Study Of Pore Evolution And Damage Mechanical Characteristics Of Coals Under The Effect Of Liquid CO₂ Cyclic Shock Fracturing

There are abundant coal resources in China,however,some features,such as high-methane concentration,complex geological structure,and low porosity and permeability,are the main factors restricting the efficient extraction of coals.CO₂ used as fracturing fluids,could achieve the dual purpose of methane recovery enhancement and CO₂ geologic sequestration by phase-transition fracturing and high-pressure flooding.However,the traditional CO₂ fracturing commonly adopts single injection into the coals,and the cold shock effect during low speed injection of liquid CO₂ is rarely considered,and the effects of cryogenic CO₂ fluids cyclical injection on the pore structure and the mechanical properties evolution of coals are seldom studied.Based on the temperature response behavior during liquid CO₂ cyclical affecting process on the coals,and the pores evolution within the small-scale coal samples under the cyclical effect of liquid CO₂ was analyzed,realizing the fine characterization of pore distribution within coal matrix;The mechanical response and acoustic wave evolution characteristics of mesoscale coal samples were studied based on the MTS uniaxial compression test and ultrasonic instrument measurement,revealing the coupling relationship between the characteristic parameters of coal fracture and the cyclical parameters of liquid CO₂;The development and expansion behavior of three-dimensional fissures in large-scale coal samples were discussed,under the sole impact of thermal cycling and of high-pressure expansion stress of liquid CO₂,and the fracture mechanism and degradation criteria of samples were built,under the“stress-temperature-damage”multi-fields coupling conditions;The quantitative characterization method of fracture parameters during the fracturing process of coals influenced by cyclical liquid CO₂ was determined based on the single elliptical crack model,and the application system of liquid CO₂ cyclic shock fracturing method was also established.The obtained conclusions were followed as:?1?Depending on the self-designed cyclic liquid CO₂ fracturing experimental simulation platform,the reactor pressure showed a periodic“increase-steady-decrease”change tendency during the liquid CO₂ cyclic injection process,and the temperature change within a single injection process showed a“U type”distribution.The surface temperature tested by infrared thermal imager of the measure lines were negative,and the unconstant temperature on the same measure line indicated that the coal heterogeneity could induce the heat transfer anisotropy within the coal matrix.The cyclically-affected coals had different coal numbers and distributions on the surface,and the coal damage degree was negatively correlated with the coal metamorphism,and was positively correlated with the cyclical parameters.The periodic“cold shock--heating”process of liquid CO₂ could cause the coal structure to undergo alternative“contraction-expansion”,a huge number of"unrecoverable"fatigue damage was accumulated weakened the coal strength,finally the coals were destroyed.?2?Under the cyclical effect of liquid CO₂,the spectrum area rates of different rank coals had positive relations with the cycle parameters,respectively;The?_AWW distribution of coals decreased continuously,and the?_AW values of coals had negative relations with CN and TCT,respectively,while the?_CWW and?_BW distributions increased,and the?_CWW and?_BWW value of coals were positively correlated with cyclic parameters,respectively;The?_t and?_e values of coals tended to increase,while the?_r values decreased continuously,and the??_t%,??_e%values had exponential increase relations with CN,respectively,and had linear increase relation with TCT,respectively,while the??_r%values had an exponential decline relation and a linear decrease relation with CN and TCT,respectively.The binding pores within the coal did not have fractal characteristics while the free pores had good fractal feature.The fractal dimension of free pores had exponential and linear relations with T_2cutoff,and?_t and?_e values,respectively;The nuclear magnetic permeability of different coals were negatively correlated with coal rank,and the?k_NMR%had an exponential growth trend with CN,and had a“increase-decrease-increase”tendency with TCT,but with a whole increase trend.The related NMR data evolution all indicated that the thermal cycling of liquid CO₂ could promote small size pore to change into larger size pore,and the semi-closed or closed pores to evolve into the open pores,finally,the free flow channels and efficient pore volume all increased.?3?The failure forms of the coal samples under the CO₂ temperature effect and the temperature-adsorption coupling effect were mainly manifested as"axial splitting"and"splitting+caving",respectively.The continuous loading process made the axial and radial strain rate,ringing count and cumulative energy of coal increased continuously,and the?_c values of the latter affected coals were smaller.Udner the influence of cyclical liquid CO₂,the compressive strength?_c and the elastic modulus E of the coal samples had linear decrease relations with the cyclic parameters,respectively,while the Poisson's ratio?and the damage variable D_v had positive relations with the cyclic parameters respectively.The damage statistical constitutive model of coals was built,combining with the Weibull distribution theory and the stress-strain curves,and the changes of brittleness index B_I were studied based on the strain energy with various?_d values.It was found that the B_I value of lignite and bituminous coals had a whole downward trend,while the anthracite had a larger fluctuation amplitude of B_I value with no obvious regularity.?4?Under the cyclic effect of the liquid CO₂,the longitudinal and shear wave velocities of coals all decreased,and the decrease amplitude was negatively correlated with the coal rank,for example,the lignite samples had an increasing dispersion of velocity scatters,while the velocity scatters dispersion of bituminous coal and anthracite did not change significantly.The wave velocities of coals were positively correlated with the related Vp/Vs value,and the accumulative fatigue damage caused by liquid CO₂ interaction with different cyclic parameters led to the heterogeneity of spatial fracture evolution.The anisotropic coefficients k_Vp and k _Vs,as well?k_Vp and?k_Vs increased with different various degrees with the increasing cyclic parameters.The?Vp_z%and?Vs_z%values of coals were negatively related with the cyclical parameters and coal rank.The fractal dimension D of different rank coals increased with the increasing cyclic parameters,and had a negative relation with the coal rank.The D values commonly had a positive relation with?k values,and negatively correlated with?V%values.The approximate uniform distribution and relatively small errors between?and?_d of different rank coals all indicated that the elastic wave velocity can effectively evaluate the degradation characteristics of the affected coals.?5?When liquid CO₂ injected the unsealed large-size specimen,the borehole wall temperature continued to drop and finally maintained around-22?,and begun to rise when the injection was finished.The acoustic emission events firstly distributed at the bottom of the borehole,and mainly distributed near the borehole along the axial direction as the injection cycles increased.The“repeatability”and“stairs type”of acoustic emission event characteristic parameters indicated that the process of repeated cold shock and temperature increase could promote the initiation of new cracks.Compared with the single fracture morphology of free samples,the effective crack number and crack surface roughness of coals were larger,under different confined conditions,and the fractured surfaces of coals had certain fractal features.There was a linear increase relationship between the?_c value and D_j value of the fracture surface in each sample,and Nc and F_c had a positive correlation with D,respectively.?6?Based on the elliptical fracture model,the quantitative characterization method was established from the relations between the relevant crack characteristic parameters,such as crack width w_c,net pressure p_n and R_c,and the affecting time t,during the liquid CO₂ cyclic affecting process.The crack evolution models were explored from two levels of crack development and crack expansion,under the cyclic effect of liquid CO₂injection.The application system of liquid CO₂ cyclic fracturing method was constructed from five levels:reservoir information collection,borehole information design,injection parameter selection,effect monitoring demonstration,and evaluation system construction.The dissertation contains 109 figures,26 tables,and 332 references.