CEMENT TRANSPORT PROPERTIES FOR WELLS IN THE PHASE 1A AREA OF THE WEYBURN CO_2 PROJECT

摘要

A task associated with the International Energy Agency (IEA) Weyburn CO_2 Monitoring and Storage Project is the determination of the long-term failure mechanisms of seals in and around wellbores penetrating the host aquifer. On this scale, the contribution to CO_2 flux along pathways associated with wells can be expected to be significant. The leakage flux of CO_2 through wells will depend on the number of wells in the CO_2-concentrated area and on the evolution with time of cement seal transport properties (annulus and plug). Thus, the durability of cement wellbore plugs and annulus seals is essential to any subsurface-based concept for the long-term storage of CO_2. The performance of the cement will depend on its resistance to attack by the CO_2 stored in the reservoir and to the different aggressive ions found in the brine aquifers that are part of the reservoir system. Although there have been some preliminary short-term experiments with the exposure of cement to CO_2, there is a need for a detailed evaluation of the performance of cement under the currently envisaged post-EOR conditions in the Weyburn reservoir. This paper addresses the computational options for assessing the potential range of cement transport properties needed for a long-term performance assessment of CO_2 storage within the Weyburn system. Cement degrades over time, usually in the form of cracking, and chemical alteration, depending on many factors, e.g., sealing quality, subsurface environment, etc. The major processes of carbonation, sulfate attack and pH of the pore solution are discussed relative to their impact on cement transport properties. Using the conditions within the Weyburn system (primarily the geosphere region about the reservoir) and computational models within the study, estimates of the initial and evolving cement transport properties are provided for the main classes of wells present within the Phase 1A project region. In general, initial cement permeabilities are estimated to be on the order of 10~(-19) m~2 and degrade to 10~(-18) m~2 adjacent to low permeability, aquitard lithologies (i.e., shales) and to degrade to 10~(-15) m~2 adjacent to high permeability, aquifer lithologies (i.e., unconsolidated sand).