Utilization of supercritical carbon dioxide for mechanical degradation of organic matters contained in shales

2021

Saad Alafnan

Fuel
Volume 316, 15 May 2022, 123427

The injection of carbon dioxide into shale gas formation has been proven effective in serving multiple purposes: 1) Recovery enhancement, and 2) Reduction of greenhouse gas emissions. The injection of carbon dioxide has an additional advantage of degrading the organic matter contained in shales for an improved fracture initiation, and propagation. Detailed findings of this study were reported in a paper published in Fuel Journal.

Supercritical carbon dioxide has been recently adopted to induce the initiation and propagation of hydraulic fractures. A rock matrix initially under reservoir conditions is subjected to high pressure by injected fluids, altering its stress state. A positive impact has been reported from the injection of supercritical carbon dioxide on the productivity of reservoirs, including in unconventional areas such as shale formations. However, the mechanisms by which supercritical carbon dioxide enhances fracture stimulation remain uncertain. Shale, which is a complex clastic sedimentary rock consisting of clay, quartz, calcite, and organic matter, responds to applied stresses based on the specific ratio of their constituents. The organic matter known as kerogen, composed of aromatic and aliphatic chains of carbon, is characteristically different from other inorganic minerals. Kerogen is scattered uniformly within a shale matrix. It is anticipated to exhibit a behavior similar to that of a polymer, and is capable of absorbing stresses without failure during fracturing jobs. Supercritical carbon dioxide injection is believed to alter the elasticity of kerogen, favoring fracture initiation. The objective of this work is to study the impact of supercritical carbon dioxide injection on kerogen’s geomechanics. At supercritical conditions, carbon dioxide interacts with kerogen, resulting in adsorption and, hence, swelling. Consequently, kerogen becomes more vulnerable to deformation under a given applied stress. The results of this research revealed a drastic change in mechanical behavior when carbon dioxide was used. The kerogen exhibited an inverse relationship between the degree of ductility and injection pressure of the carbon dioxide. The findings of this work provide nano-scale insights into the advantages of using supercritical carbon dioxide to degrade the mechanical integrity of organic matters contained in shales. These findings substantiate the value of carbon dioxide sequestration in shales. The outlined process has the benefits of both reducing greenhouse gas emissions and enhances the productivity of shales.

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