Gas-condensate banking and well deliverability - a comparative study using analytical- and numerical models

Abstract

Reservoir condensate blockage in the reservoir and in particular in the close vicinity of the wellbore may result in significant loss of well deliverability for medium to tight gas reservoirs. The dynamics of fluid flow in these types of reservoirs are investigated by way of analytical description and by numerical simulation of radial two phase flow behavior. The analytical model is based on a single phase semi steady-state solution, being revised for two phase flow of gas and condensate oil. As the liquid is dropping out of the gas, a saturation distribution of immovable oil is seen to develop radially in the reservoir with time. Condensed oil not being part of this saturation distribution is produced to the surface. The numerical model is a cylindrical, single well, fine gridded, simulation model run on the Eclipse E300 compositional simulator. The base case study comprises a rich gas condensate fluid (GOR ~ 1000 Sm3/Sm3), an initial gas-in-place volume of 750 MSm3, and reservoir permeability of 10 mD. A gas flow rate of 0.5 MSm3/day is leading to a production period of about 4 years. A comparative study is performed by varying parameters such as pressure development, development of bank of immovable oil both radially and as function of time, condensate blockage effects, and gas and condensate oil production and onset of reduced well deliverability. Sensitivity analysis are performed by studying variation in the productivity index, non-Darcy and mechanical skin, and sensitivities related to permeability. It has been confirmed in this study that condensate blockage has a direct and negative impact on well deliverability, where both the plateau period and the bottom hole pressure are reduced. We also show that the analytical model compares well with the numerical models and that many features describing gas-condensate banking and well deliverability are adequately described in the model. The analytical model also offers insight into the process of reservoir liquid storage in gas condensate reservoirs.