Status Report, December 1999
Microhole Drilling and Instrumentation Technology1 - Jim Albright, Los Alamos National Laboratory
Microhole technology development is based on the premise that because of the historic advances in electronics and sensors, large conventional-diameter wells are no longer necessary for obtaining subsurface information. Furthermore, microholes offer an environment for improved subsurface measurement.
Los Alamos, supported by the DOE in collaboration with the oil industry through the Natural Gas and Oil Recovery Partnership, has undertaken an integrated program to show that the cost of obtaining subsurface information can be drastically reduced through microhole technologies expressly developed to obtain that information. Collectively termed "Microhole Drilling and Technology Development", engineering efforts encompass: evaluating the feasibility of drilling deep microholes, miniaturization and testing of bottomhole coiled-tubing drilling assemblies, miniaturization of geophysical logging tools, and incorporation of emerging miniature sensor technologies in borehole seismic instrumentation packages.
The Los Alamos microhole drilling system, which in concept corresponds to much larger-sized commercial rigs, consists of a mechanical rotary drag bit, a hydraulically powered positive displacement motor (PDM), and a coiled-tubing drill stem (Fig. 1). For the initial feasibility test, components suitable for drilling 1.75-inch vertical bores, were either procured or fabricated, and then tested as a microhole bottomhole drilling assembly in an industrial laboratory. Motor and bit performance tests demonstrated that these assemblies were suitable for coiled-tubing drilling. Penetration rates in Berea sandstone and Carthage marble exceeded 100 ft/hr (Dreesen and Cohen, 1997). Currently, Los Alamos is drilling and casing 2-3/8-inch-diameter microholes to depths of 500 ft with the equipment shown in Figure 2. The drilling to date has been in basin-and-range valley fill and volcanic tuff (Thomson et al., 1999).
Under a separate contract to the DeepLook Collaboration (seven major oil companies and three service companies), bottomhole assemblies have been designed that will enable microholes having a 1-3/8-inch diameter to be drilled. Engineering calculations, laboratory testing, and discussions with the drilling industry have indicated that by using coiled tubing and miniaturized hardware for conventional drilling, 1-3/8-inch-diameter boreholes with depths to 10,000 ft should be achievable.
Work has begun on a basic suite of 7/8-inch-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; the sensor subassembly is ready for testing and comparison with the performance of conventional-sized logging tools.
Our studies have indicated that the radiation flux incident on a microtool sensor deployed in a microhole would always be greater than that for a conventional tool in a cased 8-1/4-inch hole (e.g., Fig. 3). As theory dealing with the gamma capture efficiency of NaI crystals has yet to effectively deal with cylinders of the high aspect ratios found in logging tools, we decided to design a prototype to compare the efficiency of the microtool with a conventional tool. Making this comparison early in the design effort is important in order to determine the relative counting time for the two tools in a constant gamma flux. If the counting time for the microtool is excessively long compared to the commercial tool, the mass of the NaI crystal will have to be increased in the final microtool design.
Microhole Seismic Packages
Two borehole seismic instrumentation packages have been tested and thoroughly evaluated. One contains miniaturized (0.395-inch) geophones (Albright et al., 1998); the second makes use of a micromachined accelerometer which is a member of the class of sensors called microelectromechanical systems, or simply MEMS devices (Albright et al. 1999). Both the geophones and the MEMS accelerometer exhibit a performance approaching, if not exceeding, the performance of conventional geophones.
As part of this project, Mark Products developed miniature (0.39-inch diameter) vertical and horizontal geophones. Los Alamos designed, fabricated, and successfully tested a wireline-deployed package for the testing and evaluation of miniature accelerometers, geophones, and hydrophones. These sensors were then field-tested at Amoco, Los Alamos, and Texaco borehole facilities. Though substantially reduced in size, the geophones achieved a sensitivity within an order of magnitude of their full-sized counterparts (Albright, 1998).
Two 2-level, 3-component seismic arrays based on a successful prototype, were designed, fabricated, and tested by Los Alamos (see below) capitalizing on Input/Output's (IOC) in-house MEMS sensors technology. The prototype 7/8-inch-diameter borehole package which provided initial information on the performance of the MEMS sensor (Albright, 1999) was substantially redesigned to serve as an interchangeable sensor pod in a multi-pod array system.
