Abstract
Nitrogen gas is used to charge the domes in gas lift valves and some downhole safety valves to provide the closing force to shut the valve. The design pressure and temperature conditions in the well must be calibrated to the shop conditions where the valve is set up and tested. Shop conditions are typically established at 60°F so the nitrogen pressure at operating conditions in the well must be corrected to this temperature; hence, the name "temperature correction factor." Currently, the use of 60°F is an industry standard, although earlier shop-conversions pre-1980's offered calibration temperatures of both 60 and 80°F. The significance of the current shop standard is examined and an explanation is offered confirming the standard.
Early downhole design injection pressure conditions were typically in the range of 500-1,000 psig which formed the basis for the development of early tables and charts used within the industry. As more challenging conditions were encountered, the accuracy of these resources diminished and new methods were developed that form the basis of the procedures that are in part used today. These methods were developed using published nitrogen Z factor data that expanded the calibaration range to 3,000 psig and 300°F. The new methods used 2nd and 3rd order polynomial equations as a basis for the model. The usage of these types of polynomial equations for models does not offer an accurate extrapolation as the boundary conditions are approached or exceeded. Currently, the industry is pursuing equipment with design pressures of 20,000 psig so the use of existing models for extreme conditions is suspect. A detailed analysis of the calculation methodology and accuracy of the published temperature correction factor methods at operating conditions ranging up to 15,000 psig and 400°F is presented. As an alternative, a new equation-of-state (EOS) based method for nitrogen PVT properties is also offered to accurately determine temperature correction factors. This method is easily implemented in existing software programs as the only the coefficients in a commonly-used, natural gas EOS are updated to accurately reflect nitrogen PVT properties to 15,000 psig and 400°F.
