Carbon Capture Storage (CCS) is the only technology that can truly decarbonise coal-fired and gas power plants. If we want to attain cost-effective CCS systems, we need to improve technologies but also get to know better the nature of the geological formations.
The potential storage of CO2 into geological formations fundamentally depends on a basic property: the amount of void spaces that remain in the rock, known as ‘porosity’. Spaces connected to one another make the movement of CO2 possible – a property of the rock called ‘permeability’, and the total connected volume of these void spaces allows the calculation of the amount of CO2 that could be stored.
Sedimentary rocks are preferentially chosen for the storage of CO2 due to their frequent high values of porosity and permeability. However, these values can abruptly change in the 3D space, because sedimentary rocks have variable fractions of grain sizes and morphologies; characteristics that define to a large extent porosity and permeability vales. Mud-rocks or salt formations act as effective barriers, whereas well-sorted sand or limestone act as conduits and storage places. Any tool that can predict the distribution of these rocks will increase the success of a CCS project. Here is where the architecture of sedimentary rocks plays a critical role.
The analysis of the architecture of sedimentary rocks is a deductive approach that consists in a comparative analysis of the geometry and texture of sedimentary rocks. Comparative analysis is done by comparing data collected from modern environments and excellently exposed ancient ones with data from a case study. The first step is to determine the palaeo-environment where the sedimentary rocks of interest were formed (the complete region is normally addressed as a basin). Was it a fluvial, lacustrine or a marine environment? We can learn all this by analysing the typically available seismic and well data. Some geometries and rock textures clearly point to fluvial environments, for example.
At this point, the analysis of fossilised biological content is also crucial to support any of these interpretations. Once the palaeo-environment has been determined, further questions arise: if the sedimentary rocks were formed by rivers, were these straight, meandered or anastomosed? These details have an impact on both the potential total storage volume but also on the pathways that CO2 and other phases will follow.
As a rule, the better we understand the former sedimentary system that formed our sedimentary rocks, the better we will define their distribution in the 3D space.
The geometry and texture of sedimentary rocks reveal the details of types of currents and geomorphology that once occurred, and this information is vital to predict the distribution of any type of rocks anywhere.
The analysis of the sedimentary architecture is later followed by other studies that allow the prediction of any geological feature that can modify preliminary calculated volumes and pathways (e.g. fractures, diagenesis).
The data from the analysis of sedimentary architecture is used in computer modelling. We can create 3D models that confidently represent the distribution of different sedimentary rocks with variable values of porosity and permeability.
These models are the key to calculations of storage volumes and the modelling of safe and effective CO2 injection.