Abstract
Abstract A conventional viewpoint is that almost all of the gas-cap reservoirs are saturated oil reservoirs. However, based on the studies and research, there exist some other special gas-cap reservoirs. What types are the gas cap reservoirs? Because of the effect of sample collection and the representation of producing data, to judge the type of a gas cap reservoir is a difficult problem. This paper analyzes and summaries different kinds of gas cap reservoirs that can be classified as saturated and equilibrilium reservoirs and special unsaturated reservoirs. The special unsaturated reservoirs can be classified as following three categories: 1) either the gas cap or the oil rim is saturated, the other phase is under-saturated (SGCR1), 2) both the gas cap and the oil rim are not saturated (SGCR2), and 3) the saturated pressures and properties of fluids vary with formation depth (SGCR3). This paper takes gas-cap reservoir RES1 (saturated and equilibrilium) and RES2 (special unsaturated) as examples to show how to determine the types of gas-cap reservoirs. And this paper analyzes the phase behaviors of all kinds of gas-cap reservoirs. In the examples, we use phase state regaining theory and phase equilibrium principles, and use a software Desktop-PVT, Landmark, USA to analyze and calculate.
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