Universal material balance equation for the analysis of condensate gas reservoirs with edgewater and bottomwater drive aquifer

The gas reservoir drive mechanism generally derives from either the fluid expansion within the reservoir, an external aquifer in contact with the reservoir, or a combination of both. The zero-dimensionless material balance equation (MBE) is a simple yet powerful tool to calculate the original hydrocarbons in place and determine the constant that characterizes aquifer performance. The purpose of this paper is to propose a universal MBE applicable to condensate (wet and retrograde) gas reservoirs with bottomwater drive (BWD) and edgewater drive (EWD) aquifers, which retains the simplicity of a straight-line plot of conventional MBE while accounting for the effects of water expansion and water influx. Black oil fluid formulation was first generated from compositional simulation via a properly tuned equation of state. By use of the temporal and spatial integral of governing equations of fluid flow in the reservoir phases, two forms of MBEs were, respectively, proposed in terms of gas/oil molar density and multiphase fluid volumes, and the external pressure support was considered through a water influx model, including either BWD or EWD. The key to new MBE is the reliability of saturation–pressure (SP) relationships. As for the volumetric MBE, SP relationships were fully derived from the limiting case of the unsteady-state path. As for molar MBE, SP relationships were investigated by either constant volume depletion or constant composition expansion, which results in the appropriate two-phase Z factor. Various MBEs along three paths of pressure depletion were compared with benchmark examples from reservoir numerical simulation. In addition, a workflow of regression analysis using a least-absolute-value-based straight-line fitting technique was provided, recapturing the simplicity of the straight line while accounting for all the drive mechanisms. The results show that the proposed MBE is straightforward to calculate reliable reserve estimations. The accuracy of reserve estimation would increase by ensuring consistency in solutions from multiple methods. These samples demonstrate the power of the universal material balance equation.