Effect of Length and Distribution of Perforated Intervals on Horizontal Wells Rates
Authors: Al Qahtani Abdullah M. , Menouar Habib , Al Majid AbdulAziz 
Published: December 2007

Abstract
Horizontal Wells are gaining popularity because of the advantages they offer, mainly high productivity and reduced coning. In this paper, the case of a high permeability reservoir producing at high rates is examined. The emphasis is specifically on how the length and selection of perforated intervals affect the production rate. The approach is to combine the inflow performance of the reservoir to the outflow performance of the horizontal section of the well to obtain the overall performance of the horizontal well. This is accomplished using a computer program that couples a theoretical solution of the productivity index to an empirical multiphase model of the flow in the horizontal section of the well. This program has been tested against actual data from some reservoirs in the Middle East. A good match between the measured and simulated pressures and rates is observed. The program has been used to generate inflow performance curves for horizontal wells drilled under different reservoir conditions. The results show that:–The performance of a horizontal well is directly affected by the length and distribution of the perforated intervals.–20% of the horizontal section of the well perforated at the heel yields twice the production of the same length perforated at the toe of the horizontal well.–The same length fraction perforated uniformly across the well yields 3 to 4 times the production of the case where this perforated length fraction is concentrated at the toe of the horizontal well.–For any given set of reservoir conditions, there is an optimum perforated length that yields maximum production rate.

Introduction
Few technical advances have marked the petroleum industry more than horizontal well technology. From less than 200 wells drilled a decade ago, horizontal drilling has become the norm in the 1990s. One single oil company reported drilling more than 2,100 horizontal wells in North America alone. This resulted in an important technology development. Significant achievements have been recorded such as TVD's up to 16,000 feet, with mud weight in excess of 15 ppg. Horizontal length up to 8,000 feet and multilateral wells with four or more side tracks are not uncommon now. All these technical advances have been realized with a reduction in cost. It has been reported for instance that in the past five years, the average cost per foot of horizontal wells drilled has been decreased by half.
The success of horizontal well technology is due mainly to the advantages horizontal wells offer such as: productivity enhancement, recovery improvement, coning reduction and sand problems attenuation. In general it is possible to take advantage of all these benefits even if the wells are drilled open-hole. Very often, either for geological reasons or for production purpose, the wells need to be cased and perforated. It is important in this case to know how much of the horizontal length should be perforated and where to locate the perforations. This is particularly important for short and medium radius horizontal wells where perforating is neither cheap nor safe. In this respect, minimizing the horizontal section to be perforated for an optimization purpose is legitimate. The present paper will be addressing precisely this subject.
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