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1 SPE-SAS-1186 Effects of Carbon Dioxide Injection in Reactive Carbonates: Computational Rock Physics Basis for Time-Lapse Monitoring Amos Nur (1,2), Tiziana Vanorio (1), Elizabeth Diaz (2). (1) The Stanford Rock Physics Program, Stanford University. (2) Ingrain Inc., Houston, Texas, USA Copyright 2011, Society of Petroleum Engineers This paper was prepared for presentation at the 2011 SPE Saudi Arabia Section Technical Symposium and Exhibition held in AlKhobar, Saudi Arabia, May This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at the SPE meetings are subject to publication review by Editorial Committee of Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and whom the paper was presented. Write Liberian, SPE, P.O. Box , Richardson, TX , U.S.A., fax Abstract Carbon dioxide injection into natural reservoirs has been used for enhanced oil recovery as well as, more recently, for geologic sequestration. In both technological applications, of major interest is remote sensing of the progress of the injected carbon dioxide through the subsurface as well as its effects on the physical characteristics of the rock, including the elastic, storage, and transport properties. Whereas it may be safe to assume that the presence of carbon dioxide does not alter the mineral matrix in most clastic reservoirs, the situation is very different in carbonates where such alteration may occur in real time, in a matter of hours, days and, certainly, months. Physical laboratory experiments have confirmed that the carbon dioxide interacts with formation water and, eventually, with the mineral matrix (Vanorio, T. et all, 2008). These chemical processes alter the pore space geometry in carbonates and can even create such flow conduits as relatively large wormholes. This pore-space and matrix alterations can definitely affect the elastic properties of carbonate rock, which are crucial in interpreting the 4D time-lapse seismic data for pore-fluid content in space and time. Because of the changes occurring in the mineral matrix during carbon dioxide injection, traditional fluidsubstitution techniques often do not work in such reactive rock and, hence, can be misleading during seismic data interpretation. Introduction To be able to quantify the changes in the pore-space geometry in terms of the variation of the elastic properties controlled laboratory experiments have to be conducted, analyzed, and then serve as input to time-lapse seismic interpretation. Two laboratory techniques can serve this purpose: (a) the traditional physical experimentation and (b) the newly developed computational experimentation where the samples are CT-scanned before, during, and after the injection to first directly reveal the alteration in the rock and then use these images to compute the rock properties, including the porosity, permeability (absolute and relative), electrical conductivity, and the elastic moduli. The latter approach reveals the pore-scale processes in all their complexity and, hence, presents a rich basis for deriving general rules and theories to be used in time-lapse seismic interpretation. Figures 1 and figure 2 illustrates the changes that can be registered using 4D X-ray CT imaging on carbonates.

2 2 [SPE-SAS-1186]!"#$%"&&'()&*+,"-.$+& /0"%&'()&*+,"-.$+& 5 mm 5 mm Figure 1: A single slice extracted from a 3D volume shows the apparent enhance of porosity in this carbonate sample. Computations of porosity, permeability, compressional and shear-velocity were done for the two digital rocks, before and after the CO2 injection.!"#$%"&&'()&*+,"-.$+& /0"%&'()&*+,"-.$+& Figure 2: An isolated pore is extracted from a CT-scanned volume to look for changes in the pore shape.

3 [SPE-SAS-1186] 3 4D-CO2 Workflow Here we present a joint physical/computational study of the aforementioned processes where a limestone sample was flooded with carbon dioxide in the physical laboratory; its porosity, permeability, and elastic properties measured before and after the injection; then its subsamples CT-scanned (before and after the flooding); and the physical properties of interest computed. Figure 3 shows the planned workflow. Results will be presented for samples scanned at resolutions corresponding to the following field of view (FOV): 8mm, 4 mm and 2 mm. FOV Work flow - Planned 27 mm 8!"##" mm &'(")*"(+,"-./012" Cut in two halves Top Sent to Stanford lab for CO2 injection Side View Bottom To obtain subsamples for different resolution tests $" 4 mm CO 4 mm 4 mm 2 mm CO 2 mm * Small samples sent to Stanford for imbibition experiment 2 %" 4 4 mm 2 mm 1 mm CO 1 mm Top View 3,4"5)1+" 4 mm 2 mm 1 mm mm CO mm Figure 3: Workflow of DRP (Digital Rock Physics at Ingrain) combined with experimental injection of CO2 (Stanford Lab)

