Every year, huge amounts of oil are left behind due to early water invasion to wells in water-drive reservoirs. Thus, efficient management of oil reserves requires an understanding of the mechanisms that control the bypassing of oil with water. This paper presents the results of a simulation study aimed at understanding the relationship between water cut development and oil bypassing. Water cut vs. time plots obtained for different reservoir conditions have been analyzed as potential diagnostic tools for the oil recovery process. The results show that there is good correlation of the rate of water cut development in time with oil bypassing. The water cut rate is also a good indicator of the oil displacement front stability and the severity of water underruning. In general, slow water cut development is associated with unstable displacement, thin and long water tongues and substantial oil bypassing.
There are more than 34,000 inactive wells in Louisiana. Most of these wells have been shut in because of water invasion, leaving behind (bypassing) substantial reserves. Oil bypassing in dipping reservoirs is normally attributed to the effect of heterogeneities. However, it has been reported that even homogeneous reservoirs may prematurely end their productive due to water invasion(1),(2). Our recent study has shown that the displacement of oil by water homogeneous-dipping systems with edge-water advancement may result in up to 62% of oil being by-passed at abandonment conditions(3).
Displacement of oil by water in homogeneous dipping reservoirs with side-water advancement is mainly controlled by viscous forces and gravity forces. These mechanisms or forces may interact with each other during the displacement, originating the formation of a stable (linear) oil-water interface or a water tongue. Knowledge of the motion of the oil-water interface is needed in reservoir engineering in order to determine the amount of oil that could be recovered by the end of the well's operation. Obviously, the motion and shape of the oil-water interface is closely related to the water cut development in time.
This paper presents the results of a simulation study undertaken to establish the relationship between water cut development and oil bypassing. Numerical simulation sensitivities were completed using dimensionless groups instead than conventional parameters. This resulted in reduced the numbers of runs and added generality to the results. Water cut vs. time plot were selected as diagnostic plot because of their simplicity and ease of use. Also, water cut can be easily calculated and is commonly available in most petroleum databases.
The numerical simulation model considers a reservoir slab with a linear drive pattern and has been created using the reservoir simulator "IMEX", developed by Computer Modeling Group (CMG) (3). The number of grid blocks has been optimized with a grid sensitivity analysis. In the analysis, finer grids were subsequently considered until there was convergence in the water cut response. This resulted in optimized CPU times and storage. As shown in Figure 1, a regular orthogonal grid was used with 40 grid blocks in the "X" direction, 11 grid blocks in "Y" and 20 grid blocks in the "Z" direction, for a total of 8800 grid blocks.
