Thermal Diffusion Characteristics in Permafrost during the Exploitation of Gas Hydrate

Abstract

Methane hydrate is the vast potential resources of natural gas in the permafrost and marine areas. Due to the occurrence of phase transition, the gas hydrate is dissociated into gas and water and absorbs lots of heat. The incomprehensive knowledge of endothermic reaction in permafrost sediments still restricted the production efficiency of hydrate commercial development. This endothermic reaction leads to a complex thermal diffusion in permafrost, which directly influences the phase transition in turn. In this research, the heat during the exploitation is transferred in two forms (specific heat and latent heat). Besides, the melting point is not constant but depends on the pore size of the reservoir rock. According to these features, a thermal diffusion model with phase transition is established. To calculate the governing equation, the pore size distribution is obtained by using the nuclear magnetic resonance (NMR) method. The heating tests are conducted and simulated to calibrate the coefficient (i.e., transverse surface relaxivity) of NMR. Then, the temperature field evolution of the hydrate reservoir during the exploitation is simulated by using the calibrated values. The results show that the temperature curves have a typical plateau related to the pore size distribution, which is effective to obtain the surface relaxivity. The heat transfer is remarkably limited by the endothermic effect of the phase transition. The hydrate recovery efficiency may depend largely on the heating capacity of the engineering operation and the rate of gas production. Compared to the conventional petroleum industry, it is significant to control the maximum temperature and temperature distribution in engineering operations during hydrate development. This research on the temperature behavior during onshore permafrost hydrate production could provide the theoretical support to control heat behavior of offshore hydrate production.

Estimation of Residual Pore Water Content in Hydrate-Bearing Sediments at Temperatures below and above 0 °C by NMR

“…NMR measurements of gas hydrates and hydrate-bearing rocks came into broad use due to diverse advanced instruments. − The NMR techniques are used to estimate fluid saturation, pore size distribution. and porosity as well as to identify the crystal structure of pore hydrates, to constrain relative permeability of hydrate-bearing porous media, − and to understand the mechanism of CH 4 -CO 2 substitution . There is successful experience in correlating field logging-while-driling and laboratory NMR data for natural gas hydrate reservoirs of the Nankai Trough, Mount Elbert, Shenhu, and Krishna-Godavari with the determination of in situ gas, water and hydrate saturations, and relative permeability. − Much success has been achieved in studying the formation and dissociation of gas hydrates at different pressures and temperatures in bulk rocks and porous media, using low-frequency NMR relaxometry, from 1 H relaxation times.…”