A: Whenever you have a measurable gas rate in the well. Regardless of the API gravity of the crude oil, its viscosity will decrease due to the gas in solution, and its density will decrease, particularly if some gas comes out of solution in the tubing. Both of these mean that in most cases the pressure losses will decrease in most cases if multiphase flow is used. Note that it is particularly important for oils heavier than 15-20°API to enter the dead oil viscosity in the advanced viscosity window.

A: Every oil field is different. These correlations are based on data from a limited geographical area. The only way to determine which correlation is most accurate would be to compare results from each correlation to actual field data. The User Guide contains references to the sources of these correlations, so you can look up the equations used and determine which ones most accurately match your data. Alternatively, if you have laboratory test data for these fluid properties, and you are currently subscribed to the PC-PUMP Maintenance Plan, send your data to C-FER and we will prepare plots comparing your data to the results from each correlation, to assist you in determining which correlations to use.

A: The best way to determine this is to have the well fluids tested in a lab in the form in which they come from the well. Even small amounts of dissolved gas can make the actual oil viscosity significantly less than the dead oil viscosity. As for the water, if the fluids are emulsified, water can cause the actual viscosity of the mixture to be significantly higher than for pure oil. The advanced viscosity window in PC-PUMP has, for single phase only, options for varying the viscosity with water. This gives you the choice of either specifying that the fluid is an emulsion or that the viscosity is to be calculated as a weighted average of the water and oil viscosities.

A: It is specified relative to vertical depth. Note that this refers to a flowing temperature gradient and not the geothermal gradient. The value for the flowing gradient will always be less than or equal to the geothermal gradient. Newer versions of PC-PUMP allow you the option of entering the flowing wellhead temperature instead of the gradient, if you prefer. In that case, it will display the calculated value for the gradient.

A: This is an option in PC-PUMP, but it uses a correlation which is based on very limited data. We suggest using this option with discretion. Try running it and see if the results make sense. When a pump is pumping a large volume of high viscosity fluid, the viscous torque can easily be 30% or more of the total torque. If the rates and viscosity are both low, the viscous torque should also be low. Due to the extremely limited data available to C-FER for checking the accuracy of this correlation, we recommend that users be careful in evaluating the results of this calculation. The program will always print an output message containing the calculated viscous torque.

If you use this calculation option built into PC-PUMP (in multiphase flow) it will do one of two things. If the pump (or tail joint) intake is located below the perforations, it will assume 100% separation efficiency—all the free gas at the perforations is assumed to go up. If the intake is located above the perforations, it will estimate a separation efficiency. This calculation assumes that the pump intake is centralized and that the well is near vertical where the intake is located—if these conditions are true, the calculation should be reasonably accurate. Times when you might not want to use the calculation option include:
a ) The intake is located below the perforations but there is high velocity and/or high viscosity, which could drag free gas down to the intake. In this case, the separation will be less than 100%.
b) The intake is not centralized. If the intake is pushed against the wall of the tubing, the separation efficiency may be improved.
c ) The intake is not in a vertical portion of the well. If the intake rests on the low side of the well, the efficiency should be improved, but if it is pushed against the high side, it could be worsened. If it is centralized, the efficiency may be increased or decreased, depending on the amount of free gas and the angle of the well.
d) A gas separation device is used. If working properly, a gas separator should increase the separation efficiency, but the amount of increase will depend on several factors.
In any of these cases, you will need to estimate a value for gas separation and enter it manually.