115
6
Exploration, Recovery,
and Transportation
6.1 INTRODUCTION
Generally the first stage in the extraction of crude oil is to drill a well into the underground reservoir. Often many wells (multilateral wells) will be drilled into the same reservoir, to ensure that
the extraction rate will be economically viable. Also, some wells (secondary wells) may be used to
pump water, steam, acids, or various gas mixtures into the reservoir to raise or maintain the reservoir pressure, and so maintain an economic extraction rate.
If the underground pressure in the oil reservoir is sufficient, the oil will be forced to the surface
under this pressure (primary recovery). Natural gas (associated natural gas) is often present, which
also supplies needed underground pressure (primary recovery). In this situation it is sufficient to
place an arrangement of valves (the Christmas tree) on the well head to connect the well to a pipeline network for storage and processing.
Over the lifetime of the well the pressure will fall, and at some point there will be insufficient
underground pressure to force the oil to the surface. Secondary oil recovery uses various techniques
to aid in recovering oil from depleted or low-pressure reservoirs. Sometimes pumps, such as beam
pumps (horse head pumps) and electrical submersible pumps (ESPs) are used to bring the oil to the
surface. Other secondary recovery techniques increase the reservoir’s pressure by water injection,
natural gas reinjection and gas lift, which injects air, carbon dioxide, or some other gas into the
reservoir.
Enhanced oil recovery (tertiary oil recovery, EOR) relies on methods that reduce the viscosity
of the oil to increase. Tertiary recovery is started when secondary oil recovery techniques are no
longer enough to sustain production. For example, thermally enhanced oil recovery methods are
those in which the oil is heated to make it easier to extract; usually steam is used for heating the oil.
Conventional primary and secondary recovery processes are ultimately expected to produce
about one-third of the original oil discovered, although recoveries from individual reservoirs can
range from less than 5% to as high as 80% of the original oil in place. This broad range of recovery
efficiency is a result of variations in the properties of the specific rock and fluids involved from reservoir to reservoir (Table 6.1) as well as the kind and level of energy that drives the oil to producing
wells, where it is captured.
Conventional oil production methods may be unsuccessful because the management of the reservoir was poor or because reservoir heterogeneity has prevented the recovery of crude oil in an economical manner. Reservoir heterogeneity, such as fractures and faults, can cause reservoirs to drain
inefficiently by conventional methods. Also, highly cemented or shale zones can produce barriers to
the flow of fluids in reservoirs and lead to high residual oil saturation. Reservoirs containing crude
oils with low API gravity often cannot be produced efficiently without application of enhanced oil
recovery methods because of the high viscosity of the crude oil. In some cases, the reservoir pressure was depleted prematurely by poor reservoir management practices to create reservoirs with low
energy and high oil saturation.
As might be expected, the type of exploration technique employed depends upon the nature of the
site. In other words, and as for many environmental operations, the recovery techniques applied to a
specific site are dictated by the nature of the site and are, in fact, site specific. For example, in areas
where little is known about the subsurface, preliminary reconnaissance techniques are necessary
6
Exploration, Recovery,
and Transportation
6.1 INTRODUCTION
Generally the first stage in the extraction of crude oil is to drill a well into the underground reservoir. Often many wells (multilateral wells) will be drilled into the same reservoir, to ensure that
the extraction rate will be economically viable. Also, some wells (secondary wells) may be used to
pump water, steam, acids, or various gas mixtures into the reservoir to raise or maintain the reservoir pressure, and so maintain an economic extraction rate.
If the underground pressure in the oil reservoir is sufficient, the oil will be forced to the surface
under this pressure (primary recovery). Natural gas (associated natural gas) is often present, which
also supplies needed underground pressure (primary recovery). In this situation it is sufficient to
place an arrangement of valves (the Christmas tree) on the well head to connect the well to a pipeline network for storage and processing.
Over the lifetime of the well the pressure will fall, and at some point there will be insufficient
underground pressure to force the oil to the surface. Secondary oil recovery uses various techniques
to aid in recovering oil from depleted or low-pressure reservoirs. Sometimes pumps, such as beam
pumps (horse head pumps) and electrical submersible pumps (ESPs) are used to bring the oil to the
surface. Other secondary recovery techniques increase the reservoir’s pressure by water injection,
natural gas reinjection and gas lift, which injects air, carbon dioxide, or some other gas into the
reservoir.
Enhanced oil recovery (tertiary oil recovery, EOR) relies on methods that reduce the viscosity
of the oil to increase. Tertiary recovery is started when secondary oil recovery techniques are no
longer enough to sustain production. For example, thermally enhanced oil recovery methods are
those in which the oil is heated to make it easier to extract; usually steam is used for heating the oil.
Conventional primary and secondary recovery processes are ultimately expected to produce
about one-third of the original oil discovered, although recoveries from individual reservoirs can
range from less than 5% to as high as 80% of the original oil in place. This broad range of recovery
efficiency is a result of variations in the properties of the specific rock and fluids involved from reservoir to reservoir (Table 6.1) as well as the kind and level of energy that drives the oil to producing
wells, where it is captured.
Conventional oil production methods may be unsuccessful because the management of the reservoir was poor or because reservoir heterogeneity has prevented the recovery of crude oil in an economical manner. Reservoir heterogeneity, such as fractures and faults, can cause reservoirs to drain
inefficiently by conventional methods. Also, highly cemented or shale zones can produce barriers to
the flow of fluids in reservoirs and lead to high residual oil saturation. Reservoirs containing crude
oils with low API gravity often cannot be produced efficiently without application of enhanced oil
recovery methods because of the high viscosity of the crude oil. In some cases, the reservoir pressure was depleted prematurely by poor reservoir management practices to create reservoirs with low
energy and high oil saturation.
As might be expected, the type of exploration technique employed depends upon the nature of the
site. In other words, and as for many environmental operations, the recovery techniques applied to a
specific site are dictated by the nature of the site and are, in fact, site specific. For example, in areas
where little is known about the subsurface, preliminary reconnaissance techniques are necessary