Fluid expulsion and microfracturing during the pyrolysis of an organic rich shale

Demand and supply of oil and gas are set to change, but world oil and gas consumption are still increasing, and it is likely that consumption will continue to rise for some time, despite mounting environmental pressures to control the emission of greenhouse gases. One reason for the increase use of fossil fuel is due to underdeveloped alternative renewable energy sources. To this end, identification of the formation process and factors controlling the flow of hydrocarbons responsible for oil and gas formation has recently attracted significant attention of researchers. This will enable a better understating of the migration pathways for easy exploration and extraction.

The chemical composition of mature kerogen determines the amount of oil and gas. Generally, kerogen (fossilized organic matter) is categorized into type I, type II, type III and type IV with the highest to lowest hydrocarbon content respectively. However, type II kerogen is highly desirable as it produces oil and natural gas in large amount. Several approaches have been developed to explore the formation of hydrocarbons from kerogen and the factors affecting the primary oil and gas migration pathways.

Recently, University of Oslo researchers: Dr. Hamed Panahi, Dr. Maya Kobchenko, Professor Paul Meakin, Professor Dag Kristian and Professor François Renard assessed the primary migration of oil and gas and the factors affecting their transportation mechanisms. In particular, they designed a system based on an externally heated pressurized autoclave to measure the fluid pressure changes due to the maturation of the organic matter and expulsion of the hydrocarbon. They wanted to characterize the fluid expulsions at different temperatures and analyses their influence on the microfractures appearances. The study has been published in the research journal, Fuel.

Briefly, shale samples were confined under low pressure and heated at a varying temperature in the range of 210 ^°C-320 ^°C. Next, changes in static pressure, and dynamic fluid pressures at constant temperature were determined using pressure and piezoelectric sensors respectively. As the shale matured, the expulsion activities were analyzed at different temperatures based on the proposed model to determine the relationship between the temperatures and the amount of gases produced.

The authors observed that the fluid accumulated in the shale until a fairly large volume enough to start the expulsion was obtained. Expulsion of hydrocarbon occurred in bursts. Microfractures formed due to thermal decomposition of kerogen were noted to be parallel to the bedding plane with a few being perpendicular. It was found that an increase in the waiting time resulted in a corresponding decrease in the bursts frequencies, and that a range of burst amplitudes could be represented by a power law..

In summary, the study by University of Oslo researchers carefully analyzed four fluid expulsion stages including early stage, second stage, third stage and production of the fluid. High resolution synchrotron X-ray tomography was used to image the microfractures, and this confirmed that the primary migration of hydrocarbons is due to percolating microfracture networks. The paper, published in the journal Fuel, will advance exploration and extraction of oil and gas.

The work reported in this article, which focuses on static and dynamic pressure measurements, is part of a larger body of research on the mechanisms of primary migration. Time-resolved X-ray microtomography, enabled by a synchrotron X-ray photon source (beamline ID19 at the European Synchrotron Radiation Facility), and recent advances in the analysis of three-dimensional X-ray tomograms have played an important role in this work on primary migration.