The widely used semi-log water-oil ratio (WOR) vs recovery semi-log plot is physically sound but should only be applied to a well stock with stable operation conditions. Otherwise the linear relationship between the logarithm of WOR and cumulative oil production can be distorted, often leading to an unacceptably high error margin. However, due to constant and varied wellworks and well activities in the field, meeting the stable operation conditions requirement is often problematic.
This article presents a method of processing the source data for the WOR vs recovery semi-log plot, developed by the author to produce a more linear relationship despite unstable well operation conditions with varying liquid rate changes distorting the actual recovery efficiency curves. In many cases the method is successful, sometimes improving the determination coefficient from 0.450 to 0.982. The method is based on reconstructing the production history of each well assuming a constant liquid rate equal to the last actual value, while retaining the individual cut vs recovery curve for that well. Reconstructing the production history prepares the data for a group of wells for the linear approximation, improving the recoverable reserves estimate accuracy and production forecast reliability. The processing of source data by the proposed method should be complemented with removing certain wells from the selected group: wells, which have been put on production or stopped during the time interval selected for approximation; also wells, which have been worked over during that time interval in a way that allows increasing recoverable reserves.
References
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