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Measuring the density of crude oil under simulated borehole conditions

What are the different steps of oil recovery?

Oil recovery is divided into 3 stages: 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.

What is Enhanced Oil Recovery?

Tertiary recovery is also called “enhanced oil recovery” (EOR) or “improved oil recovery” (IOR). It 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. 

What EOR techniques are available?

The best EOR 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.

1. Gas injection

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, which 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.

2. Thermal/steam injection

This approach uses different techniques to heat the crude oil and 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.

3. 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 – 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

Why measure the density in simulated EOR setups?

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 deciding whether to implement a technique or not. In order to prove which of the different processes for enhanced oil recovery (EOR) will stimulate the oil production in a specific reservoir and consequently give the maximum recovery factor and the most profit, it is necessary to simulate and characterize the reservoir in the laboratory.

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.

Data obtained, such as 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.

Requirements for density meters

The density meter used must be able to withstand the high pressures (up to 1400 bar) and high temperatures (up to 200 °C) in order to cope with the conditions simulated to represent borehole pressures and down-hole temperatures.

Common units

The density of crude oil is typically given in kg/m3