Abstract
Annulus pressure at depth is a primary requirement of any gas lift design. There are several methods of determining annulus pressure including reference to a monograph, density equation used to full depth with average pressure and temperature, and density equation used in small depth increments with average temperature and pressure within the increment. The choice of which method to use comes with limitations on the accuracy of the predicted pressure at depth.
When the annulus pressure at depth is calculated by use of a computer program, the choice of which correlation to use to model the critical properties of the gas and which compressibility factor correlation also affects the accuracy of the calculated pressure at depth.
As a gas lift well transitions from a geothermal temperature profile to a flowing temperature profile during the unloading and lifting process, the annulus pressure at depth will decrease even though the surface injection pressure remains unchanged.
Finally, the change in annulus pressure during the unloading process is governed by the volume of gas present in the annulus. A decrease in pressure is due to a reduction in gas volume. Traditional design techniques ignore this reality and substitute a theory of valve performance to explain annulus pressure changes during unloading. This simplified analogy of valve performance made it easy to design a gas lift well but incurred errors and misconceptions about how the annulus pressure changes during unloading.
This paper will address all of the above issues and provide advice on which method of annulus pressure prediction at depth is most appropriate for specific conditions, advise on the accuracy of the combination of different critical properties and compressibility correlations, offer an alternative design technique to account for changing annulus temperature during the unloading process, and finally, provide guidelines on how to accurately and realistically model the changes in annulus pressure during the unloading process.
