SIMPLIFIED AND RIGOROUS FORECASTING OF TIGHT/SHALE GAS PRODUCTION

In this paper, a simplified, yet rigorous, method of production forecasting for tight/shale gas reservoirs, which exhibit extended periods of linear flow, without the use of complex tools is proposed. The method is simple as it relies principally on a plot of 1/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 the hyperbolic decline during boundary‑dominated flow (BDF). 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 BDF at later times. Therefore, the method proposed in this study is practical for forecasting production data for these wells, as it considers these two important flow regimes. The model is validated by comparing its results against test cases which are built using numerical simulation to generate a 50‑year synthetic production profile. For each case, the first year of the synthetic production data was used. It is found that there is an excellent agreement between the forecast rates obtained using this method and the numerically simulated rates. Currently, analysis techniques using material balance time (MBT) are being used in industry to analyze shale/tight gas reservoirs. As MBT is superposition time during BDF, these analyses may indicate BDF data, while the reservoir is still in transient. Here are the advantages of the forecasting method proposed in this study: (1) It is not bias towards any flow regimes as no superposition time functions are used; (2) An excellent forecast 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 parameter that it needs is drainage area.