Steam-Strip Drive: A Potential Tertiary Recovery Process

Abstract

Steam-strip drive for recovering waterflood residual oil in watered-out reservoirs gives good to excellent sweep efficiencies. The stripping process is very effective in decreasing the waterflood residual oil to process is very effective in decreasing the waterflood residual oil to values well below the "distillation" residual value. Introduction In their pioneering paper on the steam-injection process, Willman et al. mentioned steam stripping, as an effective mechanism for reducing the microscopic waterflood residual oil saturation. Steam strips the residual oil and transports the oil vapors to the colder part of the reservoir. where they condense and are banked up in a movable oil bank. Since the lighter and more volatile components are preferentially stripped, the oil bank will be enriched with preferentially stripped, the oil bank will be enriched with lighter components. This also means that the residual oil directly behind the steam front will become more volatile as the process proceeds, because it has the same composition as the oil in the oil bank just ahead of the steam front. Consequently, the stripping process is becoming increasingly effective. In their core experiments, Willman et al, observed a reduction in residual oil from an initial 50-percent waterflood residual oil to as little as 8 percent after steamflooding, depending on the volatility of the crude oil. Volek and Pryor also found residual oil saturations as low as 2 percent after steam-injection experiments in waterflooded cores containing a relatively light Brea-type oil. These authors also reported low residual oil saturations in cores taken from the steamflooded part of the Brea field, averaging about 8 percent. These studies clearly indicate the potential of steam injection as a tertiary recovery process for recovering waterflood residual oil. Also, we can expect good to excellent areal and vertical sweep efficiencies when steam is injected in watered-out reservoirs, as was apparent from laboratory experiments reported by Baker. The first field-pilot test of this "steam-strip drive process" was reported by Hearn. This test was a failure, process" was reported by Hearn. This test was a failure, however, because of a poor vertical sweep efficiency. Since then, steam-strip drive has received little attention in the petroleum engineering literature. Although the precess (a combination of thermal and compositional precess (a combination of thermal and compositional effects) is rather complex, its basis seems to be sound and simple. Therefore, we conducted a laboratory study to further explore the potential of steam-strip drive as a tertiary recovery process. To direct our study we concentrated on a specific prototype reservoir and crude oil. The results obtained for prototype reservoir and crude oil. The results obtained for this reservoir served, where possible, as a starting point for more general considerations. Throughout the investigations we made extensive use of mathematical simulators. We used an existing thermal simulator for nonvolatile oil reservoirs to determine the sweep efficiencies of steam injection into watered-out reservoirs. Steam stripping and subsequent oil-bank buildup were studied with the aid of a specially developed linear, compositional, thermal simulator. Great care was taken to validate this simulator with well defined physical-model experiments. With this simulator, we physical-model experiments. With this simulator, we studied the linear displacement efficiency for various reservoir and operating conditions. JPT P. 1409