James K. Andersen
Sensor Systems Inc.
Björn N.P. Paulsson
Paulsson Inc.
The two main barriers for widespread use of time-lapse (4D) seismic technology in the oil and gas industry are data repeatability and cost. One solution for repeatability is to fix the locations of the source points and/or receiver arrays for the repeat surveys. The best way to do this is to install them permanently. Based on the cost of current systems, this often is cost prohibitive.
Marine 4D seismic data acquisition most often uses standard towed marine streamers. These surveys suffer from repeatability problems associated with errors in the location of the deployed sources and the receivers which lead to a deterioration of the image qualities. In most cases, the additional expense of redeployable ocean bottom sensor cables appears difficult to justify. Reconfiguring redeployable sensors for permanent installation likely would increase the cost further. However, 4D surveys using permanent installations of sensors can be justified by the increased resolution.
One factor in calculating the economics of permanent arrays is evidence that changes in the reservoirs due to production and/or injection are larger than expected and happen faster than expected. These large, fast changes require more time lapse surveys to properly capture the changes. This change in the monitoring paradigm favors permanent receiver arrays both technically and economically.
For over 50 years, the magnet/coil geophone has been the standard. Over the years, many replacement sensors have been developed only to fail in one way or another. The promise of a low cost solution using fiber-optic sensors in 4D applications remains on the horizon, hoping to mirror the US Navy’s successful large-scale fiber-optic submarine sonar programs. The transition of this military technology to meet commercial oilfield seismic performance and cost requirements is unrealized. The typical noise floor required for a high-resolution geophysical seismic system is on the order of 100 ng/√Hz over a frequency range of about 5 to 200 Hz.
US Sensor Systems Inc. has developed a new fiber-optic geophone (patent pending) that is superior to traditional geophones in key categories such as; sensitivity, noise floor, distortion, bandwidth, and dynamic range -- all at a lower cost. Output voltage of conventional (moving coil) seismic geophones is a function of velocity. Output of the USSI fiber-optic geophone is a 32-bit digital word representing optical phase within an interferometer which changes in response to mechanical motion. For side-by-side frequency response tests, the unamplified output of the conventional geophone was compared to the unamplified analog output of the USSI fiber-optic sensor’s optical demodulator.
What this means for 4D seismic
The major advantage fiber-optic sensors have over conventional electronic-based sensors is the ability to separate the electronics (preamplifiers, filters, ADC, multiplexing electronics, etc.) from the sensor without any significant degradation in performance. This removes the electronics from the hostile sensing environment (downhole, ocean bottom, buried, etc.), into a benign, controlled environment where they are accessible for repairs or upgrades. Thus, for permanently installed fiber-optic sensors, only the optical fiber and its associated packaging must be installed permanently. In the case of the new low-cost fiber-optic geophone, this can reduce by a large factor the cost of the permanently installed equipment. And, since the surface electronics package is needed only during an actual survey, one set of surface electronics can service several sites.
How it works
The USSI geophone system is comprised of three basic integrated building blocks:
- 1. The optical interrogator
- 2. The fiber-optic telemetry cable
- 3. Fiber-optic geophones.
The optical interrogator electronics include a laser source with a phase modulator/pulse generator for launching light down the fiber, as well as receiver electronics to demodulate return signals from the sensors and to translate them into a digital electronic signal. The optical interrogator uses optical Time Division Multiplexing (TDM). The fiber-optic telemetry cable provides the data path to and from the individual sensors. Spacing of the optical geophones corresponds to the timing needed for TDM. The optical geophone converts the ground motion into an optical phase shift, which is demodulated in the interrogator.
Field test
To verify the performance of the fiber-optic geophone, a small-scale field survey compared the performance of a state-of-the-art 15 Hz omni-directional geophone, and the USSI fiber-optic geophone.
The performance test included a repeatability study where data were concurrently recorded using both the fiber-optic geophone and the standard geophone. The repeatability study was done by dropping a 50 lb (22.5 kg) weight onto the ground five times from a height of 5 ft (1.5 m) and recording the data independently for each drop of the weight. The test shows that the repeatability of the fiber-optic sensor is excellent. The repeatability of the standard geophone was similarly excellent.
The averaged, normalized data comparison from the five records of both the fiber-optic sensor and standard geophone shows that the fiber optic sensor contains high frequency events not seen in the geophone data. Spectra of these two waveforms confirm the observation in time domain.
Acknowledgement
Eric Goldner’s and Gerald Baker’s help with the data acquisition and analysis is gratefully acknowledged.
The potential of fiber-optic geophone technology can be summarized as the following:
- The performance of the fiber-optic geophone matches or exceeds the performance of the standard geophone
- A large number of fiber-optic channels can be deployed on each fiber, making large channel count system possible
- No electronics need to be deployed with the sensor, making the sensor system robust with a potentially long survival time (as evidenced by deployment of fiber-optic sensors by the US Navy).
- No electric power needs to be transmitted to the sensor, nor does the fiber-optic sensor generate any electric signal, making the sensor intrinsically safe and immune from EMI/RFI
- A high-temperature version of the fiber optic geophone can be manufactured using commercially available high temperature fiber.
