ABSTRACT:

In the presence of an active aquifer, water coning (vertical wells) and water cresting (horizontal wells) phenomena may cause a significant reduction of the well productivity. Water coning phenomena affect the efficiency of the oil drainage from the reservoir, thus reducing the ultimate oil recovery, increasing the cost of production operations, and arising environmental problems related to water disposal. After water breakthrough at a well, the water phase may become the dominant one, and the well must be shut in because oil production becomes uneconomical. Historically several efforts were concentrated on controlling water coning. A wide number of analytical and semi-analytical approaches were developed to analyse coning phenomena and to define a valid methodology to evaluate the critical rate. The critical rate is the maximum oil rate that can be produced before water breakthrough occurs. However, due to the complexity of the problem, a rigorous approach of general validity could not be identified. The present study was aimed at providing an exhaustive comparison among the most used analytical methods and the results obtained by numerical simulation for water coning and water cresting prediction for a number of possible scenarios. In particular, the results provided by the analytical and numerical models were validated by comparison against production data of some real cases. In general, in the case of vertical wells the comparison showed that the critical rate obtained from the analytical methods underestimates the critical rate obtained from numerical models, especially for high anisotropy ratios and low oil viscosities. On the opposite, in the case of horizontal wells the critical rate obtained from analytical methods strongly overestimates the critical rate obtained from numerical models. The errors in predicting the critical rate for vertical wells with analytical models with respect to numerical simulation can be acceptable in most cases, whereas a reliable evaluation of the critical rate for horizontal wells should only rely on numerical modeling

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