An Experimental Investigation of Immiscible CO2 Flooding Efficiency in Sandstone Reservoirs: Influence of Permeability Heterogeneity

Abstract

This paper presents the results of an experimental study on the effects of various CO2-injection modes on immiscible flooding performance in heterogeneous sandstone porous media. Core flooding experiments were conducted for n-Decane – synthetic brine – CO2 systems at 9.6 MPa backpressure and 343 K to attain immiscible flooding condition (Minimum miscibility pressure (MMP) of CO2 in n-Decane is 12.4 MPa). For this purpose, two sets of heterogeneous sandstone core samples were assembled with heterogeneity in either parallel (layered samples) or perpendicular (composite samples) to the flow. The results obtained for both composite and layered core samples indicated that heterogeneity tremendously influences the outcome of the CO2 enhanced oil recovery (CO2–EOR). Oil recovery decreases dramatically with increase in the heterogeneity level or permeability ratio (PR). For instance, a recovery of 76.04% in a homogenous sample decreases to 72.15%, 65.97% and 54.71% when in layered samples the PR increases to 2.5, 5, and 12.5, respectively. In addition, the crossflow in layered core sample is found to have a noticeable effect on the ultimate oil recovery (increasing oil recovery up to 5%). It is also worth noting that for the composite samples when we arranged the plugs by putting the low permeability segments closer to the sample outlets the recovery factor increased (e.g. the recovery of 68.32% for Low-High-Low arrangement versus 64.37% for High-Low-High configuration). However, regardless of the segment arrangements, the recoveries in composite cores are less than that obtained from the homogeneous core sample (76.04%)). Reservoir heterogeneity plays a critical role in determining the successes of the EOR processes, but its effect has rarely been comprehensively quantified in the laboratory. The limited experimental studies conducted to date seem to suffer from a number of deficiency mainly associated with sample preparation and experimental setup. In the present work, in addition to investigating a number of factors rarely studied experimentally before (e.g. effect of crossflow), attempts have been made to overcome the deficiencies of previous studies. Thus the results of this study can be insightful in overcoming the current challenges in capturing the importance of geological uncertainties in the current and future EOR projects.