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Opportunities for underground geological storage of CO2 in New Zealand; Report CCS-08/5, Onshore Taranaki Basin overview



Opportunities for underground geological storage of CO2 in New Zealand; Report CCS-08/5, Onshore Taranaki Basin overview



GNS Science Report: 134



Reducing the amount of carbon dioxide (CO2 ) emitted into the earth's atmosphere from industrial sources can be achieved by carbon capture and storage (CCS). One method of storage is geological disposal of CO2 underground (geosequestration). This report presents some background information on different types of geosequestration and factors affecting them, followed by an introductory review of Taranaki Basin geology, and analysis of some geosequestration options in the basin. The Taranaki Basin is well-defined geologically, in part because of a wealth of open-file data available from petroleum industry activities. The basin has been faulted, folded and partially uplifted as a result of plate boundary-related tectonism over the past 20 million years. However, it has not been extensively uplifted or eroded, so that much of the basin's depositional fill remains preserved, either beneath the sea bed or beneath the ring plain of the Taranaki volcanoes. The sedimentary succession underlying the onshore Taranaki peninsula is up to c. 8000 m thick, and contains many different reservoir stratigraphic levels. Each of these reservoir units is usually overlain by relatively impermeable fine-grained rocks with moderate or good sealing capability. With the exception of isolated petroleum accumulations, most permeable reservoir rocks contain salt water, although freshwater aquifers occur near to the surface, and down to depths of c. 800 m in the southwest peninsula. Many ancient fault-bounded structures exist, but only a handful of active (modern) faults are known. The onshore Taranaki region is an obvious venue for CO2 sequestration in New Zealand for a number of reasons. Firstly it is the locality of several industrial emitters of CO2 (Methanex methanol manufacturing plants at Waitara and Motunui, New Plymouth power station (although this was mothballed in 2007), Taranaki combined cycle (TCC) gas turbine at Stratford, the Vector natural gas processing plant at Kapuni, Waitara ammonia-urea plant, and the Hawera dairy factory and co-generation plant). Secondly, it has an established petroleum industry, with infrastructure, technology and practices that are compatible with sequestration operations, not to mention considerable subsurface data and knowledge. In addition, there are several different types of storage and several different underground reservoir storage levels possible in onshore Taranaki. Moreover, CO2 occurs naturally in target storage reservoirs in the region, which provides confidence that injected CO2 will be chemically stable under reservoir conditions. One possible storage option is sequestration in unmineable coal seams (below approximately 350 m depth), which may be tenable in the Mokau, Ohura-Tangarakau and/or Waitewhena Coalfields in the King Country Basin (inland northeast Taranaki). However there are several factors that may count against using Taranaki's shallow unmineable coal seams for CO2 sequestration. Firstly, the coal resource is small, especially compared to that in the Waikato, West Coast, and Southland coal regions. Secondly, the coalfields are located in remote, rugged, hilly terrain and, thirdly, there is a lack of nearby infrastructure such as pipelines. The coal bed methane potential of the Maryville Coal Measures in eastern Taranaki is currently being assessed (by Solid Energy), and if a commercial extraction project should arise, there may be a parallel opportunity to trial CO2 injection into coals, for both CO2 storage and enhanced coal bed methane recovery. Sequestration options in shallow coal seams are discussed in more detail in a companion report (Edbrooke et al. 2009b). Two main types of repository are identified for possible geological sequestration in the onshore Taranaki region. The first main CO2 storage option in Taranaki is saline aquifers below depths of about 800 m (which is considered to be a safe depth threshold to prevent any potential contamination of potable water resources and to also ensure efficient storage of CO2 in a supercritical state). Saline water-bearing reservoirs are present at virtually all stratigraphic levels beneath the peninsula. The second main CO2 storage option is in depleted or near-depleted oil and gas reservoirs in mature producing fields. In this type of reservoir CO2 can be simply stored, or it can be injected to enhance oil recovery (EOR) or gas recovery (EGR). The use of CO2 for EOR is a mature technology, and requisite threshold values for various physical parameters and oil properties have been empirically defined. New Zealand oils fall mostly within the range deemed suitable for CO2 sequestration. CO2 injection for EGR is not a widely practiced technology at this stage. (auth/DG)

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