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Massive Methane hydrate (hereinafter referred to as "MH") is interpreted to be widely distributed in the offshore region of the Japanese islands. Trial calculations indicate the amount of MH is estimated to be enough to supply the domestic natural gas demand for some a hundred years. The Advisory Committee of the METI (Ministry of Economy, Trade and Industry) prepared "Japan's Methane Hydrate Exploitation Program, July 2001" with the aim of producing natural gas from the MH. The action plan "MH Resources Research and Development" (2001-2016) is currently being carried out.The existence of MH in sandy beds in the offshore region of the Japanese islands was confirmed for the first time in 2000 through the METI Exploratory Test Well "Nankai Trough".The well clarified that the net thickness of MH-rich layers is 12 to 14m in total. Several thin MH layers are also present at a depth between 1, 100-1, 140 m deep from the sea surface. Some thicker sand layers (12-14m thick) associated with a submarine fan that are characterized by the high concentration of MH were identified between 1, 140 m and 1, 213 m deep from the sea surface. In the lower interval of these thicker sand layers, each sand bed is less than lm in thickness and was correlated between test wells. Wire line logging with Combinable Magnetic Resonance* (CMR) data showed that the MH saturation was 60 to 80% in the MH containing layer. The bottom depth of high MH concentration deposits coincides with the depth of BSR on the basis of Vertical Seismic profile* (VSP).*: Sclumberger trademark

This study reports a new process of constructing a discrete fracture network (DFN) model, which reflects various statistical properties of fractures extracted from static data such as borehole images and dynamic data such as pressure derivative curves. A DFN model is constructed by generating a number of disc-shaped fractures based on their statistical properties. The process of constructing DFN model proposed consists of two streams. One stream is for the fractures with wide apertures, which work as main flow path of fluid. Another stream is for the fractures with narrow apertures, which work as main storage of fluid. In the each stream, a DFN model is constructed by applying fractal theory and geostatistics that define the size and spatial distribution of fractures, respectively. The overall DFN model is obtained by merging the two types of DFN models. Then, the DFN model is converted to a continuum model with equivalent permeable blocks based on cubic law and is evaluated by the fluid-flow behavior. Japex has been operating the Yufutsu fractured basement gas reservoir since 1996. A total of approximately 8,000meters of electrical borehole images from eleven wells is measured to understand the fracture system. According to the proposed process, the DFN models are constructed. To evaluate the DFN models by comparing the simulated pressure derivative curves with the observed curve, flow simulations are carried out as a simulation of a single porosity type. The observed pressure derivative curve shows a concave shaped curve like a dual porosity behavior, but the curve never reaches the second flat state. Through the parameter studies of the statistical properties, which relate to the size distribution and the spatial distribution of fractures, the observed curve was successfully reproduced and the quantitative relationship between the fractures with wide apertures and the fractures with narrow apertures was found.

Fractures in the Yufutsu reservoir may be divided into two sets based on apparent aperture measured in electrical image data. Large-scale fractures having centimeter-order apparent apertures (termed "mega"-fractures) are considered to contribute to gas production by acting as conduits, while relatively small-scale fractures contribute to gas storage and migration. The interaction between fractures and the state of stress may also control productivity. Analysis of wellbore failure revealed a NE-SW orientation of the maximum horizontal compressive stress (SHmax), with a small variation from the west (SHmax oriented N30E) to the southeast and north (SHmax oriented N60E). Modeling indicates that the general stress state is strike-slip (Shmin < Sv < SHmax). Coulomb failure analysis of natural fractures in the current strike-slip stress state reveals that steeply dipping fractures striking to the northeast-southwest are critically stressed and therefore most likely to be permeable. Wells in which this is the dominant fracture set are therefore most likely to be productive. The predictions agree closely with observed production in the wells. The results from this study are sufficient to provide guidance for future drilling and completion decisions for the Yufutsu Field.