Jitendra Sangwai

Alkalis such as sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)2] find lots of applications in oil and gas industry where formation of methane hydrates (and resulting flow hindrance) is a common occurrence. While many hydrate inhibitors have been identified to maintain flow assurance, effects of these alkalis on the phase stability of methane hydrate have not yet been explored in detail. In this study, the phase behaviour of methane hydrate has been investigated in the presence of aqueous solutions of NaOH and Ca(OH)2 with varying concentrations of 0.005, 0.01, 0.02, and 0.04 mass fractions in a high pressure stirred tank reactor. The phase equilibrium of methane hydrate in aqueous alkali solutions has been generated in the pressure and temperature ranges of 4.27–7.90 MPa and 276.60–283.22 K, respectively. Both the alkalis have found to exhibit inhibition of methane hydrate formation, and the inhibition effect becomes more pronounced at higher concentrations of the alkalis, with NaOH performing as a better inhibitor than Ca(OH)2. A plausible mechanism for the same has also been discussed. The heat of dissociation has been calculated to show that the presence of the alkalis does not influence the structure (sI) of methane hydrate. In addition, a simple model based on the van der Waals-Platteeuw (vdW-P) thermodynamic model consisting of single parameter has been used to predict the phase equilibrium of methane hydrate in the presence of aqueous alkali solutions. The absolute average relative deviations (AARD%) in pressure for the model were measured for the 40 experimental data points generated in this work. The predictions are well within 4 % of the experimental values, which suggests that the model can adequately predict the methane hydrate phase stability in the presence of alkalis. These alkalis are used in various operations during exploration and production of oil and gas. This study therefore will assist in designing an effective and economic thermodynamic inhibitor for methane hydrate based on alkalis for the oil and gas industry.

Phase equilibria of semiclathrate hydrates are important for their successful engineering applications due to more favorable process conditions compared to classical gas hydrate systems. Though sufficient information on the phase equilibria of semiclathrate hydrates of methane (CH4) in tetra-n-butyl ammonium bromide (TBAB) seems to be available, there are pronounced disagreements on the phase equilibrium data, particularly for 0.05 and 0.20 mass fraction (w) of TBAB. In this work, experimental studies are carried out to generate the equilibrium pressure (P) and temperature (T) for hydrates and semiclathrate hydrates of CH4 in an aqueous solution containing wTBAB=0.05 and 0.20 at P and T range of 1.02-13.73MPa and 281.63-294.54K, respectively. This study tries to clarify the discrepancy of published data in the literature and their reliability. Additionally, we present interesting insights into the phase behavior of semiclathrate hydrate of methane in TBAB based on the formation and dissociation curves observed in the experiments. It is observed that there existed two equilibrium points during the dissociation of semiclathrate hydrates of methane in TBAB; one closely corresponds to the pure methane hydrate phase stability curve and the second one to the semiclathrate hydrate system of methane. In addition phase equilibrium data is generated for the quaternary system of CH4+TBAB+H2O+NaCl for wNaCl=0.03 and 0.10 and wTBAB=0.05 and 0.20 in an aqueous solution at a P and T range of 1.65-20.71MPa and 281.19-296.38K, respectively. This is not yet available in the open literature. It is observed that NaCl inhibits the semiclathrate hydrate formation of CH4 in TBAB for wNaCl=0.03 and 0.10 in wTBAB=0.20 in the system. However, a promotion effect is observed for wNaCl=0.03 in wTBAB=0.05. This study calls for more detailed investigations on the effect of salts on semiclathrate hydrate systems, which may find potential use in engineering applications. © 2014 Elsevier B.V.

