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Estimating reservoir properties with nonseismic techniques
In general, the dominance of seismic technology in reservoir geophysics is because of three factors:
- Seismic waves respond fairly well to reservoir and host-rock properties of interest
- They provide high-resolution images
- There is a wide and deep base of knowledge of seismic techniques in the petroleum industry
However, other technologies can often be employed to investigate properties of the earth that correlate better with the properties of interest. If the images from these technologies can be provided at appropriate resolution, and if the knowledge required for interpretation and wise application of these technologies is available within the industry, they should be used. For example, electrical methods are extremely sensitive to variations in saturation, yet surface-based methods provide very poor resolution. Reservoir compaction can be directly observed from surface deformation, and pore-volume or gas-saturation changes can be detected from changes in the gravitational field.
Surface-based methods of reservoir geophysics include:
- Reservoir characterization by gravity and electromagnetic techniques
- Monitoring of deformation (by releveling surveys, satellite interferometry, gravimetry, or tiltmeters)
- Monitoring of fluid migration by gravimetry, electrical, and electromagnetic techniques
Dramatic examples of surface deformation induced by reservoir compaction have been provided by releveling studies (involving repeated high-accuracy surveying) and satellite-based interferometry. These technologies are directly applicable only to onshore fields, although extensions to bathymetric observations are possible. As pressure in a reservoir decreases during primary production, the overburden load causes a compaction in the reservoir rock. In some instances, encroachment of water can also cause weakening of the matrix and subsequent pore collapse, particularly in some chalk reservoirs. Virtually any reservoir compaction will ultimately be reflected in subsidence at the surface, although in many cases the elastic properties of the overburden rock will delay this for years, perhaps millennia, and may distribute the stresses and strains over such a large area that the actual amount of subsidence in any one location is miniscule. In some cases, however, the subsidence is nearly immediate and profound and should be monitored for a number of reasons. Direct measurements of surface deformation can be obtained by detailed bathymetric surveys, relevelling surveys, or satellite-interferometry surveys. Fig. 1 shows an example of satellite-based observations of ground deformation.
Fig. 1—Examples of satellite-observed deformation. This subsidence map was obtained using satellite interferometry over the Lost Hills and Belridge oil fields for a period covering 105 days (after Xu and Nur).
The gravitational field at the surface of the earth responds to the masses of the objects near it. The distribution of density beneath the surface gravimeter determines the gravitational attraction it senses. If that density distribution changes, for example through subsidence or the displacement of gas by water, a time-lapse high-resolution gravity survey may be able to determine the geographical distribution of that deformation or fluid migration. Surface-based gravity measurements have found some application in exploration geophysics, particularly in aiding the recognition of gas zones. Time-lapse gravity surveys show promise for monitoring gas-cap changes and reservoir deformation. Gravity gradiometry measurements (in which two or more gravimeters or accelerometers are deployed a fixed distance apart) can increase the resolving power.
Surface-based electrical or electromagnetic methods have application to reservoir geophysics through their strong response to saturation and ability to penetrate salt and igneous rocks. In general, their resolution is poor in comparison with seismic methods (see Fig. 2), although there may be instances in which they are appropriate for reservoir management. In some applications, an electrical source is used, and in others, the naturally varying electromagnetic field of the earth is used.
Fig. 2—Example of results of a 2D electromagnetic survey over a known salt body in the Gulf of Mexico. The white line represents the outline of the salt as determined from seismic data; various symbols indicate different aspects of the inversion constraints, while the color indicates the resistivity (after Hoversten et al.).
Because of the proximity of tools located in a borehole, the resolution problem associated with some of the nonseismic geophysical techniques is reduced. In particular, electrical, electromagnetic, and gravity studies find application in borehole-based reservoir geophysics projects, but currently not all of them are in common use.
Electrical and electromagnetic borehole-based methods are extensions of comparable logging technologies (see the Reservoir Engineering and Petrophysics section of the SPE Handbook) but with significant differences that involve imaging at greater distances and through casing. The presence of steel casing in most producing environments seriously limits the capabilities of current methods, but techniques have been developed that can operate in one or more steel-cased wells (see Fig. 3 for an example). These methods have their greatest application in monitoring changes in fluid saturation, for determining proximity to bed boundaries while drilling, or for observing the “streaming potential” created by fluid flow.
Fig. 3—Crosshole electromagnetic (EM) profiling for waterflood monitoring. The upper diagram shows, schematically, the different conductivity paths crossing the reservoir target when a borehole EM source is used in one well, and a set of receivers in another. The lower diagram shows a three-dimensional representation of the change in resistivity as a result of waterflooding operations over eight years for a reservoir in California (from Wilt and Morea).
Borehole gravity measurements can be used to characterize reservoir density (and therefore porosity); monitor fluid movements (particularly gas vs. liquid); and, to a lesser degree, monitor changes in porosity because of compaction. Fig. 4 shows the application of borehole gravity.
- Rhett, D.W. 1998. Ekofisk Revisited: A New Model of Ekofisk Reservoir Geomechanical Behavior. Presented at the SPE/ISRM Rock Mechanics in Petroleum Engineering, Trondheim, Norway, 8-10 July 1998. SPE-47273-MS. http://dx.doi.org/10.2118/47273-MS.
- Xu, H. and Nur, A. 2001. Integrating Reservoir Engineering and Satellite Remote Sensing for (True) Continuous Time-Lapse Monitoring. The Leading Edge 20 (10): 1176, 1198. http://dx.doi.org/10.1190/1.1487250.
