Reliable, early determination of long term production and ultimate recovery in oil and gas reservoirs is of utmost importance to E&P companies, reserves auditors and investors. In conventional reservoirs, the EUR can be reliably estimated once the drainage volume (hydrocarbon pore volume) has been established. This can be done using Rate Transient Analysis (RTA) if the presence of boundary dominated flow can be observed in the data. Unfortunately this approach is not easily applied to tight, fractured reservoirs because of the complexity of these reservoirs (which leads to non-unique reservoir characteristics) and the presence of persistent transient flow (which leads to non-unique estimations of ultimate recovery). In some instances, boundary dominated flow may not be observed until several years have elapsed during the producing life of the well.

In recent years, there have been numerous contributions to the science of well performance-based methods for estimation of ultimate recovery of unconventional resources. Wattenbarger et al. (1998) and Brown et al. (2009) propose analytical techniques while Ilk et al. (2008), Valko and Lee (2010) and Duong (2011) have each proposed new empirical decline curve equations. While each of these methods has value, they do not specifically address the problem that underpins all unconventional well analysis, uncertainty. In 2011, the authors proposed the use of probabilistic rate transient analysis to help quantify this uncertainty. This approach acknowledges the non-uniqueness inherent in the RTA model inputs and allows for the systematic investigation of an allowable parameter space based on acceptable ranges of inputs such as the conductivity, spacing, complexity, length and height of the fractures and the matrix permeability. The result is the full set of possible production forecasts (in as much as the model can be said to capture the physics of the problem) from which the "most likely" production profile can be extracted and that help define the uncertainty in long-term recovery for the play. In this paper, we will further explore probabilistic rate transient analysis by presenting a case study from the Montney play in Canada. The primary objective of this work is to illustrate that a probabilistic approach can be practical, reliable and systematic, offering a viable alternative (or complement) to the standard deterministic techniques.

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