As the length of a horizontal well is increased, its contact with the reservoir increases. But at the same time, the resistance to flow in the well also increases, which has a direct negative effect on the productivity of the well. The overall performance of a horizontal well depends on the balance of these two opposing factors. No reliable tools are currently available that account for both these factors in the evaluation of horizontal well performance. A semi-analytical well-model is developed which can quantify the effects of both single phase oil and two-phase oil/gas flow pressure loss in the well on the overall well performance. The model is quite flexible and can incorporate any friction factor correlation. A methodology is developed to show the effects of various reservoir, fluid and well parameters on well productivity. We demonstrate that ignoring frictional effects could lead to unrealistically higher production estimates and low breakthrough times for water or gas. As a result of pressure drop in the well, breakthroughs occur first at the heel of the well. A methodology is also developed to calculate the optimum horizontal well length.

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