Question

If a reservoir is undergoing a strong water drive, how does the slope of the Havlena-Odeh material balance plot change compared to a depletion-drive reservoir?

Accepted Answer

The Havlena-Odeh material balance equation is expressed as F = (N * Eo) + (We), where F represents the underground withdrawal, N is the initial oil in place, Eo is the oil expansion factor, and We is the cumulative water influx. When rearranged for a graphical plot, the equation is written as F/Eo = N + (We/Eo). In a depletion-drive reservoir, there is no water influx, so We equals zero. Consequently, the term We/Eo disappears, and the plot of F/Eo versus Eo results in a horizontal line with a slope of zero, indicating that the production is solely driven by the expansion of reservoir fluids. In a reservoir undergoing a strong water drive, the We/Eo term is significant and varies as water enters the reservoir over time. As pressure declines, the cumulative water influx We increases, causing the F/Eo ratio to rise as the reservoir is supported by this external water. Because the water influx is not constant and increases relative to the depletion, the slope of the plot of F/Eo versus the cumulative influx function (We/Eo) becomes positive and non-zero. The steeper the slope of this line, the stronger the influence of the water drive relative to the depletion mechanism, confirming that the change in slope from zero to a positive value is a direct indicator of the shift from a depletion-drive mechanism to an active water-drive mechanism.

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If a reservoir is undergoing a strong water drive, how does the slope of the Havlena-Odeh material balance plot change compared to a depletion-drive reservoir?