Page 21 - Introduction to Petroleum Engineering

WHAT IS PETROLEUM ENGINEERING? 5
TAbLE 1.2 Rules of Thumb for Classifying Fluid Types
Separator GOR Behavior in Reservoir due
Fluid Type (MSCF/STB) Gravity (°API) to Pressure Decrease
Dry gas No surface liquids Remains gas
Wet gas >50 40–60 Remains gas
Condensate 3.3–50 40–60 Gas with liquid dropout
Volatile oil 2.0–3.3 >40 Liquid with significant gas
Black oil <2.0 <45 Liquid with some gas
Heavy oil ≈0 Negligible gas formation
Data from Raymond and Leffler (2006).

where the temperature and pressure are lower. There are a variety of terms for
describing hydrocarbon fluids at surface conditions. Natural gas is a hydrocarbon
mixture in the gaseous state at surface conditions. Crude oil is a hydrocarbon mixture
in the liquid state at surface conditions. Heavy oils do not contain much gas in solu-
tion at reservoir conditions and have a relatively large molecular weight. By contrast,
light oils typically contain a large amount of gas in solution at reservoir conditions
and have a relatively small molecular weight.
A summary of hydrocarbon fluid types is given in Table 1.2. API gravity in the
table is defined in terms of oil specific gravity as

141 5 .
API 131 5 . (1.5)
o

The specific gravity of oil is the ratio of oil density ρ to freshwater density ρ :
o w
o (1.6)
o
w
The API gravity of freshwater is 10°API, which is expressed as 10 degrees API. API
denotes American Petroleum Institute.

Example 1.2 API Gravity
The specific gravity of an oil sample is 0.85. What is its API gravity?

Answer
.
.
141 5 141 5
.
.
API gravity 131 5 131 535 API
.
085
o
Another way to classify hydrocarbon liquids is to compare the properties of the
hydrocarbon liquid to water. Two key properties are viscosity and density. Viscosity is
a measure of the ability to flow, and density is the amount of material in a given volume.