Selection of PVT Properties For BLack Oil Simulation

Abstract

Abstract It is generally accepted that the reservoir depletion process is a differential liberation process where the gas is removed from contact with the oil as soon it is formed, and the production of fluids through tubing and separators is a flash liberation process where the gas liberated from solution remains in contact with the oil. laboratories commonly perform differential liberation and separator tests on the reservoir fluid and then use a correlation described by Amyx et al.(1) to arrive at a composite liberation curve for an optimum separator pressure. Engineers are often confused as to which of the two sets of PVT data, i.e, differential or composite, should be used in a black oil simulation of the reservoir. The differences between the two sets of the data becomes more substantial as the API gravity of oil increases. The black oil simulator used in this report has a compositional formulation and has the ability to rigorously model the PVT behaviour by entering differential liberation data for reservoir conditions and using a surface separator option for the surface flash process. This paper describes a critical comparison of the three methods to enter PVT properties in a black oil simulator. The results of using the simulator in three ways:differential liberation at reservoir, surface separator for flash,composite liberation data,differential liberation data, are described in this paper for a medium gravity crude. It is shown that the composite liberation data most closely represents the rigorous representation of phase behaviour as in (a). Use of inappropriate liberation data could result in significant error in estimates if oil-in-place as in (c). The paper compares the differences between composite liberation (b) and the rigorous representation (a), such as different initial pressure and saturaturation gradients due to initial density differences. Introduction When studying reservoirs with moderately high API gravity and gas-oil-ratio values, it becomes more important to incorporate both the differential and flash components of the PVT behaviour. In order to demonstrate the differences between using only the differential data as opposed to including the effects of the flash data, actual PVT data from the model study which is referred to as reservoir A, with an API gravity value of 34, is incorporated into the SPE Second Comparative Solution Project(2). The data of the Second Comparative Project remains the same except that the original PVT data set was replaced by the PVT data from reservoir A as shown in Table 1. Note that the differential solution gas (Rs) value if reservoir A was significantly highr that the separator flash data. Problem Statement Potential loss of intermediates, number of stages in the compressor design and an accurate forecast of oil and gas production are among the major concerns of any simulation study with PVT properties similar to Reservoir A. To accommodate these requirements, it would be necessary to accurately model the PVT behaviour of the fluids from the reservoir to the tanks.