8 Case study - The Sleipner Gas field
8.3 Tasks accomplished
8.3.10 Determination of the geochemical impact of injected CO 2
The impact of injected CO2 can only really be assessed once there is a sufficiently good understanding of the baseline conditions. Once these have been defined, then changes from them can be more readily identified. There are a variety of approaches that can be used. They combine numerical modelling and observations from laboratory experiments, field monitoring, and natural analogues.
Observations from laboratory experiments - During the SACS project both static and dynamic experiments were assembled. A number of identical static (batch) experiments were undertaken. These simple and relatively low cost experiments used fixed amounts of Utsira Sand and synthetic Utsira porewater, plus fixed temperature and pressure of CO2. Individual experiments were terminated after different timescales. Detailed analysis of the reaction products provided ‘snapshots’ of reaction progress over a 2 year period. The above experiments were compared to similar experiments pressurised with nitrogen. These latter
‘blank’ experiments were also useful to simulate conditions prior to CO2 injection, and hence helped to fix baseline conditions. Dynamic (flowing) experiments were conducted to investigate how geochemical reactions impacted upon fluid flow and vice-versa. Standard
‘core flood’ equipment was used for several of the tests. However, also used were non-metalic (PEEK) tubes that were joined to create a column of Utsira sand 2.4 m long.
The experiments on the Utsira sand have revealed changes in fluid chemistry, associated mainly with dissolution of primary minerals. The experiments pressurised by CO2 led to large and rapid increases in concentrations of Group II metals (and in particular Ca, Sr and Fe), as well as slow and slight increases in silica concentrations. This suggested fast partial dissolution of carbonate phases, while dissolution of silicate or aluminosilicate minerals was a much slower but real process. However, direct evidence from mineralogical observations has never been possible despite the high water-rock ratio used for these experiments (10:1), their relatively long duration (up to 2 years) and the higher temperature (70°C) used for some of them. This is because the reactivity of the Utsira sand was low and changes were below the resolution of the analytical technique or below the natural mineralogical variation within the sand.
Observations from field monitoring - The most obvious way to obtain direct geochemical information would be by direct sampling of a CO2 injection site. Once baseline conditions are established, longer-term monitoring of the injection process would be required. Access to samples over a range of timescales would be important. This approach would require observation boreholes with repeat fluid sampling to monitor fluid chemical changes. Sidewall coring, or the drilling of boreholes through the CO2 ‘bubble’ could be necessary to obtain samples of rock that had been in contact with CO2 for a variety of timescales. Such an approach would be useful in providing highly relevant ‘real time’ information about a large- scale system.
During the SACS project, the lack of observation boreholes and related samples made it impossible to monitor directly the geochemical processes occurring within the Utsira at Sleipner. However such an approach is being used in another industrial CO2 sequestration project - the Weyburn oil field in Canada4.
Observations from natural analogues - This approach utilises relevant information from other sources than the selected site to generate a better understanding of the CO2 injection system.
Natural accumulations of CO2 exist in many parts of the world and have many analogous features to any CO2 injection operation, although these may not be exactly comparable. As such, these ‘natural analogues’ can provide much useful information, especially about long- term processes as the CO2 can, in many cases, be proved to have been trapped for thousands or millions of years. Study of natural accumulations of CO2 has the advantage of similar physical size and timescale of reaction. This can build confidence in models that predict likely responses of reservoirs to geological sequestration. However, costly studies including the drilling of boreholes are needed to gain a reasonable understanding of the analogues. Several research projects on natural CO2 accumulations are presently underway in the world, such as the European NASCENT project5, the American NASC project (Stevens et al., 2001) and the Australian GEODISC project6. One of the objectives of these projects is to study the geochemical effects of CO2 on reservoir rocks and caprocks, for various geological contexts.
Although only analogous to any sequestration system, natural accumulations of CO2 also have the advantage that they are a good way to demonstrate that certain rocks can safely contain CO2 for geological timescales.
Numerical modelling - Computer simulations are very useful way to rapidly scope a range of different scenarios. They can predict the effects of CO2 addition to formation porewaters, and the consequent changes in fluid chemistry and reservoir mineralogy.
Within the SACS study, numerical modelling was used to interpret, and hence to better understand the laboratory experiments, based on thermodynamic, kinetic, flow and transport processes. Batch experiments were modelled using geochemical models while coreflood experiments were modelled using coupled reactive-transport models. At this stage, 1D simulation was sufficient to describe the coreflood experiments. The codes used were EQ3/6 (Wolery, 1995); DIAPHORE (Le Gallo et al., 1998), MARTHE (Thiéry, 1990) and Specific Chemical Simulators (Kervévan and Baranger, 1998; Kervévan et al., 1998) constructed using the ALLAN/NEPTUNIX code generator package (Fabriol and Czernichowski-Lauriol, 1992).
For most of the major elements, the predicted trends were in reasonable agreement with the experimental observations on the Utsira sand. However sensitivity calculations were necessary to fit at best the experimental results. This proves that experiments are essential to assess the key site-specific processes relevant to the natural system being studied.
Within the SACS project, the objective of the geochemical investigations was to assess the potential for geochemical reactions between injected CO2, formation water and the Utsira sand, based on direct observations from laboratory experiments under simulated reservoir conditions for timescales up to 24 months.
Unfortunately, only limited geochemical baseline data were available within the SACS project. This necessitated the use of certain (logical) assumptions in the design of the experimental programme and in the modelling work. In general, the Utsira sand showed only limited reaction with CO2. Most reaction occurred with carbonate phases (shell fragments), but these were a very minor proportion (about 3%) of the overall solid material. Silicate minerals showed only slow and minor reaction. Then, in terms of geochemical reactions, the Utsira sand would appear to be a good reservoir for storing CO2. However further studies are needed to assess the long term storage behaviour within the Utsira formation. In particular, numerical modelling at reservoir scale should be carried out, such as initiated by Johnson et al. (2001). This implies feedback between reservoir simulations and geochemical modelling.
Another key area that still remains highly uncertain is the behaviour of CO2 with the reservoir seal (both caprock and borehole cement seals). Analysis of borehole core material from the caprock at Sleipner is the only way to provide sufficiently detailed information on caprock mineralogy and porewater chemistry. Acquisition of such material should be a priority.
4 (http://www.ieagreen.org.uk/weyburn.htm)
5 (http://www.bgs.ac.uk/nascent)
6 (http://www.apcrc.com.au/Latest%20Releases/geodisc.htm)