The Characteristics Of Orientated Fine-grained Structure And Its Mechanism Research

The deformation,displacement,and arrangement of particles during the formation of fine-grained rocks play a pivotal role in affecting the mechanical properties,petrophysical properties,and brittleness of shale.The fine-grained rocks possess various geometry,complex combination,and arrangement,which exhibit layer,layer couple,and directional arrangement at the macroscale,mesoscale,and microscale,respectively.This study focuses on the formation mechanisms of the directional arrangement of fine grains and thoroughly investigates the internal structure of shale,which could help the development of shale resources.In this study,we employ the Chang #7 shales of Erodes basin as the subject.Based on a large amount of SEM characterizations,the particle size entropy,particle arrangement entropy,and pore arrangement entropy are selected as parameters to quantify the oriented structure of the sample.Combining with the concept of structureoriented entropy,an improved structure-oriented entropy model(E)is proposed to determine the oriented structure of the sample.Applying the proposed model,we analyze the structure-oriented entropy of samples retrieved from different depositional environments.The effects of water and paleoclimatic conditions on affecting the directional arrangement of fine grains are specified.From the perspective of sedimentary dynamics and diagenesis,we conduct experiments to investigate the effects of overburden pressure and structure stress on influencing the directional arrangement degree of fine-grained rocks.The diagenesis pattern is established by considering the dual effect of initial sedimentary dynamics and late diagenesis transformation,explaining the mechanisms behind it.Lastly,we investigate the correlations between directional arrangement features mechanical properties,permeability,and brittleness by using nanoindentation and permeability measurement experiments.The engineering implications of the directional arrangement of fine grains are discussed.The following conclusions can be drawn:(1)Results calculated by the improved structure-oriented entropy model show that the value of Chang #7 shale ranges from 0.72 to 0.96.For the samples that exhibit structure-oriented-entropy values that are smaller than 0.85,the directions of grains exhibit a spindle-liked distribution with a clear indication of directional arrangement;on the contrary,for those that are larger than 0.85,the grains are randomly distributed.We then calculate the value of five different lithofacies belonging to the Chang 7formation,including medium-high organic matter clayey shale,medium-high organic sandy shale,low organic matter clayey shale,low organic matter mixed shale,and low organic matter sandy shale.Results show that the medium-high organic matter clayey shales possess the minimum(0.78～0.85)exhibiting a strong directional arrangement.(2)The fine-grains particles show different particle size,shape,and directional arrangement features for the samples retrieved from deep lake-semi-deep lake,shallow lake,and delta front facies.The calculated results of structure-oriented entropy show that the sequence of the tendency of forming a directional arrangement is: deep lakesemi-deep lake>shallow lake> delta front facies.The depositional environment including alkalinity,high salinity,deep water environment,warm and humid climate is beneficial to the formation of directional arrangement.Among the tested depositional environments,the deepwater condition of deep lake-semi-deep lake facies provides a good environment for fine grains to directionally distribute.(3)The mechanical analyses of the formation of directionality show that as the overburden pressure increases,the value of decreases,and the sample tends to be directional.The effect of overburden pressure is pore shrinkage and particle deformation.The structure stress can break the inherent structure,which enlarges the pore and makes the pore more long and narrow through the rotation of shale particles.However,it exhibits an insignificant effect on the particle itself,which leads to a large value of and consequently a random distribution of particles.At the same stress condition,the medium-high organic matter clayey shale is more susceptible to forming a directional structure based on the calculated values.(4)The correlations of structure-oriented entropy and engineering properties including elastic modulus,permeability,and brittleness show that a smaller value of structure-oriented entropy leads to a reduced elastic modulus and increased permeability.The permeability along the arrangement direction is more sensitive to the value.Based on the directional structure features of shale,both composition and are used to establish a new brittleness index.The evaluated results by this new index exhibit a good correlation of productivity,which can shed light on the brittleness evaluation of shale with a directional arrangement.