Semiclathrate hydrates of natural gas have shown potential applications in natural gas storage and transportation. Promoters, viz., tetra-n-alkyl ammonium bromide (TBAB) and tetrahydrofuran (THF) have positive impacts on the phase stability condition in lowering the required pressure for hydrate formation. As part of this work, a predictive model for the phase stability of gas hydrate, which are necessary to understand the phase behavior of methane (CH4) hydrate in promoters, has been proposed. The fugacity of hydrate former in the gaseous phase is calculated from Peng-Peng-Robinson equation of state (PR-EoS), while the fugacity of water in the liquid phase is computed from recently proposed Pitzer-Mayorga-Zavitsas-Hydration (PMZH) model for TBAB system and non-random two liquid (NRTL) model for THF system. The van der Waals Plattew model is employed for the hydrate phase. The vapor pressure of water in the empty hydrate lattice as well as Langmuir adsorption constants have been expressed in terms of concentration of the promoters. The predictions of the proposed model are found to be match well with experimental data on phase stability of CH4 hydrate formed using TBAB and THF aqueous systems. Furthermore, the developed model is employed for the prediction of phase stability conditions of the semiclathrate hydrates of CH4 in TBAB + NaCl system. The developed model is found to interpret the promotion effects of both TBAB (with or without NaCl) and THF on phase stability conditions of CH4 hydrate. AARD-P% with PMZH model are observed to be 3.21% and 8.73% for semiclathrate hydrates of CH4 in TBAB and TBAB + NaCl, respectively, and 8.56% for clathrate hydrate of CH4 in THF. The model may be extended to evaluate the phase stability conditions of hydrates of multicomponent gas systems in TBAB/THF which are necessary for real field applications.

With the growing need to explore non-conventional energy sources, the hydrates of natural gases offer a realistic solution in the need for alternative energy sources. Gas hydrates are typically entrapped in the porous media showing sensitive phase stability conditions. Models for phase stability for gas hydrate have not yet been extensively investigated for porous media and thus need attention. In this paper, the phase stability model is developed from the basic Chen and Guo model (Chem Eng J, 1998, 71:145) to accurately predict the phase behavior of the clathrate hydrates of CH4 and CO2 in porous media of varying pore sizes from 6nm to 100nm which mimics the naturally occurring porous environment. We also propose a new equation for calculating the activity of water in porous media as a function of the pore size, the wetting angle, the surface tension, and the shape factor of the pores for varied temperature conditions. The model results are validated against experimental data available in open literature and found satisfactory. The proposed model uses very few input parameters (data intrinsic) and thereby is very beneficial in predicting the stability of the hydrates in virgin gas reservoirs wherein the characteristics of the gas reservoir are largely unknown. The developed model may further be applied to the hydrate systems of other natural gases in porous medium with suitable modifications. © 2014 Elsevier B.V.

Natural gas hydrate is a potential source of methane which needs to be extracted from under the sea bed. For the economic recovery of methane from natural gas hydrates, production approaches such as depressurization, thermal stimulation, and inhibitor injection are being investigated. However, studies involving hydrate-bearing clayey sediments and recovery of methane from such reservoirs are rare. This work investigates in detail the potency of hydrate dissociation methods such as depressurization by constant rate gas release, thermal stimulation and the combination of two for energy recovery from hydrate bearing clayey sediments underlying a free gas zone. Pure water and two different mud samples containing 3 and 5 wt% of bentonite were used for methane hydrate formation and dissociation studies. Thermodynamic study of methane hydrate in the presence of bentonite clay was also conducted for the above two concentrations. No considerable effect of clay on the inhibition or promotion of methane hydrate formation was observed. Initially, methane hydrate formation has been investigated using pure water, 3 and 5 wt% bentonite mud at an initial hydrate formation pressure of 8 MPa and at a temperature of 278.15 K. Subsequently, methane hydrate dissociation experiments were carried out using depressurization, thermal stimulation and their combination. The effect of the rate of gas release on hydrate dissociation by depressurization was investigated using two different rates of 10 mL/min and 20 mL/min. Thermal stimulation experiments were carried out for ΔT = 15 K at the rate of 7.5 K/hr and the results on methane recovery were recorded. The detailed investigation shows that the combination of the two methods is more efficient for methane production than the standalone method in clayey hydrate reservoir. This study provides important insights into the hydrate production methodology from clayey hydrate reservoirs.