Prediction of Gas Productivity From Eastern Nankai Trough Methane Hydrate Reservoirs

Abstract

Abstract MH21 Research Consortium of Japan has been evaluating methane hydrate (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 applicability of a variety of MH dissociation/production methods to these MH reservoirs and on the feasibility of the future development 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 reservoir), while in the others MH is deposited sparsely (MH non-concentrated reservoir). Detailed numerical reservoir models were constructed for both MH concentrated and non-concentrated reservoirs, consulting the well log and seismic interpretation results. MH dissociation/production performances were then predicted through numerical simulation assuming the application of various MH dissociation methods such as depressurization, wellbore heating, hot water huff & puff and hot water flooding. The simulation studies clarified the difference in the gas production between MH concentrated and non-concentrated reservoirs. These studies also revealed that the permeability not only of sand layers but also of clay layers significantly affected the gas productivity from MH concentrated reservoirs. Furthermore, it was suggested that the hot water injection was effective when it was applied as a secondary recovery method after depressurization. Simple economic analysis based on 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. Introduction Background. The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium), which was organized to accomplish the exploration and exploitation of methane hydrate (MH) in the Nankai Trough of offshore Japan, has been implementing a variety of research projects toward the assessment of MH resources, establishment of MH production methods and examination of impacts of MH development on the environment. As part of such research projects, we have been developing the state-of-the-art numerical simulator (MH21-HYDRES) for rigorously predicting MH dissociation and production behaviors both at core and field scales. Using this numerical simulator, we have been implementing numerical simulation studies predicting MH reservoir performances in order to examine the effectiveness of a variety of production methods in terms of the gas producability from MH reservoirs with diverse characteristics (Kurihara et al., 2005a). These studies, however, have been conducted targeting imaginary reservoirs as well as those roughly mimicking Eastern Nankai Trough reservoirs by simplifying reservoir properties.