Figure 3.8b, it can be seen that the DMOF‐based fiber optic DAS system recorded the borehole seismic data with a good SNR and correct amplitude, as well as a clear downgoing tube wave. The output receiving data spacing is 2 m. The tube wave is the dominant component in the water‐filled shallow borehole, and the first arrival of the tube wave has an SNR of about 10 dB. In addition, Figure 3.8c presents the F‐K domain spectra of the recorded DMOF‐DAS borehole seismic data, where the linear event clearly indicates the strong downgoing tube wave and weak upgoing tube wave.
3.3.2. Walkaway VSP Survey in Suning Oil Field
A walkaway VSP survey using the DMOF‐DAS system was conducted in the Suning oil field of CNPC in China. A 1.4‐km‐long armored DMOF cable was permanently cemented behind the casing, which resulted in an excellent coupling between the formation and the sensing fiber cable. Both vibrator (28 ton) and dynamite (16 kg charge) sources were used to generate seismic energy on the surface with different offset distance to the wellhead. The spacing of seismic sources was 40 m, and the farthest source was 8 km away from the wellhead. The DMOF‐DAS VSP data with 2 m spacing were recorded by the DMOF‐DAS system and are presented in Figures 3.9a and 3.9b. It can be seen that high‐quality raw DMOF‐DAS VSP data are obtained, which include clear upgoing and downgoing waves with a very high SNR. The estimated SNR of direct arrival for the zero‐offset DAS VSP data recorded from the armored optical cable cemented behind the casing is about 25 dB. A strong seismic reflector is presented, along with direct shear wave arrivals. Figure 3.9b is the zoomed‐in view on part of raw DMOF‐DAS VSP data and proves the high consistency of polarity and seismic energy attenuated in each channel with depth as expected.
Figure 3.9 Recorded seismic data in well using DMOF‐DAS: (a) DMOF‐DAS VSP data at zero offset with stronger reflector and direct P‐wave and S‐wave arrivals; (b) zoomed‐in view on part of the downgoing wave of (a); (c) VSP raw data display at the offset of 2.5 km; (d) the amplitude spectra of (a); and (e) the amplitude spectra of (c).
Moreover, we measured the raw DMOF‐DAS VSP data for different offset source locations. Figures 3.9a and 3.9c illustrate the raw DMOF‐DAS VSP data when the source locations were near the wellhead and 2.5 km away from the wellhead, respectively. The raw zero‐offset DMOF‐DAS VSP data show a very high SNR (25 dB). While even the source was far away from the well, the direct P‐wave and the direct S‐wave were also observed with lower signal strength. The corresponding amplitude spectra of Figures 3.9a and 3.9c are plotted in Figures 3.9d and 3.9e, respectively, where the red curves represent the spectra of all the fiber section, and the green curves depict the spectra of the effective signal regions. It can be seen that the recorded signals have a wide frequency spectrum and correct amplitude.
3.4. DISCUSSIONS
The field test data have proved that the DMOF‐based fiber optic DAS system can successfully acquire borehole seismic data with good quality. Through the deployment of the DMOF cable in a well as the sensing device, which is connected to the interrogator of the DAS system at surface, the seismic signals can be recorded along the full length of the well for each shot. A longer duration is not required for rigging up and down the conventional borehole geophone array; hence, the survey efficiency is significantly improved. Moreover, the DMOF‐DAS system offers the opportunity to achieve a much higher spatial resolution (typically of 2 m) and lower cost than current technologies. In addition, the coupling method is very important for the VSP survey. For the water‐injection coupling scheme, the tube wave will be the main noise. The cementation method will provide the strongest coupling, resulting in better VSP data with a higher SNR.
3.5. CONCLUSIONS
The general benefits of fiber optic DAS, such as a large number of channels and being free of power supply in the sensing area, make it more suitable for long‐distance detection (sensing) at shortest time, significant cost saving, and without a need to interrupt other activities. Thus, fiber optic DAS is increasingly being recognized as a viable alternative to downhole geophone arrays for the acquisition of borehole seismic data. To increase the SNR and eliminate the random fading of the sensing fiber, DMOF was proposed as the sensing fiber and fabricated through the UV laser light exposure. By employing the coherent detection and IQ demodulation scheme, a DMOF‐based fiber optic DAS system with a wide frequency range from 0.01 Hz to 60 kHz and an ultrahigh strain resolution of 3.4 pε/√Hz around 10 Hz was explored and demonstrated. The field zero‐offset VSP, offset VSP, and walkaway VSP tests proved that the DMOF‐DAS system can acquire borehole seismic data with good quality. Because of the benefits of the long‐distance sensing and distributed monitoring capabilities, the fiber optic DAS system can dramatically reduce the operating time required to complete a normal borehole seismic survey and can achieve much higher full well spatial sampling than current technologies. The ability to acquire borehole seismic data in a producing well without the need to disrupt production also offers significant benefits to the operators.
ACKNOWLEDGMENTS
The authors would like to acknowledge the National Natural Science Foundation of China (NSFC) (Nos. 61922033 and 61775072), the Project of Economic Development of Guangdong in China (No. GDNRC 2020‐045),the Science Fund for Creative Research Groups of the Natural Science Foundation of Hubei in China (No. 2018CFA004), the Major Projects of Technical Innovation of Hubei in China (No. 2018AAA040), and the Innovation Fund of Wuhan National Laboratory for Optoelectronics (WNLO).
The authors also thank BGP Inc. for its support in the discussion and field test.
The authors declare no competing financial interests.
Most of the material in this chapter has neither been published nor is under consideration for publication elsewhere.
REFERENCES
1 Ai, F., Sun, Q., Zhang, W., Liu, T., Yan, Z., & Liu, D. (2017). Wideband fully‐distributed vibration sensing by using UWFBG based coherent OTDR. Paper presented at Optical Fiber Communication Conference, San Diego, CA.
2 Chen, D., Liu, Q., & He, Z. (2017). Distributed fiber‐optic acoustic sensor with sub‐nano strain resolution based on time‐gated digital OFDR. Paper presented at Asia Communications and Photonics Conference, Optical Society of America.
3 Daley, T. M., Freifeld, B. M., Ajo‐Franklin, J., Dou, S., Pevzner, R., Shulakova, V., et al. (2013). Field testing of fiber‐optic distributed acoustic sensing (DAS) for subsurface seismic monitoring. The Leading Edge, 32(6), 699–706. doi: 10.1190/tle32060699.1
4 Follett, J. L., Wyker, B., Hemink, G., Hornman, K., & Lumens, P. (2014). Evaluation of fiber‐optic cables for use in distributed acoustic sensing: Commercially available cables and novel cable designs. Paper presented at SEG Annual Meeting. Society of Exploration Geophysicists.