Key Factors for Depressurization-Induced Gas Production from Oceanic Methane Hydrates

Abstract

Oceanic methane hydrate (MH) deposits have been found at high saturations within reservoir-quality sands in the Eastern Nankai Trough and the Gulf of Mexico. This study investigates the key factors for the success of depressurization-induced gas production from such oceanic MH deposits. A numerical simulator (MH21-HYDRES: MH21 Hydrate Reservoir Simulator) was used to study the performance of gas production from MH deposits. We calculated the hydrate dissociation behavior and gas/water production performance during depressurization for a hypothetical MH well. Simulation runs were conducted under various initial reservoir conditions of MH saturation, temperature, and absolute permeability. A productivity function (PF) was introduced as an indicator of gas productivity, which is a function of gas production rate, water production rate, and discount rate. The simulations showed that recovery factors over 36% and maximum gas production rates over 450 000 Sm 3 /d were expected for the most suitable conditions of a class 3 deposit (i.e., an isolated MH deposit that is not in contact with any hydrate-free zone of mobile fluids). However, gas productivity was affected by formation temperature and initial effective permeability. The values of PF increased with increasing formation temperature when the initial permeability of the deposit was higher than a threshold value (the threshold permeability); however, it decreased for the deposit below the threshold permeability. The threshold permeability was estimated to be between 1 and 10 mD in the class 3 deposit. These results suggest that key factors for the success of depressurization-induced gas production from oceanic MH are as follows: (1) The initial effective permeability of the MH deposit is higher than the threshold value, and (2) the temperature of the MH deposit is as high as possible.

Numerical Simulation of Methane Production from Hydrates Induced by Different Depressurizing Approaches

“…where h i is the enthalpy for phase i and the subscript "s" means the sand phase, T is the local temperature, and k c is the heat conductivity of hydrate bearing medium, which is a function of local composition of the medium as expressed in Equation (17). The terms of g q and w q (mass per unit volume per unit time) are the source/sink in terms of injection or production of gas and water, respectively.…”

“…Several models have been proposed for gas production from hydrates under the condition of depressurization in the past few years [8][9][10][11][12][13][14][15][16][17][18]. In the earlier models, hydrate decomposition was treated as a Stefan moving boundary problem and the dissociation process was assumed to be isothermal.…”

“…This code can model the flow of gas and water, the nonisothermal gas release, phase behaviour and heat under conditions typical of common natural gas hydrates in complex geological media at any field class of hydrate accumulations. Konno et al [17] investigated the key factors for the success of depressurization-induced gas production with the MH21-HYDRES numerical simulator and obtained that gas productivity was affected by initial effective permeability and hydrate-bearing sediments should have effective permeability higher than the threshold value. Liu et al [18] developed a one-dimensional two-phase model for hydrate dissociation in porous media by depressurization described as a moving boundary ablation-like process.…”

Development of methane hydrate production method

“…In such a case, the higher the initial reservoir temperature, the higher the rate of methane gas production and recovery. [12] As gas hydrate dissociation is an endothermic reaction, the gas production rate gradually decreases as the reservoir temperature decreases. Therefore, to guarantee continuous gas production by maintaining reservoir temperature at a certain range, the development of a combined production method coupling depressu r ization with the other production methods is being investigated.…”

Numerical Investigation of the Production Behavior of Methane Hydrates under Depressurization Conditions Combined with Well-Wall Heating

“…Numerical analysis is an effective approach in modeling and simulating hydrate dissociation in porous media, many numerical models have been systematically developed to simulate natural gas production from hydrates and predict the possible behaviors [8,26,29,[35][36][37][38][39]. Sung et al [35] developed a three-dimensional, multi-phase flow finite-difference numerical model to evaluate the gas recovery performances with Kim-Bishnoi kinetic dissociation model.…”