No abstract

This paper presents the measurement of the thermal constants of natural methane hydrate-bearing sediments and mud layer samples recovered from wells. Core samples were recovered from the Tokai-oki test wells (Nankai Trough, Japan) in 2004. The thermal conductivity, thermal diffusivity, and specific heat of the samples were simultaneously determined using the hot-disk transient method. The thermal conductivity of natural hydrate-bearing sediments decreased slightly with increasing porosity. In addition, the thermal diffusivity of hydrate-bearing sediments decreased as the porosity increased. Moreover, we also used simple models to calculate the thermal conductivity and diffusivity. Estimations of the distribution model (geometric mean model) were relatively consistent with the measured results, suggesting that sand grains and hydrates should be independently distributed for hydrate-bearing sediments, which exhibit a pore-filling pattern. The measurement results were also consistent with the thermal diffusivity, which was estimated by dividing the thermal conductivity obtained from the distribution model by the specific heat taken from the arithmetic mean. Finally, our estimate of the thermal conductivity of silt soil was much lower than that for sand soil in hydrate-bearing sediment, which suggests that the small grains influence thermal conductivity.

We study the formation of tetrahydrofuran (THF) clathrate hydrate from polyvinylpyrrolidone (PVP) aqueous solution as a function of growth rate V and adsorbed PVP concentration c using the unidirectional growth technique.

To gain insight into the hydrate crystal growth inhibitor (HCGIs) mechanism, we have studied various quaternary ammonium ionic liquid salts (QAILs) and phosphonium salts for their ability to prevent structure II (sII) hydrate crystal growth. We tested polyvinylpyrrolidone (PVP), polyvinylcaprolactam (PVCap), tetrapropylammonium bromide (TPrAB), tetrabutylammonium bromide (TBAB), tetrapentylammonium bromide (TPeAB), tetrahexylammonium bromide (THexAB), and tetrabutylphosphonium bromide (TBPB) in a tetrahydrofuran (THF) sII hydrate system using a unidirectional growth apparatus to determine the concentration and growth rate dependence. The growth rates of TPrAB, TBAB, and THexAB were found to be critical in the region of low growth rate, with the degree of supercooling ΔT becoming 0 for these additives in the low-growth-rate region. In contrast, PVP, PVCap, TPeAB, and TBPB remained effective at the lowest growth rate of V = 0.04 μm s–1. These trends can be explained by the residence time on the crystal surface of the HCGI, τ, and duration time of crystal growth.

We tested poly(N-vinylpyrrolidone) (PVP) K-12, K-15, K-30, K-90, poly(N-vinylcaprolactam) (PVCap), tetrapentylammonium bromide (TPeAB), tetrabutylammonium bromide (TBPB), polyacryloylpyrrolidine (PAP), poly(N-isopropylmethacrylamide) (PNIPMAM), and hyperbranched polyesteramide (PEA) as hydrate crystal growth inhibitors (HCGIs). We used a tetrahydrofuran (THF) sII hydrate system using a unidirectional growth apparatus. The HCGI concentration c = 0.5 wt % and the growth rate V ranged from 0.005 to 10 μm s −1 . The growth rates of TPeAB, PVP K-12, K-15, TBPB, and PEA were found to be critical in the region of a low growth rate with the degree of supercooling ΔT approaching 0. However, PVP K-30, K-90, PAP, PNIPMAM, and PVCap remained effective at V = 0.005 μm s −1 . These results enhance the persuasiveness for the model that the HCGI effect is explained by the residence time on the crystal surface of HCGI, τ, and time interval t* required to increase the interface curvature. In addition, we determined the threshold growth rate of THF hydrate for examining HCGI performance on sII gas hydrate. Thus, when 0.005 ≤ critical growth rate V* < 0.01 μm s −1 , the HCGIs are moderate inhibitors on sII gas hydrate. When V* < 0.005 μm s −1 , the HCGIs are strong inhibitors on sII gas hydrate.