Study On Rock Mechanical Properties And Evolution Law Of Deep Coal Measures Gas Reservoir In Yushe-Wuxiang Block

Coal-bed methane is the abbreviation of coal-based natural gas,which mainly refers to the unconventional natural gas stored in the coal geology,mainly"coal triple gas".Shanxi Province has abundant coal-system gas resources and is not only a large industrial province,but also ranks first in the country in terms of coal-bed methane exploration and development technology.In recent years,due to the impending depletion of shallow coal resources,China has gradually extended the exploration and exploitation of resources to the deeper part,where the geological environment of deep coal reservoirs is more complex than that of the shallow part,with the unique characteristics of"three highs and one disturbance"（high ground stress,high temperature,high karst water pressure or gas pressure and mining disturbance）.Some basic mechanical parameters,such as strength characteristics,rheological properties and deformation modulus of coal rocks,also vary greatly with the depth of burial,so it is of great significance to carry out rock mechanics tests on deep coal rocks for the establishment of deep rock mechanics theory and guidance of deep resource exploitation.In this paper,we mainly focus on the deep coal gas reservoir in the Yushe-Wuxiang block at a depth of 1300m,and use a combination of indoor tests and theoretical analysis to conduct an experimental study on the single and composite coal rocks of mudstone,coal and sandstone in this deep reservoir,including the determination of physical properties,hydrostatic laws and rheological characteristics of the deep coal rocks,revealing the basic physical properties,hydrostatic and rheological characteristics of the deep coal rocks between the single and combined bodies.The basic physical properties,hydrostatic and rheological characteristics of single and combined deep coal rocks were revealed,the mechanical laws and fracture mechanisms of different rock combinations were analyzed,the rheological intrinsic model was constructed,the reservoir fracturability was quantitatively characterized by mechanical features,and different permeability modification schemes were proposed for different lithological reservoirs.The main research contents and conclusions are as follows:（1）While obtaining the basic physical properties of mudstone,coal and sandstone,the relationship between water absorption and soaking time of different coal rocks was studied through water absorption test.Water absorption increased with the increase of soaking time,and the speed gradually slowed down from fast.At the same time,Brazilian splitting tests of mudstone,coal and sandstone under different water contents were carried out,and it was found that water had a weakening effect on the tensile strength of coal and rock.The tensile strength decreased exponentially with the increase of water content,and the failure path of coal and rock after water content became more and the integrity became worse.（2）The uniaxial compression mechanical properties of different single coal rock and combined coal rock are tested,and the compressive strength law between single coal rock and combined coal rock is obtained.The compression process of single coal rock can be divided into four stages in the stress-strain curve.In the uniaxial compressive strength test,the uniaxial compressive strength of the combined coal rock is higher than that of the single coal.When the coal:mudstone ratio is 7:3,5:5,3:7,the uniaxial compressive strength of the combined coal rock increases by 5.77%,19.33%,and 31.33%,respectively.When coal:fine sandstone is7:3,5:5,3:7,the compressive strength of the composite increases by 8.11%,28.71%,and44.35%,respectively.When mudstone:coal:fine sandstone is 3:4:3,increased by 32.56%;in terms of the compression failure type,the compression failure of a single specimen belongs to the second type of failure.Multiple main fracture surfaces appear along the direction parallel to the loading,which is a typical plate failure.The failure of coal in the composite specimen is more severe,and the integrity is worse after failure.Moreover,some composite specimens will drive the mudstone to produce radial tensile failure.（3）The uniaxial creep test of a single coal rock at room temperature was carried out.By comparing the axial and radial deformations,it can be seen that the axial and radial deformations of the sample under different stress loading include instantaneous deformation and creep deformation,which have obvious viscoelastic aging characteristics,and the performance is more prominent under higher stress.The creep curve shows attenuation creep and stable creep characteristics,and has hysteresis in radial deformation relative to axial deformation.The ultimate strength of creep failure of mudstone,coal and fine sandstone is87.56%,83.73%and 84.87%of uniaxial compressive strength,respectively.Mudstone and fine sandstone belong to instability failure,and coal belongs to loading failure.The main fracture surface of mudstone is parallel to the axial direction,and the fracture form is roughly’Y’type failure.Coal presents the failure form of sliding along a single inclined direction,which is shear failure.Fine sandstone appears several main failure surfaces parallel to the stress loading direction,which is columnar splitting failure.（4）The triaxial creep test of single coal rock and composite coal rock under seepage pressure was carried out.In the axial deformation,the instantaneous deformation of the sample under different stress is given priority to（the instantaneous strain ratio is more than 80%）.In volume deformation,the deformation of mudstone and fine sandstone is mainly instantaneous deformation,and the deformation of coal is mainly creep deformation.The deformation of the combination is mainly instantaneous deformation under low stress and creep deformation under high stress.According to the triaxial creep characteristics of stress-seepage coupling,the nonlinear rheological model and three-dimensional creep constitutive equation considering creep stress and seepage pressure are established,which can be better used to describe the creep damage law under the underground triaxial stress-seepage coupling.（5）By analyzing the reservoir characteristics,kerogen characteristics,gas-bearing characteristics,pore-fracture characteristics and permeability characteristics of the Yushe-Wuxiang research block 15^#coal in the Qinshui basin,the brittle characteristics,fracture toughness and compressive strength of the reservoir are combined together to quantitatively characterize the fracturability of coal gas reservoirs,and different permeability enhancement schemes are proposed for the reservoirs in this region;the mudstone and sandstone reservoirs in this region The mudstone and sandstone reservoirs in the region should be modified by liquid CO₂ phase change fracturing,and the coal reservoirs should be modified by explosive blast fracturing to increase permeability.