Summary

Tests of prototype horizontal and vertical microgeophones suitable for inclusion in 0.5-inch OD borehole geophone packages show sensitivities exceeding 20% of full-size borehole geophones. Borehole testing of conventional borehole geophones, microgeophones, and cemented-in geophones indicates qualitatively comparable performances for early arriving seismic energy from explosive shots in a neighboring well. Installation of access tubing for insertion of microgeophone packages in the annulus between production strings and casing in a pumped well was shown to be feasible. In the only case studied, microgeophones inserted in the access tubing appear to be well coupled and in a low noise environment.

Introduction

With the ultimate goal of deploying miniaturized seismic packages for the entire spectrum of seismic applications in low-cost holes approaching 1-3/8 inch in diameter (Dreesen and Cohen, 1997), seismic sensors and borehole-rated instrumentation packages are being miniaturized and prototyped. Boreholes of these dimensions and the instrumentation packages used in them are referred to as microholes and microtools, respectively. Dedicated microholes offer an optimal environment for arrays of miniaturized sensors since borehole noise can be controlled if not reduced to an absolute minimum. Miniaturization is expected to provide a very substantial reduction in the cost of drilling and instrumenting boreholes dedicated to taking measurements, enabling vastly more subsurface sensors to be emplaced in critical locations without interfering with production. The capability to inexpensively drill and instrument microholes will be a powerful tool in exploring for new reserves and improving production from existing, producing fields. While awaiting the first drilled microhole, borehole packages are being tested in simulated microholes and large diameter wells.

Microgeophones

A prototype horizontal microgeophone is shown in Figure 1 along with a vertical microgeophone previously reported (Albright et al., 1996). The horizontal microgeophone is sensitive perpendicular to the axis of its case. A conventional borehole geophone (CBG) manufactured by Mark Products is included for comparison. Both of the microgeophones have dimensions suitable for inclusion in 0.5-inch borehole instrumentation packages. Table 1 gives the specification for each geophone. Figure 2 gives a comparison of the impulse response of the horizontal microgeophone and the CBG. Except for amplitude, the impulse waveforms are very similar. Even though the total weight of the horizontal microgeophone is lighter by a factor of 28 from that of the CBG, the voltage sensitivity is less by only a factor of about 5.

Fig. 1: Sizes of vertical and horizontal microgeophones in relation to a conventional 30 Hz borehole geophone. A U.S. 1-cent coin is shown for scale.

Fig. 2: Impulse response of horizontal microgeophone and a conventional 30 Hz geophone.

Table 1. Specifications and performances of microgeophones and conventional borehole geophones.

Production well microtool access

In a first-of-its-kind installation, the ability to re-enter a well under pumped production without repetitive and extensive well preparation was demonstrated at the Texaco Humble field located northeast of Houston in November, 1997. The installation is illustrated in Figure 3. Using a contract workover rig, the rod, pump, and 2-3/8-inch production tubing were removed from well CO&G 57. A Texaco subcontractor, provided coiled tubing services and hardware and set up a coiled tubing unit (CTU) at a 90 degree angle to the workover rig. One-inch OD coiled tubing, with a plug installed at the bottom end was run in the annulus between the 2-3/8-inch tubing and the 5-1/2-inch casing to a depth of 2,365 feet and filled with diesel fuel. The coiled tubing was attached to the 2-3/8-inch tubing by means of stainless steel banding installed approximately 10 feet above and below each tubing joint using a pneumatic bander. The CTU is well suited for this operation because its injector/gooseneck assembly is integral to the main unit and can be suspended well above the rig floor with a minimal offset from the wellbore by means of a hydraulically angled mast. Following installation of the coiled tubing, the pump and rod were re-installed and the well was returned to production. The coiled tubing was hung off in the wellhead using a submergible tubing head. A minor modification to the head was required to accommodate the diameter of the coiled tubing through the passageway which is normally used for power cable to a submerged electric pump. A full-opening valve was installed on the top of the coiled tubing using a compression fitting. One problem encountered was that the large outer diameter of the rod stuffing box impaired access to the top of the coiled tubing. The well must be blown down and the stuffing box unthreaded and hoisted up several feet with the pump jack before instruments can be lowered into the coiled tubing. This inconvenience may be avoided in the future by using a rod stuffing box with a smaller outer diameter.

