• Energy Research
• chemical engineering close
• 14. Life underwater close
• Applied Energy close

Abstract The decrease in the oil discoveries fuels the development of innovative and more efficient extraction processes. It has been demonstrated that Enhanced Oil Recovery (EOR, or tertiary recovery technique) offers prospects for producing 30 to 60% of the oil originally trapped in the reservoir. Interestingly, oil extraction is significantly enhanced by the injection of low salinity water into oilfields, which is known as one of the EOR techniques. Surface Reverse Osmosis (SRO) plants have been adopted to provide the large and continuous amount of low salinity water for this EOR technique, especially in offshore sites. In this article, we outline an original solution for producing low salinity water for offshore EOR processes, and we demonstrate its energy convenience. In fact, the installation of reverse osmosis plants under the sea level (Deep-Sea Reverse Osmosis, DSRO) is found to have significant potential energy savings (up to 50%) with respect to traditional SRO ones. This convenience mainly arises from the non-ideality of reverse osmosis membranes and hydraulic machines, and it is especially evident – from both energy and technological point of view – when the permeate is kept pressurized at the outlet of the reverse osmosis elements. In perspective, DSRO may be a good alternative to improve the sustainability of low salinity EOR.

Abstract The abundant methane hydrates stored in marine sediments have been widely evaluated as a potential energy source. Understanding the gas and water production characteristics of methane hydrate-bearing marine sediments is critical for hydrates commercial exploitation. In this study, confining pressure was applied to simulate a sub-seafloor environment. Methane was repeatedly injected into cores to remold hydrate-bearing marine sediments with different hydrate saturation. The hydrate saturation increased from 10.6% to 21.6% as the water content increased from 8.9% to 22.2%. The results indicate that the higher the core water content, the greater the hydrate saturation and the longer the hydrate dissociation time. The gas production characteristics of hydrate-bearing sediments were severely affected by water and hydrates in pores. The results indicated that some hydrates and free gas were easily trapped by the surrounding soil, meaning that the hydrates were isolated and disconnected with the pore channels under confining pressure during depressurization. Thus, a second depressurization was conducted to achieve further gas production. For cores with different water contents, their water conversion percentage is approximately 20%. When the water content exceeded 16.7%, the water production was observed. The results of this study are meaningful for further related research and field production of marine hydrates.

Abstract Global climate change is a pressing problem caused by the accumulation of anthropogenic greenhouse gas emissions in the atmosphere. Carbon dioxide (CO2) capture and storage is a promising component of a portfolio of options to stabilize atmospheric CO2 concentrations. Meaningful capture and storage requires the permanent isolation of enormous amounts of CO2 away from the atmosphere. We investigate the effectiveness of heterogeneity-induced trapping mechanism, in potential synergy with a self-sealing gravitational trapping mechanism, for secure CO2 storage in marine sediments. We conduct the first comprehensive study on heterogeneous marine sediments with various thicknesses at various ocean depths. Prior studies of gravitational trapping have assumed homogeneous (deep-sea) sediments, but numerous studies suggest reservoir heterogeneity may enhance CO2 trapping. Heterogeneity can deter the upward migration of CO2 and prevent leakage through the seafloor into the seawater. Using geostatistically-based Monte Carlo simulations of CO2 transport in heterogeneous sediment, we show that strong spatial variability in permeability is a dominant physical mechanism for secure CO2 storage in marine sediments below 1.2 km water depth (less than half of the depth needed for the gravitational trapping). We identify thresholds for sediment thickness, mean permeability and porosity, and their relationships to meaningful injection rates. Our results for the U.S. Gulf of Mexico suggest that heterogeneity-assisted trapping has a greater areal extent with more than three times the CO2 storage capacity for secure offshore CO2 storage than with gravitational trapping. These characteristics offer CO2 storage opportunities that are closer to coasts, more accessible, and likely to be less costly.

Gas mixing in the subsurface could have crucial implications on CO2 storage capacity and security. This study illustrates the impact of gas mixing in the “Captain X” CO2 storage site, an open saline aquifer and subset of the greater Captain aquifer, located in the Moray Firth, North Sea. The storage site hosts several abandoned hydrocarbon fields where injected CO2 could interact and mix with any remaining hydrocarbon gas left in the depleted structures. For this study, compositional simulation of CO2 injection into the Captain X storage site reservoir model was conducted to quantify the impact of mixing. Results show mixing of CO2 with the remaining trapped hydrocarbon gas makes the plume considerably less dense and more mobile. This increases the buoyancy forces acting on the plume, causing it to migrate faster towards the shallower storage boundaries and therefore, reduces the storage capacity of the site. Mixing also compromises the storage security as it mobilises the structurally trapped hydrocarbon gas from within the abandoned fields. Informed injector placement helps to manage and reduce the impact of mixing. Correct assessment of mixing is also considerably dependent on the volume and property of the trapped hydrocarbon gas. To provide a correct long term understanding of storage capacity and security, the impact of mixing, therefore, needs to be correctly considered in all large-scale CO2 storage operations. Information taken from the Strategic UK CCS Storage Appraisal Project, funded by DECC, commissioned by the ETI and delivered by Pale Blue Dot Energy, Axis Well Technology and Costain [28]. Information contains copyright information licensed under the ETI Open Licence [28]. The ACT Acorn consortium was led by Pale Blue Dot Energy and includes Bellona Foundation, Heriot-Watt University, Radboud University, Scottish Carbon Capture and Storage (SCCS), University of Aberdeen, University of Edinburgh, and University of Liverpool. ACT Acorn, project 271500, has received funding from BEIS (UK), RCN (Norway) and RVO (Netherland), and is co-funded by the European Commission under the ERA-Net instrument of the Horizon 2020 programme. ACT Grant number 691712. Project ACT-Acorn is gratefully thanked for funding this study. Schlumberger is thanked for this use of Petrel, PVTi and Eclipse 300 software. S. Ghanbari is currently supported by the Energi Simulation. J. Alcalde is funded by MICINN (Juan de la Cierva fellowship - IJC2018-036074-I). Energi Simulation is thanked for funding the chair in reactive transport simulation held by E. Mackay. Peer reviewed