The Yufutsu field is a new naturally-fractured reservoir in Japan with negligible matrix porosity and permeability. This large reservoir contains near-critical gas condensate fluids with unusual fluid distribution and production. The GOR shows a decreasing trend. This is perhaps the first report of such an observation in the literature. Another interesting and perhaps related observation is that measured data in some wells show the gas richness decrease with depth; methane increases while heptane-plus decreases with depth. Steady-state data from 5-km long wells at shut-in conditions show liquid in the middle, and gas in the top and bottom.

We used the expression of total diffusion flux (molecular, pressure, and thermal diffusion) to analyze and interpret the data in the Yufutsu field. One-dimensional and two-dimensional computations were performed using the Newton method and a new robust algorithm. Predictions were made in the entire reservoir using one single PVT sample in good agreement with data. Based on this work, it is concluded that thermal diffusion significantly affects the fluid distribution in the reservoir with consequence of decreasing the GOR produced.

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