Interpretation of an Unusual Bubblepoint Pressure Variation in an Offshore Field

Abstract

Abstract Bottomhole samples were obtained from eight different wells in an undersaturated offshore oil field. A total of 25 samples were obtained from these wells at various depths. The well temperature data show considerable gradient in both the horizontal and vertical directions. Bubblepoint pressure measurements reveal unusual variations. Some of the wells have a gradual increase in bubblepoint pressure with depth. Some other wells show a decrease after increase and some wells a decrease with depth. The decrease in bubblepoint pressure with depth is the common behavior. There are also vast variations in measured composition, oil density, and the molecular weight of last fraction in different wells. These variations can be due to compartmentalization and barriers or could be due to gravity, molecular diffusion, thermal diffusion, natural convection or reservoir filling history. Identifying the cause of the variations is important especially in offshore fields. Reservoir compartmentalization can significantly affect the field development plan and the reservoir management. We have made an analysis of the variations in composition and bubblepoint pressure due to all the above mechanisms in the reservoir with no barriers. The results are different than the observed variations. The reservoir filling effect cannot also explain the large measured variations since calculated mixing time for the fluid is much less than the age of the reservoir. Based on our analysis we conclude existence of barriers in certain parts of the reservoir which explains significant changes in composition and bubblepoint pressure. We also show that thermal diffusion and natural convection can result in an unusual increase in bubblepoint pressure with depth. To the best of our knowledge this is the first report of this unusual trend in bubblepoint pressure and its interpretation based on the effects of gravity, molecular diffusion, thermal diffusion, natural convection, and reservoir filling history. Introduction Bottomhole samples were obtained from eight different wells in an undersaturated offshore oil field. A total of 25 samples were obtained from these wells. Fig. 1 shows the Cartesian xyz representation of the wells in the field. In this figure, thick black lines represent the wells with a square marking the sampling points. The xy Coordinates can be read with the help of dashed lines projections into the horizontal plane. Table 1 presents well designation, depth interval at which the samples were obtained, reservoir temperature, and pressure in different wells, and measured bubble point of the sample at a specified temperature. The well temperature data (Fig. 2) show considerable gradient in both horizontal (xy) and vertical (z) directions. Fig. 3 presents an estimate for temperature profile in horizontal (xy) plane (to be discussed later). Note that there is a horizontal temperature change of about 8.5 °C at the true-vertical depth of 2660 m. The vertical temperature gradient is due to the heat flow from the hot Earth's interior toward the cold surface. The horizontal temperature variation, in this field, can be due to the variable water depth and constant water temperature (approximately 4 °C) in the seabed floor.