Effects of Natural Fractures or Directional Permeability in Water Flooding
Authors: Carnes Peyton S. 
Published: December 2007

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Directional movement of injected water has been found to exist in many oil reservoirs in which water floods are being conducted.

This paper discusses many of the ramifications of natural fractures and directional permeability in oil reservoirs being water flooded. Suggested steps to be taken in installing water floods in suspected areas are presented, and the importance of proper pattern selection after establishing the directional orientation of the fracture system or directional permeability is discussed.

Presented in this paper is the case history of the W. A. Moncrief, Trousdale Unit Water Flood Project in Pottawatomie County, Okla., in which a directional fracture system was discovered in the 4,260-ft Calvin sand soon after the waterflood program had been installed. The adverse effects of the fracture system during the initial phase of the program and the results obtained by revising the injection pattern are shown graphically and are discussed in detail.

Introduction

Water flooding, in its simplest definition, is the displacement by water of the mobile or movable oil from an oil reservoir. A waterflood program should be installed only after sufficient study of an oil reservoir has been made to determine that an economically feasible amount of recoverable oil exists in the reservoir. The actual waterflood operation involves the injection of water into properly selected wells at a sufficient rate of injection to insure proper displacement of the oil from the reservoir in a reasonable length of time so that the optimum profit will be obtained. There are many natural and artificial conditions in an oil reservoir which can affect this displacement mechanism.

If an oil sand were simply a homogeneous uniform sand matrix, it would be an easy task to fill the oil sand with water and to displace the mobile or movable oil to the producing wells. However, nearly all oil reservoirs are heterogeneous in nature. Within a single oil reservoir there is usually a wide variation in porosity, permeability, water saturation, and gas saturation, and also there may be a wide variation in composition throughout the sand section such as shale inclusions, lime streaks, gradations in lithology, etc. In addition to this, the oil sand may be overlain by a gas cap; it may be uderlain by a water zone; it may be faulted; it may contain natural fractures; it may be composed of a series of lenses which may or may not be in communication with one another; it may contain weak zones or there may be weak formations immediately above or below the sand; and a multitude of other adverse natural environmental factors may exist. In addition, artificial conditions such as very large sand frac treatments, poor cement bond between casing and formation, poor completions, mud invasion, and others may be present which could adversely affect the successful displacement by water of oil in the desired reservoir interval. The possible presence of these adverse conditions must be recognized from the start, and a program should be installed which will eliminate or at least minimize as many of these adverse conditions as possible so that a successful water injection program can be accomplished.

DIRECTIONAL MOVEMENT OF INJECTED WATER

This paper deals with one of the many possible adverse phenomena which can affect the success of the water injection program.