Modeling CO2 Injection in a Fractured Chalk Experiment

Abstract

This paper presents a modeling study of CO2 injection in a chalk core based on experimental data reported by Karimaie (2007). The experiment consisted of a vertically-oriented 19.6 cm long chalk outcrop core initially saturated with reservoir synthetic oil consisting of C1 and n-C7 at a temperature of 85°C and pressure of 220 bar. After saturating the core with the oil mixture by displacement a small "fracture" volume surrounding the core was created by heating the solid Wood's metal which originally filled the volume between the core and core holder. Gas injection was first conducted using an equilibrium C1-n-C7 gas at 220 bar, resulting in no recovery by (thermodynamic) mass transfer but only from immiscible displacement with Darcy flow driven by induced pressure gradients and a minor impact of gravity-capillary equilibrium. Once oil production ceased in this first displacement, a second period with pure CO2 gas injection followed. Our modeling was conducted with a compositional reservoir simulator. The 2-dimensional r-z model used fine grids for the core matrix and surrounding fracture. Automated history matching is used to match experimental data. The match to reported production data gave a high degree of confidence in the model. Oil recovery improved significantly by CO2 injection. Our model study indicates that the recovery mechanism in the Karimaie experiment was dominated by Darcy displacement because of a low conductivity in the surrounding fracture. Another observation made in our study was the strong influence of surface separator temperature on surface oil production. Finally, gas injection rate changes had a significant impact on recovery performance for CO2 injection. Gravity-capillary recovery mechanism was of minor importance in the Karimaie experiments. Introduction CO2 injection has been considered as potentially enhancing oil recovery from naturally fractured reservoirs. Alavian and Whitson (2005)study the IOR potential for CO2 injection in the naturally-fractured Haft Kel field, Iran, based on detailed compositional simulations of the matrix-fracture system. Obviously, it would be useful to experimentally investigate the efficiency of gas injection in naturally fractured reservoirs, followed by CO2 injection, before this procedure is applied to a reservoir. Few experiments are reported in the literature to study gravity drainage mechanism in CO2 injection in fractured reservoirs. Li et al (2000) perform CO2 injection after water flooding in a dead oil system. They study water imbibitions followed by CO2 gravity drainage experiment on artificially fractured cores. They report that CO2 gravity drainage could significantly increase oil recovery after water flooding. They found that CO2 gravity drainage declines as the rock permeability decreases and initial water saturation increases. Asghari and Torabi (2008) conduct miscible and immiscible CO2 gravity drainage experiments with dead oil (n-C10). They show miscible CO2 injection improves oil recovery, but they could not match laboratory experiment with a simulation model. Karimaie (2007) performed equilibrium gas injection followed by CO2 experiments on chalk and carbonate cores at reservoir conditions where cores were saturated with live synthetic oil. The experiments were designed to illustrate CO2 injection in a fractured reservoir. The matrix permeability essentially controls the rate of recovery, and the ratio between the matrix and fracture permeability determines if viscous displacement (Darcy flow by pressure gradients) is governing the displacement or not. The pressure gradients over the fractures are negligible for high permeability fractures where most injected gas flows through the fracture space and the main production mechanism from matrix is gravity drainage. Darcy flow might play a role when fracture permeability is low. Fracture permeability should be adequately high to eliminate viscous displacement in the core, because it is difficult to achieve efficient recoveries by viscous dominated mechanism in low permeability fractured reservoirs. Uncertainties and error sources are analyzed to understand and simulate the Karimaie experiment.

Fickian Diffusion in Discrete-Fractured Media from Chemical Potential Gradients and Comparison to Experiment

“…This was investigated in detail using the Eclipse simulator, where agreement between the experimental data and Eclipse simulations could only be achieved by introducing an artificial two-phase grid cell between the fracture and the matrix, with an “inert” component to keep this element in two-phase. Alavian and Whitson also attempted to model this experiment with the Eclipse 300 simulator but chose to increase the surface (separator) temperature from 15 to 30 °C and decrease the fracture permeability to a low 80 md, which increases the simulated oil recovery through viscous forces to compensate for the limitations in the diffusion modeling. Alavian and Whitson also simulated a similar experiment and again found that the results with and without Fickian diffusion are similar using both the SENSOR and Eclipse 300 simulators and that the measured recovery could only be reproduced through viscous forces by lowering the fracture permeability to 26 md.…”

The Role of Diffusion for Nonequilibrium Gas Injection Into a Fractured Reservoir

“…On the basis of the results of laboratory experiments and on the basis of fluid-flow dynamics, several additional papers investigating the effect of diffusion in fractured reservoirs have been published (Grogan and Pinczewski 1987;da Silva and Belery 1989;Coats 1989;Uleberg and Høier 2002;Alavian and Whitson 2009).…”

Pore network modelling of molecular diffusion in a single‐block model during lean gas injection, a comparative study on calculation approaches

“…Molecular diffusion has also been investigated through various analytical and numerical modelling works. Different modelling approaches (such as single porosity, double porosity, or statistical modelling) have been used with various solution techniques (including implicit, explicit, finite element, or finite volume) to analyze a wide range of problems such as empirical modelling of gravity drainage, developing matrix‐fracture transfer functions, assessing block size effects, reservoir simulation of non‐equilibrium gas injection, studying cross flow equilibrium concept, realizing effect of gas flow rate in fracture, investigating displacement patterns, sensitivity analysis of matrix and fracture parameters, and dissolution of residual non‐aqueous phase liquids in porous media …”