Effect of Pore Structure on Miscible Displacement in Laboratory Cores

Abstract

Summary. Observations of pore structure in thin-sections are related to the performance of stable, first-contact-miscible displacements in reservoir cores and then to simulations of displacement performance of CO2 corefloods. Results of effluent composition measurements are reported for miscible displacements in seven core samples-three sandstones and four San Andres carbonates from west Texas or eastern New Mexico. Those displacements are interpreted by fitting the measured effluent compositions to the Coats-Smith (C-S) model, which represents the flow as occurring in flowing and stagnant fractions with mass transfer between them. Observations of thin-sections, including measurements of pore-size distributions and a simple measurement of spatial correlation of pore sizes, are also reported. Comparison of displacement results pore sizes, are also reported. Comparison of displacement results and thin-section data indicates that wide pore-size distributions and preferential flow paths are characterized in the C-S model by high dispersion coefficients and low flowing fractions. Simulations of the interactions of phase behavior and flow in nonuniform pore structures indicate that wide poresize distributions and preferential flow paths can significantly increase residual oil saturations (ROS's) in CO2 floods over those for uniform pore structures. Thus, heterogeneities observable at the scale of a thin-section have significant effects in laboratory core but much smaller effects in displacements at field scale. Large-scale heterogeneities present in field floods, however, probably cause similar increases in residual saturation in some probably cause similar increases in residual saturation in some fields. Introduction Mixing between injected fluid and that present in a reservoir plays an important role in many EOR processes. In CO2 floods and other multiple-contact-miscible gas injection processes, for example, it is the transfer of components between phases that leads to high local displacement efficiency. If the zone in which mixing takes place is confined to a narrow region, as is the case in slim-tube place is confined to a narrow region, as is the case in slim-tube displacements, then the displacement is efficient as long as the pressure is high enough that CO2 extracts hydrocarbons relatively pressure is high enough that CO2 extracts hydrocarbons relatively efficiently from the oil. Comparison of numerical simulations of CO2 floods with Helfferich's analytic solution for the interactions of phase behavior and flow in the absence of dispersive mixing indicates that when the transition zone is broad, as when the level of dispersion is high, displacement efficiency is reduced. Gardner and Ypma argued, on the basis of numerical simulations of the growth of a viscous finger, that mixing between CO2 in a finger with oil in adjacent unswept regions also reduces local displacement efficiency. Dai and Orr used simulations of the effects of phase behavior on flow in a porous medium consisting of flowing and stagnant fractions to show that the broadening of the transition zone caused by the presence of the stagnant fraction has a similar effect. They used their model to interpret the CO2 coreflood experiments performed by Spence and Watkins, who found that cores with a wide pore-size distribution showed higher ROS's after the CO2 displacements. Thus, there is both experimental and theoretical evidence that mixing effects have a significant impact on CO2 flood performance at the laboratory scale. In this paper, we examine the influence of pore structure on the mixing that occurs during miscible displacements in laboratory cores. We present the results of San Andres carbonates. Of those samples, one sandstone and one carbonate are outcrop samples; the remainder are reservoir core samples. The displacements were performed with fluids at matched density and viscosity to eliminate performed with fluids at matched density and viscosity to eliminate the effects of gravity segregation and viscous instability and hence to isolate the effects of the pore space, We report the displacement results in terms of the parameters of the C-S model, which represents the pore space as flowing and stagnant fractions with mass transfer between them. To characterize the geometry of the pore space, we present poresize distributions obtained from thin-sections from the same poresize distributions obtained from thin-sections from the same rock samples for which the displacements were performed. We argue that long transition zones, characterized in the C-S model by flowing fractions less than one, require not only a wide pore-size distribution but also that the pores be connected in such a manner that preferential flow paths are formed. We present a simple method for qualitative detection of the existence of preferential flow paths. The method is based on measurements of the mean size of paths. The method is based on measurements of the mean size of pores neighboring randomly selected reference pores. Thus, we use pores neighboring randomly selected reference pores. Thus, we use the comparison of thin-section observations and coreflood results to argue that features of the pore structure observable at thin-section scale offer clues to the causes of miscible displacement behavior at coreflood scale. To illustrate the influence of mixing on displacement performance of CO2 corefloods, we report results of one-dimensional simulations made with the model developed by Dai and Orr. Results of those calculations confirm that pore structures that lead to preferential paths also produce lower displacement efficiency in preferential paths also produce lower displacement efficiency in laboratory corefloods. Finally, we comment on the problems associated with scaling laboratory coreflood results to field scale. Displacement Apparatus and Procedure. Fig. 1 is a schematic of the apparatus used for displacement experiments. The fluids used and the apparatus varied slightly over the course of the experiments. In a typical experiment, the positive displacement pump pushes brine [1% NaNO, 1% KNO, 1% Ca(NO ), 0.1 % NaN, pump pushes brine [1% NaNO, 1% KNO, 1% Ca(NO ), 0.1 % NaN, CaSO to saturation] through a sample valve, the core, and a differential refractometer and into a collection vessel placed on an electronic balance. Injection of a slug of miscible fluid is achieved by redirecting the flow by means of the sample valve through a sample loop of known volume filled previously with the same brine containing 0.4 % sucrose as a tracer. The tracer concentration exiting the core is monitored continuously by the refractometer. Concentration and fluid weight data are acquired automatically with a microcomputer. The smallest available tubing, fittings, end caps, and refractometer cell were used to minimize dead volume in the system. Additional details of the apparatus and procedure may be found in Orr and Taber. procedure may be found in Orr and Taber. SPERE P. 857