Depending upon initial reservoir pressure p classified into the following categories

Oil reservoirs
Depending upon initial reservoir pressure P classified into the following categories:
1. Undersaturated oil reservoir. If the initial reservoir pressure P_i (as represented by point 1 on figure 7.1.), is greater than the bubble-point pressure P_b of the reservoir fluid, the reservoir is labeled an undersaturated oil reservoir.
2. Saturated oil reservoir. When the initial reservoir pressure is equal to the bubble-point pressure of the reservoir fluid, as shown on Figure 6.18 by point 2, the reservoir is called a saturated oil reservoir.
3. Gas-cap reservoir. If the initial reservoir pressure is below the bubble-point pressure of the reservoir fluid, as indicated by point 3 on figure 7.1, the reservoir is termed a gas-cap or two-phase reservoir, in which the gas or vapor phase is underlain by an oil phase. The appropriate quality line gives the ratio of the gas-cap volume to reservoir oil volume.
Crude oils cover a wide range in physical properties and chemical compositions, and it is often important to be able to group them into broad categories of related oils. In general, crude oils are commonly classified into the following types:
• Ordinary black oil
• Low-shrinkage crude oil
• High-shrinkage (volatile) crude oil
• Near-critical crude oil
The above classifications are essentially based upon the properties exhibited by the crude oil, including physical properties, composition, gas-oil ratio, appearance, and pressure-temperature phase diagrams.
7.2.1. Ordinary black oil. A typical pressure-temperature phase diagram for ordinary black oil is shown in Figure 7.2. It should be noted that quality lines which are approximately equally spaced characterize this black oil phase diagram. Following the pressure reduction path as indicated by the vertical line EF on figure 7.2, the liquid shrinkage curve, as shown in figure 7.3, is prepared by plotting the liquid volume percent as a function of pressure. The liquid shrinkage curve approximates a straight line except at very low pressures. When produced, ordinary black oils usually yield gas-oil ratios between 200-700 scf/STB and oil gravities of 15 to 40 API. The stock tank oil is usually brown to dark green in color.
7.2.2. Low-shrinkage oil. A typical pressure-temperature phase diagram for low-shrinkage oil is shown in figure 7.4. The diagram is characterized by quality lines that are closely spaced near the dew-point curve. The liquid-shrinkage curve, as given in figure 7.5, shows the shrinkage characteristics of this category of crude oils. The other associated properties of this type of crude oil are:
• Oil formation volume factor less than 1.2 bbl/STB
• Gas-oil ratio less than 200 scf/STB
• Oil gravity less than 35° API
• Black or deeply colored
• Substantial liquid recovery at separator conditions as indicated by point G on the 85% quality line of Figure 7.4.
7.2.3. Volatile crude oil. The phase diagram for a volatile (high-shrinkage) crude oil is given in figure 7.6. Note that the quality lines are close together near the bubble-point and are more widely spaced at lower pressures. This type of crude oil is commonly characterized by a high liquid shrinkage immediately below the bubble-point/
The other characteristic properties of this oil include:
• Oil formation volume factor less than 2 bbl/STB
• Gas-oil ratios between 2,000–3,200 scf/STB
• Oil gravities between 45–55° API
• Lower liquid recovery of separator conditions as indicated by point G


7.2.4. Near-critical crude oil. If the reservoir temperature T is near the critical temperature T_с of the hydrocarbon system, as shown in figure 7.8, the hydrocarbon mixture is identified as a near-critical crude oil. Because all the quality lines converge at the critical point, an isothermal pressure drop (as shown by the vertical line EF in figure 7.9) may shrink the crude oil from 100% of the hydrocarbon pore volume at the bubble-point to 55% or less at a pressure 10 to 50 psi below the bubble-point. The shrinkage characteristic behavior of the near-critical crude oil is shown in figure 7.9 The near-critical crude oil is characterized by a high GOR in excess of 3,000 scf/STB with an oil formation volume factor of 2.0 bbl/STB or higher. The compositions of near-critical oils are usually characterized by 12.5 to 20 mol% heptanes-plus, 35% or more of ethane through hexanes, and the remainder methane.