Main Factors Controlling The Wettability Of Shales

Wettability plays an important role in the exploration and exploitation of shale gas.However,it is challenging to evaluate the wettability of gas shale due to the complexity and heterogeneity of the mineral components,physical properties,and pore structures.In this study,we conduct rock composition analysis,field gas desorption,and air-liquid contact angle measurements for 15 core samples,and analyze the petrophysical properties of 8 core samples from the YY3 well drilled in the Longmaxi Formation,Hunan Province,Southern China.A strong parabolic correlation was found between water contact angles and total organic carbon(TOC)content.We divided the samples into two analysis groups using the symmetry axis with TOC?1.5%.Scanning Electron Microscope(SEM)analysis demonstrated that migratory organic matter is sparsely distributed in samples with TOC<1.5%.However,in the samples with TOC>1.5%,the migratory organic matter appeared to be like a uniformly distributed network.This suggests that migratory organic matter increases the water contact angle and is a significant factor explaining strong oil-wetting with high TOC.A negative correlation was found between the content of illite and the water contact angle(R~2=0.9070),while a positive correlation exists between the illite/smectite mixed layer and the water contact angle(R~2=0.8048).This relationship is likely due to the large inner surface area of the structure layer in the illite/smectite mixed layer,allowing for chemical adsorption of organic matter.When discussing the effect of inorganic minerals on the wettability of shale,it was found that quartz has a negative effect on the oil-wet characteristic,and illite/smectite mixed layer has a positive influence on the lipophilic feature of shale.Therefore,inorganic minerals can't all be considered as hydrophilic components.The illite/smectite mixed layer has a large inter-layer structure which can provide the main space for the chemical adsorption of organic matter.Therefore,the illite/smectite mixed layer can has an important influence on the oil-wet wettability of shale.In addition,porosity and gas content can be the indicators of shale wettability.Generally,the shale with the high TOC content and high clay mineral,relatively has the large porosity and high gas content,so its wettability is also more lipophilic.However,for samples with a little difference in total gas content,there was a negative correlation between the contact angle and gas content.The analysis is that the higher gas content is generated by the cracking of oil and degraded bitumen and the gas occupies the pore space which previously stored degraded asphalt.Eventually,the main controlling factors affecting the wettability of natural gas shales were determined and a new mode of the influence of organic matter on shale wettability was proposed.Moreover,based on the the research of rough surface contact angle model and the surface topography features of the samples,the contact angle model of the shale surface was established,which provided a new idea for the analysis of wettability of over-mature marine shales.