Coning in dual completed systems

The effectiveness of dual completions at mitigating water coning in thin oil-column reserves is examined in several ways. An analytical model valid for gravity-dominated dynamics is given, and yields useful approximate design formulae. Some representative exact numerical calculations are also presented, showing zones of `safeʹ operation where pure oil is produced from the upper completion, and clean water from the lower. In symmetric designs, water-free production of oil is possible at rates many times those available in single-completion systems (a factor of 5 or so is possible). Less dramatic improvements occur in asymmetric arrangements; perhaps only a factor of 2 is achievable in such designs. The effect of flat-bedded heterogeneity on critical coning rates is investigated numerically, and reveals that coning is critically controlled by the near-well layers. In particular, highly permeable layers at the completion or in between the completion and the oil–water interface assist greatly in reducing coning.