In general, reservoirs are conveniently classified on the basis of thelocation of the point representing the initial reservoir pressure pi and temperature
T with respect to the pressure-temperature diagram of the reservoir
fluid. Accordingly, reservoirs can be classified into basically two
types. These are:
• Oil reservoirs—If the reservoir temperature T is less than the critical
temperature Tc of the reservoir fluid, the reservoir is classified as an oil
reservoir.
• Gas reservoirs—If the reservoir temperature is greater than the critical temperature of the hydrocarbon fluid, the reservoir is considered a gas reservoir.
Oil Reservoirs
Depending upon initial reservoir pressure pi, oil reservoirs can be subclassified
into the following categories:
1. Undersaturated oil reservoir. If the initial reservoir pressure pi (as
represented by point 1 on Figure 1-1), is greater than the bubble-point
pressure pb 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.
3. Gas-cap reservoir. If the initial reservoir pressure is below the bubblepoint
pressure of the reservoir fluid, as indicated by point 3 on Figure1-1"first post", 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 intobroad 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.
1. Ordinary black oil. A typical pressure-temperature phase diagram for
ordinary black oil is shown in Figure 1-2. 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 1-2, the liquid shrinkage curve,
as shown in Figure 1-3, is prepared by plotting
the liquid volume percent
as a function of pressur
e. The liquid shrinkage curve approximates
a straight line except at very low pressures. When produced,
ordinary black oils usually yield gas-oil ratios
between 200–700scf/STB and oil gravities of 15 to 40 API. The stock tank oil is usually brown to dark green in
color.
2. Low-shrinkage oil. A typical pressure-temperature phase diagram for low-shrinkage oil is shown in Figure 1-4. The diagram is characterized by quality lines that are closely spaced near the dew-point curve. The liquid-shrinkage curve, as given in Figure 1-5, 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 1-4.
3. Volatile crude oil. The phase
diagram for a volatile (high-shrinkage)
crude oil is given in Figure 1-6. 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 as shown in Figure
1-7. The other characteristic properties of this oil include:
• Oil formation volume factor less than
2 bbl/STB
• Gas-oil ratios between 2,00
0–3,200 scf/STB
• Oil gravities between 45–55° API
• Lower liquid recovery of separator conditions as indicated by point
G on Figure 1-6
• Greenish to orange in color
Another characteristic of volatile oil reservoirs is that the API gravity
of the stock-tank liquid will increase in the later life of the reservoirs.
4. Near-critical crude oil. If the reservoir temperature T is near the critical
temperature Tc of the hydrocarbon system, as shown in Figure 1-8,
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 1-8) 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 bubblepoint.
The shrinkage characteristic behavior of the near-critical crude oil
is shown in Figure 1-9. 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.
Figure 1-10 compares the characteristic shape of the liquid-shrinkage
curve for each crude oil type.
