Water Coning And Critical Rates In Vertical And Horizontal Wells

Producing oil at a water cut higher than necessary has always been a preoccupation for practicing engineers. The existence of a critical rate above which excessive water production occurs has been mentioned earlier in Muskat`s work on bottom water drive and in Dietz`s paper on edge water drive. Since these pioneer studies, numerous papers have been published on critical rate. Some of these studies were based on laboratory experiments, others were based on analytical derivations; both approaches have limitations. In this study, a simple and accurate method to calculate the critical rate is presented. This method can be implemented either by numerically solving the equations of flow or by directly using a numerical simulator. The calculations are consistent with the present literature. However, there is a disagreement with existing literature in at least two instances. For horizontal wells, most of the studies present the critical rate as an increasing function of the anisotropy ratio. The present study shows that this is true only for 0.5 < a < 1. In fact, for 0.01 < a < 0.1, critical rate is a strongly decreasing function of anisotropy ratio. This is not a mere theoretical case. Many reservoirs with discontinuous thin siltmore » barriers are believed to display very low anisotropy ratios. Most of the studies describe the vertical well critical rate as a decreasing function of the anisotropy ratio, opposite in behavior to horizontal wells. Although the present study agrees with the general trend, it shows that there is in fact a harmonious continuity between vertical and horizontal wells and short horizontal wells display a behavior in between. Other results of this study present relations in dimensionless form that describe the effect of anisotropy ratio, horizontal well lengths, well penetration and reservoir geometry on critical rates.« less