Modelling of Critical Gas Velocities based on the Entrained Droplet Model for Gas Wells and the Effect of Heat Loss on Gas Production
Research output: Research › Master's Thesis
Natural gas is one of the most important energy sources. In 2014, it accounted for about a quarter of the world energy consumption. Accordingly, gas production operations take place on a global level, and potential improvements of these operations may possibly have a significant effect. This holds especially true when those improvements are straightforward and easy to implement. Additionally, if these measures eliminate or postpone the need for workover interventions, there is a reduced risk of incidents which directly leads to an improved health, safety and environment record. Thus, this thesis aims to investigate the ubiquitous issue of liquid loading which at some point affects every gas well. The liquid loading of gas wells occurs in many cases due to heat loss into the surrounding formation and decreasing flow velocities within the production tubing. Heat loss into the formation is facilitated by the circumstance that the use of appropriately insulated tubings is usually neglected. The absence of insulation permits unnecessary heat loss and thus a subsequent reduction in gas temperature. The reduced temperature in turn may lead to condensation of liquids, adding to the possibly already existing amount of fluids in the reservoir. Moreover, flow velocities decrease due to two main causes. One is the inevitable decline of reservoir pressure. The other is the reduced gas volume which is caused by the reduced gas temperature. Therefore, the understanding of heat flow and methods to reduce loss are desirable to tackle this issue. In this thesis, the leading formula for the determination of critical gas flow velocities which must not be undercut by the actual flow velocity is investigated. This formula is colloquially known as ”Turner’s equation” and allows the calculation of the minimum flow velocity needed to lift liquid droplets all the way up to the surface. This “entrained droplet model” contains several parameters which ultimately depend on the prevailing pressure and temperature conditions. Hence, a heat transfer model was built in order to formulate and determine the pressure and temperature values for every point within the wellbore. The heat transfer model builds on concepts such as equation of state correlations, heat capacity models, density models, viscosity models, the Joule-Thomson coefficient and water density and water surface tension models. As a result, it was found that insulated tubings lead to higher temperature and pressure readings, and that these effects can be quantified for different types and thicknesses of insulation materials. Moreover, the temperature increase also leads to higher actual gas velocities. Thus, critical gas velocities are exceeded more easily and for a longer period of time, and the margin between them is more pronounced. In the end, this allows an increased amount of gas production and the postponing of the economic limit of a gas well.