Effects of hydrostatic pressure on strain measurement with ... - Core

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ScienceDirect Energy Procedia 63 (2014) 4003 – 4009

GHGT-12

Effects of hydrostatic pressure on strain measurement with distributed optical fiber sensing system Ziqiu Xue a,*, Hyuck Park a, Tamotsu Kiyamaa, Tsutomu Hashimotoa, Osamu Nishizawaa and Tetsuya Kogureb b

a Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto, 619-0292, Japan Department of Geoscience,Interdisciplinary Faculty of Science and Engineering,Shimane University, 1060 Nishikawatsu-cho,

Matsue, Shimane, 690-8504,Japan

Abstract This paper presents effects of hydrostatic pressure on strain measurements by using distributed optical fiber sensing system. Laboratory experiments on strains of metallic plates and a cylindrical sample of Berea sandstone were performed under hydrostatic pressure by using a system that measures strains by the frequency shifts of Brillouin and Rayleigh scattering in an optical fiber, together with conventional strain gauges. The measurements revealed that the radial strain of the optical fiber due to the hydrostatic pressure considerably affect the frequency shifts as well as the strain along the fiber line. The effects of two strains on the Brillouin or Rayleigh frequency shifts were clearly confirmed and provided important insights when converting both frequency shits into the fiber strains along the linear and radial directions. We have established the method to determine the Brillouin or Rayleigh frequency shift-strain coefficients and our experimental technique successfully applied to strain measurement sandstone under static pressure and to monitor deformation of a high permeable target layer during injection of fluids. These results strongly suggest the usefulness of distributed strain measurement with an optical fiber sensing system.

© 2014 Published by Elsevier Ltd. This © 2014The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of GHGT Peer-review under responsibility of the Organizing Committee of GHGT-12

Keywords: CO2 injection; optical fiber; strain; deformation; Brillouin shift; Rayligh shift

* Corresponding author. Tel.: +81-774-75-2312; fax: +81-774-75-2316. E-mail address: [email protected].

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12 doi:10.1016/j.egypro.2014.11.430

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Ziqiu Xue et al. / Energy Procedia 63 (2014) 4003 – 4009

1. Introduction In Salah is an inductrial-scale carbon capture and storage projected in central Algeria. It provides the first example using InSAR (Interferometric Synthetic Aperture Radar) satellite data to monitor surface deformation resulting from pressure buildup caused by CO 2 injection. Onuma and Ohkawa [1] analyzed the surface deformation around the three CO2 injection wells and estimated the deformation (uplift) rate was up to 7mm/year. Figure 1 shows a few snapshots from September 2004 to May 2008. Areas around KB-501 and KB-503 deformed soon after the commencement of CO2 injection. Many research works have been done on the surface deformation analysis and several hypotheses have been put forward by various groups. These hypotheses include fault leakage, flow through preexisting fractures, or the possibility that injection pressures hydraulically fractured a portion of the lower seal [2, 3, 4]. Recently White et al. [5] suggested the most likely explanation for the observation results is that the lower caprock was hydro-fractured, although interaction with preexisting fractures may have played a significant role.

Figure 1. Deformations around CO2 injection wells at In Salah during September 2004 to May 2008.

As pointed out by White et al. [5] that In Salah CO2 injection project benefitted greatly from the diversity of monitoring techniques applied at the site. It is groundbreaking in its use of InSAR as a storage monitoring technology. It is not so difficult to understand that formation pressure buildup due to injection of CO2 and the pressure buildup will cause deformation within overburden layers. From a geomechanical point of view, we argue how to relate surface deformation to pressure buildup in deep formation. A conventional strainmeter measures local change in the volume immediately surrounding the instrument while distributed optical fiber sensors is commonly used for spatial resolution of strain and temperature. In this study we propose the distributed optical fiber sensing technology to monitor the deformation profile within overburden layers caused by pore pressure buildup due to CO2 injection into the target reservoir. Liu et al. [6] demonstrated a multiplexed Fiber Bragg Grating sensor with static strain resolution of 10 nanostrain for crustal strain monitoring. Optical fiber sensing system has attractions and advantages in small size, low cost, easy installation, high stability and good linearity over a large strain range [6]. We carried out laboratory measurements with metallic plates and a cylindrical Berea sandstone sample by using hybrid Brillouin-Rayleigh sensing system and then applied our technique to monitor deformation along the wellbore above a target layer during fluid injection, while the optical fiber cables were cemented outside of well casing.


2. Laboratory Measurements with Metal Plates and Sandstone Sample 2.1. Metal Plates Under Atmospheric Pressure Conditions The first simple experiment was performed to measure both strains of conventional strain gauge and optical fiber [7]. Two strain gauges and one optical fiber cable were glued to the Aluminum and Copper plates. Pet bottle filled with water was used to apply dead weight to plates. Figure 2 shows the configuration of this simple test and we measured strains when changing the dead weight. Figure 3 shows the results of strain measurements. Strains measured by optical fiber and strain gauge are in good agreement. The results also indicate the two different methods share same accuracy for strain measurements.

Figure 2. Schematic view of strain measurements under atmospheric pressure conditions.

Figure 3. Results of strain measurements of aluminum (a) and copper plates (b) by Brillouin and Rayleigh scattering and strain gauges under atmospheric pressure condition. BR for Brillouin; RAY for Rayleigh; SG for strain gauge.

