Hydrate Plugging and Flow Remediation during CO2 Injection in Sediments

Successful geological sequestration of carbon depends strongly on reservoir seal integrity and storage capacity, including CO2 injection efficiency. Formation of solid hydrates in the near-wellbore area during CO2 injection can cause permeability impairment and, eventually, injectivity loss. In this study, flow remediation in hydrate-plugged sandstone was assessed as function of hydrate morphology and saturation. CO2 and CH4 hydrates formed consistently at elevated pressures and low temperatures, reflecting gas-invaded zones containing residual brine near the injection well. Flow remediation by methanol injection benefited from miscibility with water; the methanol solution contacted and dissociated CO2 hydrates via liquid water channels. Injection of N2 gas did not result in flow remediation of non-porous CO2 and CH4 hydrates, likely due to insufficient gas permeability. In contrast, N2 as a thermodynamic inhibitor dissociated porous CH4 hydrates at lower hydrate saturations (<0.48 frac.). Core-scale thermal stimulation proved to be the most efficient remediation method for near-zero permeability conditions. However, once thermal stimulation ended and pure CO2 injection recommenced at hydrate-forming conditions, secondary hydrate formation occurred aggressively due to the memory effect. Field-specific remediation methods must be included in the well design to avoid key operational challenges during carbon injection and storage.publishedVersio

Unlocking multimodal PET-MR synergies for geoscience

The recent combination of positron emission tomography (PET) and magnetic resonance (MR) imaging modalities in one clinical diagnostic tool represents a scientific advancement with high potential impact in geoscientific research; by enabling simultaneous and explicit quantification of up to three distinct fluids in the same porous system. Decoupled information from PET-MR imaging was used here, for the first time, to quantify spatial and temporal porous media fluid flow. Three-dimensional fluid distribution was quantified simultaneously and independently by each imaging modality, and fluid phases were correlated with high reproducibility between modalities and repetitive fluid injections.publishedVersio

Multiscale investigation of CO₂ hydrate self-sealing potential for carbon geo-sequestration

Storage of liquid CO2 in shallow geological formations is a recently proposed concept that can facilitate increased storage capacity and improved mobility control. If stored below the gas hydrate stability zone (GHSZ), unwanted vertical migration of CO2 can be effectively inhibited by the formation of solid hydrate layers. Lowering the risks of CO2 leakage to the atmosphere is instrumental to accelerate the implementation of full-scale carbon sequestration in the North Sea and elsewhere. In the laboratory, we have successfully visualized CO2 trapping phenomena, measured CO2 leakage rates, and demonstrated that the integrity of the hydrate seal strongly depends on fluid-rock interactions and initial water distribution. CO2 propagation in water-filled core samples has been monitored over a total of 140 days inside the GHSZ. Solid CO2 hydrate formed and sealed the pore space in both homogeneous sandstone and heterogeneous limestone cores. However, the physical flow barrier developed considerably faster in sandstone (after 1.8 pore volumes – PV) compared to limestone (after 7.4 PV), with a factor ten reduced CO2 leakage rate through the seal in favor of sandstone. Furthermore, pore-scale images of upward CO2 migration verified trapping of CO2 both as solid hydrate precipitation and as liquid CO2 clusters made discontinuous and stabilized by capillary forces. Small-scale hydrate rearrangement followed initial formation, and caused temporarily dissociation of local hydrate structures without affecting the overall integrity of the seal. Our study suggests that a homogeneous, water-filled GHSZ directly above a CO2 storage site can provide a secondary safety mechanism and significantly reduce the risk of CO2 leakage.publishedVersio

Measurements of CH4 and CO2 relative permeability in hydrate-bearing sandstone

This paper reports measurements of relative permeability to methane (CH4) and carbon dioxide (CO2) in hydrate-bearing sandstone core samples. The CH4 (or CO2) permeability was measured at reservoir conditions for different hydrate and brine saturations. The saturation span ranged from 0.18 to 0.60 (frac.) for CH4 gas and from 0.37 to 0.70 (frac.) for liquid CO2. The hydrate saturation ranged from 0.18 to 0.61 (frac.). The growth of hydrates within sandstone pores reduced the permeability for both the CH4 and CO2 system significantly, and the relative reduction was more pronounced for lower gas saturations. This effect is currently not included in numerical models of relative permeability in hydrate-bearing sediments and should be considered. The reported measurements are relevant to production-forecasting of methane gas from hydrate reservoirs and CO2 storage schemes where CO2 hydrates may provide self-sealing in cold aquifers.publishedVersio

