Oilfield crude oil production shrinkage factor and equation of state measurement, testing and simulations.
Intertek exploration and production laboratories provide reliable crude oil shrinkage factor analyses and EOS (Equation of State) simulations.
The shrinkage factor of crude oil from separator conditions varies, dependant upon the pressure and temperature of the separator and the individual fluid properties. The more volatile the separator liquid phase, the more impact separator conditions and shrinkage will be. Shrinkage value will be very dependant upon the separator pressure and temperature and will change as these values vary.
Measuring the crude oil shrinkage factor:
Good oil shrinkage measurement is best approached by collecting the primary separator liquid and performing a separator test on it. This involves simulating shrinkage in the laboratory at each stage of separation (pressure and temperature) from primary separator to stock tank conditions. Different liquid streams (with different compositions) require laboratory analysis for each stream. The only problem with this approach is if separator conditions change so will the shrinkage. There are a couple of ways to take this into account.
Intertek recommends collecting a separator liquid sample at the maximum pressure a separator will be operated (preferably at a lowest temperature) to allow the maximum amount of gas in solution. The sample is compositionally analyzed and subjected to a separator test, duplicating standard separator conditions from primary separator through stock tank conditions. Examples are of the primary separator at 1100 psia and 100°F, second stage separator at 750 psia and 85°F, third stage separator at 340 psia and 75°F, fourth stage separator at 120 psia and 105°F, and stock tank at 15 psia and 85°F. Utilizing the composition and the results from the separator test, an equation-of-state (EOS) computer model is "tuned" to the measured shrinkage data. This tuned model can then be used to predict shrinkage values at different separator pressure or temperature conditions with the resulting data well within a 5% error band.
Alternatively, a series or matrix of separator tests at separator conditions covering the anticipated spread of pressures and temperatures can be performed in the laboratory. These tests generate a matrix of shrinkage values covering the anticipated range. The data can establish a table or equation to yield shrinkage value as a function of separator conditions. Typically, only primary separator conditions are varied. This approach can be used with the equation-of-state computer model instead of physically performing the matrix of separator tests — it is important to perform one experimentally to tune the EOS, however.
Measuring shrinkage factor at the wellsite:
The "shrinkage tester" suggested for wellsite installation provides a rough shrinkage value, one that probably isn’t is sufficient if your system is a 30°API oil and your separator conditions are not subject to significant change. Typically such equipment employ a vessel filled with separator liquid at pressure (although not necessarily at temperature). The volume of this vessel is known and calibrated. The vessel is then drained into a non-pressurized graduated container while the entrained gas is allowed to escape. The shrinkage value is simply the non-pressurized volume divided by the volume of the pressurized vessel (dead oil divided by live oil volumes). Petroleum table values can correct the non-pressurized volume to standard conditions (15°C or 60°F) although this is often not done, adding yet another error.
This method does not take into account changes in shrinkage value caused by multiple pressure/temperature changes of the liquid as produced by secondary and tertiary separators. On an offshore platform and all the way to the shore base, typically there will be several downstream separators in line from the primary separator. Each one of these separators will have an impact on the shrinkage value and this is part of the reason they are in place. Not taking temperature into account, there will be less shrinkage with more stages of separation as the oil pressure is reduced to atmospheric pressure.
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