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2.2. Metal Plates Under Hydrostatic Pressure Conditions For the practical use optical fiber cable will be deployed to deep formation, consequently effects of pressure and temperature needed to be considered when converting Brillouin or Rayleigh frequency shift into strain. To verify such effects we set the metal plates inside a high pressure vessel and measured strains by optical fiber and strain gauge when changing the hydrostatic pressure (Figure 4).

Figure 4. Schematic view of strain measurements under hydrostatic pressure conditions.

Figure 5. Comparison between measured and calculated frequency shift for Brillouin (a) and Rayleigh (b).

Kogure et al. [7] concluded that under hydrostatic pressure conditions both linear and radial deformations appeared in optical fiber and such effects must be considered when determining the Brillouin or Rayleigh frequency shift-strain coefficients. They used strains measured by strain gauges to


estimate both Brillouin and Rayleigh frequency shift with the new coefficients and then compared the measured frequency shifts with the calculated ones. Results of the comparison suggest that they have strong correlation with each other (Figure 5). 2.3. Berea Sandstone under Hydrostatic Pressure Conditions A cylindrical Berea sandstone sample was set into the high pressure. Optical fiber and strain gauges were glued to the sample and strains were measured by both of them when increasing hydrostatic pressure up 12 MPa. We used the new frequency shift-strain coefficients to estimate strains from both Brillouin and Rayleigh scatterings. Figure 6 shows the results comparing strains measured by optical fiber and strain gauge. Strains are in good agreement between the two different methods and one of strain gauge possibly damaged during loading hydrostatic pressure was detected from this results.

Fig. 6. Results of strain measurements for Berea sandstone by optical fiber and strain gauge.

3. Field Results from a Shallow Well Test We drilled a vertical shallow well with depth of 300 m and installed three fiber cables outside of well casing (Figure 7). Gaseous CO2 was injected from the perforated zone at the depth of 277.4 m to 279.4 m and the total injected CO2 was about 320 kg. The well head pressure was 2.5 MPa as we did not use any boost pumps to increase the injection pressure. Generally the pressure due to the weight of the column of CO2 added to the well head pressure and the bottom pressure was a little higher than the pore pressure within the target zone. The Rayleigh scattering frequency shift appeared soon after started injection of CO2 and impacts of CO2 injection was also detected above the CO2 injection zone. From the preliminary results obtained from optical fiber cables we clearly confirmed that the width of impacted zone was about 10 meters which almost doubled the width of perforated interval. The strain resolution of this distributed optical fiber cables was a few microstrain. There is still room for improvement of optical fiber cable but a few microstrain is almost enough to monitor the caprock integrity.

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Fig. 7. Schematic view of the shallow well for CO2 injection. Optical fiber cable was installed outside of well casing.

Fig. 8. Time lapse changes of Rayleigh frequency shift during CO2 injection and roughly estimated strains.


4. Conclusions We demonstrated the usefulness of distributed strain optical fiber sensing system from both laboratory and field measurements. Results of metal plates strain measurements under atmospheric pressure and hydrostatic pressure conditions enabled us establishing a new method to determine Brillouin and Rayleigh frequency shift-strain coefficients. With these new coefficients we successfully estimated strains of Berea sandstone under hydrostatic pressure and the strains are in good agreement with that measured by conventional strain gauge. We also successfully installed optical fiber cables outside of well casing at the vertical shallow well. Rayleigh scattering frequency shift was clearly observed at the perforated and the above zones. The frequency shift increased during CO2 injection and maximum strain estimated from the Rayleigh frequency shift was less than 20 microstrain. Strains within the impacted zone due to CO 2 injection and will not make any influence to the overburden integrity.

Acknowledgements This work was supported by the Ministry of Economy, Trade and Industry of Japan (METI) under the contract research of “Development of Safety Assessment Technology for Carbone Dioxide Capture and Storage”. We thank staff of NBX and RITE involved in this project. References [1] Onuma T. and Ohkawa S., Detection of surface deformation related with CO2 injection by DInSAR at In Salah, Algeria, Energy Procedia 2009, 2177–2184 [2] Vasco DW, et al. (2010) Satellite-basedmeasurements of surface deformation reveal fluid flow associated with the geological storage of carbon dioxide. Geophys Res Lett 37(3): L03303.. [3] Shi JQ, Sinayuc C, Durucan S, Korre A (2012) Assessment of carbon dioxide plume behaviour within the storage reservoir and the lower caprock around the KB-502 injection well at In Salah. Int J Greenh Gas Control 7:115–126. [4] Verdon JP, et al. (2013) Comparison of geomechanical deformation induced by megatonne-scale CO2 storage at Sleipner, Weyburn, and In Salah. Proc Natl Acad Sci USA 110(30): E2762–E2771.. [5] White J., Chiaramonte L., Ezzedine S., Foxalla W., Hao Y., Ramirez A. and McNab W., Geomechanical behavior of the reservoir and caprock system at the In Salah CO2 storage project. 6] Liu Q., Tokunaga T., Mogi K., Matsui H., Wang H., Kato T and He Z., Ultrahigh resolution multipleaxed fiber bragg grating sensor for crustal strain monitoring, IEEE Photonic Journal, 3, 2012, 996-1003. [7] Kogure T., Horiuchi Y., Kiyama T., nishizawa O., Xue Z. and Matsuoka T., Fiber optic strain measurements using distributed sensor system under static pressure conditions (in Japanese), submitted to Explaoration Geophysics of Japan.

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