粗糙单裂隙换热特征及全局灵敏度分析研究

Study on heat transfer characteristics and global sensitivity analysis of a rough single fracture

干热岩开采时,往往需要借助于压裂手段对储层改造形成换热通道,因此,热储裂隙中的对流换热过程对干热岩采热有重要影响.借助于巴西劈裂试验得到粗糙单裂隙,利用三维激光扫描仪扫描裂隙面得到裂隙的点云数据,经处理之后导入至COMSOL Multiphysics数值模拟软件,建立二维粗糙单裂隙对流换热模型,分析其对流换热特性并开展Morris全局灵敏度分析.结果表明:出口温度和平均对流换热系数随粗糙度增加而增加.局部对流换热系数与裂隙起伏度和波动程度呈强相关性,局部粗糙度较高,换热效率可有效得到提高.在裂隙开度较大(大于40μm)或初始流速较高(大于50 mm/s)时,平均和局部对流换热系数在粗糙度越高时增幅越明显.体积流速与裂隙起伏度有强相关性,起伏度越大的位置对应体积流速越大.压力差随着粗糙度的增大而平缓增加.裂隙开度、初始流速、流体比热容和岩石热导率是花岗岩粗糙单裂隙对流换热过程中的灵敏参数,这些参数对模型的换热效率影响较大,并且与其他参数的相互作用也越强.该研究能够揭示流体在粗糙裂隙中的换热特征,并且找到影响对流换热的主控因素,为增强型地热系统优化设计提供科学依据.

During Hot Dry Rock(HDR)developing,it is necessary to form a penetrating heat transfer channel by means of hydraulic fracturing,supercritical CO_(2) fracturing and other fracturing methods.Therefore,heat transfer in thermal reservoir has an important influence on the heat extraction of HDR.The rough single fracture is obtained by means of Brazilian splitting test.Threedimensional laser scanner is used to scan the fracture surface to obtain the point cloud data.After processing the data,the point cloud data is imported into the numerical simulation software COMSOL Multiphysics.The 2D rough single fracture convective heat transfer model is established to analyze the convective heat transfer characteristics and carry out the Morris global sensitivity analysis.The results show that the outlet temperature and average heat transfer coefficient increase with the roughness increasing.The local heat transfer coefficient is strongly correlated with the undulating and fluctuating degree.The higher the local roughness is,the higher the heat transfer efficiency can be effectively improved.The increase of the average and local heat transfer coefficient is more obvious when the fracture aperture is larger(>40μm)or initial flow rate is higher(>50 mm/s).There is a strong correlation between the volume flow rate and the undulating degree of the fracture.A higher undulating degree corresponds to a higher volume flow velocity.Additionally,the pressure difference increases gently with the increase of roughness.It can be obtained by the Morris global sensitivity analysis,the fracture aperture,initial flow rate,specific heat capacity of rock and fluid,thermal conductivity are sensitive parameters in the convective heat transfer process.There parameters have a great influence on the heat transfer efficiency of the model and the interaction with other parameters is also greater.This study can reveal the characteristics of convective heat transfer in rough fractures and find out the main controlling factors affecting the convective heat transfer,providing scientific basis for the optimization of enhanced geothermal systems.