We present experimental structure-I clathrate hydrate (methane, carbon dioxide, and methane-carbon dioxide) equilibrium and ice-melting data for mesoporous silica glass. In both cases, high capillary pressures result in depressed solid decomposition temperatures (clathrate dissociation and ice melting), as a function of pore diameter. Clathrate dissociation data show a significant improvement over existing literature data, which is attributed to the improved experimental techniques and interpretative methods used. Through application of a melting (or clathrate dissociation) modified Gibbs-Thomson relationship to experimental data, we determine similar values of 32 ( 2, 32 ( 3, and 30 ( 3 mJ/m 2 for ice-water, methane clathrate-water, and carbon dioxide clathrate-water interfacial tensions, respectively. The data are important for the accurate thermodynamic modeling of clathrate systems, particularly with respect to subsea sedimentary environments, and should prove useful in the simulation of potential methane hydrate exploitation and carbon dioxide sequestration schemes.
The characteristics of clathrate hydrate equilibria in mesoporous media are discussed in terms of a conceptual model, with the aim of resolving current inconsistencies concerning experimental and interpretative methods employed in studies of such systems. This conceptual model is used as the basis for an analysis of experimental results from our own work and that of others. From this review, we conclude the following: (1) the GibbsThomson (or Kelvin) relationship used to model clathrate inhibition in porous media must be modified correctly to reflect the hysteresis between growth and dissociation; (2) step heating provides more-reliable data than continuous heating techniques; (3) if equilibrium dissociation data cover the complete pore size distribution, then, contrary to what has previously been proposed by some researchers, inhibition can be interpreted in terms of the mean pore diameter; and (4) the enthalpy of clathrate dissociation is not a strong function of pore size (crystal size), as has been suggested in other studies.
We present modifications to a previously reported statistical thermodynamics model that facilitates the prediction of capillary pressure effects on hydrate equilibria in narrow pores. The model uses the Valderrama modification of the Patel and Teja Equation of State (VPT EoS) for fugacity calculations in fluid phases, while the hydrate phase is modeled using the solid solution theory of van der Waals and Platteeuw (1959), as implemented by Cole and Goodwin (1990). The Kihara model for spherical molecules is applied to calculate the potential function for hydrate-forming gases. To account for capillary pressure effects on phase fugacities, we apply a correction similar to the Poynting correction for saturated liquids. This correction can be applied to any model capable of predicting bulk (unconfined) hydrate phase equilibria. The only new parameter required is hydrate-liquid interfacial tension-values for which we have derived previously from experimental data. The model assumes cylindrical pores, although differs from the majority of existing literature models in how the curvature of the solid-liquid interface is considered; we assume a curvature of 2/r for growth and 1/r for dissociation, in accordance with accepted capillary theory. Model predictions are validated against previously published experimental hydrate dissociation data for binary CO 2 -H 2 O, CH 4 -H 2 O and ternary CH 4 -CO 2 -H 2 O systems, and newly reported data for CH 4 -H 2 O-CH 4 O (3.5 mass% methanol aqueous solutions representing the salinity of seawater) systems, confined to mesoporous silica glass. Good agreement between predictions and experimental data is observed.
Considering the ever‐increasing importance of marine gas hydrates, it is crucial to gain a better understanding of clathrate formation and decomposition in porous media. It is well established that, due to capillary effects, small‐diameter pores – similar to those found in natural sediments – act to inhibit hydrate stability. However, accurate data constraining these effects are still lacking. Here, we present experimental methane clathrate dissociation data for 3.5 mass% methanol aqueous solutions in confined silica glass pores of narrow distribution (30.6, 15.8, and 9.2 nm mean diameters). These data have been used to validate a thermodynamic model for clathrate stability porous media. Experimental data show a marked improvement on literature data – which we attribute to the experimental and interpretative methods used – and are in good agreement with the model predictions. Results suggest that mass transfer of inhibitors (methanol) and dissolved gas during clathrate formation/dissociation within the porous network plays an important role in controlling gas hydrate equilibria.
Acid gases production, such as hydrogen sulfide and carbon dioxide, from heavy oil reservoirs in Venezuela is generally associated with the application of thermal enhanced oil recovery methods. These undesired gases, especially H2S, can be removed by injecting chemical additives that promote chemical reactions with oxidative or nonoxidative mechanisms in the producing system to generate fewer toxic byproducts. According to the literature, H2S scavengers evaluated in the oil industry are amines, alkaline sodium nitrite, hydrogen peroxide, triazine, among others. To mitigate both H2S and CO2 from a reservoir, some novel proposals are under study to offer alternatives to control them from the reservoir and reduce their production in surface. This article presents a review of the key parameters that play a role in the generation of acid gases, mainly H2S and CO2, in Venezuelan oil reservoirs. The operational field data, the main reactions and mechanisms involved in the process (e.g., aquathermolysis, hydro pyrolysis), and the type of byproducts generated will be reviewed. The results and knowledge gained will assist in identifying the main insights of the process, associating them with other international field cases published in the literature, and establishing perspectives for the evaluation of the most convenient techniques from health, safety, technical and economic points of view. Lab and field results have shown that the application of thermal EOR methods in reservoirs of the main Venezuelan basins promote the generation of acid gases due to physicochemical transformations of sulfur, and/or fluid-rock interactions. Sulfur content in Venezuelan viscous oil reservoirs, together with rock mineralogy (clay type) has a significant impact on H2S production. Reported lab results also indicated that H2S scavengers reduce the amount of sulfur, and the presence of CO2 also affects the H2S removal mechanisms, depending on which type of scavenger is selected (e.g., amines, triazine, etc.). Solubilization, hydrolysis, adsorption, absorption, and complex sequestrant reactions (oxidation, neutralization, regeneration, and precipitations) are the main mechanisms involved in the removal of H2S. The literature reported that the application of triazine liquid scavengers is found to generate monomeric dithiazine byproducts (amorphous polymeric dithiazine) which might cause formation damage or inflict flow assurance issues upstream and downstream. This work presents a state of the art review on H2S generation mechanisms and new technologies for the mitigation of acid gases in Venezuelan reservoirs. It also provides perspectives for the application of the most convenient technologies for the reduction of greenhouse gas emissions (mostly CO2), which is critical to producing hydrocarbons with low environmental impact.
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