The onshore Nakajo oil and gas field in Niigata Prefecture is composed of three different types of hydrocarbon accumulations: non-associated natural gas, black oil, and natural gas dissolved in water (GDW). The natural GDW is produced together with formation water as gas in solution under in-situ condition. While the formation water is lifting to surface, the dissolved gas is separated from the water. With this reason, well log response from GDW sandstone was believed to be almost the same as normal sandstone which does not contain any hydrocarbon.

In the recent in-fill well drilling campaign for GDW reservoir, a full suite of wireline logging was carried out. With a success of good quality data acquisition, an advanced sonic and borehole image analysis was conducted. As a result, gas-effect-like sonic log response such as significant P-wave slowness slowing down and unchanged S-wave was newly observed in GDW sandstone formation, while other logs such as Neutron and Density did not show any hydrocarbon response. In addition, the sonic velocity radial-profiling analysis result indicated slow P-wave formation property radially continuing from near wellbore into the formation. Based on these observations and following desktop study, we successfully reached to the conclusion that gas-water 2 phase fluid model is a reasonable assumption rather than a single-phase fluid model for GDW formation. These new findings on sonic log response will bring new aspect to the formation evaluation of GDW reservoir in Nakajo oil and gas field.

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