Fig 3: Schematic of the seismic microtool access installatation in well CO&G 57 at the Texaco Humble Field.

Comparison testing

During and following the installation of the coiled-tubing, microtool access noted above, tests were conducted using conventional borehole geophones, microgeophones, and cemented-in geophones. The objective was to evaluate coupling, signal-to-noise (S/N), and noise environments in a normally pumped well in which production was interrupted. Of particular importance was the new acquisition environment presented by the existence of access tubing. Signals were generated by 8 gm primacord shots in a well located approximately 250 ft from Well CO&G 57. Primacord shots were used to acquire relatively high frequency crosswell signals propagated through highly attenuating sediments. Signals were acquired (i) using a conventional 1-11/16 inch slim hole geophone package (SHGP) in 4-1/2-inch casing free of any tubing; (ii) using the SHGP inside of the production tubing which itself was inside of casing; (iii) using geophones cemented in a nearby well; and (iv) using a microtool containing only vertical geophones. Borehole packages were placed at a depth of 1337 feet. Source locations were not replicated but varied over approximately 20 feet to avoid casing damage in the source well. Although limited data were acquired in the experiments reported here, the results were very encouraging. Crosswell seismic signals and an icon representing the acquisition geometry for each signal are shown in Figures 4 and 5. Qualitatively, the S/N of the waveforms, the sharp first motions, and the high frequency content of the first arriving waveforms, are very similar for the four waveforms shown in each figure. Figure 6 shows a spectral comparison between the microgeophone package and the cemented-in geophones in which the respective spectrum of each is normalized to its peak frequency amplitude. Across the frequency band, the S/N of the respective geophones is within 10% of each other. Ringing is apparent after 100 msec in the waveforms of the microtool package that is not apparent in the cemented-in geophone and the SHGP. The source of this ringing, which is not excited by the first arriving energy, is not now known with certainty, but may be a package resonance excited by tube waves. Whether in the casing, in the production string, or in the microtool access, coupling of the packages to the formation qualitatively appears to be good. Further testing with horizontal components included in the microgeophone package and an airgun source is planned to obtain a much more complete characterization of the performance of the microgeophone package.

Fig. 4: Vertical component output for incoming signals at horizontal incidence for the SHGP, cemented-in geophones and microgeophone tools. 400 msec total time. Corresponding icons represent acquisition geometry. Open circles represent casing or tubing. Solid circle and bar represent borehole package and locking mechanism. Fig. 5: Vertical component output for incoming signals at 30 degree incidence for the SHGP, cemented-in geophones and microgeophone tools. 100 msec total time. Fig. 6: Signal and noise spectra for the cemented-in geophone and the microgeophone package. Both signals and their respective, corresponding noise spectra are normalized by the peak amplitude of the signal spectra.

Conclusion

Miniature vertical and horizontal microgeophones have been prototyped that have sensitivities exceeding 20% of their full-sized equivalents. Initial tests of the vertical geophones packaged for borehole deployment in pumped wells qualitatively exhibit performance comparable to retrievable slimhole geophone packages deployed in either production tubing or casing and to cemented-in geophones. A method was demonstrated for efficient, low-cost microtool reentry of pumped wells through access tubing installed between a production string and casing.

Acknowledgments

The authors wish to acknowledge the support of the Texaco Humble Field Operations Office and its subcontractors in the design and installation of the microtool access in CO&G 57. This work was sponsored and funded by the Natural Gas and Oil Technology Partnership of the United States Department of Energy.

References

Dreesen, D.S. and Cohen, J. H., 1997, Investigation of the feasibility of deep microhole drilling: ASME/API Energy Week Conference, January 28-30, Houston, TX.

Albright, J. N, Dreesen, D., Harbin, P., Woo, D. M., Tan, H., and Fairbanks, T.D., 1996, Retrievable 1/2-inch OD borehole seismic package: SEG 66th Annual Meeting, Technical Program, pp.114-117, Denver, CO.