Visualization of CO2 EOR by Molecular Diffusion in Fractured Chalk

Fig. 1—2D porosity distribution (left) and voxel distribution (right). The core sample has been digitally cut to show the center of the sample. The average porosity is 46.8%, and the sample is homogeneous.

This work demonstrates that molecular diffusion may be a viable oil-recovery mechanism in fractured reservoirs during injection of carbon dioxide (CO2) for enhanced oil recovery (EOR). The oil-production rate from diffusion alone, however, depends heavily on the distribution of CO2 within the fracture network and fracture spacing. A numerical sensitivity analysis, using a validated numerical model that reproduced the experiments, showed that the rate of oil production during CO2 injection declined exponentially with increasing diffusion lengths from the CO2-filled fracture and the oil-filled matrix.

Introduction

Compared with other EOR methods based on gas injection, CO2 injection has many beneficial properties, among them that CO2 lowers the gas/oil interfacial tension and reduces oil viscosity and density, resulting in increased oil mobility and oil swelling. The main drawback with injecting gas is the high mobility, a factor especially true in fractured reservoirs.

The success of a potential CO2 EOR project increases when key driving forces for oil displacement during a CO2 injection are identified. In a highly fractured reservoir, molecular diffusion could be an important driving mechanism; however, it would require high fracture density. Molecular diffusion is the mixing of fluids caused by random motion of molecules, and its calculation is described in detail in the complete paper. Diffusion is often neglected as a production mechanism during modeling of CO2 injection in oil reservoirs because it is computationally expensive to handle and is assumed to be of minor importance. While this might be true for most conventional reservoirs, it is not true for heavily fractured reservoirs and in laboratory experiments, where the diffusion distances are much smaller.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 170920, “Visualization of CO2 EOR by Diffusion in Fractured Chalk,” byØyvind Eide, SPE, Martin A. Fernø, SPE, and Arne Graue, SPE, University of Bergen, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed.
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Visualization of CO2 EOR by Molecular Diffusion in Fractured Chalk

01 July 2015

Volume: 67 | Issue: 7

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