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CLIMIT-Forskning, utvikling og demo av CO2-håndtering

CO2 permeability along fragmented shale interfaces

Alternative title: CO2-permeabilitet langs fragmenterte grenseflater av skifer-stein

Awarded: NOK 8.0 mill.

Carbon Capture and Storage (CCS) is identified as one of the implementable methods for reducing the amount of Carbon in the atmosphere. CCS can be divided into the sub-categories Capture, Transport and Storage. The storage is done by injecting CO2 through wells into a suitable underground reservoir. The long-term storage of CO2 is made possible by an impermeable caprock situated above the reservoir. However, to get access to the reservoir, wells will have to be drilled through the caprock. If the drilling of the wells is not planned, designed and executed properly, it is possible that the wells will not provide sufficient zonal isolation, and the injected CO2 can migrate along the wellbore up to the surface. The Norwegian Research Council has through the CLIMIT-programme supported a project with the objective to study how the drilling operation can affect the shale interface. The INTERFACCS project focused on the storage aspect of CCS, and in particular focus on the drilling phase. The planning, design and execution of the drilling will need to be carefully executed to avoid having a well with a poor well integrity and enhanced probability of CO2 leakage along the wellbore and to the atmosphere. The scope of the project is to gain insight into how the drilling operation can affect the rock wall in the well, investigation of drilling-induced damage on shale, the effect on cement placement, and finally to assess the probability of leakage of CO2. The review study show that certain shale types have properties that can lead to borehole instability that later can lead to difficulties in acheving complete cement coverage. The initial experiments in the project focused on the mapping of the extent of damage the shale rock wall could exhibit in the experiments. The initial experimental testing showed that in the worst case scenario, the cement placement in a complex fracture network should be engineered carefully. Without proper execution of the cement job, it can be difficult for the cement to penetrate the smaller fractures, and thus, increasing the probability impaired well integrity. The second part of the project focused on the behaviour of different shale types during the drilling process. The purpose of this is to try to be able to predict how cement will behave at different shale types. The third and last part of the project focused on the properties of the shale wall will influence the cement placement. The work on modeling and upscaling focused on improving current models to be able to predict accrately the flow in 1) a relevant (i.e. complex) fracture network, rather than in simplified networks and 2) incorporating relevant rheological behavior of the fluid into the model. The secondary experiments used a dedicated cell enabling placement of cement under pressure and controlled flow. The cell is X-Ray CT transparent enabling complete visualization of the rock-cement sample to properly assess the quality of the cement placement. The results show that the quality of the cement placement is dependent on the shale properties and also on the pre-conditioning of the wellbore wall. A more water wet wall gave better cement bonding compared to an oil wet wall. The project was a cooperation between SINTEF, NORCE, the Norwegian University of Science and Technology (NTNU) and Los Alamos National Laboratory (LANL) of New Mexico, USA. The experimental work was performed by researchers from SINTEF and NTNU. The role of NORCE is to perform modelling and calculations on upscaling. LANL contributed with their expertise to the discussion based on their work from similar North American projects.

Review study show that certain shale types have properties that can lead to borehole instability that later can lead to difficulties in acheving complete cement coverage. The initial experimental testing showed that in the worst case scenario, the cement placement will not be sufficient to achieve zonal isolation The secondary experiments used a dedicated X-ray transprent cell enabling placement of cement under pressure and controlled flow. The results show that the quality of the cement placement can dependent on the shale properties and also on the pre-conditioning of the wellbore wall.

The integrity of the interface between cement and caprock is a crucial barrier towards leakage from CO2 storage reservoirs. It is present both in active and abandoned wellbores. The quality of zonal isolation along this interface has previously only been studied using defect-free shale rocks, even when it is well-known that shale fragments/fractures during drilling operations. The fragmenting is caused by a combination of mud circulation, drillstring/stabilizer rotation and the lack of filter cake formation on the low-permeability shale wall. To ensure that realistic data on the permeability of the cement-caprock interface in wells can be gathered, this project will use existing experimental equipment available at SINTEF to create drilled shale cylinders, cement them and flood CO2 through the composite samples. This will give information on: (i) how to minimize fragmenting (e.g. by optimizing circulation rate, drillstring rotation speed or the angle with which the shale is penetrated), (ii) how cement can be optimized to seal against the fragmented shale interface, (iii) how to reliably predict leakage rates through wells penetrating CO2 storage reservoirs.

Funding scheme:

CLIMIT-Forskning, utvikling og demo av CO2-håndtering