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Evaluation Method for Hydrocarbon Cleaning Effect in Kerogen-Rich Gas Shale

Abstract

Researchers have strived for years to study the rock properties of gas shale, but little attention has been paid to the cleaning effect of the hydrocarbon. Lack of proper evaluation criterions for the cleaning effect makes it hard to prove the reliability of rock physics parameters, such as porosity, permeability and electrical parameters etc. In rocks with no organic matter (i.e. sandstone), the cleaning usually aims to remove all hydrocarbons. However, some organic matter compounds in gas shale have fluid and solid dual characteristics. A proper core cleaning for it should be able to remove all the fluid but keep the solid organic matter as it is classified as a part of the matrix. Consequently, the evaluation method for gas shale needs to be able to clear the integrity of solid organic matter matrix. The recently proposed Rock-Eval II pyrolysis evaluation method, which can discern the solid and fluid organic matters in gas shale and give a visual reflection of hydrocarbon cleaning effect, solves the problems in fluorescence detection methods. However, researches indicate that the pyrogram from this progressive pyrolysis method has problems in differentiating free hydrocarbons and organic matter components in kerogen-rich gas shale. It provides an insufficient cleaning result because a portion of fluid-like hydrocarbon residue (FHR, which naturally considered as free hydrocarbon) might be inappropriately classified as the solid organic matter matrix. Therefore, the ESH (Extended Slow Heating) pyrolysis evaluation method is proposed to evaluate the hydrocarbon cleaning effect in kerogen-rich gas shale. The ESH pyrolysis technique solves those problems in Rock-Eval II cycle by providing sufficient pyrolysis time, lower heating rate and lower initial temperature. The ESH pyrogram divides the hydrocarbon into three finer fractions: S1E, light free hydrocarbon; S2aE, FHR; and S2bE, solid organic matter. The completion of hydrocarbon cleaning is evaluated by comparing the differences between ESH pyrograms measured on the uncleaned and cleaned aliquots. The evaluation criterions for the complete cleaning are expressed as the disappearance of S1E and S2aE fractions and the invariant of S2bE fraction on the cleaned sample's ESH pyrogram. The experimental results show that the ESH pyrogram is more reasonable in evaluating the free hydrocarbon cleaning level and the integrity of solid organic matter in kerogen-rich gas shale contrast to the Rock-Eval II pyrogram.