Page 22 - Introduction to Petroleum Engineering

6 INTRODUCTION

TAbLE 1.3 Classifying Hydrocarbon Liquid Types Using
API Gravity and Viscosity
Liquid Type API Gravity (°API) Viscosity (cp)
Light oil >31.1
Medium oil 22.3–31.1
Heavy oil 10–22.3
Water 10 1 cp
Extra heavy oil 4–10 <10 000 cp
Bitumen 4–10 >10 000 cp

Water viscosity is 1 cp (centipoise) and water density is 1 g/cc (gram per cubic
centimeter) at 60°F. A liquid with smaller viscosity than water flows more easily
than water. Gas viscosity is much less than water viscosity. Tar, on the other hand,
has very high viscosity relative to water.
Table 1.3 shows a hydrocarbon liquid classification scheme using API gravity and
viscosity. Water properties are included in the table for comparison. Bitumen is a
hydrocarbon mixture with large molecules and high viscosity. Light oil, medium oil,
and heavy oil are different types of crude oil and are less dense than water. Extra
heavy oil and bitumen are denser than water. In general, crude oil will float on water,
while extra heavy oil and bitumen will sink in water.

1.2 LIFE CYCLE OF A RESERVOIR

The life cycle of a reservoir begins when the field becomes an exploration prospect
and does not end until the field is properly abandoned. An exploration prospect is a
geological structure that may contain hydrocarbons. The exploration stage of the
project begins when resources are allocated to identify and assess a prospect for
possible development. This stage may require the acquisition and analysis of more
data before an exploration well is drilled. Exploratory wells are also referred to as
wildcats. They can be used to test a trap that has never produced, test a new reservoir
in a known field, and extend the known limits of a producing reservoir. Discovery
occurs when an exploration well is drilled and hydrocarbons are encountered.
Figure 1.2 illustrates a typical production profile for an oil field beginning with the
discovery well and proceeding to abandonment. Production can begin immediately
after the discovery well is drilled or several years later after appraisal and delineation
wells have been drilled. Appraisal wells are used to provide more information
about reservoir properties and fluid flow. Delineation wells better define reservoir
boundaries. In some cases, delineation wells are converted to development wells.
Development wells are drilled in the known extent of the field and are used to optimize
resource recovery. A buildup period ensues after first oil until a production plateau is
reached. The production plateau is usually a consequence of facility limitations such
as pipeline capacity. A production decline will eventually occur. Production continues
until an economic limit is reached and the field is abandoned.