Water Cresting Behavior Under High Angle Wells: An Experimental Investigation
Authors: Permadi P. 
Published: December 2007

Abstract
Most of the published papers related to bottom water movement deal with vertical and horizontal wells. The behavior of water cresting under high angle wells is not well understood. this paper presents and discusses results of an experimental investigation of water crest development under high angle wells. A transparent physical model was employed to enable us to visually abserve the water/oil interface movement. Three wells with different high angle and a horizontal well were used. Two oils with different viscosity were employed and initial thickness oil column was varied.
Results demonstrate that water breakthrough in high angle wells can occur at two points close to the down-stream/heel end and at the toe of the well, but it always occur at close to the heel end in the horizontal well case. Thinner oil leads to bottom water invading oil around the toe after breakthrough and results in increased displacement efficiency. This investigation might give engineers some insight about planning completion strategy for high angle and horizontal wells.

Introduction
The application of horizontal wells as one of the options to recover more oil from bottomwater drive reservoirs has been widely accepted. Slanted or high-angle wells have also been used to increase reservoir productivity. Physical models as wells as mathematical analysis have been employed to theoretically show their advantages over conventional vertical wells. Field implementation has demonstrated the capability of improving hydrocarbon recovery. Classification of those wells is principally based on the angle made between the wellbore axis within the productive zone and a vertical line intersecting the axis. The angle is about 900 for horizontal well, while any well with deviation higher than about 800 is usually considered as a high angle well. They are mostly used to hit targets or productive intervals which are not economically viable or environmentally workable to locate a rig to drill a conventional vertical well. Placement of a such non-vertical well within a thick productive zone can be long enough so that the drawdown required can be lowered for a higher rate of production. In a bottom water drive reservoir, the stand-off for the whole length of a horizontal wellbore section cant be maximized by placing the wellbore as close to the uppermost of the oil zone as possible. In the case of a high-angle well, the tip end tends to be close to the plane of water/oil contact and thus one may expect the the breakthrough of water takes place at the tip end and occurs earlier. In fact, the growth of water conning under a high-angle well is not well understood.
In the pertinent literature available, much attention has been focused on problems of water coning in vertical wells and water cresting in horizontal wells. It is known that wellbore hydraulics can affect production performance of horizontal well. When this happens in a bottomwater drive system, an interesting phenomenon is that the water breakthrough occur at the heel end. The presence of discontinuous impermeable layers in the oil zone, however, controls the contact point of breakthrough, depending on the position of the heel end with respect to the impermeable layers distribution.
Although wellbore hydraulics within long high-angle wells is not well studied, we can expect that it might have some effects on reservoir flow behavior. Gravity forces might not be neglected due to the differential vertical position of the hell end and the tip end. In this regard, the development of a mathematical model to elaborate reservoir flow behavior in a such system would be too difficult.
The purpose of this study was therefore to experimentally investigate the movement of water/oil contact under high-angle wells and the production performance prior to and after water breakthrough. A. horizontal well case was also considered for comparative purposes. A simple, transparent physical model was employed in this effort. Results of the present work might be helpful in considering and planning whether a high-angle or a horizontal well to be drilled in a bottomwater drive reservoir and what completion method would be appropriate for handling water coning problems.
P. 641