the Dual-Spacing TDT to identify gas in shallow, low-salinity formations (Fig. 7).
4. Indication of gas in low-resistivity sands by the Dual-Spacing TDT (Fig. 8).
5. Comparison of Dual-Spacing TDT responses in gas sands with those in low-porosity sands (Fig. 9).
6. Use of the Dual-Spacing TDT to identify depleted zones in gravity-drainage reservoirs (Fig. 10).
7. Use of the Dual-Spacing TDT to monitor the gas-oil contact in a pressure-maintenance project (Fig. 12).