Pore-to-Core Laboratory Upscaling and Visualization of Enhanced Oil Recovery and CO2 Storage

The global energy demand increases, and the need for hydrocarbon reserve growth is evident. The maturation of hydrocarbon formations worldwide combined with declining rate of major oil and gas discoveries, have caused a renewed focus on implementing enhanced oil recovery (EOR) methods in hydrocarbon reservoirs. The success of an EOR project relies on identifying the key driving forces. Non-invasive, non-perturbing imaging of fluid dynamics in laboratory opaque systems can identify recovery mechanisms beyond material balance experiments. Furthermore, flow experiments should be conducted at a variety of scales in the laboratory to couple small-scale phenomena and basic mechanisms to the complexity of fluid flow in the field. This thesis visualizes and identifies EOR mechanisms from pore- to core-scale in order to improve fluid flow characterization in porous media. A novel imaging approach is presented in Paper 1 where positron emission tomography (PET) to image fluid flow was combined with structural information acquired from computed tomography (CT). Superimposed images described how rock discontinuities affected labeled water fronts and overall sweep efficiency. Paper 2 is an extension of Paper 1 and involves explicit tracking of the gas phase and evaluates the synergy between EOR and permanent CO2 storage. Molecular diffusion and viscous displacement were identified as recovery mechanisms in respectively fractured sandstone and tight shale. Furthermore, a large fraction of injected CO2 was effectively retained in the pores by capillary forces, demonstrating the potential for safe CO2 sequestration. Explicit flow information during waterfloods and CO2 injection for EOR and storage was successfully used to evaluate size dependence on developed flow patterns. The pore-to-core scale approach was experimentally verified in Paper 3, where similar displacement systems were studied at the pore-scale. Capillary and dissolution trapping of CO2 by water were directly observed in etched-silicon micromodels. CO2 was trapped in single pores and in larger clusters, and the residual phase was poorly connected throughout the network. In pore-level observation of CO2 EOR, high recoveries were observed due to a spreading oil layer between the water phase and the non-wetting gas phase. Building on Paper 1-3, it was evident that CO2 injection for EOR in fractured systems needed to be improved. Therefore, Paper 4 evaluates mobility control in fractures. Co-injection of gas and surfactant solution was compared to wateralternating- gas (WAG) and continuous gas injection (CGI), and was the preferred method in terms of areal sweep and mobility reduction factor in 2D fracture networks as a result of foam generation. Foam generation was studied at the pore-scale in Paper 5, where rectilinear snap-off and snap-off at permeability discontinuities were identified as important lamella creation mechanisms. Low salinity waterflooding (LSW) was evaluated in Paper 6 as an alternative to gas injection in oil-wet carbonates. Wettability alteration and interfacial tension reduction between crude oil and water were effects attributed to LSW, resulting in enhanced secondary and tertiary oil recovery at reservoir conditions. Osmotic pressure was discarded as a dominant LSW mechanism in corefloods based on pore-level observations

Storing CO2 as solid hydrate in shallow aquifers: Electrical resistivity measurements in hydrate-bearing sandstone

