Water-Oil Ratio Forecasting Theory
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Chesca Mae Arzaga RapirapWater-Oil Ratio Forecasting Theory
Uploaded by
Chesca Mae Arzaga RapirapWater-Oil Ratio Forecasting Theory
Water-oil ratio (WOR) forecasting is a method of trending future water production for the purpose
of forecasting oil production and determining expected ultimate recoverables (EUR). Water-oil
ratio forecasting is an empirical analysis method; there is no formal model or equations. Plots of
both water-oil ratio and water-oil ratio +1 are used in forecasting, and the plots are typically semilog with cumulative oil production as the x-axis. Water-oil ratio, water-oil ratio +1, and water cut
are defined below:
Ratio interpretation in combination with rate interpretation (traditional decline analysis) can
increase confidence in the forecasts. In exponential or harmonic rate decline, slope dependencies
are often observed between the rate and ratio trends. By plotting a forecast on both rate and ratio
data, the slope dependencies can be used to match historical data on both the rate and ratio
trends. These interpretations are less likely to show personal bias, as the forecast would only look
reasonable if it matched both trends. Exponential decline is best analyzed on a semi-log plot of
water-oil ratio or water-oil ratio +1 vs. time. Harmonic decline is best analyzed on a semi-log plot
of water-oil ratio or water-oil ratio +1 vs. cumulative oil. The plot below shows slope dependence
(rate slope is inverse of ratio slope) between oil rate and water-oil ratio +1 while the total fluid rate
(oil + water) is constant.
Water-oil ratio and water-oil ratio +1 are often used interchangeably. The advantage of water-oil
ratio +1 in some situations is that periods with zero water production can be plotted. In some
cases, the ratio trend will appear more linear on water-oil ratio or water-oil ratio +1. It is suggested
that water-oil ratio behaves linearly when water rates are constant, and water-oil ratio +1 behaves
linearly when total fluid rates are constant. These may guide the user in which plot to select, but
both should be considered uncertain. To create the forecast, a linear trend is placed on the wateroil ratio or water-oil ratio +1 semi-log plot, and a maximum water-oil ratio or water cut is used to
truncate the forecast. An oil rate forecast is calculated by combining the ratio forecast with a total
fluid rate.
Water-oil ratio forecasting will provide the most useful result in water drive or waterflood scenarios
(e.g. scenarios with high water production), however linearizing the data removes the diagnostic
aspects that may be desired for these reservoirs and production scenarios. Similar to decline
forecasts, the trends created with water-oil ratio plots should be considered under constant
operating conditions. Changing conditions such as infill drilling or recompletions often appear as
changing slope in the water-oil ratio trend.