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
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.
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.
Technically the EOR uses different solutions, such as water or gas injection, chemical injection, thermal methods
In addition these gases reduce the viscosity of the oil when mixed together.
Chemical injection may use alkaline or caustic solutions to combine with organic acids contained in the oil to produce soap reducing the capillary pressure of the oil and improving the its flow across the reservoir.
Similar effect may be obtained with polymers injection or surfactants such as the sulfonates.
More recently these surfactants could be developed by the injection of microbes digesting hydrocarbon molecules and producing bio-surfactants in-situ.
Thermal methods are mostly based on steam injection vertically or horizontally in order to increase the pressure in the reservoir and heat up the oil to reduce its viscosity.