Geothermal Field Condition Of Coal Reservoirs In The Qinshui Basin And Its Influence On The Permeability Of Coal Reservoir

The storage and transport of coalbed methane（CBM）produced in coal reservoirs are subjected to a variety of energy fields,such as in situ stress fields,groundwater pressure fields,and geothermal fields.The stress and temperature sensitivities of coal reservoirs are critical geological factors affecting CBM well exploitation,and in particular it is important to reduce or eliminate their influence on coal reservoir permeability.Taking No.3 and No.15 coal reservoirs as the research object,this thesis comprehensively uses the means of field data statistical analysis,indoor experimental research,theoretical analysis and numerical calculation to study the current geothermal field distribution and its influencing factors of main coal reservoirs in Qinshui Basin.Meanwhile it is revealed the evolution of coal reservoir permeability under the coupling control of temperature and stress;The stress sensitivity and temperature sensitivity of coal reservoir have been evaluated;The effect mechanism of temperature on coal permeability has been revealed from the meso point of view.The main research results have been achieved are as follows:（1）Taking ground temperature,geothermal gradient and geothermal heat flow as the characteristic parameters of geothermal field,the spatial distribution of present geothermal field in No.3 and No.15 coal reservoirs have been systematically analyzed.The results indicate that the ground-temperature of No.3 coal reservoir ranges from 14.61to 56°C,that of No.15 coal reservoir is 15.3 to 60°C,the temperature of No.15 reservoir is 0.08 to 4.3°C higher than that of No.3 coal reservoir.The geothermal gradient of the no.3 coal reservoir ranges from 0.008 to 3.77°C/hm with an average of 1.6°C/hm,and the geothermal gradient of the no.15 coal reservoir ranges from0.046 to 5.35°C/hm with an average of 1.6°C/hm.In addition,the terrestrial heat flflow of Qinshui Basin ranges from 0.93 to 94.6 m W/m² with an average of 41.5 m W/m²,which is lower than the average heat flow in mainland China（63m W/m²）.It has the characteristics of“cold”basin in stable structural area.（2）The classification standard of geothermal field in Qinshui Basin has been proposed,then the range of geothermal negative anomaly area of coal reservoir in Qinshui Basin has been delineated,and the control mechanism of negative anomaly area has been analyzed.The results show that the negative geothermal anomaly area of coal reservoir in Qinshui Basin accounts for nearly 30%of the total basin area,which is distributed in strips in the northeast and east of the basin,including Estern Shouayang-Yangquan,Hengling-Heshun,Estern Qinnan-Xiadian,Changzi,Zhaozhuang and Estern Changping blocks.The negative geothermal anomaly area of coal reservoir may be influenced by the uplift and denudation of the crust and the strong runoff of low-temperature water.（3）Seepage experiments have been conducted on coal permeability at various stresses,pore pressures and temperatures,the coal permeability evolution with effective stress under different temperature conditions and the coal permeability evolution with with temperature under different stress conditions have been revealed.The results show that under five different temperatures（25–65°C）,the coal permeability decreases in a negative exponential function with the increase of external stress,it basically decreases in a negative exponential function with the decrease of pore pressure.Under different external stress conditions,the coal permeability decreases in a negative exponential function with the increase of temperature.At low pore pressure,the coal permeability decreases linearly with the increase of temperature.While at high pore pressure,the coal permeability decreases in a negative exponential function with the increase of temperature.（4）The phased characteristics of stress sensitivity and temperature sensitivity of coal have been analyzed by the permeability damage rate PDR,stress sensitivity coefficientα_k and temperature sensitivity coefficientα_T.A new stress sensitivity index S has been proposed to evaluate the stress sensitivity of coal permeability under different temperatures,meanwhile a new temperature index S_Thas been proposed to evaluate the temperature sensitivity of coal permeability under different stresses.The results show that the PDR of coal samples increases with the increase of effective stress and temperature,α_k andα_Tdecreases with the increase of effective stress and temperature.The S of coal samples decreases first and then increases with the increase of temperature.At low stresses,the S_Tof coal sample increases with the increase of effective stress because the fracture aperture is greater than the linear strain due to thermal expansion.Under higher stress,when the fracture aperture is less than the thermal expansion deformation,the S_Tof coal sample decreases with the increase of effective stress.When the effective stress further increases,new cracks are generated in the coal,which provides a new development space for thermal expansion,and the S_Twould continue to increase.（5）The coal samples pore size distribution,porosity and permeabilityat various temperatures have been characterized by low field nuclear magnetic resonance NMR technology,and the control mechanism of temperature on coal reservoir permeability has been revealed from the meso point of view.Combined with the steady-state seepage experimental test data,the existing nuclear magnetic resonance permeability models have been modified,and the NMR permeability model which can predict the permeability of different rank coal under different temperatures has been deduced.The results show that with increasing temperature,the T₂ spectrum area of micro pores in anthracite is basically unchanged,and the T₂spectrum area of micro fractures decreases;The T₂spectrum area of micropores increases and the T₂ spectrum area of microcracks decreases in lean coal and coking coal samples;The T₂ spectrum areas of micropores and microcracks of fat coal and lignite coal samples all decreased.The results of permeability model validation show that TC model can better predict the permeability evolution of anthracite,lean coal and coking coal samples with temperature.For fat coal and lignite,either TC model,generalized SDR model or modified SDR model can accurately predict the permeability under different temperature conditions.