Plots of water/oil ratio (WOR) versus time can provide a valuable indication of when an excess water problem develops.2,24 Along with other information, such plots can also aid in identifying the cause of the problem. However, these “diagnostic plots” (of WOR or WOR derivative versus time) should not be used alone to diagnose excessive water production mechanisms and problems.25,26 This method was said to be capable of distinguishing whether a production well is experiencing premature water breakthrough caused by water coning or channeling through high permeability layers.24 According to this method, gradually increasing WOR curves with negative derivative slopes are unique for coning problems, and rapidly increasing WOR curves with positive derivative slopes are indicative of a channeling problem. As far as we are aware, this method has not been used to distinguish between linear flow (fracture or flow behind pipe) and radial flow for either channeling or coning. As mentioned above, the linear/radial distinction is extremely important—much more so than whether the problem is due to generic channeling or coning.
Recently, reservoir models were built for water coning and channeling, respectively, and a sensitivity analysis was performed using numerical simulation.25,26 Reservoir and fluid parameters were varied to examine WOR and WOR derivative behavior for both coning and channeling production problems. The results from this study demonstrated that multi-layer channeling problems could easily be mistaken as bottomwater coning, and vice versa, if WOR diagnostic plots are used alone to identify an excessive water production mechanism. Hence, WOR diagnostic plots can easily be misinterpreted and should therefore not be used alone to diagnose the specific cause of a water production problem.