Summary

Side-by-side field tests of borehole seismic packages containing either micromachined analog servo accelerometers or conventional piezoelectric accelerometers currently used in borehole seismic applications, show important differences in their relative performance. Laboratory testing of the micromachine shows dramatically improved distortion and low frequency response compared to conventional geophones (Gannon et al., this volume).

Introduction

Laterals having a diameter of 2-3/4-inches are now being routinely drilled from deep wells on the North Slope (Baker Hughes, 1999). Feasibility studies show that deep boreholes starting from the surface of half this diameter at their terminal depths are possible (Dreesen and Albright, 1999). Because of the anticipated low cost of boreholes of this size, a tremendous opportunity is in the offing for the proliferation of subsurface exploration and characterization. Aside from their low cost, boreholes and laterals with diameters of these dimensions are more favorable for the acquisition of borehole seismic data than conventional wells drilled for the high-rate production of fluids. Instrumentation deployed in these small boreholes, if dedicated to measurement, can be economically completed in optimal locations without the possibility of flow, thus creating an ideal low noise environment for acquisition of high quality seismic data.

Initial attempts at simple miniaturization of conventional technology for microhole applications have not succeeded in achieving the capability of existing borehole seismic instrumentation. Albright et al. (1996, 1998) show that 1-gm-mass vertical geophones with a diameter as small as 0.345 inch exhibit a performance of the same order of magnitude, but still less than conventional geophones (30% of the sensitivity of a 30-Hz conventional geophone), when deployed in 1/2-inch packages in small boreholes. A 0.425-inch horizontal geophone has been fabricated and bench tested, but is still less sensitive and has yet to function reliably for an extended period of time. These shortcomings will be overcome with further development.

Reliable microelectromechanical systems (MEMS) have undergone rapid development and, consequently, new small sensors capable of accomplishing seismic acquisition tasks are becoming available. This paper reviews the laboratory performance and field testing of a MEMS configured to make 3-component particle acceleration measurements in microholes using 7/8-inch-diameter borehole instrumentation package.

The sensor, a MEMS analog servo accelerometer, and its associated electronics are shown in Figure 1 in comparison with a conventional geophone. In concept, this device is similar to several such sensors now under development, but this sensor has been specifically designed for seismic systems. The device is fabricated by selectively etching a silicon wafer. The sensor consists of a mass suspended by a spring and centered between two electrodes. The sensors used for these tests are incorporated into analog feedback loops. In response to movement of the sensor case, the feedback loop provides an electrostatic force to rebalance the mass and maintain centering. The output of the circuit provides an analog output linearly proportional to the acceleration of the case.

Figure 2 shows the frequency response and the total harmonic distortion (THD) of the MEMS accelerometer compared to a conventional geophone. Within measurement error, the distortion of the MEMS sensor exhibited by the presence of harmonic peaks, is less than or equal to the geophone throughout the entire spectrum. The THD of the MEMS accelerometer and the geophone is 0.02% and 0.09%, respectively. Additional testing is underway to characterize cross-axis response of the devices and compare them to conventional sensors (Gannon et al., this volume).

Borehole package

The sensor and electronics were packaged for borehole use in a 7/8-inch-diameter tool using a design in which threaded joints are replaced by easily connected tubing fittings and effective coupling is achieved by a spring loaded lever arm (Albright et al., 1996).

Field tests

The 7/8-inch instrumentation package was deployed at 90 feet in a 2-1/4-inch borehole completed with 1-1/4-inch PVC tubing cemented in place. The boreholes penetrate low velocity, highly attenuating volcanic tuff from the surface to the measurement depth. Within 16 feet of this location, a conventional piezoelectric accelerometer used in borehole seismic measurements was grouted into a 4-1/2-inch borehole at the same depth. As a seismic source, a 2-ton weight was repeatedly dropped using a crane. The source location was 80 feet from the sensor wells.

The response of the conventional accelerometer peaks at 80-100 Hz and decreases uniformly through 1 kHz where little signal appears to remain. The noise spectra of the conventional accelerometer are rather flat throughout the spectrum. Substantially more signal appears in the 800-1000-Hz band of the conventional accelerometer than is apparent in the MEMS sensor. In the case of the MEMS device, little signal is seen above 600 Hz while substantial signal energy appears below 100 Hz. A broad peak characteristic of both signal and noise appears around 200 Hz. Noise in the MEMS device decreases to 600 Hz and monotonically increases thereafter as is characteristic of accelerometers with low resonant frequencies.

Conclusion

MEMS devices offer the potential of effective performance in small boreholes while maintaining or even improving the performance of conventional sensors. Laboratory tests show promising results for the MEMS analog servo accelerometer. Side-by side field tests of a MEMS borehole package and a conventional piezoelectric accelerometer show differing responses. More careful testing in a controlled environment is necessary to understand the differences. However, reliable micromachined accelerometers, which are deployable in microboreholes, are now a reality.

Acknowledgements

This project was completed with the first successful drilling of a 2-1/4-inch microhole using a shallow microhole-drilling rig. We would like to thank Input/Output, Inc. for providing the MEMS accelerometers used in this project. The work at Los Alamos was sponsored and funded by the Natural Gas and Oil Technology Partnership of the United States Department of Energy.

References

Albright, J. N., D. Dreesen, P. Harben, D. W, Woo, H. Tan, and T. D. Fairbanks, 1996, "Retrievable 1/2-inch OD borehole seismic package," 66th Ann. Mtg., Soc. of Explor. Geophys., Expanded Abstracts, 114-117.

Baker Hughes, 1999, "TinyTrak‹an overview, product literature," http//:www.bakerhughes.com.

Dreesen, D. S. and J. N. Albright, 1999, "Microholes for exploration and reservoir measurements, Part 2. Promise for high performance, deep microhole drilling, Los Alamos," National Laboratory Technical Memorandum.

Gannon, J. C., M. G. McMahon, H. T. Pham, K. E. Speller, 1999, "A seismic test facility," 69th Ann. Mtg., Soc. of Explor. Geophys., Expanded Abstracts (this volume).