| The deep coalbed methane(DCBM)resources are rich in China,and the breakthrough in the exploration and development of DCBM in some areas indicates that its development potential is great.However,the overall development situation is not optimistic.Low-porosity and low-permeability reservoir characteristics are important factors that restrict the high production of DCBM,which also determines that the fracturing reconstruction of deep coal reservoirs is essential.At present,the formation and evolution of porosity and permeability of deep coal reservoirs and the fracturing reconstruction feasibility under high temperature,high reservoir pressure,and high in-situ stress environment are still not clear.In view of the above problems,this paper reveals the evolution mechanism of porosity and permeability in deep coal reservoirs in terms of coal rank,structure evolution and burial depth,and discusses the fracturing feasibility of deep coal reservoirs taking coal reservoir permeability,mechanical properties of coal and surrounding rock,and spatial distribution of in-situ stress as the dominant controlling factors.The porosity and permeability development characteristics of deep coal reservoirs controlled by coal rank was clarified.Based on vitrinite reflectance and fluid inclusion test,combined with burial history simulation,the evolution process of coal metamorphism in Linxing area was identified.It is pointed out that magmatism only accelerates the metamorphism process of coal reservoirs outside the Zijinshan,and the maximum burial depth of the geological history controls the present coal metamorphism degree.Through the maceral,approximate analysis and pore structure test of different coal metamorphic samples,the control characteristics of coal metamorphism on coal material composition and pore structure development were analysed.It is revealed that with the increase of coal metamorphism(0.75-1.35%),the proportion of microspores increases;the proportion of small pores decreases first and then increases;the proportion of mesoporous and macrospores decreases.The fractal dimension of the pore is divided into two segments by 10nm,and each segment has different response characteristics to the degree of metamorphism.As the degree of coal metamorphism increases,the pore connectivity deteriorates and the seepage capacity decreases.The evolution mechanism of the physical properties of coal reservoirs under multi-stage tectonic movements was identified.Through field geological survey and log interpretation,combined with tectonic curvature,the paleotectonic stress field and paleotectonic traces in the key tectonic periods were restored,and the deformation degrees of coal reservoirs in the key tectonic periods were determined.It is concluded that the direction of the maximum principal stress of Yanshanian period was N W direction(134°-314°),and the direction of the maximum principal stress of Himalayan period was NEE direction(53°-233°).There are six superposition methods for the two-stage tectonic movements.The different superposition methods lead to different degrees of deformation of coal reservoirs.The deformation degree of coal reservoirs in Himalayan period is greater than that in Yanshanian period.Based on GSI and logging curves,a quantitative characterization model of coal structure was established.It was found that the primary-cataclastic coals were mainly developed in the study area,followed by the cataclastic coals.The maximum deformation degree of coal reservoir in geological history and coal reservoir physical property were comprehensively analysed.The corresponding relationship between coal structure and the maximum deformation degree of coal reservoir in geological history was determined.With the increase of historical maximum deformation degree,the seepage capacity of coal reservoirs was enhanced.The pore and permeability characteristics of coal reservoir under the coupling of temperature and stress were revealed.Based on the high temperature and high pressure nuclear magnetic resonance and high temperature overpressure permeability tests,the control characteristics of temperature and stress on the pore-fracture structure of coal reservoirs were identified,the response characteristics of pore and fracture with different pore sizes to temperature and stress were clarified,the variation law of coal reservoir permeability under the coupling effect of temperature and stress was revealed,and the permeability prediction model suitable for deep coal reservoir considering variable pore compression coefficient and temperature coefficient was constructed.The results show that the temperature and stress have negative effects on pore and permeability of coal reservoir,and stress is the key controlling factor.The variation of porosity and permeability with stress shows obvious stages,which can be divided into stress sensitive stage,low stress sensitive stage and stress insensitive stage with the boundary of 5 MPa and 10 MPa.The negative effect of temperature on porosity and permeability of coal reservoir is small,and that is related to stress.When the stress is small,the negative effect of temperature on permeability is relatively obvious.When the stress is large,the negative effect of temperature on permeability is not obvious.Microcrack and large-medium pores are sensitive to temperature and stress.The permeability model fits well the experimental data of permeability.The model shows that the permeability of coal reservoir decreases exponentially under the coupling of temperature and stress.The hydraulic fracturing reconstruction feasibility of deep coal reservoirs in study area was discussed.Through the full stress-strain experiment under high temperature conditions,it is revealed that the stress has a positive and significant effect on the mechanical properties of coal,and temperature has a negative and indistinctive effect on the mechanical properties of coal.By analysing the coal and surrounding rock combination characteristics and mechanical parameters of the study area,the differences of coal and surrounding rock mechanical properties were clarified.Through hydraulic fracturing data and imaging log interpretation,the spatial distribution characteristics of in-situ stress in coal reservoirs in study area were identified,and it is pointed out that the direction of maximum horizontal principal stress in the study area is near EW direction.There are two critical transition depths in the vertical distribution of in-situ stress in the study area,and the in-situ stress combination of deep coal reservoirs(1500-200m)is characterized by ?hmax>?V>?hmin.When the burial depth exceeds 2000m,the vertical principal stress gradually dominates.The plane distribution of in-situ stress is controlled by both buried depth and local structures.Through numerical simulation,the effects of coal reservoir permeability,mechanical properties of coal and surrounding rock,and in-situ stress on coal reservoir fracturing were studied,and he hydraulic fracturing reconstruction feasibility of deep coal reservoirs was discussed.The results show that the smaller the permeability of coal reservoir is,the more favourable it is for fracturing;the larger the elastic modulus of coal,the easier to form high,narrow and short cracks;the larger the difference of elastic modulus of coal,the easier to form short,long and wide cracks;when the horizontal principal stress difference is less than 3 MPa,the fracture cracks expand along the natural fracture;when the horizontal principal stress difference exceeds 6MPa,the fracture cracks expand along the direction of the maximum horizontal principal stress;when the horizontal principal stress difference exceeds 10MPa,the fracture cracks are basically single cracks along the principal stress;the larger the minimum horizontal principal stress difference of coal and surrounding rock,the easier it is to form short,long and wide cracks. |
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