Water Cresting and Oil Recovery by Horizontal Wells in the Presence of Impermeable Streaks
Authors: Permadi P. , Gustiawan E. , Abdassah D. 
Published: December 2007

Abstract
The trend of water/oil contact movement in a reservoir containing impermeable layers and producing oil through a horizontal well is investigated. A transparent physical model is employed and the model represents two-dimensional observation. The physical model used permits variation in horizontal well length, changing flow direction within the wellbore oil viscosity, and initial thickness of oil column.
Results demonstrate thatthe entry point of bottom water breaking through a well depends on the wellbore flow direction,above critical rates, non-uniform influx occurs in all the systems, andan increased oil recovery is obtained when the heel end of a long horizontal well is located above the upper layer of the streaks.
The right direction and placement of a horizontal well in a bottomwater drive reservoir having discontinuous impervious streaks therefore should be carefully planned.

Introduction
A bottomwater drive reservoir is a reservoir in which oil or gas contained has at least partly pressure support from the underlying bottom water. In most cases, the use of conventional vertical wells to produce oil from such reservoirs always create severe coning problems. This is because the actual rate of production highly exceeds the critical rate, defined as a rate above which the flat surface of water/oil contact (WOC) starts to deform. Water coning itself is a phenomenon of bottom water intruding the oil zone such that the invaded oil zone resembles a cone. This is usually termed water cresting for horizontal wells. Basically, these are vertical and diagonal movement of bottom water, displacing oil toward the producing wells. In these respects, vertical permeability of the reservoir definitely affect the critical rate. In fact, a critical rate is inversely proportional to vertical permeability and oil viscosity. If the well is produced at a rate much higher than its critical rate then bottom water will reach the wellbore in a relatively short time. A typical water break through time in a vertical well may be in the order of days or weeks, depending on not only the rate of production but also production interval opened. vertical distance from WOC to the most bottom limit of the open interval, oil viscosity, vertical permeability, etc. Once water breakthrough occurs, usually water cut increases rapidly with time.
The presence of discontinuous impermeable layers or streaks in a bottomwater drive reservoir can influence the mechanism of water intrusion into the oil zone. Such layers in the oil zone may act as obstacles to vertical reservoir flow, or in other words, the effective vertical permeability is reduced. This condition leads to breakthrough time delayed and the well performance with respect to oil production could be much better.
The use of horizontal wells to enhance oil production from waterdrive reservoirs has been widely appreciated around the world. A much larger area of a long horizontal wellbore wall exposed to the reservoir significantly increases influx to the well. Therefore, for a given rate, a horizontal well requires a lower drawdown, resulting in longer water breakthrough time. This advantage combined with wider displacement coverage provided by the invading bottom water can improve the oil recovery. The improved oil recovery that will be obtained through the use of a horizontal well, for a homogeneous reservoir in this case, depends on many factors, such as the length of horizontal section opened, rate of production, position of the wellbore in the oil zone, etc. There are some analytical and numerical studies that have been conducted for homogeneous reservoir relating to the oil recovery from horizontal wells. Lien et. al. also performed a numerical study on reservoirs with and without discontinuous shales. They conclude that the randomly distributed discontinuous shales may increase oil recovery. As such, in fact in many cases, a geologic unit or a reservoir that can be considered as single layer may contain discontinuous permeability barriers. If it does, the barriers can be randomly distributed or just single or two layers. However, no study has been specifically directed toward elaborating the mechanism of fluids flow in such systems.
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