Study On Imbibition Mechanism And Flowback Pattern Of Shale Gas Reservoir After Fracturing

Shale gas plays an important role in unconventional oil and gas reservoir development and production.While large area hydraulic fracturing can make shale gas developed effectively,the view of the influence on gas well productivity and recovery taken by imbition and flowback mode after fracturing are various.Studying about imbition and flowback mode after fracturing has theoretical significance and application value.This paper establishes a type of double-permeability,gas-liquid two-phase and hydraulic fracturing single-fracture model,combining the actual reservoir characteristics and production mode of shale gas reservoir,studies the influence on imbition and the fluid dynamic in shale reservoirs taken by different factors(porosity,permeability,wettability,initial water saturation,artificial fracture width).On this basis,this paper analyzes the force condition in the process of fluid intrusion and flowback after fracturing,establishes a micromechanical displacement model including capillary force and osmotic pressure in shale pores,and studies the flowback law and microdistribution characteristics of residual fracturing fluid.Finally,basing on the mechanism of imbition,characteristics and rules of flowback,this paper also builds a multi-stage fracturing production model with shale gas wells and studies the effects of different production or flowback mode on productivity,which can be applied to shale gas well example to realize the comprehensive optimization scheme design.This paper comes to the following conclusions:1.The double-permeability,gas-liquid two-phase and hydraulic fracturing single-fracture model can simulate and analyze the influence of different factors on imbition,the simulation results are consistent with the existing experimental results.Meanwhile,the model can judge the proportion between capillary force and gravity during the process of osmosis after hydraulic fracturing.2.porosity,wettability and water saturation are the main factors affected imbition.The larger the porosity,the greater the imbition rate and invasion range.The stronger the hydrophilicity is,the more the imbition is,but the imbition rate changes little.The more severe the phenomenon of "ultra-low water saturation",the faster the imbition speed and the higher the imbition efficiency.Permeability has little effect on the imbition of low permeability shale reservoirs.Artificial fracture width can also affects osmosis action,The larger the crack width,the faster the infiltration rate,the smaller invasion scale.3.Capillary forces play a dominant role in fluid dynamics in shale reservoirs,having a certain inhibitory effect on gravity.The greater porosity and ultra-low water saturation phenomenon,the greater inhibitory effect.The larger hydrophilicity,permeability and artificial fracture width,the smaller inhibitory effect.4.The main factors affected the fluid intrusion and flowback after fracturing of shale reservoir is capillary force and osmotic pressure caused by Salinity difference.On this basis,this paper establishes a micromechanical displacement model including capillary force and osmotic pressure.This model can simulate the flrid distribution rules after displacement and flowback in shale pores.After flowback,residual fracturing fluid will fill the micropores and form water films in macropores.5.The retention of fracturing fluid forms a water lock and pollutes the reservoir,on the other hand,reverse imbition during flowback caused by the capillary force and osmotic pressure can due to the exchange of air and water,then improve the productivity within a certain time.The multi-stage fracturing production model established in this paper can study the effect of operation with different long shut-in operations in different production stages on productivity and flowback rate.Using Shut-in operation in the middle-later stages of production can affects the gas well production,there exists a limit shut-in days,in this time scale,the longer the shut-in time is,the better the productivity the gas well is,but the productivity will decrease beyond that time scale,the flowback rate declines during the whole process.6.Through the simulation and fitting of a production example well in Fuling shale gas field,this paper form a productivity chart with different output and shut-in time.In order to improve the productivity,the best production plan is optimized and used to guide production.The results shows,shut-in for 30 days and 70,000-cubic meters daily gas production can reach the ideal yield quickly,shut-in for 180 days and 30,000-cubic meters daily gas production can miximize the cumulative gas production when the time is suitable.