Effect of Pressure and Rate on Steam Zone Development in Steamflooding

Experimental studies have been carried out to determine the effects oil injection pressure and rate on formation heating by steam flooding. Heat losses, vertical sweep efficiency, and steam zone volume were determined for steam displacing water at different rates and pressures. A radial flow model was used that consisted of reservoir, overburden, and substratum, all composed of unconsolidated sand. Following are some of the results and conclusions. Heat loss to overburden and substratum, as percent of the total injected, is almost solely a percent of the total injected, is almost solely a function of time for given formation thickness, a conclusion that tends to agree with the theoretical result that percent loss is a function only of dimensionless time at/ 2 (where a is thermal diffusivity, t is time, and h is formation thickness). The volume of the steam zone was found to be a function of time and the dimensionless injection parameter will hkhf (Tb-Ti) (where Lv is parameter will hkhf (Tb-Ti) (where Lv is heat of vaporization, wi is mass injection rate, khf is thermal conductivity, Tb is saturation temperature, and Ti is initial reservoir temperature). Vertical sweep efficiency, defined in the text, depends mostly on injection rate - improving at higher rate - and bas minimal dependence on pressure and time. pressure and time. A few floods were carried out with an initial oil saturation and residual water saturation and using oils with viscosities of 18,100 and 900 cp at initial reservoir temperature. Results arc presented. A radio-frequency capacitance probe was used in some runs in an effort to measure water (liquid) saturation changes in the steam zone. Introduction Reports appearing in the literature on oil recovery by steamflooding now show that almost every aspect of the process has been studied by a variety of methods. Early experimental work 1 by Willman and colleagues demonstrated that steam is an effective oil-displacing agent in a linear flood system; theoretical methods have been developed for calculating the thermal efficiency of reservoir heating by steamflooding; and several field trials have been reported that attempted to test the over-all economics of the process. Shutter, in two papers, published work on a numerical model that includes heat loss, gravity effects, and oil recovery. Reported results of heat flow studies in an experimental steamflood model have shown that a significant portion of the injection heat is contained in the "hot water zone"; i.e., in the flooded formation but outside the steam zone. With gravity override (steam overrunning) much of this heat would be under the steam zone. Gravity override has been observed in steamflood field trials and in Shutler's numerical model. In the experiments described in Ref. 10, gravity override was noted, but was not quantitatively measured. in a new experimental project, using a new model, the pressure range was extended upward to 100 psig. At the same time, more detailed definition of psig. At the same time, more detailed definition of the steam front was obtained, providing a quantitative measure of steam zone volume and gravity override, or vertical sweep efficiency. Model floods were carried out at different pressures between approximately 1 psig and 100 pressures between approximately 1 psig and 100 psig, and at rates between 0.1 and 1.0 lb/min psig, and at rates between 0.1 and 1.0 lb/min (576 and 5,760 lb/D-ft in the 3-in.-thick model formation). In most runs the reservoir was initially saturated with water because such a series of experiments would not be practical to carry out in a reasonable period of time with viscous oil. The results of these runs, which are given in detail in the text, show the effects of pressure and rate on gravity override, steam zone volume, and heat losses under ideal fluid flow conditions. /L few runs were made with an initial oil saturation, using oils with viscosities of 18,100 and 900 cp at initial reservoir temperature. Results with 18- and 100-cp oil differed very little from those with water only; but with the 900-cp oil, gravity override increased and the steam zone pattern became definitely nonradial. This indicates pattern became definitely nonradial. This indicates that the observed pressure and rate effects should be valid for initial oil viscosities up at least 100 cp in a medium as homogeneous as the model. SPEJ P. 274