A new material balance equation for the analysis of liquid rich natural gas reservoirs

In this study, we analytically cross examine 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 (i.e. tank) material balance equation that is directly applicable to the analysis of liquid-rich (wet and retrograde) gas reservoirs by expression of the equations in term of an equivalent gas molar density. Following model development, proposed model predictions of gas reservoir behavior with varying condensate content (lean, intermediate and rich) are investigated and critically compared to previous zero-dimensional models. 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 the ability of the proposed models to provide reliable reserve estimations using straight-line methods. The proposed density-based equation is proven to be straightforward to implement since is written in terms of density. This, in turn, allows it be directly expressed as an extension of the dry gas MBE, while not requiring the implementation of two-phase Z-factors.