Fluid Classification

Fluid classification is the somewhat ambiguous task of labeling a reservoir fluid from a range of categories like volitile oil, gas condensate or dry gas. This task is essentially the same as classifying a reservoir type. There are several potential metrics used to classify a fluid, and in this article the solution gas-oil ratio (GOR) and phase envelope will be used to do fluid classification.

To show some of the ambiguity in classifying a reservoir fluid, two different definitions of different fluid classes are given below for the solution GOR.

Reservoir Type	GOR (scf/STB)
Black Oil	<1000
Volatile Oil	1000-3000
Near-Critical	~3000
Gas Condensate	3000-30000
Wet Gas	30000-100000
Dry Gas	\(\infty\)

Excerpt 1: Figure taken from the SPE Monograph Phase Behavior¹ page 12 showing range of GOR and OGR for different types of fields

In the following sections, another common definition (with somewhat less ambiguity) is given by associating a reservoir fluid class with its relationship to the reservoir fluid's critical temperature.

Black-Oil

Figure 1: An example of a PT phase envelope (bubble point in green and dew point in red) for a black-oil reservoir. The reservoir condition is marked by a black square marker.

A black-oil fluid is defined by having a reservoir temperature much less than the critical temperature of the reservoir fluid (i.e. \(T_{R}<<T_{c}\)). An example is given in Figure 1. For the black-oil classification, a GOR of less than 1000 scf/STB is also a good indication as the definition of "much less" is ambiguous.

Volatile Oil

Figure 2: An example of a PT phase envelope (bubble point in green and dew point in red) for a volatile oil reservoir. The reservoir condition is marked by a black square marker.

A volatile oil fluid is defined by having a reservoir temperature somewhat less than the critical temperature of the reservoir fluid (i.e. \(T_{R}<T_{c}\)). An example is given in Figure 2. For the volatile oil classification, a GOR of 1000 to 3000 scf/STB is also a good indication as the definition of "somewhat less" is ambiguous.

Near-Critical Fluid

Figure 3: An example of a PT phase envelope (bubble point in green and dew point in red) for a near-critical oil reservoir. The reservoir condition is marked by a black square marker.

A near critical fluid is defined by having a reservoir temperature close to the critical temperature of the reservoir fluid (i.e. \(T_{R} \approx T_{c}\)). An example is given in Figure 3. For the near critical fluid classification, a GOR of around 3000 scf/STB is also a good indication as the definition of "close" is ambiguous.

Gas Condensate

Figure 4: An example of a PT phase envelope (bubble point in green and dew point in red) for a gas condensate reservoir. The reservoir condition is marked by a black square marker.

A gas condensate is defined by having a reservoir temperature between the critical temperature and the cricondentherm of the reservoir fluid (i.e. \(T_{c}<T_{R}<T_{cric}\)). An example is given in Figure 4. The general idea of this classification is that if we reduce the pressure, the original gas-like fluid will condense retrograde fluid.

Wet Gas

Figure 5: An example of a PT phase envelope (bubble point in green and dew point in red) for a wet gas reservoir. The reservoir condition is marked by a black square marker.

A wet gas is defined by having a reservoir temperature somewhat more than the cricondentherm of the reservoir fluid (i.e. \(T_{R}>T_{cric}\)). An example is given in Figure 5.

Dry Gas

Figure 6: An example of a PT phase envelope (bubble point in green and dew point in red) for a dry gas reservoir. The reservoir condition is marked by a black square marker.

A dry gas is defined by having a reservoir temperature much more than the cricondentherm of the reservoir fluid (i.e. \(T_{R}>>T_{cric}\)). An example is given in Figure 6. A dry gas has essentially no produced condensate.

---

1. C. H. Whitson and M. R. Brulé. Phase behavior. Volume 20. Henry L. Doherty Memorial Fund of AIME, Society of Petroleum Engineers …, 2000. ↩