Using Difference Factor Thermodynamics Models in Hydrate Formation Prediction During Surface Production Well Testing

Abstract

Abstract All oil fields under water-drive, including gas-cap and depletion reservoirs may produce some water along with oil. This co-production of water causes an additional set of challenges: corrosion, scale/salts deposition, disposal of the water itself, and gas-hydrate formation. Hydrates are ice-like solids that form when free water and natural gas combine at high pressure and low temperature. This can exist in gas and gas/condensate wells, as well as in oil wells. Hydrate formationis a very serious challenge and can result in a huge loss especially during production. It's also one of the major challenges encountered during surface production well testing and it is controlled by several factors which include thermodynamics and kinetics of the system. This study presents a thermodynamic model under operating field conditions, which centers on changes in the Langmuir constant of the guest molecule(s) which is an important parameter in the hydrate formation process. In this study, real-time production data acquisition system was used to acquire surface production well testing data such as pressure, temperature, and choke size, from a hydrate well. The acquired real-time data were used to determine the effect of the variation in choke size on Langmuir constant and therefore on hydrate formation. Evaluating the Langmuir constant for large cavity under field conditions using a generalized correlation, the Difference Factor method was developed and expanded to incorporate the choke sizes variation. The correlation shows that Langmuir constant (decreases) is inversely proportional to the choke size (increases) causing a decrease in the rate of hydrate formation. Due to an increase in choke size, Langmuir constant approaches zero, therefore the product of fugacity and Langmuir constant which is the driving force for hydrate formation approaches zero, leading to zero hydrate formation.