Modeling of the Unusual GOR Performance in a Fractured Gas-Condensate Reservoir

Abstract

Abstract The Yufutsu gas-condensate field in Hokkaido, Japan, is an unusual reservoir. The fluid in the reservoir is heavier at the top, and the C7+ fraction decreases with depth despite pressure communication. Another unusual aspect of the reservoir is the decrease of the GOR from an initial value of around 1350 vol/vol to around 950 vol/vol in the first five years of production. In this work, we present the results of compositional simulation. The simulation is based on the initialization from a model that takes into account molecular, pressure, and thermal diffusion. The GOR predicted from the compositional simulation is in agreement with measured GOR. Introduction The Yufutsu field in Hokkaido, Japan, is a large gas-condensate reservoir. The reservoir is naturally fractured with a very tight matrix of granite/conglomerate rock of negligible porosity. The fractures provide both the storage and conductivity. 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 sub-sea level (mSSL). No distinct gas-water contact (GWC) or gas-oil contact (GOC) has been found. 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 temperature is 150° C at 4500 mSSL. Until early 2000, there was only one producer in the field (well MY1). Fig. 1 depicts the measured GOR produced from the well. This figure reveals that the GOR decreases gradually from around 1350 vol/vol to less than 1000 vol/vol. Under the same size bean, gas production has a decreasing trend while the condensate rate stays constant. The GOR from another well (MY2) which started producing in early 2000 shows a similar trend. The GOR decrease is believed to be due to the heavier fluid at the top of the formation at initial conditions (prior to depletion). In a previous work,1,2 we have studied compositional variation in the Yufutsu field. A new numerical algorithm based on a diffusion model from irreversible thermodynamics3,4 was developed to predict composition and pressure in the entire reservoir using a single PVT sample as a reference point. The results from the model were in good agreement with compositional data from different wells. It was demonstrated that thermal diffusion is the main phenomenon affecting compositional variation in the Yufutsu field. Due to thermal diffusion, a heavy fluid floats on the top of a light fluid. The model results also indicate the existence of liquid (in the near critical region) on the top of the vapor column in some upper part of the Yufutsu field. The predictions from the model are in agreement with pressure data from the shut-in wells tubing which show a high pressuregradient region between two low pressure-gradient regions. At the bottom of the tubing, the density is less than 400 kg/m3; it gradually increases to over 500 kg/m3 in the middle; then there is a sharp density decrease to about 350 kg/m3. A density of 500 kg/m3 corresponds to a liquid state while a density of 400 kg/m3 and less corresponds to a vapor state. The GOR decrease in the Yufutsu field is believed to be related to the state of initial fluid distribution: the liquid dropout at the top is higher than that in the bottom (see Fig. 2). Constantvolume depletion (CVD) data show that the retrograde-liquid dropout decreases substantially with depth. Data from some of the wells in the formation also show that the heavy fraction (C7+) decreases with depth.1,2 On the other hand, methane mole fraction increases with depth. The vertical compositional variation of heptane-plus in the Yufutsu formation is similar to that reported by Temeng et al. from the Ghawar Khuff reservoirs.5