Effect of H₂S Content on Thermodynamic Stability of Hydrate Formed from CO₂/N₂ Mixtures

Abstract

Huge resources of energy in the form of natural gas hydrates are widely distributed worldwide in permafrost sediments as well as in offshore sediments. A novel technology for combined production of these resources and safe long-term storage of carbon dioxide is based on the injection of carbon dioxide injection into in situ methane hydrate-filled sediments. This will lead to an exchange of the in situ methane hydrate over to carbon dioxide-dominated hydrate and a simultaneous release of methane gas. Recent theoretical and experimental results indicate that the conversion from natural gas hydrate to carbon dioxide hydrate and mixed carbon dioxide/methane hydrate follows two primary mechanisms. Direct solid state transformation is possible, but very slow. The dominating mechanism involves formation of a new hydrate from injected carbon dioxide and associated dissociation of the in situ natural gas hydrate by the released heat. Nitrogen is frequently added in order to increase gas permeability and to reduce blocking due to new hydrate formation, and will as such also reduce the relative impact of the fast mechanism on the conversion rates. In addition to carbon dioxide also other sour gases, such as hydrogen sulfide, may follow the carbon dioxide from the sour gas removal process. Hydrogen sulfide is a very aggressive hydrate former. It is abundant in various amounts in thermogenic hydrocarbon systems. In this work we investigate the sensitivity of possible additions of hydrogen sulfide in carbon dioxide/nitrogen mixtures, and how the ability to form new hydrate changes with the additions of hydrogen sulfide. This analysis is applied to four case studies: (1) Bjørnøya gas hydrate basin, (2) the Nankai field in Japan, (3) the Hikurangi Margin in New Zealand, and (4) a gas hydrate basin in South-West Taiwan. The hydrate saturations found in these fields vary over a range from 25−80%. Pressures range from 4−22.6 MPa and temperatures from 275.15−292.77 K. For all these ranges of conditions, even 1% H 2 S will substantially increase the ability to form new hydrate from an injected CO 2 /N 2 mixture containing H 2 S. Except for the most shallow of the reservoirs (Bjørnøya) 1% H 2 S results in formation of a new hydrate for all concentrations of CO 2 in N 2 above 1%. Implementation of results from this work into a reservoir simulator is a natural follow-up which can shed light on the macroscopic consequences in term of possible local blocking of the flow due to content of H 2 S. The mass transport, mass balances, and energy balances in a reservoir simulator are also needed for a more detailed evaluation on how the content of H 2 S and CO 2 changes over time and location in the reservoir due to various processes in addition to hydrate formation. H 2 S and CO 2 dissolves significantly in pore water, and also adsorbs well on various sediment minerals.

Clathrate hydrates: recent advances on CH4 and CO2 hydrates, and possible new frontiers

“…43 Many studies, including field campaigns, continue to be conducted to determine the optimal operation conditions (T, P, addition of liquid vs. gaseous CO2) to enhance CH4 removal from hydrates. [44][45][46][47][48][49][50] Because it is a very aggressive hydrate former, H2S is proposed as an additive for this application, 48,51 and it could help improving the kinetics of the CO2/CH4 swap process.…”

New approach for evaluating the risk of hydrate formation during transport of hydrocarbon hydrate formers of sI and sII

“…It is also vital to remember that a number of gas mixtures form more than one hydrate with different densities, composition, and free energies. From the first and second laws of thermodynamics, we know that the most stable hydrates form first, after which a variety of hydrate compositions will form . In a natural gas mixture containing hydrate formers of both Structures I and II as in this work, Structure II hydrate guest molecules should form hydrates first before those of hydrate Structure I.…”

“…Therefore, the hydrate that would finally form as given in Figures could be a mixture of Structures I and II hydrates of varying compositions of the initial guest molecules. Thus, for several experimental gas mixtures, it is not specified that the final hydrate is uniform . Rather, it is more likely that it is a mixture of several hydrates with varying compositions of the initial hydrate formers from gas or liquid will result …”

“…Thus, for several experimental gas mixtures, it is not specified that the final hydrate is uniform . Rather, it is more likely that it is a mixture of several hydrates with varying compositions of the initial hydrate formers from gas or liquid will result …”

Hydrate Formation and Decomposition Regularities in Offshore Gas Reservoir Production Pipelines

“…In 2017, Kvamme et al [20] analyzed the effect of hydrogen sulfide on the thermodynamic stability of the carbon dioxide-nitrogen hydrate mixture based on the process of gas hydrate extraction by carbon dioxide replacement. Hydrogen sulfide presents a net positive electrostatic field on the outside when rotating in the cavity, and since the average electrostatic field from water in the cavity is negative inward, there is an electrostatic stabilizing component for H 2 S [21]. In 2018, Kvamme et al [22] studied the effect of methanol on hydrate stability.…”