In benchtop testing of the prototype, the MEMS pod qualitatively exhibited sensitivity comparable to a commercial geophone. The redesign reduced the complexity of each pod and streamlined the assembly into an array. Included in the redesign were: (1) improvements in the reliability of the locking arms; (2) specially designed and fabricated feed-thrus and connectors to accommodate up to 41 electrical conductors; and (3) a flex board circuit to pass power and telemetry through the electronic assemblies of each pod to the pods lower in the array.
The principal objectives of the current phase of microhole seismic work are not only to incorporate MEMS sensor technology into a borehole array, but also (1) to demonstrate that the arrays could be deployed and successfully retrieved, and (2) to evaluate the potential contribution that data from microhole arrays could contribute to seismic reflection surveying. With respect to Objective 1, four 2-3/8-inch-diameter microholes were drilled to depths of between 300 and 500 ft using a Los Alamos coiled-tubing rig. These wells were cased by grouting-in 1-1/4-inch, inside-diameter flush joint PVC tubing. A subcontractor to Phillips collected 2D reflection data (Fig. 4) simultaneously from conventional surface geophone arrays and the two MEMS-borehole arrays using IOC System 2, data acquisition equipment.
The arrays were successfully deployed and retrieved without incident. So far, field records indicate that single channels of borehole data exhibited a lower signal-to-noise ratio than 9-geophone array gathers used in the reflection line, array noise levels gradually declined with the depth of each array level, and the horizontal array elements recording the elastic wave showed lower amplitude motion than the verticals. To the best of our knowledge, this development represents the first reported use of MEMS technology for a borehole seismic array.
For exploration and instrumentation access wells, reduction of scale to decrease costs becomes economically much more attractive when carried to microhole dimensions. Two examples of the savings due to miniaturization are shown in Table 1, which gives comparisons between a hypothetical 1-3/8-inch-diameter microhole and commonly drilled 8-3/4-inch production wells in terms of well volume and drill string weight.
Both the absolute value and the ratio of the microhole value to that of the 8-3/4-inch hole are also shown. For a microhole, the volume of the drilling fluid and the weight of the drill can be as little as 1/42th and 1/77th of that for a nominal production well. This compares with a 2- to 5-fold reduction in scale for a slimhole. Because of scaled-down weight and material requirements for microholes, there is a potential savings in nearly every aspect of a hypothetical microdrilling system.
In concept, components of a microdrilling system are miniaturized versions of what is for the most part familiar conventional drilling and coiled tubing technology. Consequently, to the first approximation, microhole drilling will have the same characteristics and limitations of conventional drilling technology, but will have the savings inherent in the scale reduction. Savings also accrue with the reduced material requirements associated with logging tools and equipment. As with drilling equipment, small, microhole instrumentation and data gathering systems can be produced with a comparatively small investment and on a short development cycle.
At the November Partnership Review a proposal was presented by Los Alamos to begin preparations with industry for the drilling of a 5000 ft microhole to demonstrate the capability to drill a deep microhole and obtain reservoir information using the microhole instrumentation developed under Partnership funding. If funded, the demonstration microhole would be drilled after three years of development.
Albright, J.N., J. C. Gannon, T.D. Fairbanks, and J.T. Rutledge, 1999, "Borehole testing of a micromachined silicon accelerometer," Proceedings of the Annual Meeting of the Society of
Exploration Geophysicists, Houston, TX.
Albright, J.N., Woo, D.W., Fairbanks, T.D., Rutledge, Thomson, J.C., Howlett, D., and Barge, D.,
1998, "Development and testing of a 0.5-inch microhole geophone package," Proceedings of
the SEG Annual Meeting, September 14-17, New Orleans LA.
Dreesen, D.S. and J.H. Cohen, 1997, "Investigation of the Feasibility of Deep Microborehole
Drilling," Proceedings of 8th Annual Energy Week Conference and Exhibition, Houston, TX,
Vol. I, Book III, p. 137-144.
Thomson, J.C., J. Hufford, and D.S. Dreesen, 1999, "Coiled-tubing Microdrilling Drilling
Demonstration in Basin and Dry Lake Sediments," Los Alamos Report (LAUR-99-5310).
Contact: Jim Albright, LANL, 505-667-4318.