Locate suitable storage sites

CO2 storage cannot start tomorrow. We must first overcome the challenge of finding the appropriate storage sites and develop a number of new techniques and methods.

An underground storage site for CO2 must consist of two different rock types. The reservoir must contain a porous rock that can be used for storage and a tight cap rock to prevent CO2 escaping after storage, says Elin Skurtveit in NGI's Department for offshore geotechnics.

There are many potentially suitable storage sites in the North Sea, and several countries in Europe may have a potential for CO2 storage onshore. The possibilities for CO2 storage onshore in Norway are limited. However, the conclusion is that the supply of deposits worldwide is great enough for CO2 storage to be a viable technology, says senior physicist Eyvind Aker at NGI's Department of instrumentation and geophysics.

NGI is involved in several projects aimed at the identification and mapping of potential storage sites.

Focus on cap rocks
Porous rock in the target reservoir will typically consist of sandstone or limestone, while the dense (tight) cap rocks could be mud shale, shale or salt deposits. Petroleum reservoirs that have been tapped for oil or natural gas are natural candidates for CO2 storage. Another possibility is freshwater reservoirs.

The project SSC Ramore focuses on cap rocks. CO2 storage in the subsurface depends on finding and identifying good cap rocks. We must therefore develop a methodology for characterization and testing of such rocks, explains NGI's project manager Elin Skurtveit.

Good test results
NGI has already tested a cap rock from a potential storage site in Svalbard, as noted in parentheses will be developed into a so-called CO2-neutral society.

NGI has also tested shale from the Draupne formation in the North Sea, and the results suggest that this shale is ideal as a cap rock or "ceiling" for CO2 storage in the underlying porous Johansen formation. The Johansen formation is a known aquifer, i.e. it is water-bearing, and consists of an up to 80 meter thick layer of sandstone that covers a large area from the Norwegian coast to the Troll field.

Testing the shale from Draupne area was conducted at NGI's laboratory in Oslo. The material, in the form of a specimen 38 mm in diameter and 40 mm height, was thoroughly characterized in advance with permeability measurements, helium picnometry for mapping the pore structure, as well as investigations by X-ray equipment (XRD) and scanning electron microscope to map the specimen's mineral content.

The mineral content of shales is important since the permeability and other mechanical properties are governed to a large extent by it. After preliminary investigations, (index testing), the specimen was mounted in a flow cell, designed to measure the permeability of geological specimens. The measurements in the flow cell formed the basis of geology student Magnus Soldal's thesis at the University of Oslo.

The permeability testing of the shale in the flowmeter took place after the shale specimen was first saturated with salt water, and prior to an all round pressure corresponding to 800-900 meters of water head was applied. It was set on a gradient so that the pore pressure was higher on one side of the specimen. This gradient was maintained for about a week until a steady stream was measured through the shale allowing the material's permeability to be calculated.

Pressure increase until breakthrough
The next step was to cancel out the pore pressure before exposing the specimen to CO2 on one side. CO2 pressure was then gradually increased until reaching critical pressure (breakthrough pressure) where the gas started to flow through shale. The breakthrough, (resulting in deformation of the specimen), and the corresponding measurements, showed that the seismic velocities through the specimen were greatly reduced. This information is important when future CO2 reservoirs are monitored and checked for the containment of gas.

This is the start of the development of a methodology, and the goal is for us to arrive at guidelines for optimal characterization of cap rocks. Running a sufficient number of such tests on different types of shale will enable us to create a numerical model that can be used to characterize cap rocks. It should not the be necessary to perform long-term tests to establish the breakthrough point for each cap rock. Instead, we should be able to input all available data and let the computer calculate how much the cap rock resists, concludes Skurtveit.
 

Facts SSC Ramore

SSC Ramore (Sub surface storage of CO2: Risk assessment, monitoring and remediation) is a KMB-project (Competence project with user interaction) with support from Climit program in RCN.

KMB projects should always have a partial industry funding, which in this case comes from StatoilHydro, ConocoPhillips, RWE Dea, Schlumberger and Shell. University of Oslo is the project manager. In addition, participates NGI, University of Bergen and the Institute for Energy Technology (IFE). SSC Ramore website: http://www.geo.uio.no/ssc-ramore/

Project period: 2007-2011, Project SSC Ramore: Professor Per Aagard, University of Oslo, NGI's Project Manager: Geologist and cand. Elin Skurtveit.