Abstract

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 un-usual fluid distribution and production. The GOR shows a de-creasing trend. This is perhaps the first report of such an obser-vation 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 reser-voir using one single PVT sample in good agreement with data. Based on this work, it is concluded that thermal diffusion sig-nificantly affects the fluid distribution in the reservoir with con-sequence of decreasing the GOR produced.

Introduction

The Yufutsu field, located in Hokkaido, Japan, is a new, large gas-condensate reservoir operated by Japan Petroleum Explo-ration Co., LTD. (Japex). The reservoir is naturally fractured with a very tight matrix of granite/conglomerate rock of negligi-ble porosity. The fractures provide both the storage and conduc-tivity. The gas condensate is rich and has a high wax content. The field has an areal extent of 4 km × 8 km with a maximum hydrocarbon column of 1 km. Top of the reservoir is at 3800 m vertical depth (VD). No distinct gas-water contact (GWC) or gas-oil contact (GOC) has been observed.

The Yufutsu field was discovered in 1989. A total of twelve wells have been drilled. Production started in February 1996. The initial reservoir pressure is about 550 bar and the tempera-ture is 150°C at 4500 mVD. Pressure and temperature gradients in the field will be discussed later.

There has been only one producer in the field. Fig. 1 shows the measured GOR produced for the well. This figure shows that the GOR decreases gradually from 1350 m3/m3 to about 1000 m3/m3. Under the same size bean, gas production has a decreasing trend while the condensate rate stays constant. The decreasing GOR behavior is a unique feature of the Yufutsu field. This is perhaps the first report of such an observation; in all other gas-condensate reservoirs, the GOR is reported to increase.

Another interesting and perhaps related observation in the Yufutsu field is that in some wells, the GOR at upper depth is lower than that in lower depth. Constant-volume depletion (CVD) tests show that the retrograde-liquid dropout decreases substantially with depth. Data from some wells in the formation also show that the heavy fraction (say heptane-plus fraction) de-creases with depth. On the other hand, methane mole fraction increases with depth. The vertical compositional variation of heptane-plus in the formation has similarities with that reported by Temeng et al1.

Pressure data from the shut-in well tubing also showa very in-teresting behavior; a high pressure-gradient region is observed between two low pressure-gradient regions. This behavior is shown in Fig. 2 for four wells. The data in this figure are measurements after a long shut-in period except for well MY1 where the shut-in period is about nine days. Fig. 3 shows mea-sured pressure data for well MY2 at shut-in conditions for var-ious times. Detailed description of Figs. 2 and 3 is provided later in this paper.

This content is only available via PDF.
You can access this article if you purchase or spend a download.