Decline curve analysis for tight gas and gas condensate reservoirs

To understand the reservoir flow behavior and estimate its parameters (e.g., permeability, skin), either transient pressure or rate data are usually used. However, in tight reservoirs, due to the economic and technical difficulties of transient pressure well test operations and data analysis, working with transient rate data, i.e., decline curve analysis, sometimes, is considered to be more attractive.

This study focuses on the application of the widely accepted Fetkovich type curves for tight gas and gas condensate reservoirs. Initially, a synthetic reservoir model was constructed to replicate one of the case histories of a tight oil reservoir studied by Fetkovich et al. (1987). Various sensitivities on permeability, skin, reservoir radius, and fluid type were performed to ensure the validity and generality of the model. The application of Fetkovich type curves was then investigated for three gas condensate fluids with various richness levels. Here, implications and limitations of this extension are highlighted when various reservoir parameters (i.e., skin and reservoir radius) are varied.

Our results demonstrate that Fetkovich type curves can be used to derive reservoir parameters for tight reservoirs, but caution needs to be taken for different fluid types and production constraints. For dry gas, the Fetkovich method can directly be applied. In gas condensate systems, this method together with the gas equivalent concept gives reliable results if bottomhole pressure is above the dewpoint. Extension of this approach when bottomhole pressure is below the dewpoint, leads to erroneous results. If the preferred two-phase pseudo-pressure approach is considered in the interpretation; the results are more accurate but still not fully acceptable. The findings of this study allow better evaluation of production potentials and improved management of unconventional gas reservoirs.