Nanopore Development, Chemical Structure And Mechanical Properties Of Organic Matter In Highly Matured Shale

As a main form of orgainc matter?OM?in shale at high maturity,solid bitumen is not only an important source of shale gas,but also a great contributor of nanopore development.The chemical structure and nanomechanical properties?including Young's modulus,hardness and creep behavior?of solid bitumen could affect the physical behavior of shales at macroscale.The oil shale from Maonming,Guangdong province is thermally simulated by using semi-closed pyrolysis systems to study the effects of oil expulsion and pressure on nanopore development at high maturity with the applicaiton of low-pressure N₂/CO₂ gas adsorption;a camparison of atomic force microscopy and nanoindenter in nanomechanical characterization is conducted by using a shale sample form Longmaxi Formation,Sichuan Basin;by using closed pyrolysis system,solvent-extracted organic matter is thermally simulated to obtain solid bitumen samples,of which the dependence of mechanical properties on chemical structure is characterized with the applicaitons of atomic force microscopy and solid ¹³C nuclear magnetic resonance;a natural solid bitumen sample is reviewed as object to study the dependences of mechanical properties and creep behavior on indentation depth and loading rate with the application of nanoindenter.The main conclusions are as follows:?1?Oil expulsion efficiency can strongly affect organic gas generation and nanopore development in highly matured shales.As exemplified here,bitumen remained in shales with low oil-expulsion efficiency,which accounts for 60%of the TOC in the shales,contributes 52%-72%of pore?diameter<10 nm?volumes at high maturity.?2?The evolution trend of micropores and fine mesopores?diameter<10 nm?in highly matured shales can be divided into two stages:Stage I,corresponding to wet gas generation;and Stage II,corresponding to dry gas generation.For shales with low oil expulsion efficiency,pore evolution increases rapidly in Stage I,whereas slowly in Stage II.Comparatively,for shales with high oil expulsion efficiency,the evolution trend grows slightly in Stage I,and decays in Stage II.However,the evolution trend of medium-coarse mesopores and macropores?diameter>10 nm?remains flat at high maturation.?3?Higher pressure could promote the development of micropores and fine mesopores in shales,whereas it has weak effect on the evolution of medium-coarse mesopores and macropores.?4?The results from nanoindenter and atomic force microscopy on mechanical characterization of shales are in good consistency.?5?Both chemical structure and mechanical properties of solid bitumen show two-staged evolution trends during maturation.In the wet gas stage,the chemical structure changes greatly,as C/H atomic ratio and carbon aromaticity increase rapidly;the whole sample is relatively compliant and structurally homogeneous.However,in dry gas stage,the variation of chemical structure turns to be minor,during which the C/H ratio and carbon aromaticity do not change significantly.And the sample evolves to be stiffer and structurally heterogeneous.It is suggested that chemical structure of solid bitumen may govern its nanomechanical properties,which is demonstrated by a strong positive correlation between carbon aromaticity and Young's modulus.?6?Nanomechanical properties of organic matter may potentially influence the development of OM-hosted nanopores.With increasing maturity,the modulus of OM increases,and the state of OM turns from a viscous-elastic one into a glassy one.The gaseous hydrocarbon generated could not be retained within the brittle OM,as OM is prone to crack and subsequently leading to the destruction,merging and collapse of OM-hosted nanopores.?7?The hardness of solid bitumen decreases with increasing indentation depth and indentation loading rate.The creep strain rate sensitivity shows a relatively weak indentation depth and loading rate dependency.