Experimental Investigation of the Interaction of Phase Behavior With Microscopic Heterogeneity in a CO2 Flood

Abstract

Summary This paper reports results of an experimental investigation of the effects of microscopic heterogeneity on local displacement efficiency in a CO2 flood. Flow-visualization experiments for first-contact miscible displacements are described and compared with effluent composition measurement for the same models. High-pressure flow-visualization experiments for multicontact miscible CO2 floods are also described. The displacements were performed in two-dimensional (2D) etched glass models made from thin-sections of San Andres carbonate core from the Maljamar field. Techniques used in preparation of the models are described briefly. Observations of first-contact miscible displacements in heterogeneous models indicate that early breakthrough and a long transition zone occur when preferential flow paths exist in the pore structure. This behavior corresponds to a flowing fraction less than 1 in the Coats-Smith model. Because the 2D models contained no dead-end PV, the results presented indicate that a low flowing fraction can occur if flow velocities in different portions of the pore space differ significantly. Coats-Smith parameters obtained for models were comparable with values obtained in parameters obtained for models were comparable with values obtained in floods performed in reservoir samples. Visual observations of the CO2/crude-oil displacements indicate that there is an interaction between phase behavior and microscopic heterogeneity. Mixing of nearly pure CO2 in the preferential flow path with oil in adjacent regions leads to a residual oil saturation (ROS) that forms in the preferential path after oil initially present has been displaced. Introduction Dense CO2 can displace oil very efficiently if the pore space is uniform and the effects of viscous instability are suppressed. For example, oil recoveries in excess of 95% are commonly observed in slim-tube displacements at pressures above the minimum miscibility pressure (MMP). Many reservoir rocks do not have uniform pore structures, however, and viscous instabilities will occur in most CO2 floods. Thus, a question of some importance in the interpretation of CO2 floods in laboratory cores and in the prediction of the performance of larger-scale CO2 floods is the prediction of the performance of larger-scale CO2 floods is the following: to what extent is CO2 flood displacement efficiency influenced by interactions of phase behavior, viscous instability, and local heterogeneity of the pore structure? Local displacement efficiency in a CO2 flood is sensitive to the composition path of fluid mixtures that are created as injected CO2 encounters crude oil in the pore space. Composition path is determined partly by the phase behavior of the CO2/crude-oil mixtures and partly by the mixing that takes place within the pore structure. Theoretical results suggest that both viscous instability and heterogeneity at the pore level affect local displacement efficiency by altering local mixing and hence changing composition path. For example, Gardner and Ypma predicted, on the basis path. For example, Gardner and Ypma predicted, on the basis of numerical simulations of the growth of a viscous finger, that mixing of nearly pure CO2 in the finger with crude oil from adjacent unswept regions creates a higher ROS in zones first penetrated by a finger. Dai and Orr also used simulation results penetrated by a finger. Dai and Orr also used simulation results to argue that microscopic heterogeneity has a similar effect. Mixing between CO2 in fast-flowing regions of the pore space and oil in other regions causes phase compositions to fall deeper into the two-phase region, with the result that ROS's exceed those obtained in simulations for uniform pore structures. Experimental evidence concerning those interactions is limited. Campbell and Orr described flow visualization experiments that confirmed qualitatively the predictions of Gardner and Ypma, who also reported results of coreflood experiments that supported their arguments. Spence and Watkins performed stabilized CO2 floods in reservoir core samples in which ideal miscible displacements were also performed. They interpreted the miscible displacements in terms of the Coats-Smith model, in which represents the pore space as a flowing fraction in which dispersion occurs and pore space as a flowing fraction in which dispersion occurs and a stagnant fraction with mass transfer between the two fractions. They found that cores for which Coats-Smith flowing fractions were less than one showed higher ROS's than did more uniform cores. Thus, there is experimental evidence that supports the interpretations of Gardner and Ypma and Dai and Orr, but there has been no previous attempt to observe directly the effects of heterogeneity previous attempt to observe directly the effects of heterogeneity at the pore level. In this paper, we report results of flow-visualization experiments that provide additional evidence concerning the interplay of phase behavior, microscopic heterogeneity, and viscous instability. The displacement experiments were performed in etched-glass pore networks made from thin-sections of carbonate reservoir core material. We describe preparation of the micromodels and then compare results of both stable and unstable first-contact miscible displacements with multicontact miscible CO2/crude-oil displacements. We also report results of effluent composition measurements for stable miscible displacements in the same models, along with interpretations of those displacements in terms of the Coats-Smith model. Finally, we discuss the connection between heterogeneities observed at the scale of thin-sections with displacements at laboratory core scale. The experiments described here are part of an investigation of mixing behavior in reservoir core samples. In the course of that study, miscible displacements were performed in several sandstone and carbonate cores, and the resultant effluent composition data were fit to the Coats-Smith model. In addition, thin-sections from each core were examined in an attempt to determine whether features of the pore space observable at the scale of a thin-section correlate with mixing behavior obtained in the core displacements.