Abstract

Prior to implementation of a horizontal well in a bottom water drive reservoir, a production prediction model is usually needed to support its economic evaluation. The available published correlation mostly assume uniform flux along the wellbore, which is not the case in the most field situation where wellbore hydraulics should be considered.

A new method for horizontal well coning calculation is presented here. The approach used to develop the correlation is simply the application of linear displacement concepts. To account for both non-uniform flux and flow geometry, a factor that contains drainage area, well length, and well position, is introduced based on results of production history matching of a field case. Also investigated in this study is the effect of relative permeability characteristics on coning behavior.

The method is simple for fast calculations and the applicability is demonstrated by using two different sets of field data. The present work also shows that the assessment of correct end-point relative permeability, in particular, is of importance in coning predictions.

Introduction

A bottom water drive reservoir indicates that the oil zone is underlain by an interconnecting water zone. As the oil is being produced, the bottomwater region might be affected to cause bottomwater to move toward the perforated interval. The bottomwater will only move if an appreciable pressure differential exists just across the plane of the water-oil interface/contact (WOC). The plane moves upward uniformly when the rate of production is below a certain critical value. This critical rate of production of a well depends on the reservoir characteristics, the fluid properties, and the length of opened interval. When the well is produced at a rate higher than its critical rate, due to economic reasons usually, the plane of WOC deforms.

Deformation of presumably flat surface of WOC is generally referred to as water coning in conventional vertical wells and water cresting in horizontal wells. The shape of the crest formed has been subject of several investigations. In a horizontal well producing oil at rate much higher than its critical rate, the advancing water-oil interface would tend to be a conelike shape during early period and then change toward a crestal shape afterward.

For the purposes of quantitative discussion, however, either the term coning or "cresting" used would not be materially important. Even in horizontal well cases, most engineers probably get used to adapt the term "coning" for bottomwater encroachment problems.

Background

The application of horizontal well technology has been widely used in many oil producing countries. The common ultimate objective was to improve oil recovery from water drive reservoirs. The advantages of using a horizontal well over a conventional vertical well are that a larger capacity to produce oil at the same drawdown, a longer water breakthrough time at a given same rate, and a larger area of water-invaded oil zone may be obtained.

As the costs of horizontal well drilling and completion are usually higher than that of vertical ones, a tool of production performance prediction is needed to support the economic evaluation prior to implementation.

Many correlations to predict coning behavior in horizontal wells are available in the literature. Most of them were derived analytically for determining critical rates and water breakthrough time in bottomwater drive reservoirs. For post-breakthrough periods, an analytically derived method based on an extension of two-dimensional gravity drainage model has been recently presented.

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