UCG Theme 5: Geo-Energy The effect of carbon dioxide injection on the creep of granular calcite aggregates Researcher: E. Liteanu Within the Geo-Energy theme, our research investigates the effect of carbon dioxide (CO2) long term storage in carbonate reservoirs, as subsurface storage of CO2 offers one of the most promising options for reducing CO2 emissions in the coming decades. To evaluate possible CO2 escape scenarios, and to model the long-term behavior of such reservoirs, data concerning the response of carbonate rocks to carbon dioxide introduction are needed. Therefore, we performed uniaxial compaction tests on dry and wet granular calcite aggregates (crushed and sieved Carrara Marble) to systematically investigate under controlled conditions similar to typical reservoir condition the effect of CO2 on the compaction creep of calcite. In addition to these tests we have performed uniaxial compaction creep experiments in presence of supercritical CO2 using pore fluids of varying salinity, similar to brine filled reservoirs. Uniaxial compaction experiments in absence of salts were conducted on dry and wet calcite samples of various grain sizes (1, 1-20, 20-40, 28-37, 37-50, 50-75 m) at temperatures between 25- 150º C, applied effective stress of 4-50 MPa, and 5-10 MPa CO2 pressures. The tests conducted in presence of salts were carried out under the same conditions as stated previously, but using saline pore fluid solutions with 0, 0.6, 1, 2, 3 M NaCl concentrations, and concentrations of 0,1 and 2 M MgCl2 with and without supercritical CO2 at a pressure of 10 MPa. The result of the experiments showed that compaction of calcite in presence of water is dependent on grain size, applied stress and pH of the solution. Injection of CO2 into the system determined an increase of the strain rates up to 3 orders of magnitude in comparison with CO2 free cases. Furthermore, tests performed without CO2 and without added salts showed a systematic increase in strain rate with increasing grain size. Addition of salts to the pore fluid led to the same type of dependence on grain size, however at strain rates some 5 times slower. In the experiments conducted without added salts, but in the presence of CO2 at 10 MPa, the strain rates are increased strongly (20 times compared with CO2 “free” samples) and vary inversely with increasing grain size. Addition of salts in this case, led to a decrease of strain rate by a factor of 3-5, and produced a switch from inverse dependence of the strain rates on the grain size to a direct dependence. In the experiments performed without supercritical CO2, the mechanical results and microstructural evidence are interpreted to indicate that the mechanism responsible for compaction of our calcite aggregates is microcracking, whereas in the experiments with CO2, but without added salts, these data lead us to conclude that pressure solution creep controlled compaction. When salts are added, a switch from pressure solution creep to microcracking is inferred to take place, suggesting that addition of salts inhibits pressure solution. The results of the present work imply that storage of CO2 under geological conditions, will lead to significantly slower compaction creep, due to presence of salts in the pore fluid, than predicted in previous modeling studies. However, it remains difficult to quantify the rate of compaction creep of specific carbonate reservoirs due to CO2 storage, as compaction behaviour can change strongly as a function of variables such as pore fluid pH, the presence of salts, grain size, porosity and other microstructural and compositional factors. The findings of our research will be further integrated within the Geo-Energy theme, as input data for future numerical models to predict, at a reservoir scale, the long-term effects of CO2 storage. Publications: The results of these experiments are to be submitted for publication by Dec 2007 under the following titles: a. “The effect of carbon dioxide injection on creep of granular calcite aggregates” to be submitted to Journal of Geophysical Research b. “Effect of impurities on compaction creep of calcite in presence of supercritical carbon dioxide” to be submitted to Chemical Geology. Presentations: 1) The effect of carbon dioxide injection on creep of granular calcite aggregates, 3rd CATO day, Ready to take off?!, June 15 2007, Congrescentrum Elektrum, Arnhem UCG Theme 5: Geo-Energy 2) Effect of CO2 Injection on Compaction of Carbonate Rocks- AAPG Hedberg Research Conference Basin Modeling Perspectives: Innovative Developments and Novel Applications, May 6-9, 2007, The Hague 3) The effect of carbon dioxide injection on creep of granular calcite aggregates - Euroconference of Rock Physics and Geomechanics , 25 – 30 September 2007,Erice, Italy.
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