Slim Hole Logging and Analysis for Uranium Exploration
- Eddie P. Howell (Arco Oil and Gas Co.) | Orland J. Gant Jr. (Arco Oil and Gas Co.) | Terry J. Crebs (Anaconda Copper Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1980
- Document Type
- Journal Paper
- 1,144 - 1,150
- 1980. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating, 7.2.2 Risk Management Systems, 5.2 Reservoir Fluid Dynamics, 5.1.5 Geologic Modeling, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 6.5.4 Naturally Occurring Radioactive Materials
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Significant improvements have been made in uranium logging equipment and analysis techniques. Logging systems now provide geological, geophysical, and geochemical data from routine measurements. Through computer analysis, these systems also can determine porosity, density, permeability, lithology, formation-water resistivity, and thermal and mechanical properties.
Deeper targets for uranium exploration warrant more complete logging systems than those used previously. Such systems can produce adequate data for a geologic understanding of the ore deposition processes with fewer drill holes. As the formation depth increases, data extrapolated from surface outcrops become less reliable and make log-derived data more important. An alternative to a complete logging package is extensive coring of ore-bearing and related formations, but this is expensive and time consuming. We are using reasonably sophisticated logging systems that provide data for geologic models, which are used to understand the sedimentary environment, groundwater, and other physical and chemical properties of uranium deposition. These models, based largely on log-derived data, are important when designing infill drilling for exploration, evaluation, or production.
Uranium log interpretation is simplified somewhat by short drilling times that restrict invasion and by lack of hydrocarbons that complicate fluid properties. However, less-sophisticated small-diameter downhole tools can have the opposite effect.
The digital logging systems that we use have been designed for relatively simple operation. Analog records are produced during logging to verify equipment operation and allow operator adjustments if necessary.
For most logging tools, digital data are generated uphole from a depth encoder, two count-rate meters, and four 4.5-digit volt meters. Signals are recorded from all of these devices at operator-selectable digitization intervals. Recordings are made with cassette tape decks using a 1,200-baud transfer rate. With this recording rate, logging speed is limited by the downhole instruments themselves. Up to 2,000 ft (600 m) of multiplexed logs can be recorded on one side of a cassette.
Each truck is designed to run at least 12 separate downhole tools with minimal operator intervention. At present, three sondes are used routinely and several others are used on an experimental basis. These include a gamma-ray log that uses a 0.75×1.25 in. (1.91×3.18 cm) Nal crystal with downhole scaling in areas where count rates are high. This tool also generates 16- and 64-in. (41- and 163-cm) normal calibrated resistivity curves. A single-point resistivity curve can be recorded for correlation with older uranium logs. Also, a spontaneous potential (SP) curve is measured but is difficult to interpret in mineral logging since this potential is the sum of several variable potentials including chemical, membrane, and filtration.
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