Water saturation of natural gas dissolved in water sandstone and its interpretation, Nakajo oil and gas field, Japan

Abstract

Natural gas dissolved in water（GDW）is a common form of hydrocarbon occurrence in Japan, and gas production from the GDW accounts for 23％ of total natural gas production in the country. Nakajo field located in Niigata prefecture 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 behavior, it was a new finding that gas-effect response was observed from well logs in the GDW sandstone. In this paper, we review petrophysical analysis result and demonstrate rock physics modelling, fluid substitutions, synthetic seismogram calculations, wedge modelling and AVO modelling for GDW sandstone. The result of petrophysical analysis indicated presence of isolated gas in GDW sandstone, and the rock physics modelling revealed presence of isolated gas in there. Besides, water saturation estimated from rock physics modelling was regarded as useful tool to detect GDW reservoir with gas isolation according to similarity of the water saturation estimated from rock physics modelling and the mud gas readings（total gas and methane）. An application to seismic data is also encouraged to detect isolated gas in GDW sandstone 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. Finally, although conclusion remains equivocal, we discuss a few possible situations to explain the existence of isolated gas in GDW sandstone reservoir.