Retrievable 1/2-inch OD Borehole Seismic Package James N. Albright* and Donald Dreesen, Los Alamos National Laboratory; Philip Harben, Lawrence Livermore National Laboratory; Daniel M. Woo, Mark Products; Henry Tan, Amoco Production Research; Thomas D. Fairbanks, Nambe Geophysical, Inc.

Summary

Initial laboratory and field tests show that a miniaturized borehole seismic package can approach the performance of its larger-sized predecessors. The package is designed so that it can either be used singly or as a unit in a multi-level array deployed in a production tubing annulus or small diameter well. Interchangeable subassemblies of vertical geophones and 3-component accelerometers enable quick field substitution. A 0.3-inch diameter geophone was developed to meet the size limitations imposed by space requirements for feed through of conductors and pressure protection. The package is deployed using light weight, small-diameter multi-conductor cable.

Introduction

The need exists for a retrievable borehole seismic package or array that can be repeatedly deployed in wells under production without pulling production tubing for each use. Modern electronics and advances in sensor technology makes practical the miniaturization of borehole packages so that it is now feasible to deploy seismic packages in a variety of settings and at a reduced operations cost not formerly thought possible. We are exploring the engineering and operational feasibility of miniaturization of conventional borehole instrumentation technologies used in seismic applications and the increased deployment opportunities and applications that these borehole tools may represent.

1/2-inch OD borehole seismic package

A 1/2-inch, outside-diameter (OD), retrievable geophone package has been fabricated and tested. Referred to as a microhole tool, the package has been designed to be used as a single unit or as an element in an array in a variety of borehole seismic applications where economic deployment is in a cemented annulus, in coiled tubing installed in a production tubing annulus, or in a small diameter well.

A schematic of the microhole package is shown in Figure 1. The tool's mechanical subassemblies include those for top and bottom cable heads (for 4-conductor, 0.18-inch wireline or a 5-pair 0.30-inch cable, respectively), spring-loaded locking mechanisms, and electronics. The external housing and individual parts were machined from 304 stainless steel tubing. Tubing unions with roll pins, o-ring seals, and fluid ports are used for quick substitution or exchange of geophone and accelerometer subassemblies. The locking mechanism is engaged on command by heating a tungsten filament that breaks a polymer thread holding a spring-loaded locking arm in a retracted position in the subassembly body. The package in its current version has a length of 40 inches and a weight of l lb. The minimum total length as an array element having 4 geophones and 2 locking subassemblies is 32 inches. A combined locking force-to-tool weight ratio of 18 is obtained with helical extension springs. Because of its light weight, the package is easily retrieved without retracting the locking arms.


Figure 1. Schematic of the 1/2-inch seismic package. An accelerometer subassembly (not shown) may be substituted for the geophone subassembly.

Geophone and accelerometer subassemblies

Four Mark Products 0.3-inch OD geophones, wired in series, and a low noise amplifier are used in the geophone subassembly. The geophones have a natural frequency of 30 Hz. Figure 2 shows the output of individual geophones compared with a single Mark Products Model L15 in benchtop shake table testing. As these data were used for relative comparison only, no effort was made to obtain an absolute value for excitation amplitude. Generally, the combined output of the 0.3-inch geophones is within 15 dB re mV of the L15 through the measured frequency range. The accelerometer subassembly consists of three orthogonal EGA-5g piezoresistive silicon micromachined accelerometers. The three accelerometers have temperature compensation modules and are connected to a custom 3-channel signal conditioning board that consists of an instrumentation amplifier and a line driver for each channel.

Figure 2. Relative spectral response of Mark Products 0.3-inch and L15 geophones.

Amoco's Mounds tests

Initial field tests of the microhole tool were conducted at Amoco's Mounds facilities. Figure 3 shows the field installation used for comparison of the output of various sensors and instrumentation packages. A dual completion string of 3/4- and 2-inch pipe was cemented in a 4-1/4-inch diameter, 147-foot deep well. The bottom 20 feet of the 3/4-inch pipe was filled with water. The microhole tool and a 3-component, slimhole (1-11/16 inch) geophone package made by Los Alamos, were installed in the test well. One Oyo Model SMC 1850 geophone was used for each component in the slimhole package.

A shallow grouted installation of Wilcoxon Model 731-20 accelerometers next to the Los Alamos test well, was made available by Amoco for the tests. 4 and 8 gram primacord explosions were used as a signal source in a shot hole 260 feet away. Simultaneous recordings were made of telemetry from either the microhole tool geophones or the accelerometers, the slimhole geophone package, the Wilcoxon accelerometers, and a geophone near the explosion that was used for a shot break. A typical output from the vertical geophones including a shot break signal is given in Figure 4. A high amplitude ringing is apparent on the microhole tool geophones that is not associated with any natural mechanical resonances of the unclamped package. The cause of the resonance is being isolated and will be corrected for subsequent tests. The amplitude spectra of the output of the microhole tool geophones is given in Figure 5. The amplitude spectra shows that source signals with an energy content through 1 kHz are detected by the geophones in the microhole tool. A typical output from the horizontal accelerometers is given in Figure 6. As a first test of the microhole tool, results are encouraging. Qualitatively, the signal-to-noise of the first arrival detected by either microtool geophones or accelerometers is comparable or greater than that on the slim line tool or cemented-in accelerometers.

Figure 3. Test well with dual completion at the Amoco Mounds test site used in comparison testing of the 1/2-inch and slimhole geophone packages.	Figure 4. Vertical geophone output from microhole tool and slimhole packages.
Figure 5. Amplitude spectrum of 1/2-inch package geophone output.	Figure 6. Microhole tool and Wilcoxon horizontal accelerometer output.

Future tests

A number of application tests for which 1/2-inch microtools are deployed either singly or in multilevel retrievable configurations, are proposed. Los Alamos is seeking industrial collaborators on any test of mutual interest. Well installation possibilities for the package include deployment in a production tubing annulus, in a cemented casing annulus, or in a shallow small diameter well. The former type of deployment is illustrated in Figure 7 and is the installation proposed for application testing. Shown is 3/4-inch OD tubing strapped to 2-3/8 -inch OD tubing inside 4-1/2 -inch casing. The coiled-tubing installation and wellhead fixtures for access to the tubing are patterned after submersible pump power cable installations. Once installed, the performance of the miniature seismic package or array will be evaluated for background noise levels, detection of reservoir and/or hydraulic fracture seismicity, vertical seismic profiling, or 3- and possibly 4-D seismic imaging. Collaborators should expect to provide a host site, modification of well equipment, and support for installation of the Los Alamos equipment, and to participate in an analysis of the quality and value of the data. Los Alamos will provide all necessary borehole and surface instrumentation at the field sites where the application is to be evaluated.

Figure 7. Schematic of deployment configuration of microhole package in 3/4-inch tubing strapped to 2~3/8-inch production tubing.

Conclusions

First testing of miniaturized borehole seismic packages show that these tools can have a performance approaching that of larger diameter tools while reducing hardware and operations costs. Several methods of deploying the miniaturized instrumentation appear to be feasible and need to be demonstrated and evaluated.

Acknowledgments

The authors wish to acknowledge the assistance of John Beck of Amoco Production Research who helped with all of the preparations for the Mounds activities including the drilling and completion of the test well, the planning and execution of the field activities, and interpretation of anomalous test data. This project was funded in part by the US Department of Energy through the Natural Gas and Oil Recovery Partnership.

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