Page 24 - Introduction to Petroleum Engineering

8 INTRODUCTION

Primary Secondary Tertiary
Oil production rate Abandonment

Time

FIGURE 1.3 Sketch of production stages.

EOR processes include miscible, chemical, thermal, and microbial processes.
Miscible processes inject gases that can mix with oil at sufficiently high pressures
and temperatures. Chemical processes use the injection of chemicals such as
polymers and surfactants to increase oil recovery. Thermal processes add heat to the
reservoir. This is achieved by injecting heated fluids such as steam or hot water or by
the injection of oxygen‐containing air into the reservoir and then burning the oil as a
combustion process. The additional heat reduces the viscosity of the oil and increases
the mobility of the oil. Microbial processes use microbe injection to reduce the size
of high molecular weight hydrocarbons and improve oil mobility. EOR processes
were originally implemented as a third, or tertiary, production stage that followed
secondary production.
EOR processes are designed to improve displacement efficiency by injecting fluids
or heat. The analysis of results from laboratory experiments and field applications
showed that some fields would perform better if the EOR process was implemented
before the third stage in field life. In addition, it was found that EOR processes were
often more expensive than just drilling more wells in a denser pattern. The process of
increasing the density of wells in an area is known as infill drilling. The term improved
oil recovery (IOR) includes EOR and infill drilling for improving the recovery of oil.
The addition of wells to a field during infill drilling can also increase the rate of
withdrawal of hydrocarbons in a process known as acceleration of production.
Several mechanisms can occur during the production process. For example, pro-
duction mechanisms that occur during primary production depend on such factors as
reservoir structure, pressure, temperature, and fluid type. Production of fluids without
injecting other fluids will cause a reduction of reservoir pressure. The reduction in
pressure can result in expansion of in situ fluids. In some cases, the reduction in
pressure is ameliorated if water moves in to replace the produced hydrocarbons.
Many reservoirs are in contact with water‐bearing formations called aquifers. If the
aquifer is much larger than the reservoir and is able to flow into the reservoir with
relative ease, the reduction in pressure in the reservoir due to hydrocarbon production
will be much less that hydrocarbon production from a reservoir that is not receiving
support from an aquifer. The natural forces involved in primary production are called
reservoir drives and are discussed in more detail in a later chapter.