Storage in subsurface carbonates

It is widely believed by the scientific community and a huge proportion of society that the rise in anthropogenic emissions since the industrial revolution has been forcing climate change at a rate that can not be explained by natural causes. CO2 has risen from 350ppm to over 450ppm since the industrial revolution. An increase of 2oC is seen as the threshold of global warming, anything greater than a 2oC in temperature could pass this tipping point with devastating consequences. (ref needed) It is widely recognised that this increased rate of climate change needs to be slowed and eventually stopped. Climate change experts are predicting that emissions of CO2 need to be stabilised at 550ppm to prevent the change of catastrophic climate change. (ref needed) There are many options available to help mitigate climate change, with simple solutions like being more energy efficient in our day to day lives, to the more elaborate solutions such as Carbon Capture & Storage (CCS). CCS has the potential to reduce future world emissions from energy by 20% (Haszeldine, 2009). CCS allows the continued burning of fossils fuels whilst mitigating climate change by capturing the CO2 emissions. The CO2 emissions are then stored underground in geological formations, such as depleted oil and gas reservoirs or saline aquifers. The geologic and chemical properties of each reservoir or aquifer will be unique. The successful sequestration of CO2 depends on the properties of the storage site; this requires detailed modelling and data collection of the proposed site to highlight potential problems. This essay will concentrate on the problems of CO2 storage in subsurface carbonates.

The properties that make a good site for potential CO2 storage are very similar to those properties that make a good oil or gas reservoir. A seal or cap rock is an essential component for CO2 storage, without which storage would not be possible. If the potential site for storage is a depleted oil/gas field then it is very important to make sure that the seal is still intact. The seal could have been destroyed through exploration of the reservoir. This problem is universal to every reservoir and is not specific to just carbonate reservoirs. The integrity of the seal will not be covered in this essay, but it is a very important factor for CO2 storage. Assuming that the seal or cap rock is intact and suitable for storage, the next important factor for CO2 storage is the properties of the reservoir. Each reservoir will have its own unique properties, but there are several conditions that need to be met for a reservoir to be suitable for CO2 storage. The properties that make a reservoir suitable are the same for sandstone and carbonate reservoirs, the required properties will be mentioned here and then investigated later on in regards to the problems associated with carbonate reservoirs. The two essential attributes governing the quality of the reservoir is the porosity and the permeability of the rock. The size and geometry of the pores and the diameter and tortuosity of the connecting throat passages all affect the suitability of the reservoir for CO2 storage. The porosity and permeability are greatly affected by many different factors; this will be explored in relation to carbonates in the next section.

As already discussed reservoirs, whether they are sandstones or carbonates require similar properties for CO2 storage. However, to be able to fully evaluate carbonates potential as a reservoir for CO2 storage an understanding of the uniqueness of the carbonate regime is needed, such as; the biological origin of most carbonates, the bathymetric framework of carbonate depositional facies and environments, the response of carbonate depositional systems to changes in relative sea level, and the diagenetic consequences of the high chemical reactivity of carbonates (Moore, 2004). When studying carbonates it is necessary to remember that modern carbonate deposition is very different to ancient carbonate deposition. We have to use our knowledge of modern carbonates, from areas such as the Bahama Platform and the Florida Shelf, and our knowledge of ancient carbonates from the rock record to modal the depositional environments of ancient carbonates. Three important considerations have to be taken into account; Sea level, Biology and evolution, and sedimentation rates. To understand ancient carbonates it's important to recognise the controls on modern carbonate deposition. These controls are; tectonics and climate which in turn affect sea level. Tectonics affects the quantity of terrigenous material entering a carbonate system and along with climate can affect water circulation patterns, sea temperature, salinity, nutrient supply, wave activity, turbulence, and storm and tidal current strengths (Tucker, 1990). Along with these controls, the evolution of carbonate secreting organisms through time and changing seawater chemistry will affect carbonate deposition and patterns of early diagenesis (Moore, 2004).

As the controls on modern and ancient carbonate sedimentation have been highlighted and briefly discussed it is now possibly to move onto the diagenesis of carbonates. Diagenesis has a massive impact on the quality of a carbonate reservoir. Diagenesis can be divided up into different mechanisms, many studies have divided them up slightly different, but here are the main 5 mechanisms; 1. Compaction - overburden pressure on the buried carbonates causes tighter grain packing, thus reducing pore size. 2. Dissolution - the destruction of carbonate material through chemical dissolution and micritization. In Limestones, vugs, potholes, caverns and cave systems may develop as a result of dissolution. 3. Cementation - the precipitation of cement between grains, and recrystallization, such as limestone to dolomite.

40% of all remaining oil reserves are in carbonate reservoirs (Wood, 2010),

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