Viscous and Gravitational Fingering in EOR and Carbon Sequestration

Abstract

Enhanced Oil Recovery (EOR) through immiscible, or multi-contact miscible, gas injection can be unstable to viscous flow instabilities when the mobility ratio between injected gas and displaced oil-in-place is high. Additionally, there is a risk of gravitational flow instabilities associated with, for instance, CO₂ injection. Gravitational fingering can occur both when a denser fluid is injected on top of a lighter fluid (e.g. supercritical CO₂ may have a higher density than oil in some reservoirs), and also when multiphase compositional effects result in local density variations that may trigger instabilities (i.e. due to evaporation and dissolution of species). In the context of EOR, both viscous and gravitational fingering are detrimental to hydrocarbon recovery, because the instabilities can result in early breakthrough of injection fluids. Carbon sequestration in the top of saline aquifers can also be affected by gravitational fingering when CO₂ dissolution locally increases the aqueous phase density. In this scenario the flow instability is beneficial. The gravito-convective mixing of CO₂ throughout the aquifer is more efficient than Fickian diffusion alone. A reliable model for viscous and gravitational flow instabilities is critical for EOR and carbon sequestration studies, as well as for various other flow problems. The early onset of fingering instabilities has been investigated analytically by perturbation theory, and the non-linear regime has been considered in numerous simulation studies. However, these studies have generally relied on a range of simplifying assumptions on dimensionality, compositional and phase behavior effects, and boundary conditions. Additionally, simulation results may be model dependent. Particularly, the small-scale onset of fingering is often delayed or suppressed altogether by numerical dispersion when lowest-order methods are used. In this work, we present a unified study of both viscous and gravitational fingering for fully compositional, three-dimensional, multiphase flow, modeled with advanced higher-order finite element methods. Moreover, we compare our simulations on structured and unstructured grids to results from other academic and commercial reservoir simulators. We demonstrate that higher-order methods are essential in predicting fingering behavior, and its potentially disastrous impacts on hydrocarbon recovery, on feasible grid sizes.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015