Reservoir factors and how they affect waterflooding predictions are discussed in this paper. Results of model studies are used to illustrate the effect of various factors and how they affect decisions in evaluating the optimum depletion method. Some of the factorsstudied include, the effect of well rate and reservoir depletion rate on recovery, significance of capillary forces, the effect of associated gas caps and underlying water on recovery, water injection profiles, viscous oil, fluid migration and lease line drainage. Several pools located in Alberta are used to illustrate these factors and the effect they may have on waterflood recovery and in some cases show that waterflooding may not be the optimum depletion scheme.
The many and varied engineering considerations to be resolved before instituting a waterflood encompasses too many topics for a single paper. Many of the specialized details such as water treating, environmental considerations, formation sensitivity to injection water, design of water injection systems etc. have been the topic of other papers and monographs. The discussion in this paper is limited to the reservoir factors which should be of concern in a waterflood engineering study. Only those subjects which are considered to be of fundamental importance are discussed and examples are used to illustrate the effects of omitting or varying some of these factors.
The most important pre-requisites of any reservoir depletion study are the best possible understanding of the following:
Method of primary depletion.
It is important to have a clear understanding of the foregoing before realistic predictions can be made for any reservoir depletion scheme. Since depletion schemes other than waterflooding may be more economic accurate predictions of these schemes are also required before a waterflood is implemented. The effect of producing rate on economics and ultimate oil recovery must also be considered.
For these reasons the majority of waterflood studies will justify the use of reservoir simulators as an aid in forecasting future production rates, recoveries and the optimum location for injection and producing wells. Cross-section models are used to evaluate the effects of stratification, gravity and capillary forces. Pseudo relative permeability curves can be generated from cross-section studies for use in areal model studies. The use of a cross-section model will result in dynamic curves which will cake into account any sensitivity to producing rate. Some studies may justify, the use of more costly three-dimensional model studies.
A waterflood study should also consider the effects of producing rate on both ultimate recovery and economics. Reservoir simulation will account for all the variables that may be sensitive to producing rate and therefore result in more accurate estimates of oil recovery for any foreseeable producing rate. Although the ultimate recovery for lost schemes will probably not be sensitive to rate except for economic limit considerations the number of producing and injection wells should be optimized based on economics.