Two- and three-dimensional simulation studies have been done to evaluate waterflood oil recovery in a 40-acre 5-spot pattern using horizontal and vertical well systems. The three-dimensional simulation results indicate that the parameters considered, i.e. vertical permeability-horizontal permeability ratio, injection and production rates, and reservoir thickness have little effect on waterflood oil recovery for a particular water-oil mobility ratio. For both vertical and horizontal well systems, oil recovery decreases with increasing mobility ratio. It is concluded that the mobility ratio is the dominant parameter affecting waterflood oil recovery, in line with the experimental results of Craig, Geffen, and Morse1 for conventional waterfloods. Waterflood oil recovery using a horizontal well system is shown to be higher than that obtained by using a vertical well system. A correlation has been developed which expresses volumetric sweep efficiency in a hypothetical 5-spot pattern as a function of mobility ratio. This correlation would be useful for estimating waterflood oil recovery for the ranges of reservoir parameters considered.
Oil recovery and production rate from a reservoir may be improved by waterflooding. The conventional waterflood technique, which has been used for decades, involves flooding the reservoir using various patterns of vertical injection and vertical production wells. The most common waterflood pattern is the five-spot. Extensive literature exists on the types, design, performance prediction and operation of conventional waterfloods.2–4
Improvements in horizontal well technology during the last decade have made the use of horizontal wells a technically and economically attractive alternative to vertical wells under certain conditions. Horizontal wells have been shown to be superior to vertical wells for purposes of:
intersection with and drainage of vertical-fracture networks,
reduction of gas or water coning, and
increase in sweep efficiency.5
Theoretically, production from all reservoirs can be improved through the use of horizontal wells. However, to benefit from the use of horizontal wells, both vertical and lateral reservoir characteristics must be well known, including the reservoir drive mechanisms and fluid characteristics.
Horizontal wells can be applied in any phase of oil recovery: primary, secondary, and tertiary. The drilling of horizontal wells may be considered as an alternative to:
infill drilling with vertical wells, and
In all cases, the objective is to increase the economic recovery and production rate of oil and gas.
Previous researchers have carried out simulation studies to investigate several important aspects of horizontal wells. These include the partial penetration of the well in the horizontal direction within the drainage area, the positioning of horizontal wells between the top and the bottom of the reservoir, and the effect of reservoir permeability anisotropy. However, most previous studies have focused on the performance of a single horizontal well, omitting the interaction of the horizontal well with other wells.6–10
In spite of numerous papers on horizontal wells, especially during the past six years, little information is available on waterflooding with horizontal wells. Norris et al11 presented a review on the reservoir engineering aspects of horizontal wells. Of the 84 references cited in the paper, eight were in EOR applications involving steamflooding, but none involved waterflooding. There is, therefore, a need to study how horizontal waells, used as injectors and/or producers, would affect waterflood performance.