The Oil Compressibility Below Bubble Point Pressure Revisited - Formulations and Estimations
- Muhammad Ali Al-Marhoun (King Fahd University of Petroleum & Minerals)
- Document ID
- Society of Petroleum Engineers
- SPE Middle East Oil and Gas Show and Conference, 15-18 March, Manama, Bahrain
- Publication Date
- Document Type
- Conference Paper
- 2009. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics, 5.5 Reservoir Simulation, 4.1.4 Gas Processing, 5.6.4 Drillstem/Well Testing, 5.7 Reserves Evaluation, 4.6 Natural Gas, 5.2.2 Fluid Modeling, Equations of State, 4.1.5 Processing Equipment
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Oil compressibility plays an important role in reservoir simulation, material balance calculations, design of high-pressure surface-equipment and the interpretation of well test analysis, specifically for systems below the bubble point pressure. Accurate information on the oil fluid compressibility above and below bubble point pressure is very important for reservoir evaluation.
The conventional definition of the isothermal oil compressibility below bubble point pressure is being questioned on its scientific merit and challenged against the basic compressibility definition and the general trend of physical behavior of compressibility. This paper presents a new derivation based on the basic compressibility definition to calculate the oil compressibility below bubble point pressure. The experimental data obtained from flash and differential liberation tests is utilized to determine the isothermal oil compressibility values.
It is found that the oil and gas compressibilities below original oil bubble point pressure computed in the conventional method may lead to errors in reserve estimation for saturated reservoirs and skin factor in well test analysis.
By definition, the single phase isothermal compressibility or the reciprocal bulk modulus of elasticity is defined as the unit change in volume with pressure. The compressibility can be calculated from the slope of relative volume versus pressure or from differentiation of a fitted equation to the relative volume curve and is defined as:
Definition of Co above Pb
At pressures above the bubble point the volume of oil decreases with the increase in pressure. This is a normal physical behavior for fluids. Typical relationship of isothermal oil compressibility co with pressure above the bubble point is shown in Fig. 1. In equation form, the co is defined as a point function as:
Conventional Definition of Co below Pb
At pressures below the bubble point the volume of oil decreases with reduction in pressure contrary to the behavior at pressures above the bubble point. This behavior is clearly noticed in the differential liberation test. This is due to shrinkage associated with the liberation of the solution gas with decreasing reservoir pressure or swelling of the oil with repressurizing the reservoir and redissolving the solution gas. Also the density of oil increases with the decrease in pressure due to the liberation of gases. Therefore the profile of volume and density of liquid oil are opposite to the normal trends in single-phase, constant-composition fluids.
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