TOWARDS A METHODOLOGY FOR TOP SEAL EFFICACY ASSESSMENT FOR UNDERGROUND CO_2 STORAGE

摘要

Quantitative assessment of leakage risk and leakage rates from planned underground CO_2 storage sites is a primary requirement for public acceptance, formal site approval, and credit for stored CO_2 quantities under CO_2 emission schedules. Leakage through the top seal can basically occur by three processes: (ⅰ) diffusion through the pore system, (ⅱ) capillary transport through the pore system of the seal, (ⅲ) multiphase migration through a (micro-) fracture network; or by a combination of any of these. Diffusion results in very low leakage rates; maximum rates typically attained after several 100 000 years, being in the ppm range. Multiphase capillary migration is characterized by two main parameters: capillary breakthrough pressure and effective permeability to the non-wetting phase (CO_2). The dependence of effective permeability to CO_2 on capillary pressure, which in turn is a function of CO_2 column height, is hysteretic in character with generally higher effective permeability during pressure decrease (column shrinkage) than during increase, at the same capillary pressure (CO_2 column height). Leakage is likely to stop at approximately 20 to 50% of the breakthrough pressure as suggested by the 'snap-off' theory by Vassenden et al. (2003). Capillary breakthrough pressure and effective permeability is very difficult to measure for low-permeable rocks. A recently presented method by Hildenbrand et al. (2002) shows promising results, but their data require cautious re-interpretation prior to application to CO_2-storage cases. Time-delay effects may imply that their 'maximum effective permeability' at laboratory conditions is not the maximum attainable in nature and in CO_2 storage reservoirs. Their 'minimum capillary displacement pressure' may rather be a 'snap-off pressure than a breakthrough pressure, the latter being higher by a factor of 2 to 5. Detection and prediction of the presence of microfractures, which have much larger permeability than the rock matrix, is difficult. Simulation techniques can, however, be used to estimate the likelihood for their generation by burial-induced overpressure. In general, prediction of fluid-flow parameters for the seal to CO_2 storage sites is a challenge due to the probable low data coverage. Reliable extrapolation of such parameters from punctual data at wells across the space above the whole storage site requires considerable improvements of the understanding of depositional processes of fine-grained rocks.