Reduction of Residual Oil Due to the Presence of Free Gas During Water Displacement

Abstract

Petroleum Conference on Production and Reservoir Engineering, 20–21 March, Tulsa, Oklahoma Introduction It has been proved that in many cases the presence of a free gas phase will reduce the residual oil saturation obtained in the displacement of oil by water. This holds for both natural water drives and imposed water floods. For floods at constant pressure, the effects of the presence of free gas on waterflood behavior may be evaluated from tests on short core samples. However; it may be desired to estimate these effects when the results of appropriate flow tests are not available. The purpose of this note is to assemble available information from several sources to aid in doing this. The discussion in this note pertains to the displacement of oil by water under conditions of essentially constant pressure, and with the pressure differential across the oil bank and flood front small compared to the flooding pressure. The beneficial effect of free gas in the displacement of oil by water comes from the establishment of an immobile or trapped gas saturation at the flood front. Evaluation of this effect in a particular case requires the estimation of [1] the trapped gas saturation maintained at the flood front, and[2] the reduction in residual oil saturation caused by this trapped gas. Trapped Gas as a Function of Initial Gas It is an accepted concept that, under the pressure conditions prescribed, some portion of any initial free gas is trapped in the oil bank pushed ahead of the displacing water. If a continuous mobile gas phase existed prior to arrival of the oil bank, part of the initial mobile gas is displaced by the oil bank, leaving a residual immobile trapped gas saturation. [It is assumed that pressure remains fairly constant such that gas solubility and compressibility effects are negligible.] The trapped gas, being a nonwetting phase, is in discontinuous globules or filaments. Within the oil bank ahead of the displacing water, oil fills up the pore space around the trapped gas, except for that portion occupied by connate water. The relationship between initial and trapped gas saturation obtained from tests on various materials is given in Fig. 1. These data are from References 1, 2, and 3. Pertinent information on the tests is in Table 1. Obviously the relationship depends on the core material and fluid properties. For the same core material and initial gas saturation, less gas is trapped by the more viscous oil; this is shown by the variation of the points plotted for tests 2aand 3a from the line for tests 2 and 3. For initial gas saturations of less than 10 per cent, trapped gas as a percentage of initial gas saturation ranges from 60 to 100 per cent. The percentage on each test and for the group decreases as initial gas saturation increases [Fig. 1 and Table 2]. This trend is expected, and it means that the greater the initial mobile gas saturation, the larger the portion of the gas which is displaced ahead of the oil bank.