Abstract
The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium), which was organized to attain the exploration and exploitation of methane hydrate (MH) resources, has been evaluating MH reservoirs located in the Eastern Nankai Trough from the viewpoints of geology, geophysics, petrophysics and reservoir/production engineering. As one of these studies, we have been predicting gas/water production performances from these MH reservoirs showing diverse characteristics. This paper presents the results of our examinations on the producibility of gas from MH reservoirs located in the Eastern Nankai Trough by a variety of MH dissociation/production methods and on the feasibility of the future development of MH in terms of gas production and economics.
Eastern Nankai Trough MH reservoirs, which are composed of alternating beds of sand, silt and clay in turbidite sediments, have various conditions of clay distribution as well as of initial pressure, temperature, permeability and MH saturation. Some of these reservoirs contain MH of high saturation at a certain interval (MH concentrated reservoirs), while in the others MH is deposited sparsely (MH non-concentrated reservoirs). First, the numerical near well model for one of the typical MH reservoirs was constructed mimicking this alteration of sand, silt and clay layers by simplified grid layer system, which was used for the simulation to clarify the mechanisms of MH dissociation and production by the application of the depressurization method.
The detailed numerical reservoir models were then constructed for the vicinity of the wells located both in MH concentrated and in non-concentrated reservoirs, consulting the well log and core interpretation results. MH dissociation/production performances were predicted through numerical simulation assuming the application of various MH dissociation methods such as depressurization, wellbore heating, hot water huff'n'puff and hot water flooding. The simulation studies revealed the difference in the gas production between MH concentrated and non-concentrated reservoirs. These studies also suggested that the methane recovery from MH concentrated reservoirs by depressurization methods ranged from 30 to 60% and that the energy efficiency by thermal methods was very small despite high methane recovery. Furthermore, simple economic analyses on the basis of these simulation results exhibited the promise that some MH reservoirs in the Eastern Nankai Trough could be economically developed if the well spacing and MH dissociation/production methods were appropriately designed.
