Horizontal Well IPR Calculations
Authors: Thomas L.K. , Todd B.J. , Evans C.E. , Pierson R.G. 
Published: December 2007

This paper presents the calculation of near-wellbore skin and non-Darcy flow coefficient for horizontal wells based on whether the well is drilled in an underbalanced or overbalanced condition, whether the well is completed openhole, with a slotted liner, or cased, and on the number of shots per foot and phasing for cased wells. The inclusion of mechanical skin and the non-Darcy flow coefficient in previously published horizontal well equations is presented and a comparison between these equations is given. In addition, both analytical and numerical solutions for horizontal wells with skin and non-Darcy flow are presented for comparison.

Introduction

Since the start of this decade, there has been a large increase in the number of horizontal wells drilled worldwide. In the United States and Canada alone more than 10,000 horizontal wells have been drilled since 1990. Advantages of horizontal wells over vertical wells for specific reservoirs include increased productivity, improved sweep efficiency, reduced coning of water and gas, and increased drainage areas. This latter advantage is of particular importance in fractured reservoirs such as the Austin Chalk, where horizontal wells are drilled perpendicular to the predominate fracture trend.

As a result of the success of improved and accelerated recovery utilizing horizontal wells, there is a continual effort in the industry today to minimize the cost of drilling and completing horizontal wells. Drilling technology has steadily advanced in terms of geo-steering to the point where most objectives can be met. For long wells where steering may be a problem, opposing dual laterals are being employed. Stacked laterals are being used in formations with extremely low vertical permeability between major pay zones. Coiled tubing is routinely being used to drill multi-laterals from existing wells in mature reservoirs.

Considerable effort is currently being expended to lower the cost of horizontal wells by developing technology and methods to minimize near-wellbore damage during drilling and completion operations. This is extremely important because of the increased difficulties in the cleanup of openhole horizontal wells with or without prepacked screens and the increased expense of stimulating cased hole wells. Both overbalanced and underbalanced drilling and completion techniques are being used and improved results are being reported in both areas.

During the past decade several analytical solutions have been developed for predicting the pressure and rate performance of horizontal wells. Both transient and pseudo steady state solutions have been presented and well test procedures for determining reservoir properties, anisotropy and near wellbore skin have been described. Some authors have developed solutions assuming the horizontal well is analogous to a vertical well with a vertical fracture, while other authors have developed solutions from first principles for a well drilled in the horizontal direction. All of these solutions have assumed that skin is known or can be measured from well test data.

This paper presents the calculation of near-wellbore skin and non-Darcy flow coefficient for horizontal wells based on how they are drilled and completed. In particular, the effects of drilling overbalanced versus underbalanced and completing openhole with or without a slotted liner or cased hole on near-wellbore skin are discussed. The inclusion of mechanical skin and the non-Darcy flow coefficient in previously published horizontal well equations is presented and a comparison between these equations is given. Both analytical and numerical solutions for horizontal wells with skin and non-Darcy flow are presented for comparison. Special attention is required for field simulations to assure accurate solutions.

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