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Establishing credible reaction-kinetics distributions to fit and explain multi-heating rate S2 pyrolysis peaks of kerogens and shales

David A. Wood

(Published: 2018-09-01)


Corresponding Author and Email:David A. Wood, dw@dwasolutions.com; ORCID: https://orcid.org/0000-0003-3202-4069


Citation:Wood, D.A. Establishing credible reaction-kinetics distributions to fit and explain multi-heating rate S2 pyrolysis peaks of kerogens and shales. Advances in Geo-Energy Research, 2019, 3(1): 1-28, doi: 10.26804/ager.2019.01.01.


Article Type:Original article


Abstract:

Extracting reaction-kinetic distributions, in terms of activation energies (E) and pre-exponential factors (A), from the S2 peak data generated by pyrolysis tests conducted at three or more distinct heating ramps, is a well-established technique. These reaction-kinetics distributions are of paramount importance in establishing the timing and degree of petroleum generation from shales undergoing a range of burial and thermal histories. A commonly adopted approach is to determine and define reaction kinetics using a derivative of the Arrhenius equation configured in terms of a fixed/constant A value. Although the fixed-A approach can obtain good fits to multi-rate pyrolysis data, here it is shown that a formulation of the Arrhenius equation that involves reactions with a range of E and A values provides equally good fits to the multi-rate pyrolysis data. Moreover, the kinetic distributions with variable E-A provide more credible reaction kinetics consistent with those established for a range of kerogen types known for decades. To establish accurate fits to multi-rate pyrolysis S2 peak data at 1 ◦C intervals from 250 ◦C to 700 ◦C an optimizer is applied to the preferred Arrhenius equation formulation to derive reaction increments and transformation fractions to a range of reaction kinetics (E-A pairs). The methodology applied involves two steps: Step 1 finds the single E-A pair that best matches the S2 peak temperatures (three or more for multi-rate pyrolysis data); step 2 uses the E-A pair from step 1 as its modal focus and fits the full S2 peak shape using a distribution of 11 distinct reaction. This approach can replicate the fixed-A approach but is best applied using reactions with variable E-A values. The results of applying this method to multi-rate pyrolysis data for ten published kerogens and shales show credible kinetic distributions spread along the established E-A trend for kerogen/shales.


Keywords:Kerogen-kinetics distributions, pyrolysis S2-peak shape analysis, mixing kerogen kinetics, multi-versus single-heating rate kinetics, case for varying A pre-exponential factor, applying optimizers to derive kinetic, distributions.


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