It has been more than 70 years since J.J. Arps1 firstly documented the empirical equations used to analyse production trends. With the development of the unconventional reservoirs, other more complex techniques were presented to forecast the production history and determine the estimated ultimate recovery (EUR). Arps Transient Analysis seems to be in the opposite direction.

Arps formulas are widely used and highlypopular because of their simplicity and easy understanding. However, the cited author published his research in a context where the boundary-dominated flow period described the common behaviour of a field. On the other hand, unconventional reservoirs are characterized by the transient flow, and therefore, using Arps in this context, without the appropriate knowledge of the flow regimes involved, will certainly lead to incorrect conclusions.

Many other authors have already discussed the close relationship between the hyperbolic and diffusivity equations. Once the hyperbolic formulas start to be combined with flow regimes identification plots, the Arps formulations become a reliable transient analysis methodology.

Based on the complementation of hyperbolic and diffusivity equations, this paper provides a compiled methodology for tight gas evaluation. The first part introduces the Arps Transient Analysis and its final goal: a sound EUR calculation. The second part provides a comprehensive methodology to deeply understand the flow regimes present in the reservoir, both for gas and water, and how each flow parameter can be related to the well productivity and the ultimate recovery.

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