The recovery factor of an oil reservoir without any technical support is only 10 %. As the consumption of oil-based products like fuel, lube, and plastics increases and it becomes more difficult to discover new oil fields, oil companies frequently turn to considering how more oil can be extracted from a reservoir while maintaining an economically reasonable process. This article discusses ways to enhance oil recovery.
Enhanced oil recovery
Different steps of oil recovery
The different steps of oil recovery are referred to as primary, secondary, and tertiary recovery. Primary oil recovery is the first stage of hydrocarbon production. It uses the pressure difference between the reservoir and the well bore and transports the crude oil towards the well and up to the surface. Only 10 % of the hydrocarbons in the reservoir are produced due to this decline in pressure. Secondary oil recovery using water or gas is the second stage of hydrocarbon production. The secondary production is stopped when water or gas is the main substance in the production well. Primary and secondary recovery together give an output of 15 % to 40 % of the hydrocarbon in the reservoir. Tertiary recovery is also called enhanced oil recovery (EOR).
Secondary oil recovery method
Water injection/waterflooding is the most commonly used secondary oil recovery method due to the fact that water is inexpensive and effective in increasing the oil output of a production well. Waterflooding is a method in which water is injected into the reservoir to increase the pressure in the reservoir. Ideally the increased pressure pushes the crude oil upwards into the production well and increases the output of the reservoir. The efficiency of such displacement depends on many factors like oil viscosity, the density difference between the displacing and the displaced fluid, and rock characteristics.
Enhanced oil recovery (EOR)
Enhanced oil recovery, also known as improved oil recovery (IOR) or tertiary recovery, describes various techniques which enhance the crude oil output from an oilfield by increasing the pressure in the reservoir, displacing the crude oil, and improving the flow of the oil by changing the fluid properties like density (API gravity) and viscosity. The best technique for each oilfield depends on the temperature, pressure, and depth of the reservoir as well as the crude oil properties and the ability of the crude to flow through the reservoir.
Different methods of enhanced oil recovery
Gas injection uses gases such as natural gas, nitrogen, or carbon dioxide (CO2). It is also called miscible flooding because it introduces miscible gases into the reservoir. These mix with the oil and give a single homogenous phase. The fluid mainly used is CO2, but it depends on the availability and the reservoir conditions.
This approach uses different techniques to heat the crude oil, decrease viscosity, and increase mobility by using water steam or steam flooding. The latest innovations produce the steam using solar energy. This technique is used in heavy-oil reservoirs and tar sands.
Chemical injection/chemical flooding
Chemical injection is a method which uses special chemical solutions added to water to increase the mobility and decrease the surface tension of the crude oil, and increase the viscosity of water to make it similar to the oil. A distinction between those methods is made:
- polymer flooding
- carbon dioxide flooding
- water alternating gas (WAG) injection
Polymer flooding uses soluble polymers in water to increase water viscosity for easier transportation of the crude. Carbon dioxide flooding uses CO2 in the supercritical state if the reservoir is deeper than 2000 ft. Water alternating gas (WAG) injection uses water in addition to CO2.
Simulating EOR in the laboratory
The EOR techniques increase the costs of each extracted barrel of hydrocarbons and therefore the economic efficiency of the whole process has to be evaluated before they are used.
By simulating the reservoir and reservoir characterization, it can be demonstrated which of the different processes for enhanced oil recovery will stimulate the oil production in a specific reservoir and consequently give the maximum recovery factor and the most profit prior to investing in a new injection well. The simulation in the laboratory is performed by a slim-tube apparatus. This is a stainless steel coiled tube of a certain length, packed with a specific type of sand. This sand simulates the core or the rock of the reservoir. The first step of the simulation is that the sand is saturated with crude oil from the specific reservoir. The second step is that the packed slim-tube is brought to the respective temperature similar to reservoir conditions. Then the test is performed at four to six different pressures by injecting the respective liquid or gas. After each run the volume of the effluents and the gas/oil ratio is determined. The density is measured continuously during each run. This density is determined by a high-pressure and high-temperature density meter, placed at the end of the tube.
Any residual oil produced during this process is also collected and weighed. Afterwards the coil is disconnected and weighed to determine the weight of residual oil remaining at the end of the test. The minimum miscibility pressure of the reservoir oil – that is the pressure at which no interface between the fluids exists – is calculated for the specific liquid or gas.
High-temperature and high-pressure density meter
The high-temperature and high-pressure density meter measures the density based on the oscillating U-tube principle in real-time conditions under the borehole pressure and down-hole temperature. The instrument is directly integrated into the slim-tube apparatus and gives real-time density results at flow rates of 10 L/h to 100 L/h. The maximum achievable pressure is 1400 bar and the maximum temperature 200 °C. At the end of the slim-tube an external density measuring cell is placed to measure the density of effluent fluid and to distinguish between the crude oil and the injection fluid like water, brine, CO2, and others.
The data like density decrease, changes in density, and the gas gravity is used for the decision about which of the EOR techniques will leave the maximum margin and for calculating the equation of state for the crude oil for modeling the oilfield with a computer simulation.