The Effect of CO₂ Injection on the Compaction Creep Behaviour of Granular Feldspar Aggregates

S.J.T. Hangx and C.J. Spiers
High Pressure and Temperature Laboratory, Faculty of Geosciences, Utrecht University, P.O. box 80021, 3508 TA, Utrecht, The Netherlands

Geological storage of carbon dioxide in clastic reservoirs and aquifers will result in several effects on the injected host rock: (1) mineralisation, due to water-CO₂-rock interactions; and (2) geomechanical effects, e.g. compaction due to enhanced microcracking. Mineral reactions are expected to be very slow, however, on the short term the geomechanical behaviour of the injected host rock may be influenced by the presence of CO₂.
We performed compaction creep experiments on granular aggregates of feldspar, in order to study mineralisation reactions and geomechanical effects of CO₂ injection. Aiming to maximise the mineralisation contribution experiments were performed on Ca-rich feldspars, i.e. anorthite and labradorite. During the experiments several factors were varied in order to determine their individual effect: (1) grain size, d = 15-300 μm; (2) temperature, T = 25-150°C; (3) CO₂ pressure, PCO₂ = 0-100 bar; (4) effective stress; σeff = 0-80 MPa; and, (5) chemical environment.
Systematic investigation of these factors on granular feldspar aggregates showed that mineral reactions were so slow that they made no significant contribution to the observed compaction behaviour. It appeared that all compaction observed was an effect of microcracking, which was significantly enhanced by the introduction of fluids to the sample. Increasing the experiment temperature of wet aggregates also improved compaction creep rates significantly. However, injection of CO₂ inhibited microcracking, and hence creep rates, of the feldspar samples, as did the presence of an alkaline pore fluid.
Therefore, injection of carbon dioxide into clastic reservoirs and aquifers will inhibit microcracking mechanisms, which could reduce reservoir integrity. In the future more research needs to be done on the long-term effect of CO₂ injection, in order to quantify the effect of mineralisation.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands