Effect of Fractional Flow Hysteresis on Recovery of Tertiary Oil

This paper deals with the oil/water bank propagation in a tertiary oil recovery process. Oil/water bank propagation was studied in a series of laboratory micellar floods and simultaneous oil/water flow tests using a microwave scanning apparatus for measuring in-situ dynamic oil saturation. It was observed that a high oil saturation region, or hump, developed at the leading edge of the oil/water bank and grew linearly with distance. A lower steady-state oil saturation region was observed behind the hump. As the hump was produced from the core, high initial oil fractions were observed, as often seen in laboratory micellar floods. This is the result of the observed hysteresis in fractional flow behavior. A graphical method of predicting the occurrence of a hump, its rate of growth, and saturations within an oil/water bank was developed using the observed hysteresis in fractional flow. Using this prediction procedure, it was concluded that in a tertiary oil recovery process, oil breakthrough time or rate of advance of the oil/water bank, oil saturation at the leading edge, and initial produced oil fractions are only functions of the oil-saturation-increasing fractional flow curve and are not necessarily indications of oil recovery efficiency. Introduction During a tertiary oil recovery process, a small slug of displacing fluid (e.g., a micellar fluid) mobilizes residual oil and water and forms an oil/water bank. It is important to understand the propagation behavior of the oil/water bank in a tertiary oil recovery process since it affects the oil breakthrough time and initial oil cuts. This understanding also will aid in the interpretation of oil displacement tests. Moreover, oil breakthrough time and initial oil cuts have been used for judging the efficiency of a tertiary oil recovery process. Oil/water bank propagation was studied in a series of micellar floods and oil/water flow tests using a microwave scanning apparatus for measuring in-situ dynamic oil saturation profiles. Experimental Details The microwave scanning apparatus used is similar to that discussed by Parsons1 and Parsons and Jones.2 Microwaves are transmitted through a core where they are partially absorbed by the water molecules. The measured microwave power attenuation, or degree of absorption of the microwave energy, is a direct measure of the quantity of water and, consequently, of the oil saturation in an oil/water system since the oil does not absorb the microwave energy. The microwave scanning apparatus is capable of measuring the dynamic oil saturation profiles during pressure-monitored laboratory micellar floods and other oil/water flow tests. Fig. 1 is a schematic of the apparatus. Additional experimental details are given in Appendix A. Displacement tests were conducted at room temperature in 120-cm-long rectangular Berea cores (1.91 cm thick×7.62 cm wide). The brine permeability range of these cores was from 418 to 714 md, and pore volumes varied from 377 to 395 cm3. Three tertiary micellar floods were conducted in separate Berea cores with Second Wall Creek crude oil. Table 1 shows the fluid injection sequence and compositions3 for the micellar floods. In addition, simultaneous oil/water injection tests were conducted in separate Berea cores using both Second Wall Creek crude oil and refined oils (see Table 2 for the fluid injection sequence).