Simplified Yet Rigorous Forecasting of Tight/Shale Gas Production in Linear Flow

M. Nobakht, SPE, L. Mattar, SPE, S. Moghadam, SPE, and D.M. Anderson, SPE, Fekete Associates Inc.

Copyright © 2010, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Western Regional Meeting held in Anaheim, California, USA, 27–29 May 2010.

This paper presents a simplified, yet rigorous, method of production forecasting for tight/shale gas reservoirs exhibiting extended periods of linear flow, without the use of complex tools. The method is simple as it relies principally on a plot of inverse rate versus square root time, and it is rigorous in that it is based on the theory of linear flow and combines the linear flow transient period with hyperbolic decline during boundary-dominated flow.

The dominant flow regime observed in most fractured tight/shale gas wells is linear flow, which may continue for several years. This linear flow will be followed by boundary-dominated flow at later times. Therefore, the method proposed in this study is applicable for forecasting production data for these wells, as it considers these two important flow regimes.

The method is validated by comparing its results against test cases which are built using numerical simulation. For each case, only the first year of the synthetic production data is then used for the analysis. It is found that there is excellent agreement between the forecast rates obtained using this method and the numerically simulated rates.

Currently, analysis techniques using material balance time are being used in industry to analyze shale/tight gas reservoirs. Because material balance time is actually boundary-dominated flow superposition time, these analyses may show symptoms of boundary-dominated flow even though the reservoir is still in transient flow. Here are the advantages of the forecasting method proposed in this study: (1) It is not biased towards any flow regimes, as no superposition time functions are used; (2) Reliable forecasts can be obtained without using pseudo-time-this is an advantage, as using pseudo-time introduces complexities and an iterative procedure; and (3) The only externally specified parameter required is the drainage area.

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