Research And Application Of Pore-related Mechanical Properties Of Tight Rocks At Ultra-low Temperature

With the large-scale exploitation of unconventional reservoirs of oil and natural gas using hydraulic fracturing,the problems caused by hydraulic fracturing arise more and more attention,such as waste of water resources and environmental pollutions.With the enhancement of environmental awareness,many countries have issued the related policies and regulations about forbidding Petroleum Company to exploit oil and gas resources through hydraulic fracturing.Therefore,the petroleum industry is badly in need of a waterless and environment-friendly fracturing technology,and liquid nitrogen fracturing may be a good choice.In contrast to hydraulic fracturing,liquid nitrogen fracturing does not require water resources,has no pollution to the environment,and will not cause clay hydration expansion and other issues.However,the research in the field of liquid nitrogen fracturing is still very scarce.On the one hand,the mechanical properties and damage mechanism of reservoir rocks under the liquid nitrogen environment are still unclear.On the other hand,there is no clear understanding of the fracture initiation and propagation of liquid nitrogen fracturing.Based on the above problems,we developed new experimental equipment and innovative experimental methods to study the pore-related mechanical properties of tight sandstones with different saturations under the liquid nitrogen environment.Firstly,the uniaxial compression experiments and the Brazilian splitting experiments of tight sandstones with different saturations at normal and ultra-low temperature were carried out,and the variation law of macroscopic tensile and compressive mechanical properties was investigated.With the increase of water saturation,the tensile strength,compressive strength and elastic modulus of tight sandstone decreased first,then increased and then decreased under the liquid nitrogen environment.Then,in order to clarify the microscopic mechanism behind the macroscopic mechanical properties,the digital image correlation method,nuclear magnetic resonance experiment and microscopic electron microscopy were used to study the thermal deformation characteristics,the change of pore structure and microstructure of tight sandstones with different saturations at ultra-low temperature.It is found that the band-like distribution of irreversible damage deformation zone appeared on tight sandstones with different saturations,and the pore size distribution and total porosity of the tight sandstone changed significantly after the liquid nitrogen treatment.The microscopic structure of the tight sandstones also suffered various forms of damage,after the liquid nitrogen treatment.Next,according to the experimental research results under different scale conditions above,the complex physical-mechanical coupling process occurring in the tight sandstone with different saturations at ultra-low temperature was theoretically analyzed.Under the liquid nitrogen environment,the critical phase transition temperature corresponding to the pore water in tight sandstone decreases with the decrease of pore size.The expansion pressure caused by multi-stage phase transition of pore water and the shrinkage deformation of rock matrix,combined with the influence of strong combined water phase transition and capillary force effect,are the root causes of macroscopic mechanical properties and rock structure changes of tight sandstone.It also provides theoretical support for subsequent research on liquid nitrogen fracturing and other related applications.Finally,based on the above-mentioned research on the pore mechanical properties of tight sandstone at ultra-low temperature,a three-stage model of liquid nitrogen fracturing was proposed,and the numerical simulation and physical experiment research of liquid nitrogen fracturing(depending on the self-developed liquid nitrogen fracturing equipment)were carried out.The initiation and expansion laws of the fractures in the process of liquid nitrogen fracturing were explored.In the process of liquid nitrogen fracturing,the tight sandstone formed a complex fracture network composed of main fractures,branch fractures,and volumetric fractures.The fracture morphology is much more complicated than the single fracture formed by hydraulic fracturing.This is consistent with the three-stage model of liquid nitrogen fracturing.