Empirical Estimates of Primary Oil Recovery

Several attempts have been made to correlate primary oil recovery with reservoir parameters [239-2421. Based on field data [241] from water-drive reservoirs, a statistical study [242] yielded the following empirical relationship for primary oil recovery:

where NP is oil production in STB, k is permeability in md, Sw is fractional water saturation, po is oil viscosity in cp, $ is fractional porosity, h is pay thickness in ft, A is a real extent in ft5, and Boi is the initial formation volume factor of oil in reservoir barrels per STB. Based on the first API study [240], correlations were developed for recoverable oil. For solution gas drive reservoirs, the recoverable oil (RO) in stock tank barrels per net acre-ft was:

However, the second study concluded that none of the equatirms developed in either study was statistically appropriate to provide a valid correlation. Furthermore, no statistically valid correlation was found between oil recovery and definable reservoir parameters. The second study found that when reservoirs were separated by lithology, geographical province, and producing mechanism, the only reasonable correlations that could be developed were between recoverable oil and original oil in place. Even then, the correlations were of poor quality as indicated by Figure 5-139 which presents the best correlation for Texas sandstone naturalwater-drive reservoirs.