Rock Physics Modelling of Gas Dissolved in Water Sandstone, Nakajo Field, Japan

Summary Natural gas dissolved in water (GDW) is a common form of hydrocarbon occurrence in Japan, and gas production from the GDW accounts for 17% (approximately 290 mmscf/day) of total natural gas production in the country. Nakajo field is one of the fields which has long production history from the GDW reservoir. However, as GDW sandstone itself has not been well understood in aspect of the petrophysics and rock physics behaviour, it was a new finding for us that gas-effect and gas-effect-like sonic responses were observed in the GDW sandstone. In this paper, we show result of rock physics modelling, fluid substitutions, synthetic seismogram calculations, wedge modelling and AVO modelling for GDW sandstone. The rock physics modelling results revealed presence of isolated gas in GDW reservoir. Besides, water saturation estimated from rock physics modelling was regarded as useful tool to detect GDW reservoir with gas isolation. An application to seismic data is also encouraged according to synthetic seismogram calculations and AVO modelling. These outcomes allow a better evaluation approach of the surrounding area where future exploration and development potential may exist.