Behavior of Water Cresting Under Horizontal Wells

Abstract

Abstract Complex reservoir flow problems could be better understood through a study using physical models. The purpose of this paper is to present results of an experimental study of water crest behavior under horizontal wells. This work is made in an effort to better understand the formation and growth of water crest prior to and after water breakthrough. The physical model constructed differs from others in that variation in length and position of the horizontal well can be made. Eighteen different systems with varied oil column thickness and oil viscosity were run. Particularly in systems with viscous oil, the bottom water never reached the tip end of the well even at a producing water cut of almost 100 percent. The end effects defined as the unswept oil are pro- flounced as the well length is reduced and as the oil viscosity is increased. Also, at high water cut, the portion of the wellbore with low productivity increases with well length. These physical occurrences have not been previously reported. Postbreakthrough performance will be presented and discussed. Introduction Oil production from bottom water drive reservoirs is usually followed by water produced through the same well. This situation occurs when bottom water has broken through the well due to a high drawdown applied to the reservoir. The appearance of bottom water in vertical wells is in many cases observed in a relatively short time after the wells are put on production. Once the water enters the wells, a rapid increase in water cut may lead to low oil recovery. The application of horizontal well technology has been widely used in many countries to improve oil recovery from water drive reservoirs. At a low drawdown, a horizontal well can have a larger capacity to produce oil as compared to a conventional vertical well. Thus, the critical rate, below which the flat surface of water-oil contact will not deform, in a horizontal well may be higher than that in a vertical well. But in practice, production rate is usually higher than the critical rate due to economic consideration. When this takes place, high mobility bottom water will invade into the overlying oil zone and moves toward the well. The higher the rate of production the faster the bottom water movement. The period required for the water to reach the well is called as a breakthrough time. As compared to a vertical well for a given rate, a horizontal well requires a lower drawdown and therefore gives a longer breakthrough time. This advantage combined with larger displacement cover- age provided by bottom water result in better oil recovery. A region of oil zone that has been invaded by bottom water may be in the form of a cone for vertical wells and a crest for horizontal wells. Many publications relating to water coning phenomena have been available. The water cone beneath a vertical well is assumed to develop symmetrically with respect to the vertical well axis. This is true because no pressure gradient in the well system is accounted for and the well acts as a point source, eventually. In the context of horizontal well producing oil from bottom water drive reservoirs, many researchers considered no pressure gradient exists along the horizontal wellbore. This implies that bottom water rises uniformly and the crest formed is symmetric with respect to a vertical plane passing through the wellbore axis. Later it has been shown that considerable pressure drops within a horizontal well may affect the production performance. Thus, the pressure gradients create varied potential fields beneath the wellbore The first water would then break through the well at a point below which the potential is the highest. A numerical study has recently shown this phenomenon and also reported that neglecting pressure drop in the wellbore leads to overestimation of the rate of oil production. Unfortunately, no detailed information as to the fluids flow mechanism along and in the vicinity of the wellbore has been presented. Complex mechanisms of fluids flow in a reservoir system containing a horizontal well and in the wellbore itself are not well understood. Tackling such problems through the use of physical models has been appreciated. Particularly, water cresting phenomena were recently investigated by Aulie et al. P. 431