Constrain on Oil Recovery Stage during Oil Shale Subcritical Water Extraction Process Based on Carbon Isotope Fractionation Character

Rongsheng Zhao,Zhiyong Chang,Sunhua Deng,Luquan Ren,Youhong Sun

doi:10.3390/en14237839

Rongsheng Zhao, Zhiyong Chang + Show 3 more

Open Access

https://doi.org/10.3390/en14237839

Copy DOI

Abstract

In this work, Huadian oil shale was extracted by subcritical water at 365 °C with a time series (2–100 h) to better investigate the carbon isotope fractionation characteristics and how to use its fractionation characteristics to constrain the oil recovery stage during oil shale in situ exploitation. The results revealed that the maximum generation of oil is 70–100 h, and the secondary cracking is limited. The carbon isotopes of the hydrocarbon gases show a normal sequence, with no “rollover” and “reversals” phenomena, and the existence of alkene gases and the CH4-CO2-CO diagram implied that neither chemical nor carbon isotopes achieve equilibrium in the C-H-O system. The carbon isotope (C1–C3) fractionation before oil generation is mainly related to kinetics of organic matter decomposition, and the thermodynamic equilibrium process is limited; when entering the oil generation area, the effect of the carbon isotope thermodynamic equilibrium process (CH4 + 2H2O ⇄ CO2 + 4H2) becomes more important than kinetics, and when it exceeds the maximum oil generation stage, the carbon isotope kinetics process becomes more important again. The δ13CCO2−CH4 is the result of the competition between kinetics and thermodynamic fractionation during the oil shale pyrolysis process. After oil begins to generate, δ13CCO2−CH4 goes from increasing to decreasing (first “turning”); in contrast, when exceeding the maximum oil generation area, it goes from decreasing to increasing (second “turning”). Thus, the second “turning” point can be used to indicate the maximum oil generation area, and it also can be used to help determine when to stop the heating process during oil shale exploitation and lower the production costs.

Highlights

Oil shale has become an important backup energy of conventional fossil energy in the world, and its pyrolysis exploitation mode has changed from aboveground to underground [1,2,3,4,5,6,7,8], as the former technology encounters significant challenges, such as environmental pollution and inefficiencies [2]
There are three main traditional methods for forecasting the oil shale pyrolysis process: (1) direct measurement of vitrinite reflectance (%Ro) [11], hydrocarbon generation (Tmax), and element ratios (H:C) of rock samples [12]; (2) indirect measurement through analyzing the structure and composition of rocks by wave-substance interaction using terahertz time-domain spectroscopy [13], Raman spectroscopy [14], and infrared spectroscopy et al [15], or through analyzing the product and its isotope compositions according to empirical formula, using a mass spectrometer and isotope analyzer [16]; (3) numerical modeling method, which establishes kinetics data acquired from pyrolysis experiment or actual mining data [17,18]
Our goals were as follows: (1) investigate the carbon isotope evolution characteristics below the max oil generation stage; (2) illuminate the possible mechanisms responsible for isotope fractionation; (3) evaluate if the carbon isotope fractionation characteristics based on kinetics and thermodynamic equilibrium can be used to indicate the oil generation characteristics during oil shale pyrolysis within water

Summary

Introduction

Oil shale has become an important backup energy of conventional fossil energy in the world, and its pyrolysis exploitation mode has changed from aboveground (ex situ) to underground (in situ) [1,2,3,4,5,6,7,8], as the former technology encounters significant challenges, such as environmental pollution and inefficiencies [2]. There are three main traditional methods for forecasting the oil shale pyrolysis process (organic matter decomposition): (1) direct measurement of vitrinite reflectance (%Ro) [11], hydrocarbon generation (Tmax), and element ratios (H:C) of rock samples [12]; (2) indirect measurement through analyzing the structure and composition of rocks by wave-substance interaction using terahertz time-domain spectroscopy [13], Raman spectroscopy [14], and infrared spectroscopy et al [15], or through analyzing the product and its isotope compositions according to empirical formula, using a mass spectrometer and isotope analyzer [16]; (3) numerical modeling method, which establishes kinetics data acquired from pyrolysis experiment (e.g., thermogravimetric analysis) or actual mining data [17,18] These methods are either time consuming and expensive for the routine control of complicated sample preparations and analytical techniques, or it depends on an amount of experimental data that is hard to achieve in the early mining stage. A simple and quick method for indication of the oil shale pyrolysis process is still needed

Objectives

Methods

Results

Conclusion