Fr-01-03 Reservoir Monitoring in Oil Sands Using a Permanent Cross-well System: Status and Results after 18 Months of Production R. Tondel* (Statoil ASA), S. Dümmong (Statoil ASA), H. Schütt (Statoil ASA), A. Ducrocq (Statoil ASA), R. Godfrey (Schlumberger), J. Ingham (Schlumberger) & D. LaBrecque (Multi-Phase Technologies) SUMMARY A joint research project between Statoil and Schlumberger is focusing on permanent cross-well geophysical methods for reservoir monitoring during steam assisted gravity drainage (SAGD). A feasibility study indicated detectable differences in seismic and electrical reservoir properties based on expected changes in temperature and fluid saturation during oil production. Based on these results, a permanent cross-well system was installed at Statoil s Leismer Demonstration Area (LDA) in Alberta, Canada. Baseline datasets, including cross-well seismic, 3D vertical seismic profiling (VSP) and crosswell electrical resistivity tomography (ERT), have been acquired. Comparisons between conventional surface seismic and downhole seismic data show an increase in resolution and frequency content as expected. The ERT baseline shows clear separation between zones of high and low electrical resistivity. Several time-lapse studies have been carried out over the last year, and production-induced time-lapse effects are observed in the reservoir section, both for the time-lapse 3D VSP and ERT datasets. Continuous remote ERT monitoring and annual 3D VSP acquisitions will give the potential of each data type as monitoring tools for oil sand reservoir, as well as possibilities for integrating acoustic and electric data.
Introduction Statoil entered the Athabasca oil sands region in Canada in 2007, and first oil from the Leismer Demonstration Area (LDA) was produced in early 2011. Out of several research initiatives, a joint project between Statoil and Schlumberger focuses on permanent cross-well geophysical methods for reservoir monitoring during steam assisted gravity drainage (SAGD). The feasibility study indicated detectable differences in both acoustic and electrical reservoir properties based on expected changes in temperature and fluid saturation during SAGD. Based on these results, a permanent dual-well monitoring system was developed and installed at LDA (Tøndel et al., 2011). Straddling two production/injection well pairs, the two vertical observation wells were drilled 150 m apart to approximately 550 m depth, and the following sensors were attached to the outside of electrically insulated steel casing in each well and cemented in place, as shown in Figure 1: 32 three-component geophones 32 electrical resistivity tomography (ERT) electrodes A continuous distributed temperature sensing (DTS) system 2 pressure and temperature gauges (east well only) With the exception of geophones and geophone cables, all downhole components were rated for temperatures above 250 o C. Background Key factors during the initial screening of cross-well technologies were hightemperature capabilities, possibilities for permanent deployment and costs. The most important factor was, however, how detectable an expected time-lapse contrast would be compared to the background model. After the initial screening, three permanent cross-well methods were evaluated in the feasibility study: Electromagnetics (EM), electrical resistivity tomography (ERT) and seismic. Both cross-well measurements and vertical seismic profiling (VSP) were considered as seismic solutions. Ranganayak et al. (1992) and Engelmark et al. (2007) have described that steam flooding of a heavy oil reservoir will decrease formation resistivity. This is promising for the ERT method, which is being used routinely for shallow environmental monitoring and in a few examples from the oil industry (e.g., Daily et al., 2004). Given the combination of increased temperature and reduced oil saturation, modeling indicated that the ERT technology would be able to detect production changes in the oil sand reservoir. The feasibility study indicated that EM methods are less likely to give clear time-lapse images under the current reservoir conditions; hence EM tools were not included in the well design. Using the rock physic work of Kato et al. (2008), seismic property modeling indicated detectable changes in compressional and shear velocities, mainly caused by saturation and temperature effects, respectively. Figure 1 Design of east observation well. Six cables were installed in this well, connecting 32 three-component geophones, 32 ERT electrodes, two pressure & temperature gauges and a DTS system to the recording instruments on the surface. The black rectangle marks the reservoir zone, where instruments are more closely spaced. Instruments are distributed over an interval of approximately 335 m (195 530 m MD).
