Study on the Influential Factors of CO2 Storage in Low Permeability Reservoir

As the demands of tight-oil Enhanced Oil Recovery (EOR) and the controlling of anthropogenic carbon emission have become global challenges, Carbon Capture Utilization and Sequestration (CCUS) has been recognized as an effective solution to resolve both needs. However, the influential factors of carbon dioxide (CO2) geological storage in low permeability reservoirs have not been fully studied. Based on core samples from the Huang-3 area of the Ordos Basin, the feasibility and influential factors of geological CO2 sequestration in the Huang-3 area are analyzed through caprock breakthrough tests and a CO2 storage factor experiment. The results indicate that capillary trapping is the key mechanism of the sealing effect by the caprock. With the increase of caprock permeability, the breakthrough pressure and pressure difference decreased rapidly. A good exponential relationship between caprock breakthrough pressure and permeability can be summarized. The minimum breakthrough pressure of CO2 in the caprock of the Huang-3 area is 22 MPa, and the breakthrough pressure gradient is greater than 100 MPa/m. Huang-3 area is suitable for the geological sequestration of CO2, and the risk of CO2 breakthrough in the caprock is small. At the same storage percentage, the recovery factor of crude oil in larger permeability core is higher, and the storage percentage decreases with the increase of recovery factor. It turned out that a low permeability reservoir is easier to store CO2, and the storage percentage of carbon dioxide in the miscible phase is greater than that in the immiscible phase. This study can provide empirical reference for caprock selection and safety evaluation of CO2 geological storage in low permeability reservoirs within Ordos Basin.

The impact of heterogeneous mixed siliciclastic-carbonate systems on CO2 geological storage

Three different outcrops are selected in this study, each representing a shallow marine system with varying heterogeneity provided by siliciclastic-carbonate mixing that may form a small or large stratigraphic trap. The impact of these styles of mixed facies on CO2 storage is relatively poorly known. This study demonstrates the significance of these systems for safe CO2 geological storage, as stratigraphic traps are likely to be a significant feature of many future storage sites. The three 3D models are based on the: 1. Grayburg Formation (US), which displays spatial permeability linked to variations in the mixture of siliciclastic-carbonate sediments; 2. Lorca Basin outcrop (Spain), which demonstrates the interfingering of clastic and carbonate facies; and 3. Bridport Sand Formation outcrop (UK), an example of a layered reservoir, which has thin carbonate-cemented horizons.This study demonstrates that facies interplay and associated sediment heterogeneity have a varying effect on fluid flow, storage capacity and security. In the Grayburg Formation, storage security and capacity are not controlled by heterogeneity alone but influenced mainly by the permeability of each facies (i.e., permeability contrast), the degree of heterogeneity, and the relative permeability characteristic of the system. In the case of the Lorca Basin, heterogeneity through interfingering of the carbonate and clastic facies improved the storage security regardless of their permeability. For the Bridport Sand Formation, the existence of extended sheets of cemented carbonate contributed to storage security but not storage capacity, which depends on the continuity of the sheets. These mixed systems specially minimise the large buoyancy force that act on the top seal and reduce the reliance of the storage security on the overlying caprock. They also increase the contact area between injected CO2 and brine, thereby promoting the CO2 dissolution processes. Overall, mixed systems contribute to the safe storage of CO2.Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage