Horizontal wells have been drilled in many oil and gas reservoirs. Measuring or calculating the productivity of horizontal wells has been difficult because of their long length in the formation compared with vertical wells and because of the inadequacy of measurements during drilling and production. In this paper, we examine the performance of horizontal wells. First, we review the theoretical productivity of horizontal wells and examine the sensitivity of productivity to well and reservoir parameters. We then compare theoretical and actual productivities from horizontal wells in several oil fields to show that most horizontal wells do not produce at their full potential because of the effects of wellbore and near-wellbore formation conditions right after drilling on cleanup. Second, using field examples, we show that the new integrated production logging measurements can be used to assess horizontal well productivity by identifying productive zones, fluid entries, and determining the fluid distribution (oil, gas, and water) in the wellbore. We also show how reservoir heterogeneity affects the reservoir pressure distribution.

The field examples indicate that horizontal wells have permanent water sumps, regardless of whether they produce water. In these wells, we show that when a large percentage of the wellbore cross section is filled with water, reduced oil entry occurs. To assess the effect of the water sumps, flow restrictions, and noncontributing sections along the wellbore on the well productivity, a numerical model with actual well trajectories and openhole and production log measurements is used to estimate well productivity. These horizontal well performance simulations indicate 30 to 50 percent productivity reduction when they are compared with their full potential.

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