Predicting EUR is a challenging task. Nearly all approaches that are being used in predicting said long-standing behaviors, have displayed various limitations. So, if decline curve analysis (DCA) is not the appropriate method for unconventional reservoirs, which one is it?
For about twenty years now, the oil and gas industry, has widely applied DCA in production forecasting. All rooted in the need to get realistic EUR and economic KPI’s for your wells.
However, the absence of historical data, the deep understanding of the fluid flow regime, determination of EUR and market volatility, are just some of the challenges remaining while evaluating unconventional reservoirs.
“Oil and Gas Property Evaluation,” by John Wright, 2013., has often being a basis to describe the equations needed for DCA, but it all changes when real wells, real data and real software come into play. In unconventional reservoirs, the use of DCA can lead to overestimating or underestimating the wells’ EUR. For instance, multi-frac horizontal wells typically experience prolonged transient/transitional flow regimes. Whereas, in conventional wells, the pseudo-steady-state conditions are established relatively quickly. Leading to dramatic problems in other scenarios where the NPV varies more: it’s related to the improper EUR and economic expectations estimation. It doesn’t consider the impact of reservoir and completion properties.
However, in some cases, the use of DCA does not seem to be too off.
The best, and more accurate results, come from a physics-based model. The forecasts with this model include total cumulative production, production rate, stage-specific production, and spatial evolution of quantities of interest. Taking into account the reservoir and completion properties, which results in a more precise prediction.
All methodologies have their strong and weak points. DCA can accurately match historical patterns, but, when it comes down to forecasting future events that can stray from the trend, there is a clear need for alternate methodologies. A combination of tools can, more often than not, provide a more distinct analysis of each well’s unique characteristics.
One thing to always keep in mind, is that technology and workflows evolve, being artificial intelligence (AI) a crucial part of this constant evolution, so the usefulness of all the guidelines and “rules of thumb”, usually vanishes.