EOR is the acronym of enhanced oil recovery and refers to all the ultimate technologies to compensate
the natural depletion of the oil and gas fields
Enhanced oil recovery (abbreviated EOR), also called tertiary recovery. It is a more technologically
advanced method of bringing production to surface than traditional methods of drilling.
Production of oil and natural gas requires energy to lift the fluids from the reservoir deep underground
to the surface. The reservoir's natural pressure provides much of this energy but is usually
supplemented by artificial lift equipment. The reservoir can also be re-pressurized by injecting water or
gas to mobilize and displace additional oil. Even after applying these techniques, a large quantity of oil
and gas may remain in the reservoir. It is the extraction of crude oil from an oil field that cannot be
extracted otherwise. EOR can extract 30% to 60% or more of a reservoir's oil, compared to 20% to 40%
using primary and secondary recovery. According to the US Department of Energy, there are three
primary techniques for EOR: thermal, gas injection, and chemical injection. More advanced, speculative
EOR techniques are sometimes called quaternary recovery. In developing a new oil and gas field, the
natural pressure of the reservoir is at its maximum.
For the oil fields, this natural pressure combined with the viscosity of the oil and the percentage of
associated gas will determine the producing conditions.
For the non-associated gas reservoirs, it will be the combination of the natural pressure of the gas and
the percentage of condensate.
If those natural pressures are too high, above 500 bar, the drilling operations and production will remain
always challenging to keep the control of the wells.
If too low, below 20 bar, the oil and gas production will request soon some assistance..
In this context, the oil and gas recovery is defined in three stages.
The first stage is called Primary Oil Recovery and correspond to the first period of production when the
oil and gas can flow through the reservoir up to the wellheads naturally without any assistance.
In general, the quantity of oil and gas collected during the Primary Oil Recovery period represents 5 to
15% of the Original Oil In Place (OOIP).
The second stage is the Assisted Oil Recovery and starts when the natural pressure in the reservoirs
declined that much that some additional help must be provided with extra pumps and compressors to
stimulate the production up to collecting 10 to 30% of the OOIP.
The third stage is when the operating companies want to go beyond this level of recovery, up to 60% of
the OOIP.
This third stage defines what is called the Enhanced Oil Recovery or EOR.
Since the production costs escalate exponentially for each additional percentage of recovery, the oil and
gas field must contain a significant remaining quantity of barrels of oil equivalent (boe) to justify the
corresponding capital expenditure.
At a certain point of recovery somewhere between 30% and 60% it is better to stop investing and leave
the oil and gas recoverable reserves in the ground in the meantime that technology improvements or oil
and gas market prices open new opportunities to produce again.
The challenge to EOR lies in the complex interaction of injected agents with the existing reservoir fluids
in an ever-changing downhole environment. Many of these challenges are well known from the
development of the field. The difficulty is ensuring the proper chemical interaction and subsequent flow
conformance of the EOR sweep front to recover more oil, more quickly. Making the right parametric
decisions regarding a chosen EOR technique, while evaluating dynamic economic conditions,
compounds these complex challenges.
There are two phases involved with the extraction of oil from reservoirs that are employed as the
extraction develops. Primary recovery leads to a natural depletion of the reservoir. In order to aid oil,
recovery water is pumped to sweep oil and also to maintain reservoir pressure, this is the ‘second
recovery’ phase of the operation. If water injection fails to recover all the oil reserves ‘tertiary recovery’
techniques are employed to harvest the remaining deposits.
Gas injection.
Gas injection, which uses gases such as natural gas, nitrogen, or carbon dioxide (CO2), accounts for
nearly 60 percent of EOR production in the United States.Gas injection or miscible flooding is presently
the most-commonly used approach in enhanced oil recovery. Miscible flooding is a general term for
injection processes that introduce miscible gases into the reservoir. A miscible displacement process
maintains reservoir pressure and improves oil displacement because the interfacial tension between oil
and water is reduced. This refers to removing the interface between the two interacting fluids. This
allows for total displacement efficiency.[10] Gases used include CO2, natural gas or nitrogen. The fluid
most commonly used for miscible displacement is carbon dioxide because it reduces the oil viscosity and
is less expensive than liquefied petroleum products.
In carbon dioxide flooding, injected CO2 releases trapped oil from porous rocks in the reservoir and
causes it to flow more easily to the wellhead.
During this process, a mixture of oil, natural gas and a portion of the injected CO2 flows into nearby
wells and is produced at the surface. The CO2 is recovered from this production stream and re-injected
in a closed loop process that results in additional oil recovery. Over time, virtually all of the CO2
introduced into a field becomes trapped underground, occupying the pore space left after the oil and
associated gas are produced.
Applicable to a variety of suitable oil and gas reservoirs, CO2 EOR can increase ultimate oil and
associated gas recovery by 10 to 25 percent in the fields where it is employed.
Environmental Benefits of CO2 Injection.
CO2 EOR benefits the environment in three ways:
First, recovering additional oil from existing fields requires fewer resources than installing new
infrastructure and equipment in new locations. Thus, EOR maximizes the efficient use of existing
infrastructure and reduces land and habitat disturbance.
Second, expansion of EOR operations as additional manmade sources of CO2 become available has the
potential to substantially reduce greenhouse gas emissions — by capturing the CO2 instead of releasing
it into the atmosphere. Currently, the majority of CO2 that Occidental uses is produced from natural
underground CO2 reservoirs. Occidental is actively developing projects that will capture CO2 emissions
for use in our EOR operations, and we are seeking other economic manmade sources of CO2.
Third, because carbon dioxide becomes trapped in deep, underground formations, CO2 EOR can provide
information and experience that will help foster full-scale commercial deployment of other carbon
capture and storage technologies.
Thermal injection
Thermal injection, which involves the introduction of heat, accounts for 40 percent of EOR production in
the United States, with most of it occurring in California.In this approach, various methods are used to
heat the crude oil in the formation to reduce its viscosity and/or vaporize part of the oil and thus
decrease the mobility ratio. The increased heat reduces the surface tension and increases the
permeability of the oil. The heated oil may also vaporize and then condense forming improved oil.
Methods include cyclic steam injection, steam flooding and combustion.
Steam flooding
Steam flooding is one means of introducing heat to the reservoir by pumping steam into the well with a
pattern similar to that of water injection. Eventually the steam condenses to hot water; in the steam
zone the oil evaporates, and in the hot water zone the oil expands. As a result, the oil expands, the
viscosity drops, and the permeability increases. To ensure success the process has to be cyclical. This is
the principal enhanced oil recovery program in use today.
EOR – Low Salinity
As technology evolves constantly, the monitoring of reservoirs in terms of optimum method to employ
EOR in our offshore assets.
Low salinity water (EOR LSW) highlights the effect that salt water has on oil recovery. Replacing sea
water injection with low saline water increases the recovery of oil. This is a relatively new concept but
one that is nearing maturity in terms of R&D and testing.
Solar EOR is a form of steam flooding that uses solar arrays to concentrate the sun’s energy to heat
water and generate steam. Solar EOR is proving to be a viable alternative to gas-fired steam production
for the oil industry.
The global average recovery factor for a typical oilfield is approximately 40%. This results in a large
amount of identified oil left behind despite an existing production infrastructure. The need to improve
the recovery factor and the accelerating of the associated production is the main driver behind the
many EOR schemes in practice around the world.