There are three main ways for water in crude oil to enter.
First, the original crude oil in the reservoir itself contains water.
Second, in order to maintain the reservoir pressure, water is injected into the reservoir.
Thirdly, crude oil is affected by the change of temperature during storage and transportation, and gas is alternately discharged or air is inhaled in the oil collecting tank. Due to the continuous inhalation of air, water vapor keeps entering, which increases the number of water molecules in crude oil.
There are four kinds of water in the oil sample: free water, suspended water, emulsified water and dissolved water. Among them, free water refers to water that can be separated by tilting method; Suspended water refers to the water mechanically mixed in crude oil with tiny droplets; Emulsified water refers to the liquid after oil and water are evenly emulsified together; Dissolved water refers to water dissolved in crude oil according to its ability to dissolve in crude oil, and its value is very small.
At present, there are two main methods to determine the water content of crude oil in oil fields, namely distillation and centrifugation.
(1) distillation method.
Distillation method refers to mixing a certain amount of oil sample with anhydrous solvent gasoline, heating, distilling water, recording the volume of water, and calculating the water content of crude oil according to the following formula, expressed as a percentage.
(2) centrifugation.
Centrifugation is based on the fact that crude oil and water are two immiscible liquids with different densities. After adding demulsifier, emulsified water in oil is separated. Using the centrifugal force generated by the high-speed rotation of the centrifuge, concentrated water is precipitated at the bottom of the centrifuge tube to realize oil-water separation. After reading the water level gauge, calculate the water content of crude oil.
Determination of water content in crude oil by distillation has the characteristics of power saving, simple operation and high dehydration efficiency, but the loss of light oil is more, which affects the quality of crude oil analysis. Centrifugal determination of water content in crude oil has the characteristics of high speed, large batch and high safety factor, but the determination accuracy is not high enough, especially for the determination of water content in exported oil, distillation method must be used.