A recent proposed carbon dioxide (CO2) storage scheme suggests solid CO2 hydrate formation at the base of the hydrate stability zone to facilitate safe, long-term storage of anthropogenic CO2. These high-density hydrate structures consist of individual CO2 molecules confined in cages of hydrogen-bonded water molecules. Solid-state storage of CO2 in shallow aquifers can improve the storage capacity greatly compared to supercritical CO2 stored at greater depths. Moreover, impermeable hydrate layers directly above a liquid CO2 plume will significantly retain unwanted migration of CO2 toward the seabed. Thus, a structural trap accompanied by hydrate layers in a zone of favorable kinetics are likely to mitigate the overall risk of CO2 leakage from the storage site. Geophysical monitoring of the CO2 storage site includes electrical resistivity measurements that relies on empirical data to obtain saturation values. We have estimated the saturation exponent in Archie’s equation, n ≈ 2.1 (harmonic mean) for CO2 and brine saturated pore network, and for hydrate-bearing seal (SH < 0.4), during the process of storing liquid CO2 in Bentheimer sandstone core samples. Our findings support efficient trapping of CO2 by sedimentary hydrate formation and show a robust agreement between saturation values derived from PVT data and from modifying Archie’s equation

Pore-level foam generation and flow for mobility control in fractured systems

Pore-level foam generation, propagation, and sweep efficiency were visualized using silicon-wafer micromodels with an accurate representation of sandstone pore structure, grain shapes and sizes based on thin-section analysis. Foam generation by snap-off was observed both in the interior of the porous network (rectilinear snap-off) and at permeability discontinuities between fracture and porous matrix. Lamella creation by the two snap-off mechanisms identified here resulted in different foam textures. During foam injection for enhanced oil recovery, microvisual data revealed that the aqueous phase advanced as film flow along water-wet grains whereas discontinuous gas bubbles were located in the center of pores. Foam injection significantly enhanced sweep efficiency in fractured systems in terms of greater pore occupancy by gas and larger contact area with displaced fluid compared to continuous gas injection

Hydrate Plugging and Flow Remediation during CO2 Injection in Sediments

Successful geological sequestration of carbon depends strongly on reservoir seal integrity and storage capacity, including CO2 injection efficiency. Formation of solid hydrates in the near-wellbore area during CO2 injection can cause permeability impairment and, eventually, injectivity loss. In this study, flow remediation in hydrate-plugged sandstone was assessed as function of hydrate morphology and saturation. CO2 and CH4 hydrates formed consistently at elevated pressures and low temperatures, reflecting gas-invaded zones containing residual brine near the injection well. Flow remediation by methanol injection benefited from miscibility with water; the methanol solution contacted and dissociated CO2 hydrates via liquid water channels. Injection of N2 gas did not result in flow remediation of non-porous CO2 and CH4 hydrates, likely due to insufficient gas permeability. In contrast, N2 as a thermodynamic inhibitor dissociated porous CH4 hydrates at lower hydrate saturations (<0.48 frac.). Core-scale thermal stimulation proved to be the most efficient remediation method for near-zero permeability conditions. However, once thermal stimulation ended and pure CO2 injection recommenced at hydrate-forming conditions, secondary hydrate formation occurred aggressively due to the memory effect. Field-specific remediation methods must be included in the well design to avoid key operational challenges during carbon injection and storage

Unlocking multimodal PET-MR synergies for geoscience

The recent combination of positron emission tomography (PET) and magnetic resonance (MR) imaging modalities in one clinical diagnostic tool represents a scientific advancement with high potential impact in geoscientific research; by enabling simultaneous and explicit quantification of up to three distinct fluids in the same porous system. Decoupled information from PET-MR imaging was used here, for the first time, to quantify spatial and temporal porous media fluid flow. Three-dimensional fluid distribution was quantified simultaneously and independently by each imaging modality, and fluid phases were correlated with high reproducibility between modalities and repetitive fluid injections

Combined positron emission tomography and computed tomography to visualize and quantify fluid flow in sedimentary rocks

Here we show for the first time the combined positron emission tomography (PET) and computed tomography (CT) imaging of flow processes within porous rocks to quantify the development in local fluid saturations. The coupling between local rock structure and displacement fronts is demonstrated in exploratory experiments using this novel approach. We also compare quantification of 3-D temporal and spatial water saturations in two similar CO2 storage tests in sandstone imaged separately with PET and CT. The applicability of each visualization technique is evaluated for a range of displacement processes, and the favorable implementation of combining PET/CT for laboratory core analysis is discussed. We learn that the signal-to-noise ratio (SNR) is over an order of magnitude higher for PET compared with CT for the studied processes