A density-based material balance equation for the analysis of liquid-rich natural gas systems

This study analytically cross examines the consistency among available zero-dimensional material balance equations (MBEs) for liquid-rich gas equations and derive a new simple yet rigorous MBE starting from governing equations applicable to these systems. We propose a new zero-dimensional (tank) material balance equation directly applicable to the analysis of liquid-rich (wet and retrograde) gas reservoirs expressed as a function of an equivalent gas molar density, as well as investigate and critically compare its predictions against other zero-dimensional (tank) models proposed in the past for gas reservoir cases with different amounts of condensate content (lean, intermediate and rich). All models are employed to predict reservoir performance given reservoir original-fluids-in-place and compared against benchmark examples created by numerical simulation. Actual field examples are also analyzed using existing and proposed models to test their ability to provide reliable reserve estimations using straight-line methods. The proposed density-based equation is proven to be straightforward to implement since it is written in terms of density, which allows it be directly expressed as an extension of the dry gas MBE, while not requiring the implementation of two-phase Z-factors.