Empirical and Numerical Evaluation of Mechanisms in Gas Production from CH4-Hydrates: Emphasis on Kinetics, Electrical Resistivity, Depressurization and CO2-CH4 Exchange
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Focus is shifted towards renewable energy and sources of natural gas as the demand for cleaner energy continues to increase with global awareness on anthropogenic climate change. Methane (CH4) provides advantages such as high enthalpy upon combustion and low carbon imprint compared to other fossil fuels. Natural gas is therefore predicted to play an important role as the world moves from coal dependency towards a cleaner and more sustainable energy future.
Natural gas hydrate is a solid state of gas and water, where water molecules interconnect through hydrogen bonding to form a cavity which is stabilized by a gas molecule through van der Waals interaction forces. This reaction occurs where water and CH4 coexist at low temperature and high pressure. In nature, such conditions are typically found in permafrost and sub-marine environments. Vast energy resources are associated with gas hydrates, where different models suggest that hydrates contain 1015 to 1017 m3 CH4 at standard temperature and pressure (STP). In comparison, the annual gas consumption in the US is about 7•1011 m3. Gas hydrates may therefore become a significant contributor in the future energy mix. Current technological challenges are related to in situ characterization for accurate saturation estimates, further advances in production technologies and continuous improvements of available numerical models through comparison with actual fieldand core-scale data.
A synergy between gas production and safe CO2 storage is achieved through CO2 sequestration in hydrate bearing sediments, where CO2 replaces the existing CH4 molecule within the hydrate crystal. The process occurs because CO2 offers favorable thermodynamic conditions. Salt was observed to impact the hydrate formation rate and the amount of excess water in Paper 1. Depressurization and diffusion-driven CO2 exchange were compared, where Magnetic Resonance Imaging (MRI) was used to monitor production in situ. CO2-CH4 exchange was more abundant for high residual brine, and therefore sensitive to initial salt concentration. Depressurization was assumed to be limited by permeability and heat transfer. Current opinion on geomechanical issues related to hydrate bearing sediments was addressed in Paper 2. Hydrate decomposition through depressurization resulted in production of associated water with potential loss of structural integrity, as gas hydrates enhance sediment shear strength through mineral interaction. The sediment shear strength was assumed to be maintained during the CO2-CH4 exchange process based on minor intensity variations.
Depressurization is considered a promising production method for gas hydrates. The technology is yet available from the conventional oil and gas industry, and little energy is required to promote dissociation relative to thermal stimulation. Empirical dissociation data were compared with predictions utilizing TOUGH+HYDRATE in Paper 3. Accurate predictions of heat and fluid flow within the sample were achieved by discretizing the problem into a significant number of subdomains in a Cartesian 2D and a complex Voronoi 3D model. The problem was initialized based on MRI saturation data, while temperature measured in the confining fluid was used as a time-variable boundary. Empirical decomposition was successfully reproduced numerically by employing both the kinetic and equilibrium reaction model. Heat transfer was the main controlling mechanism. Kinetic limitations may be present in rapid small-scale dissociation tests, and the choice of reaction model should therefore reflect the physical geometry of the problem.
Some of the main conclusions from Paper 1 - Paper 3 were:
- Heat transfer was the most important mechanism for sustained hydrate dissociation during depressurization.
- Kinetic modeling was required for accurate numerical reproduction of small scale dissociation.
- The sediment shear strength was assumed to be maintained during CO2-CH4 Exchange
Data logs such as resistivity and acoustics are often acquired during and after drilling through hydrate bearing intervals for evaluation of pore fluids. Accurate calibration is essential for correct interpretation of data. The complexity of the measurement is enhanced by competing processes, where increasing tortuosity increases the resistivity, while elevated ion concentration reduces the measured resistivity. These issues were addressed in Paper 4. Electrical measurements were compared to spatially resolved MRI saturation data for improved interpretation. The standard Archie model was insufficient for porosity and saturation estimates, and a dynamic empirical function that accounted for variable ion concentration was implemented. Changes in effective porosity were accurately described when employing the dynamic function. n varied during growth and was dependent upon the hydrate growth pattern. Additional techniques for determining saturation should therefore preferentially aid in the resistivity interpretation.
The CO2-CH4 exchange process was maximized in Paper 5 through constant volumetric injection rate in a fractured sample design which provided optimized flow conditions and a constant reaction interface. Five consecutive exchange sequences demonstrated enhanced exchange efficiency during constant injection, where negative effects of CO2 dilution during CH4 release were minimized. Final conversion efficiency was a function of saturation, non-uniformities and soaking time. 59-83% of the CH4 was replaced by CO2 during 2-5 days of injections.
Exchange efficiency was further addressed in non-fractured samples, where released CH4 was continuously displaced towards the producer. The probability of plugging increased, and final mixed gas hydrate compositions were observed. Flow issues were addressed through CO2/N2 binary gas injection, which resulted in excellent flow conditions. CO2-CH4 exchange was substantiated during binary gas injection which was confirmed by in line Gas Chromatography (GC) measurements. The overall variation in hydrate saturation was not quantified, but significant resistivity decrease indicates partial dissociation or rearrangement of hydrate crystals.
Main conclusions from Paper 4 and Paper 5 were:
- Interpretation of saturation and porosity estimates during hydrate growth was improved by modifying Archie’s resistivity model.
- 59-83% CO2 was safely stored in gas hydrates through constant CO2 injection while benefitting from CH4 production.
- Binary gas injection (CO2/N2) promoted further exchange while maintaining permeability.
Collaborative experimental effort between the University of Bergen and ConocoPhillips resulted in co-injection of CO2 and N2 in a recent field test in Alaska (Ignik Sikumi). Several controlled laboratory experiments were conducted in preparation of the field test. Extended CO2-CH4 studies and electrical resistivity measurements were the main contributions from this work.