Abstract

Conventionally, water coning is considered as a cause of early water production in bottom water drive reservoirs. However, in edge-water drive systems with unfavorable mobility ratios, water tongues may underrun oil and provide the conditions for water coning to partially completed wells. During initial production a water tongue commonly conforms to strike far from the well, and then forms a peak (or areal tongue), as it gets closer; finally, a water cone may form atop the tongue when it reaches the well. Water coning, tonguing, and their simultaneous interaction primarily affect the water breakthrough time, the post-breakthrough production, and therefore ultimate recovery. To date, the effects of water coning and tonguing on well production have been addressed separately. This study considers the two mechanisms as a chain of causality leading to excessive water problem and early termination of well's life.

The study demonstrates that the two combined effects may cause early water breakthrough (tonguing) and a very slow, ineffective oil recovery due to water coning. In this study, analytical expressions for diffuse and segregated flow conditions have been used to assess tonguing and coning severity while numerical reservoir simulation identified dominant reservoir conditions for water tonguing (early water production) and separately, the conditions for water coning and bypassed oil. Dimensionless scaling groups are used in a sensitivity analysis of a factorial design. Important engineering factors including well location, mobility ratio, gravity numbers, well penetration ratios, well spacing, and vertical permeability are analyzed. An example demonstrates how knowledge of factor influences can be used to select well location and completion length.

Introduction

Water serves the dual purpose of maintaining pressure and displacing oil towards the production wells. These two facts are valid for both water injection and edge water systems. Under water drive where the reservoir fluid is more viscous than the encroaching water, the water tends to bypass the oil. At low flow rates, the difference in specific gravity of fluids counterbalances the effect of difference in viscosity and keeps bypassing of oil. It strives to keep the oil-water interface horizontal, so that equilibrium develops such that the interface moves up-dip through the formation maintaining a constant slope. However, at high flow rates, this equilibrium is not achieved and a water-tongue penetrates into the oil-bearing formation resulting in pre-mature water production at the producing well. Since the water has higher mobility than the oil, it tends to channel and by-pass the oil; but since the water is also denser than the oil, it seeks the bottom of the layer creating a water tongue. (1) Water-production continues to increase until the economic limit of the well is reached. Hence, at the time of abandonment, there may be considerable oil still remaining in the formation. In reservoirs under edge water drive, oil bypassed due to gravity tongue is a significant problem.

There have been several attempts to describe analytically the oil recovery and water production trend for reservoirs under waterflood. Most of the analytical models assume flow of incompressible fluids in a homogeneous reservoir.

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