- Massonnet, D. and Feigl, K.L. 1998. Radar Interferometry and Its Application to Changes in the Earth’s Surface. Reviews of Geophysics 36 (4): 441. http://dx.doi.org/10.1029/97RG03139.
- Chapin, D. 1998. Gravity Instruments: Past, Present, Future. The Leading Edge 17 (1): 100.
- Johnson, E.A.E. 1998. Use Higher Resolution Gravity and Magnetic Data as Your Resource Evaluation Progresses. The Leading Edge 17 (1): 99.
- Huston, H.H., Sestak, H. and Lyman, G.D. 1999. Methodology for Interpreting 3D Marine Gravity Gradiometry Data. The Leading Edge 18 (4): 482.
- van Gelderen, M., Haagmans, R. and Bilker, M. 1999. Gravity Changes and Natural Gas Extraction in Groningen. Geophysical Prospecting 47: 979. http://dx.doi.org/10.1046/j.1365-2478.1999.00159.x.
- Eiken, O., Zumberge, M., and Sasagawa, G. 2000. Gravity Monitoring of Offshore Gas Reservoirs. Paper presented at the 2000 Society of Exploration Geophysicists Intl. Exposition and Annual Meeting, Calgary, 6–11 August.
- Rybakov, M. et al. 2001. Cave Detection and 4D Monitoring: A Microgravity Case History Near the Dead Sea. The Leading Edge 20 (8): 896. http://dx.doi.org/10.1190/1.1487303.
- Brady, J.L., Wolcott, D.S., and Aiken, C.L.V. 1993. Gravity Methods: Useful Techniques for Reservoir Surveillanc. Presented at the SPE Western Regional Meeting, Anchorage, Alaska, 26-28 May 1993. SPE-26095-MS. http://dx.doi.org/10.2118/26095-MS.
- Pawlowski, B. 1998. Gravity Gradiometry in Resource Exploration. The Leading Edge 17 (1): 51.
- Hoversten, G.M., Constable, S.C., and Morrison, H.F. 2000. Marine Magnetotellurics for Base-Of- Salt Mapping: Gulf of Mexico Field Test at the Gemini Structure. Geophysics 65 (5): 1476. http://dx.doi.org/10.1190/1.1444836.
- MacGregor, L. and Sinha, M. 2000. Use of Marine Controlled-Source Electromagnetic Sounding for Sub-Basalt Exploration. Geophysical Prospecting 48 (6): 1091. http://dx.doi.org/10.1046/j.1365-2478.2000.00227.x.
- Rabinovich, M. et al. 2000. Application of Array Resistivity Measurements in Horizontal Wells. The Leading Edge 19 (4): 413. http://dx.doi.org/10.1190/1.1438624.
- K., Mizunaga, H., and Tanaka, T. 1998. Reservoir Monitoring by a 4D Electrical Technique. The Leading Edge 18 (12): 1422.
- Hoversten, G. M. et al. 2001. Reservoir Characterization Using Crosswell Electromagnetic Inversion: A Feasibility Study for the Snorre Field, North Sea. Geophysics 66 (4): 1177. http://dx.doi.org/10.1190/1.1487064.
- Wilt, M. J. et al. 1995. Crosshole Electromagnetic Tomography: System Design Considerations and Field Results. Geophysics 60 (3): 871. http://dx.doi.org/10.1190/1.1443823.
- Wilt, M. and Morea, M. 2004. 3D Waterflood Monitoring at Lost Hills with Crosshole EM. The Leading Edge 23 (5): 489. http://dx.doi.org/10.1190/1.1756840.
- Nekut, A.G. 1995. Crosswell Electromagnetic Tomography in Steel-Cased Wells. Geophysics 60 (3): 912. http://dx.doi.org/10.1190/1.1443826.
- Wilt, M. and Alumbaugh, D. 1998. Electromagnetic Methods for Development and Production: State of the Art. The Leading Edge 17 (4): 487.
- Newmark, R., Daily, W. and Ramirez, A. 1999. Electrical Resistance Tomography Using Steel-Cased Boreholes as Electrodes. Paper presented at the 1999 Society of Exploration Geophysics Annual Intl. Meeting, Houston, 31 October–5 November.
- Ander, M.E. and Chapin, D.A. 1997. Borehole Gravimetry: A Review. Paper presented at the 1997 Society of Exploration Geophysicists Intl. Exposition and Annual Meeting, Dallas, 2–7 Cite error: Invalid
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- Adams, S.J. 1991. Gas Saturation Monitoring in North Oman Reservoir Using a Borehole Gravimeter. Presented at the Middle East Oil Show, Bahrain, 16-19 November 1991. SPE-21414-MS. http://dx.doi.org/10.2118/21414-MS.
- Alixant, J.-L. and Mann, E. 1995. In-Situ Residual Oil Saturation to Gas from Time-Lapse Borehole Gravity. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 22-25 October 1995. SPE-30609-MS. http://dx.doi.org/10.2118/30609-MS.
Noteworthy papers in OnePetro
Brady, J.L. Reservoir Surveillance Via Modern Gravity Methods,
Fang, S. and Zhdanov, M.S. 3-D Electromagnetic Inversion Using Quasi-linear Approximation: Case Study. Presented at the 1997/1/1/.
Liang, L., Abubakar, A., and Habashy, T. Crosswell Electromagnetic Inversion Constrained by the Fluid Flow Simulator. Presented at the 2010/1/1/. http://dx.doi.org/10.2118/135069-MS.
Patzek, T., Wilt, M., and Hoversten, G.M. Using Crosshole Electromagnetics (EM) for Reservoir Characterization and Waterflood Monitoring. Presented at the 2000/1/1/. http://dx.doi.org/10.2118/59529-MS.
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