Studying moving fluid interfaces during cementing of CCS wells

Wells have in numerous scientific publications been pointed out as the "weak links" of safe and cost-efficient CO2 Capture and Storage (CCS). They are man-made intrusions into the storage reservoir, and their sealing abilities depend on construction materials like steel and cement. The latter is an especially important well barrier which is pumped into the well to mechanically support pipes and to provide sealing towards CO2 leakage. The requirements for cement sealing are stricter in CO2 wells compared to in petroleum wells, since injection pressures are high and since CO2 is a reactive and buoyant fluid. Until now, CCS-related well cement research has been directed towards enhancing the robustness of the cement material. The problem of actually pumping cement in place kilometers below the surface has been given little attention. This is the case even when field experience from the petroleum industry shows that cement placement is one of the most critical challenges in well construction operations. There is limited information on how successful today's procedures are, since logging tools have insufficient resolution to uncover defects like cement debonding and uncemented channels along the well. Such defects might not cause problems in petroleum wells, but they can pose a risk towards long-term CO2 storage. To ensure integrity of CCS wells, the present project has aimed to improve cement placement and post-job cement evaluation. Through a set of fundamental flow experiments using non-Newtonian fluids, we have studied whether small partigles ("tags") following the flow can be used to visualize where fluids or fluid interfaces are. The goal has been to reveal not only the geometry of the cement front, but also any discontinuities within it. Applied to real wells, this concept can improve placement and monitoring of cement barriers and thus reduce uncertainty in CO2 storage projects. Numerical simulations of particle tracking of interfaces have been performed continuously throughout the project, and have resulted in several scientific publications. The work has revealed that in most cases, the fluid interfaces can indeed be tracked by small particles. Experiments to test the concept in practice were first performed in a down-scaled annular geometry, with good results, and this work has been published. Thereafter, a tailor-made Hele-Shaw cell was used for running experimental campaigns with relevant fluids and particles. This work has been finalized, and three papers describing these experiments are under submission/publishing. The project has had a fruitful collaboration with Canada, and the researchers involved in the project have presented their work at multiple international conferences. In total, 11 peer reviewed papers and 5 popular scientific texts have resulted from the project.

The project has had the following impact: - Developed novel concept for fluid interface tracking by small particles targeting well cementing applications - Developed novel numerical/mathematical methods for studying particle tracking of fluid interfaces - Developed experimental procedures and set-ups for studying fluid interface tracking by small particles - A total of 11 peer reviewed papers (6 published, 5 in process of being published) - 16 conference presentations, 4 invited conference contributions, 7 special reports/meetings with industry, 4 seminars/workshops - 5 popular scientific works - Education of 1 PostDoc

CO2 capture and storage (CCS) is crucially dependent on long-term well integrity, since wells constitute man made paths between the storage reservoir and the atmosphere. To ensure leak-free wells, it is first of all important to ensure leak-free well cement. As of today, there is no reliable way of making sure that cement pumped into a well solidifies to form a leak-tight barrier. Logging tools can still not identify all types of pockets or channels of undisplaced mud within the cement, and these can strongly reduce the robustness of the well - or constitute leakage paths for stored CO2. As a response to this knowledge gap, the present project aims to improve post-job cement evaluation. Through a set of fundamental flow experiments using non-Newtonian fluids, we will study whether "tags" can be used to visualize where fluids or fluid interfaces are after cementing. This has the potential to reveal not only the 3D geometry of the cement front, but also any discontinuities (pockets, channels) within it. If successful, the technology can significantly improve monitoring of barriers in CO2 wells - and thus CCS safety and cost-efficiency.