Gas in place/reserves estimation in unconventional (low/ultra-low permeability) reservoirs has become a topic of increased interest as more of these resources are being developed domestically and internationally. Production data from these unconventional gas reservoirs exhibit extended periods of transient/transition flow behavior that often lead to the overestimation of gas in place/reserves with the use of simple rate-time extrapolation techniques.

In this work we show that the analysis of production data (particularly rate-time data) before the onset of boundary dominated flow for these unconventional gas reservoirs leads to significant overestimation of gas in place/reserves. Consequently, we introduce the concept of "continuous estimation of ultimate recovery" (or "continuous EUR") where estimation of reserves from a single gas well is performed in a dynamic fashion — in other words we continuously estimate the reserves for selected time intervals throughout the producing life of the well.

We have applied the "continuous EUR" method by using simple rate-time relations such as the Arps' "hyperbolic" rate decline relation and the power-law exponential rate decline relation along with the straight line extrapolation technique, which provides a lower limit for EUR. We present the application of the "continuous EUR" method with a simulated tight gas data set and four (tight gas) field data sets. Our analyses show that the distinction in the flow behavior is more evident and the uncertainty in reserves estimation decreases significantly when reserves are evaluated continuously.

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