Effect Of Tectonic Compression On The Characteristics Of Pore Structure And Gas Storage Of Shale

The marine-face shales in Lower Silurian Longmaxi Formation(S1l)and the Lower Cambrian Niutitang Formation(∈1n)in South China,that are characterized by large thickness,wide distribution and high organic carbon abundance,are over-mature and have experienced strong multi-stage tectonic reformation.And the reservoir space character and gas potential of shale may be strongly modified by tectonic movement and followed variation of the temperature and pressure in geological condition.Shales were picked from S1 l,∈1nand O2 h profiles at different tectonic locations and their geochemistry,reservoir properties,pore characteristics and methane sorption capacity were investigated in this study.And then comparison study of the shales at the same layer but at different geological locations have been conducted to illustrate the influence of tectonic compression to the character of shale nanopore structure.On the other hand,low matureted Chang7 shale has been utilized in a semi-closed heat simulation system to study the evolution of shale nanopore structure with increasing maturity.And the effects of compression stress,fluid pressure to shale nanopore structure are discussed through adjustment of the system condition.Also,the triaxial compression experiment of shales at different confining pressure have been conducted to reveal the modification of shale pore system at different conditions.According to above research,the influence of tectonic compression on the pore characteristic and established the relationship between the tectonic reformation and reservoir properties.The following knowledge and results are obtained:(1)Tectonic compression is one of the controlling factors to the reservoir characteristics of shales.And the reservoir space characters of specific shales picked from tectonic stable and fault compression profiles show distinctive differences.The result suggests fault compression may decrease shale porosity,BJH pore volume and BET specific surface area,but has little effect to the micro-pore volume of shales.(2)The higher the mechanical stress provided by the system,the greater degree of damage to the meso-porous and macro-porous volume of Chang7 Member shalestreated by semi-closed high-pressure heat simulations.However,the extrusion stress has little effect to micro-pores.The generated volume and specifica area of nanopores in thermal maturation process growing large at higher fluid pressure condition induced by the hydrocarbon expulsion The pore volume and specific surface area of pores at each scale decreases obviously during the deformation and breach of shale under triaxial extrusion experiments,and the decrease level show positive correlation with the confining pressure.(3)Organic matter is the main carrier of nano-scaled pores,followed by inorganic mineral,organic nano-pores are smaller than mineral pores.Total porosities of most shale samples are about 4%,while few samples are more than 10% because of the development of micro-fractures.The total porosity of Cambrian shale is obviously lower than that of the Silurian and Ordovician shale,suggesting decrease porosity with increasing maturation level and buried depth.The porosity of specific shale is positively correlated with the TOC content,suggesting that the porosity is mainly controlled by the organic matter.And the specific surface areas are predominantly controlled by pores less than 10 nm,while the pore volume is mainly associated with meso-pores and macro-pores.Meso-pores are predominate in the Silurian Longmaxi shale,which is accounting for 65.6% of the total pore volume,while the micro-pores accounted for only 10%.Therefore,micro-pores are just work as oil and gas migration channel rather than the main storage space.And the nanopores of Cambrian shale are mostly composed of micro-pores and mesopores,that are account for 31% and 46% respectively.(4)The heat simulation result shows that the evolution of the organic micro-and meso-pores with increasing temperature are complicated,and may be divided into three stages.At the first stage,EasyRo range from 0.7% to 1.3%,the volume of micro-and meso-pores decrease with increasing maturity due to the occupation of the organic pores occupied by generated fluids.At the second stage,EasyRo range from 1.3% to 2.5%,the volume of the micro-and meso-pores increase with increasing maturity,due to forming and development of the secondary organic pores.At the third stage,EasyRo range from 2.5% to 3.6%,the volume and SSA of micro-pores decrease due to the transformation of micro-pores to meso-pores or micropore destruction.However the volume of macro-pores and porosity keep increasing trend with increasing maturation level.(5)At oil generation stage,the amount of solvent extractable material shows positive correlation with the TOC abundance of shales.Solvent extractable materials mainly stay at nanopores,and solvent extraction may dramatically increase the volumes of meso-and macro-pores with increasing maturity until VRo=2.0%,but only slightly increase the micro-pore volume at higher maturation level.In comparison,the increase of specific surface area after solvent extraction may is mostly contributed by micro-pores,and then meso-pores.(6)The TOC abundance is the dominant controlling factor of shale methane adsorption capacity.Sincelarge amount of small meso-pores afford by illite mineral,the mineral character is the second controlling factor of gas adsorption capacity.And the average pore size of shale shows negtive correlation with the adsorption of methane.Also,the temperature and pressure have distinct influence to the methane sorption capacity of shale formation.And it is concluded that the methane adsorption capacity of shales increases with increasing tectonic stress and decreasing of temperature.