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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Are Rescaled Decline Curves a Solution for Shale Gas Assessment?
(1) Talisman Energy Inc, Calgary, AB, Canada.
The intent of the study has been to find a reliable way to predict ultimate recoveries as soon as possible after the start of production in unconventional gas wells.
In order to create single decline curves that combine many wells, our approach has been to rescale the data by using the logarithm of the cumulative production instead of the traditional production parameter (Mcf or Bcf). In our graphs we refer to this parameter as the “normalized production index”. The main reason for the use of the logarithm is to better define the bottom part of a decline curve. The advantage of this rescaling is the ability to inspect with more precision the behavior of the production rate (the proxy being the log of production divided by the number of days).
The method has been first tested with data from tight sand reservoirs for which long term production is readily available and for more than 160,000 wells from the US and from Canada. The same method has then been tested for some shale gas, keeping in mind that the historic data is more limited in time, the Barnett shale being the exception.
Many wells can thus be plotted at once using rescaled decline curves, any of these curves is typical of an inverse function (harmonic behavior) that is a simple and very reliable mathematical equation; the result of which can be easily plotted and efficiently compared. Comparison for the same formation in different basins has been achieved and has demonstrated the robustness of the method. The effect of limited data has been tested by reducing the data to a selection of time periods; the results of that test indicate the need to compare production between wells using the same length of time.
The rescaled decline curves become most useful when referenced to a normal decline curve. Shale gas rescaled decline curves have been compared to normal decline curves in two ways
- to the production per day starting from the peak production month
- to the normalized production per day achieved in the peak month.
Rescaled decline curves allow a quick assessment of the type of production decline that any well should experience. They could thus be used as a first pass or as a quick test to estimate the level of success of any change in completion design as well as the expected ultimate production potential.