Developing a monitoring and verification plan with reference to the Australian Otway CO2 pilot project
Authors: Dodds Kevin , Sherlock Donald Hosking 
Published: October 2008

The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently injecting 100,000 tons of CO2 in a large scale test of storage technology in a pilot project in South Eastern Australia called the CO2CRC Otway Basin Project (Otway). The Otway Basin with its natural CO2 accumulations and many depleted gas fields, offers an appropriate site for such a pilot project. An 80% CO2 stream is produced from a well (Buttress) near to the depleted gas reservoir (Naylor) used for storage. The goal of this pilot project is to demonstrate that CO2 can be safely transported, stored underground and its behaviour tracked and monitored. The monitoring and verification framework has been developed to monitor for the presence and behaviour of CO2 in the sub-surface reservoir, near surface and atmosphere. This monitoring framework has been selected to address the areas identified by a rigorous process of risk assessment and subsequently verify conformance to clearly identifiable performance criteria. These criteria have been agreed with the regulatory authorities to manage the project through all phases addressing responsibilities, liabilities and to provide assurance of safe storage to the satisfaction of the public at large.

Many aspects of the proposed monitoring will be discussed and this paper will provide an overview of the whole plan, with reference to progress in baseline measurements. An extensive range of established direct and remote sensing technologies deployed on surface and in the borehole are being used for repeat assessments from a reservoir, containment, wellbore integrity, near surface and atmospheric perspective. These involve seismic, microseismic, petrophysical well logs and geochemical sampling including tracer and isotope analysis, plus associated forward modelling. The presence of naturally occurring CO2 in the Otway area makes it more difficult to identify injected CO2. A regional survey of the distribution, type and origin of existing CO2 will be carried out through soil gas sampling. The areal consequences of CO2 migration and trapping are being addressed through characterization of the hydrodynamic properties of the region. The connectivity and fluid migration time scales of the potential fresh water reservoirs are being established using all available (and appropriate) well pressure and geological information. The Otway project has been selected as one of the Carbon Sequestration Leadership Forum (CSLF) projects reflecting its global standing.

Introduction

The commercial oil and gas leases (tenements), in the Otway Basin in Victoria, selected for the pilot project, are in an undeveloped CO2 field (Buttress), which is the source of CO2, and a depleted gas field (Naylor), which is the injection/containment site (fig 1). The extracted and separated CO2 stream is transported by pipeline and injected into a new well (CRC-1); drilled down-dip of the existing well; into the depleted Waarre reservoir in the Naylor field at a depth of approximately 2000 metres. The existing shut-in production well (Naylor-1) is being used as the monitoring well. Characterization of the site has involved the collection of a large quantity of geological, geophysical and other regionally relevant data and the construction of static and dynamic reservoir models. The regional formations provide an excellent porous and permeable geological formation that provides a highly suitable reservoir system for CO2 storage. In summary, the site assessment results, indicating that the Waarre Formation is a suitable site for CO2 storage, conclude the following key attributes of the site. There are no significant faults evident in the wells at the Waarre C level; there is a fairly uniform Waarre C thickness. The local and regional seals have contained a number of natural CO2 accumulations in the eastern Otway Basin over geological time. The storage reservoir has enough porosity and permeability to be able to accept the injected CO2 at rates forecast. The injected CO2 is predicted to move updip from the injector location and migrate to the crest of the fault block and accumulate below the residual methane gas cap in the vicinity of the existing Naylor-1 well. The selected site has the major advantage of being onshore rather than offshore, allowing the project research teams to test and further refine the monitoring and verification techniques at a more accessible location.