|Formation Logging for Microborehole
Goal: The goal is to develop technologies that can provide significant reductions in both the cost and environmental impact of drilling.
Objectives: The objective is to develop a suite of miniaturized logging tools for measurement of formation properties with wireline tools capable of running in microholes.
Los Alamos National Laboratory – Project management and all research products
Work has begun on a basic suite of 7/8-in.-diameter logging tools that is to include both spectral gamma and electrical resistivity tools, as well as a capability for surveying the trajectory of completed microholes. Furthest along in this tool development is the gamma tool.
- Carried out laboratory studies comparing performance of small-diameter and conventional gamma ray tools,
- Fabricated a prototype inclinometer for measuring wellbore inclination and azimuth and tested it in a microhole drilled at the LANL test site, and
- Evaluated the special requirements imposed by operation in a small borehole, on porosity and resistivity logging methods.
Our studies have indicated that the gamma ray flux incident on a centralized sensor deployed in a microhole would always be greater than that for a conventional tool in an uncased, 8-1/4-in. hole. Off-normal flux components will also be greater for the microhole tool because of the closer proximity of the rock above and below to the sensor in a microhole. This may cause some loss of depth resolution. The increased gamma flux incident on the microhole tool is offset by the reduced photopeak detector efficiency inherent in its smaller sensor. In order to adequately characterize the overall performance of the microtool relative to a conventional tool, LANL has designed a test barrel to compare their performance over a range of borehole diameters, casings, and fluids.
One of the critical aspects of evaluating microdrilling technology is to demonstrate that microdrilling could first be effective in producing straight vertical boreholes and then adapted to produce directionally-drilled holes. The critical instrumentation needed to start the process is an inclinometer logging tool that fits near vertical microholes to measure the inclination and azimuth of the borehole between bit trips as the microhole is being drilled. LANL identified a commercial product that could be repackaged to produce a 1.00-inch diameter logging tool that could be run in open holes and cased holes with an internal diameter greater than 1-3/8 inches (Applied Physics Systems, Model 544, Miniature Angular Orientation Sensor package). LANL designed and fabricated a housing for the tool. The inclinometer sonde was run in San Ysidro No. 2A, a microhole drilled with the LANL coiled-tubing drill unit. A complete review of the log indicates that the tool is operating properly.
After a detailed analysis of the special requirements imposed by operation in a small diameter borehole, a combination neutron porosity/gamma density tool is recommended for microhole tool prototyping. Accuracy of the tools should compare well with conventional wireline tools because there can be less of a borehole perturbation with careful design due to the diameter reduction. The nuclear tools are constructed with a metal housing, which greatly increases the strength and reliability. Acoustic tools, by contrast, are inherently weak due to the need for a slotted mechanical structure that attenuates sound waves. Mechanical strength is considered to be particularly important because the tools will likely have a greater length/diameter ratio than usual, due to the need to maintain similar source-to-detector spacings to standard tools, while they may be required to absorb axial loads when mounted on coiled tubing.
After a similar analysis, a high-frequency induction tool, also known as a wave-propagation tool, was recommended as the best option for resistivity measurement in microboreholes. Signal levels are high because sensitivity increases as the square of frequency, more than compensating for a serious loss of signal due to the diameter reduction. This type of tool can be constructed on a metal mandrel, which greatly increases the strength and does not require oil filling. While this is the preferred solution, an alternate proposal for an electrode type of tool is made that may be less complex to design but has some operational limitations.
Current Status and Remaining Tasks: This project has been completed.
Project Start: April 1, 2001
Project End: April 30, 2003
Anticipated DOE Contribution: $300,000
Performer Contribution: $0
NETL – John D. Rogers (email@example.com or 304-285-4880)
LANL – James N. Albright (firstname.lastname@example.org or 505-665-0640)