Among in-situ microbes within depleted oil-gas reservoir, the species dominant in CO2 rich environment produce methane much faster than those dominant in CO2 poor environment. CO2 acts as a catalyst in the reaction. If we maintain preferable conditions for methanogenic microbes during geological CCS, we will be able to abate greenhouse gas emission and produce natural gas as one of natural energy resources at the same time. We named the technological concept as‘ Microbial Associated Geological CCS (Bio-CCS)'. In Bio-CCS, CO2 will be injected from a well for two purposes: to abate greenhouse gas emission and to cultivate methanogenic geomicrobes. CH4 gas will be produced later using other wells. The procedure is similar to the Enhanced Oil/Gas Recovery (EOR/EGR) operation, but in Bio-CCS, the target is production of methane gas out of residual oil in depleted oil/gas reservoir CO2 abatement. To evaluate the basic feasibility of the new conceptual technology, we conducted preliminary risk assessment of Bio-CCS conceptual process. First of all, based on result of numerical calculations using geological model of Bio-CCS process, we assumed a procedure of Bio-CCS site: 1 million CO2 will be injected into depleted oil reservoir in 10 years; the reservoir will be kept still for 90 years and 0.5 million t CH4 will be produced; after 100 years from the first CO2 injection, CH4 production will be started. We developed hazard scenarios by way of literature survey and statistical analysis of accident statistics. Then we applied the hazard scenarios to the assumed Bio-CCS procedure. As the result, the preliminary risk assessment assures that the Bio-CCS process will be safe. Even it happens any leaking accidents, most impacts on peripheral area of Bio-CCS site will be negligible.
