Study On The Mechanism Of Pollutant Migration In Heterogeneous Shale Gas Reservoirs

With the development of unconventional shale gas resources,large-scale hydraulic fracturing technology is often used in the transformation of shale reservoir.Some scholars have suggested that fracturing may extract some heavy metals from shale,which will cause the pollution of groundwater.The subsurface consists of matrix pores,discrete natural fractures(NFs)and fault,which makes it has a strong heterogeneity.Besides,the high-pH drilling fluid has been widely used in the drilling process to prevent wellbore instability.It would cause the alkali erosion of siliceous minerals,such as clay and quartz.In this paper,the Weiyuan gas field,in southwest Sichuan,is investigated to study the mechanism of pollutants migration and assess the pollution of aquifers by means of theoretical research and numerical calculation.The main works of this paper are as follows:(1)Darcy's law,cubic law and equivalent Darcy's law are used to describe the flow mechanisms of pollutants in matrix,NFs and fault,respectively.Based on the theory of damage mechanics,the damage parameter of rock under the alkaline erosion of drilling fluid is obtained from the perspective of chemical kinetics.Based on the Blake-Kozeny equation,the relationship between permeability and time is obtained,and the erosion mechanism of high-pH drilling fluid on the rock is revealed.Therefore,the basic control equaitons of pollutants migration model of the heterogeneous shale gas reservoirs are obtained.(2)The model of pollutants migration in the heterogeneous shale gas reservoirs is established through four simplified models:(i)the homogeneous single fracture unit;(ii)the cross-layer fractured rock mass simplified model;(iii)the heterogeneous fractured rock mass simplified model;(iv)the corrosion effect of high-pH drilling fluid.Then,the effectiveness of this model is validated by a series of numerical simulation and comparison with the previous research results.It is found that the heterogeneity of the overburden will affect the migration of pollutants,NFs can increase the risk of aquifers contaminated,and the erosion of the high-pH drilling fluid makes the effect more serious.(3)The Weiyuan gas field,in southwest Sichuan,is investigated to assess the aquifers pollution according to the two useful metrics:(i)the concentration of pollutants at the top of the fault;(ii)the accumulation of pollutants in aquifers.Pollutants form a preferential flow in the fault due to the high permeability.A thousand years later,although most of the pollutants are trapped in subsurface,a small amount of polltants still migrate to aquifers.Among them,the concentration of Cd at the top of the fault will be 562 times of the water quality standard,which will seriously pollute the shallow aquifers and then endanger people's health.(4)Factor transformation method,known as a local analysis method,is used to perform a sensitivity analysis of various parameters,including fault parameters(height,width,dip and permeability),longitudinal dispersivity,NFs parameters(density and aperture)and pH.Where,the fault height is the most important factor affecting the migration of pollutants.The accumulation of pollutants in aquifer is quite sensitive to the pH values,which is followed by the fault permeability.While,the accumulation of pollutants seems to insensitive to the longitudinal dispersivity.In the end,a design scheme to reduce the pollution of shallow aquifers is proposed,which can provide a theoretical and practical guidance for the assessment of shallow aquifers pollution.