Control Mechanisms for Gas Hydrate Production by Depressurization in Different Scale Hydrate Reservoirs

Abstract

The methane hydrate was formed in a pressure vessel 38 mm in id and 500 mm in length. Experimental works on gas production from the hydrate-bearing core by depressurization to 0.1, 0.93, and 1.93 MPa have been carried out. The hydrate reservoir simulator TOUGH-Fx/Hydrate was used to simulate the experimental gas production behavior, and the intrinsic hydration dissociation constant (K 0 ) fitted for the experimental data was on the order of 10 4 mol m -2 Pa -1 s -1 , which was one order lower than that of the bulk hydrate dissociation. The sensitivity analyses based on the simulator have been carried out, and the results suggested that the hydrate dissociation kinetics had a great effect on the gas production behavior for the laboratory-scale hydrate-bearing core. However for a field-scale hydrate reservoir, the flow ability dominated the gas production behavior and the effect of hydrate dissociation kinetics on the gas production behavior could be neglected.

Kinetics of Methane Hydrate Replacement with Carbon Dioxide and Nitrogen Gas Mixture Using in Situ NMR Spectroscopy

“…The results indicate that 36% of CH 4 in small cages was replaced with k 1 of 0.0007 min −1 , while 31% of CH 4 in large cages was replaced during 17 h with k 1 of 0.0002 min −1 (Figure 2). This is in a good agreement with 1 H NMR data as the amount of replaced methane is 25% during 17 h. As the large cages outnumber the small cages by a factor of 3 in structure I hydrate, the amount of replaced CH 4 should be much larger in large cages than small cages. Figure 3(a),(b) shows the amount of methane in large and small cages, respectively, which was obtained from the integrated intensities in Figure 2 and the unit cell composition of structure I methane hydrate.…”

“…This phenomenon implies that the CH 4 is replaced steadily by the gas mixture of 20 mol % CO 2 , balance N 2 . The change in CH 4 peak intensity was monitored in order to identify the amount of CH 4 recovered from hydrate cages. When the experiment ran for 65 h, it was confirmed that 42 mol % of CH 4 was replaced.…”

“…Therefore, in order to gain additional insight, we have observed the 13 C NMR spectra of CH 4 in hydrate cavities and the changes in occupancy of CH 4 in small and large cages of structure I hydrate, 13 C NMR spectra were monitored during the replacement reaction, as shown in Figure 2. The results indicate that 36% of CH 4 in small cages was replaced with k 1 of 0.0007 min −1 , while 31% of CH 4 in large cages was replaced during 17 h with k 1 of 0.0002 min −1 (Figure 2).…”

“…Prepared in this fashion, water-containing silica gels were placed in a high pressure cell, and pressurized with CH 4 up to 140 bar. After confirming hydrate formation in silica gel pores, CH 4 hydrates in the different pore-sized silica gels were contacted with a CO 2 /N 2 gas mixture at 100 bar for 60 h. After that, the compositions of both vapor and hydrate phases were measured. C NMR spectra, (b) methane in small cages, and (c) the ratio of methane in small to large cages from 13 C NMR spectra during the replacement process in silica gel pores of nominal size 15 nm at 273 K and 100 bar.…”

“…Eventually, the chemical potential of the vapor and hydrate phases will be in a pseudosteady state. Schicks et al reported that that all of the CH 4 can be replaced by flowing pure CO 2 over CH 4 hydrate, but that the reaction is reversible, implying that the CO 2 hydrate can be converted to CH 4 hydrate with a steady stream of CH 4 . Therefore, in order to maintain the driving force, the vapor phase composition needs to be replenished with fresh CO 2 /N 2 mixture while recovering methane from the vapor phase.…”