Baseline datasets After successfully testing the downhole instruments, baseline datasets were acquired prior to production start-up. For comparison purposes, Figure 2A shows a section from the baseline surface seismic dataset. In Figure 2B, a section from the baseline 3D VSP cube can be found, while Figure 2C shows the cross-well seismic section. These three seismic sections show an increase in resolution and frequency content as would be expected, due to use of downhole receivers and increased source point density for the seismic data in Figures 2B and 2C. While the source frequency is comparable for the surface seismic and 3D VSP datasets (8-210 and 8-180 Hz, respectively), a sweep of 30-800 Hz was used for the cross-well seismic section. Figure 2 Baseline data from permanent cross-well system at LDA together with conventional surface seismic data (A). The three panels to the right show seismic from 3D VSP (B), cross-well seismic (C) and an ERT section (D). The blue arrow in the middle represents the reservoir zone. Sections are 150 m wide and approximately 240 m deep. Warm colors in the ERT section are zones with high electrical resistivity (~100 m), while cold colors are zones with low electrical resistivity (~1 m). Prior to acquiring the ERT baseline section shown in Figure 2D, extensive acquisition parameter testing was carried out. During this time, an instrument cabinet was installed at site, allowing for permanent cross-well ERT acquisitions without personnel on site. The 3D VSP surveys still require a moving source on the surface, but seismic recording and quality control of data can be done remotely. The ERT baseline section shows clear separation between zones of high and low electrical resistivity, and the reservoir zone (indicated by the blue arrow) is imaged as a high resistive body as expected. Some noise can be observed along the edges, likely due to minor electrical leakages through the well casing. Careful preparation of a reliable background resistivity model is paramount for ERT timelapse imaging since the baseline model serves as start model for each repeat inversion, and the inverted repeat data are finally referenced to the baseline model in order to prepare images of the resistivity changes. Time-lapse data examples After 10 months of production, including ramp up of the SAGD process, the first repeat 3D VSP was acquired in January 2012. Approximately 3,400 source points distributed over a 1 km 2 area were once again visited. During processing of the individual vintages, first arriving turning waves represented a challenge, as did the non-conventional geometrical setup of the dual-well 3D VSP (Dümmong et al., 2012). These challenges could be mitigated by careful hand picking of the direct arrivals in the data and the use of a general radon transform migration algorithm during imaging (Miller et al., 1987). Another challenge for the time-lapse processing is annual thickness variation of frozen ground. This needs to be treated with care to avoid removing subtle production effects in the reservoir section, and most of this was overcome by velocity model calibration via traveltime tomography. As shown in Figure 3, a clear velocity decrease can be observed in the reservoir interval when displaying interval
velocities calculated from near-offset checkshots for the two 3D VSP vintages, in line with the predictions from the feasibility study. As shown in Figure 4, production-induced time-lapse effects can be seen in the reservoir section, partly biased by acquisition footprints. Here, seismic sections from the baseline and repeated 3D VSP can be found. The repeat survey shows distinct amplitude brightening and lower frequencies above the horizontal injection/production pairs at approximately 225 m in Figure 4B. Acquisition footprints can be observed in the shallower parts. These are recognized as high amplitudes close to the wellbores and lower amplitudes as the distance increases. Figure 3 Comparison of near-offset checkshots for the 3D VSP baseline (blue line) and repeat (red line) surveys for the two observation wells. A clear velocity decrease can be observed in the reservoir zone indicated by yellow color. In the time-lapse ERT data, a clear decrease of electrical resistivity is observed around the two steam injection wells. This decrease is gradual over time and reaches a resistivity change of more than -50% to date. As for the seismic data, this change is caused by a combination of temperature increase and steam/water replacing the oil. The order of magnitude of the inverted resistivity change is within the limits of Archie estimates for reasonable temperature and saturation changes (Mavko et al., 1998). Given a good data aperture, the two steam chambers are separated during the inversion, and the steam chamber growth can be observed week by week using the permanent system setup. The noise along the observation wells (as seen in Figure 2D) is still present, but has decreased during the time-lapse processing, indicating that the majority of this noise is repeatable. Figure 4 Images of the baseline 3D VSP (A) and the repeated survey (B). In both figures, surface seismic data is in the background. The two observation wells with the permanent geophone locations and four horizontal injection/production well pairs are shown. The yellow arrow in Figure 4B indicates the reservoir section.
Conclusions A permanent cross-well geophysical system has been installed on Statoil and PTTEP s Leismer Development Area in the Athabasca oil sands region in Alberta, Canada. Baseline datasets have been acquired through an established remote link, allowing for frequent data acquisitions throughout the year independent of surface conditions. Several time-lapse studies are initiated, aiming at understanding how acoustic and electric reservoir parameters vary during SAGD. Preliminary results from these studies show that the individual steam chambers are observed in both the 3D VSP and ERT time-lapse data. A planned repeat of the 3D VSP acquisition in Q1-2013 and continuously ERT monitoring will increase our understanding of each technology s potential and how different data types can be integrated to allow for robust and reliable remote monitoring of oil sand reservoirs. Acknowledgements The authors would like to thank Statoil, PTTEP and Schlumberger for permission to present this work. We would like to acknowledge WesterGeco s GeoSolutions group for their contribution to this project. In addition, valuable input has been given by Andrew Williams and Ole-André Eikeberg from Statoil. References Daily, W., Ramirez, A., Binley, A. and LaBrecque, D. [2004] Electrical Resistivity Tomography. The Leading Edge, 23, 438-442. Dümmong, S., Tøndel, R., Barrett, A., Nutt, L., Campbell, A. and Godfrey, R. [2012] Baseline processing of a dual-well 3D VSP: A case study from monitoring steam-assisted gravity drainage in the Canadian Oil Sands, 82 nd Annual International Meeting, SEG, Expanded Abstracts. Engelmark, F. [2007] Time-Lapse Monitoring of Steam Assisted Gravity Drainage (SAGD) of Heavy Oil using Multi-Transient Electro-Magnetics (MTEM), CSPG CSEG Joint Convention. Kato, A., Onozuka, S. and Nakayama, T. [2008] Elastic property changes in a bitumen reservoir during steam injection. The Leading Edge, 9, 1124-1131. Mavko, G., Mukerji, T. and Dvorkin, J. [1998] The Rock Physics Handbook, Cambridge, 282-287. Miller, D., Oistaglio, M. and Beylkin, G. [1987] A New Slant on Seismic Imaging: Migration and Integral Geometry. Geophysics, 52, 943-964. Ranganayaki, R.P., Akturk, S.E. and Fryer, S.M. [1992] Formation resistivity variation due to steam flooding: A log study. Geophysics, 57, 488-494. Tøndel, R., Ingham, J., LaBrecque, D., Schütt, H., McCormick, D., Godfrey, R., Rivero, J.A., Dingwall, S. and Williams, A. [2011] Reservoir monitoring in oil sands: Developing a permanent cross-well system, 81 st Annual International Meeting, SEG. Expanded Abstracts, 30, 4077-4080.