Study On The Effect Of Freeze-thaw Weathering On Rock Mechanical Properties And Oil And Gas Reservoir Conditions

The impact of long-term freeze-thaw weathering on rock mechanical properties in cold regions is a frontier issue in rock mechanics and engineering geology research.At the same time,the fissures(cracks)produced by the weathering of freezing and thawing have an important influence on the geological conditions of oil and gas basins.However,the mechanical properties of rocks,the evolution of cracks and their impact on oil and gas reserves after freeze-thaw weathering are still in the exploratory stage,and are the frontier issues in international petroleum geology and engineering geology research.In response to the above problems,through systematic tests and simulations,the thesis explores the mechanical properties of rock and the mechanism of crack development by freezing and thawing weathering.Combined with the analysis of Qiangtang oil and gas basin in the high-cold frozen soil area,the influence and control of freezing and thawing cracks on gas hydrate reservoir conditions are discussed.In this dissertation,non-destructive testing methods are used to observe the internal microstructure changes,fracture initiation and expansion of rocks at different stages of freezing and thawing,and analyze the effect of water-thermal-mechanical coupling on micro-fractures.Through the high-speed impact compression test,the mechanical properties and changes of the strength,deformation,and failure modes of granite and sandstone under complex stress conditions after freeze-thaw weathering are studied.The thesis is summarized by the methods of freeze-thaw test,non-destructive testing,impact compression test,numerical simulation,constitutive equation establishment,core observation and other methods.The thesis obtained the following main understandings through the above research:(1)By studying the causes of freeze-thaw weathering and the meso-mechanism of freeze-thaw damage,it is proposed that the freeze-thaw weathering of rocks is a complex degradation process coupled by comprehensive factors such as lithology,water saturation,number of freeze-thaw cycles,time and temperature.Its macroscopic performance is the development of pores and micro fissures,and the relationship between the freeze-thaw cycle and the fissure development is established.(2)Through the static compression and impact compression tests on the freeze-thaw damaged rock specimens,it is believed that the freeze-thaw weathering has a significant weakening effect on the compressive strength of the rock.Strain rate and confining pressure have obvious strengthening effect on compressive strength.(3)Through the numerical simulation of the rock impact compression test,it is concluded that the uniaxial impact compression specimen is compressed axially while expanding radially,and its tensile strain reaches the limit and cracking occurs.Furthermore,it is proposed that the effect of confining pressure can inhibit or delay the breakage of specimens,and can effectively improve the compressive strength of rocks.(4)Based on the damage evolution theory of rock materials and combined with the results of experimental research and numerical analysis,a statistical damage evolution equation is to be established.Based on the idea of the element model,a set of a visco-elastoplastic dynamic constitutive model including uniaxial and confining pressure in consideration of freeze-thaw damage factors is established.The improved differential evolution and gradient descent methods are used to identify the parameters of the established dynamic constitutive equation,and the inverse analysis is used to determine the characteristic parameters,and the accuracy of the model is verified by test data.(5)Through freeze-thaw tests and cracks development and evolution studies,combined with the analysis of cracks,permeability and porosity of drilling cores in the Qiangtang Basin,four types of cracks related to freeze-thaw are identified.The identification signs and methods of various types of fissures have been preliminarily obtained,and it is proposed that the cracks caused by freeze-thaw weathering have a significant impact on the reservoir physical properties.The rapid increase of rock permeability and porosity in the basin at 310-350m is mainly controlled by the cracks generated by freezing and thawing,and it is universal in the basin.