4 4 [SPE-SAS-1186] DRP (Digital Rock Physics) and Experimental Results The scanned samples were selected at two different scales (tens of mm and several mm) and registered to each other. The bigger sample provided a general framework for the flood-related changes while the smaller sample showed these changes at the pore scale. In the latter case, we observed well discernable porosity enhancement in the inlet portion of the sample but essentially no change at the outlet portion. The permeability increase computed on these images went far beyond that anticipated due to porosity increase (Figure 4). This means that preferential flow conduits formed in the sample under investigation. #!!!$ 7.89,:./$ Post_CO2 #!!$ Post_CO2 Pre_CO2 Permeability (md) #!$ ()*+,-./$ Pre_CO2 #$ ()*+,-./$0123(4&$ ()*+,-./$04563(4&$ 7.89,:./$0123(4&$ 7.89,:./$04563(4&$!"#$!"!!$!"!%$!"#!$!"#%$!"&!$!"&%$!"'!$!"'%$ Porosity (fraction) Figure 4: Permeability vs. Porosity computed on a digital sub-sample (green circles- Ingrain DRP) exhibit a similar behavior as the properties measured in the whole plug (red circles - Stanford Lab). This happened because the chemical reactions associated with the injection, combined with drag due to the flow, acted to dissolve and remove the fines that originally impeded the flow. We also discovered that such changes were most pronounced in the micritic part of the sample that had large surface area and, hence, was more prone to chemical alteration. We also registered significant changes in the computed (dry-rock) elastic moduli of the samples, which were reduced by 5 to 10% of the pre-injection values (Figure 5). These elastic property variations showed the same trend as observed during physical measurements.

5 [SPE-SAS-1186] 5 (" '" &" Computed Vp (Km/s) %" $" +,-"./0-12"345678#" +,-"./0-12" #" 9:;<./0-12"!"-=2678#" Measured 9:;<./0-12"!"-,.>678#" 9:;<./0-12"#"-=2678#" #" 9:;<./0-12"#"-,.>678#"?2/.:=2@" #"?2/.:=2@" #"!" )" )*)&" )*!" )*!&" )*#" )*#&" )*$" )*$&" Porosity (fraction) Figure 5: Compressional Velocity vs. Porosity computed at several resolutions on digital sub-samples exhibit a similar behavior as the properties measured in the whole plug. Symbols without black marker line represent values before CO2 injection, while the symbols with black marker line correspond to the values after CO2 injection. Summary We show recent results of a newly developed method for time-lapse digital rock physics to monitor the changes within a carbonate rock upon CO2 injection. We observe a self-enhancing process in porosity and permeability. The change in permeability was higher than the one occurring in porosity. CO2 is removing the fine particles which were probably part of a blocking mechanism in the system and has been now removed. We illustrate via high-resolution CT-scanning that the main changes occur in the micritic phase (which has the highest surface area). Most important, we find changes (5 to 10 %) in elastic moduli and velocities. Velocity decreases as a function of porosity enhancement. These changes significantly deviated from Gassmann s predictions. One significant value of computational experimentation is that it allowed us to rigorously decouple the effects of the mineral matrix alteration from the static pore-fluid effect on the elastic properties. Moreover, computational experiments allowed us to measure various rock attributes on the same physical objects as they were changing in time. Arguably, this is very difficult to achieve in the physical laboratory as the measurement setups vary for different types of measurements. These computational experiments make us conclude that computational rock physics is a new and promising technique for quantitative interpretation of time-lapse seismic data in reactive rock subject to carbon dioxide flooding. Acknowledgements The authors wish to acknowledge Avrami Grader and Jack Dvorkin for all their input during this project. Special thanks to Meghan Armbruster, Sneha Bakhta and Kellen Kanak, Ingrain-Houston, for their hard work on getting all the samples scanned and computed.

6 6 [SPE-SAS-1186] References Vanorio, T., Nur, A.M., Diaz, E., 2011, The rock physicochemical basis for time-lapse seimsic reservoir monitoting of CO2 injection Vanorio, T., Scotellaro, C., Mavko, G., 2008, The effect of chemical and physical processes on the acoustic properties of carbonate rocks, The Leading Edge, 27 (8), Mavko, G., Mukerji, T., & Dvorkin, J., 1998, The Rock Physics Handbook - Tools for seismic analysis in porous media. Nur, A.M., Tosaya, C., and Thanh, D.V., Seismic monitoring of thermal enhanced oil processes: 54th Ann. Internat.Mtg., Soc.Expl.Geophys., Expanded Abstracts, session RS6 Dvorkin, J., Armbruster, M., Baldwin, C., Fang, Q., Derzhi, N., Gomez, C., Nur, B., Nur, A., and Mu, Y., 2008, The future of rock physics: Computational methods vs lab testing: First Break, 26,