Sweep Efficiency Improvement Using Horizontal Wells Or Tilted Horizontal Wells In Miscible Floods (1985)

Abstract Two factors determine oil recovery:displacement efficiencysweep efficiency For a miscible flood, high displacement efficiencies may be obtained. However, field performance (i.e. early sol vent breakthrough) shows that sweep efficiencies are generally low and may be attributed to the vastly different properties between the miscible and reservoir fluids. This paper discusses the improvement in performance that may be expected if horizontal injectors and producers are used in a miscible flood scheme. A numerical simulation model was used to quantify the increase in areal sweep efficiency that will result from using horizontal wells of various lengths at different mobility ratios in a 5-spot or staggered line-drive pattern. The study showed that the greatest percentage increase in areal sweep efficiency occurred at conditions with the most adverse mobility ratios. The increase in productivity at various wellbore lengths was also calculated and displayed in graphical form, showing that the productivity index may increase by nearly 5 times as the well bore length is extended. The improvement in flood front stability, effect on solvent dilution, and the potential uses of tilted horizontal wells in layered reservoirs, as well as some of the disadvantages and limitations involved with using horizontal wells have also been reviewed. Introduction There are two factors that determine the recovery from an oil reservoir: 1) displacement efficiency and 2) sweep efficiency. The use of miscible fluids to displace reservoir oil results in very high displacement efficiencies. reducing residual oil saturations to a small fraction of the original hydrocarbon pore volume. However, the differences in mobilities between the reservoir and the injected fluids has led to very poor sweep efficiencies, as evidenced by field performance. The more mobile solvent sweeps only a small portion of the project pattern before breaking through at the producing wells. It would seem that the generally poor sweep efficiencies that can be expected by using a fluid of such vastly different properties to displace oil is one of the prime factors preventing field scale projects from being technically and economically successful. The purpose of this paper is to investigate the improvement in areal sweep efficiency and general field performance that may be expected if horizontal wells, rather than vertical wells, are used in a miscible flood scheme. The concept of using horizontal wellbores, so-called 'horizontal wells', as a means of improving recovery and productivity dates back to the 1950's(1,2), but the most recent applications have generally been confined to the field of heavy oil. Little attention has been paid to the potential advantages to be gained by utilizing horizontal wellbores in miscible flooding schemes. This paper presents the results of a simulation study that was done to quantify the improvement in areal sweep efficiency that may be achieved by using horizontal wells instead of vertical wells. The potential uses of tilted horizontal wells, and the effect of horizontal wells on flood front stability and solvent dilution will also be discussed.