Analytical Techniques for Evaluating Seal Capacity for Carbon Dioxide Storage in Selected Australian Basins

Abstract

The geological storage of carbon dioxide (CO₂) is a mitigation strategy for the continued use of fossil fuels without the release of CO₂ to the atmosphere. The successful geological storage of CO₂ requires, among other properties, adequate caprocks; to provide a seal for the safe storage of CO₂ on a geological time scale. This study identifies and evaluates various technologies to determine the suitability of caprocks from selected Australian Basins to contain stored CO₂. The primary technique for assessing the amount of CO₂ that can be contained in a geological formation by the overlying caprock is mercury injection capillary pressure (MICP) analysis. MICP is used to test sealing lithologies to determine the maximum column height of carbon dioxide that can be retained. The study has been broken down into four aspects critical to evaluating and improving MICP analyses. These are (i) Evaluation of the accuracy, repeatability and comparability of MICP analytical techniques; (ii) Reviewing the influence of different sample types on MICP analysis; (iii) Investigating the effects of warehousing (long-term core storage) on MICP data and (iv) Developing a technique to produce synthetic MICP curves from nuclear magnetic resonance (NMR) data over caprock (sealing) intervals. The results of the study demonstrate that MICP analysis is accurate and by using a method derived from this study, a viable correction for conformance is possible. The MICP derived porosity and permeabilities (poroperms) were compared to poroperms from both helium pycnometry and NMR analyses. The porosity results indicate that there is a trend between measurements from different techniques within the same well, but this trend does not extend to other wells tested. The sample type and subsequent effects on MICP analysis were evaluated by analysing conventional core (CC) samples using (i) ―Routine‖ preparation techniques, (ii) Samples where mercury was allowed to enter by vertical intrusion only, and (iii) Synthetic cuttings prepared from crushed CC. Also, drill cuttings from equivalent depths as the conventional core were analysed. The MICP analyses of all CC samples show no significant differences from one another other than a consistent and predictable change in porosity across sample types. The analyses of the drill cutting samples, however, indicate significant variation in the MICP analyses to the conventional core samples. The investigations into warehousing of CC samples demonstrate that the MICP analyses performed on the warehoused samples differ from results of the original samples by varying degrees, suggesting that the storage of samples does lead to alteration of the pore networks. The degree of variation appears to be formation specific; significant changes are observed in MICP analyses of the Muderong Shale while only minor differences are observed in MICP analyses of Belfast Mudstone. Synthetic MICP curves were generated from NMR analyses using both laboratory and well data. The synthetic MICP curves from the laboratory NMR data are similar to the CC MICP curves. The extension of this technique to wellbore NMR data produces synthetic MICP curves too variable to that of CC MICP curves to be considered reliable.