Study On The Transport Law Of Fracturing Fluid In Shale Gas Reservoirs

At present,hydraulic fracturing technology is extensively employed in the shale gas reservoir reconstruction,wherein substantial volumes of fracturing fluid are injected into the subsurface,inducing fractures in the reservoir rocks and establishing fracture networks for facilitating natural gas flow into the wellbore.The transport of fracturing fluids within the reservoir is often intricate,influenced by factors such as geological conditions,fracture network structure,pressure variations,and fluid composition.Without an accurate understanding of the underground transport processes of the fracturing fluid,optimizing the fracturing operations and predicting the recovery rate of the gas reservoir becomes challenging.Consequently,investigating the transport law of fracturing fluids in complex fracture networks is of significant technical importance for shale gas reservoir development.In this thesis,the unclear transport law of fracturing fluids in complex fracture networks are addressed by conducting research in three areas:investigating the factors influencing proppant-supported fracture conductivity,examining the imbibition and invasion characteristics and mechanisms of fracturing fluids in fracture networks with varying morphologies,and studying the flowback patterns and influential factors of fracturing fluids.Furthermore,a microscopic visualization experimental method and a multi-physics coupled model with two-phase flow have been established for evaluating the transport of fracturing fluid.The main contents and findings of this thesis are as follows:(1)The artificial fractured rock samples are selected to conduct the test of fracture conductivity.The following factors affecting fracture conductivity have been investigated:proppant placement concentration,laying method,particle size,and fracturing fluid viscosity.The results show that placing larger proppant particles in the fractures is more likely to generate flow advantages.Increasing the proppant concentration can increase the equivalent width of fracture and pore space of the rock core.By optimizing the proppant laying method and reducing the mixing area of the proppant,the conductivity of fractures can be improved.When the confining pressure of the reservoir changes up and down,the damage to the conductivity of fractures is irreversible.(2)The transport process of fracturing fluid is qualitatively and quantitatively studied using microscopic visualization glass model technology.Considering four combinations of primary,branch and natural fractures,the interlaced hydraulic fracture networks in shale reservoirs are simulated.The transport law and imbibition mechanism of fracturing fluid are as follows:the branch fractures promote the imbibition of fracturing fluid into the reservoir,and natural fractures can serve as an extension point for hydraulic fracturing fractures,making the fracturing fluid more efficiently enter the pores.In different fracture networks,the imbibition degree of high-concentration fracturing fluid is generally lower than that of low-concentration fracturing fluid.During the imbibition process of fracturing fluid,the forces acting on it are injection pressure,capillary force,viscous force,and gas pressure.And the main driving forces are injection pressure and capillary force,while the main resistance is viscous force.(3)A model with a realistic fracture network structure is used to perform microscopic visualization experiments on the fracturing fluid flowback in shale gas reservoirs.The effects of fracturing fluid concentration and displacement pressure on the fracturing fluid flowback in the model are investigated,and the retention and flowback mechanism of fracturing fluid are revealed:As fracturing fluid concentration decreases and gas drive pressure increases,the ultimate flowback rate increases.The retention of fracturing fluid during flowback is influenced by multiple factors such as pore and fracture microstructure and resistance.The retention of fracturing fluid in the micro model can be divided into four types:viscous retention,Jamin effect,gas blocking effect,and dead-end retention.Viscous force and capillary force are two important resistances to the fracturing fluid flowback.The ultimate flowback rate of fracturing fluid in the micro model increases with the increase of capillary number Ca,and the two also show a high linear positive correlation.(4)Based on experimental research and fully considering the complex mechanisms involved in the reservoir,a multi-physics coupled mathematical model with two-phase flow for shale gas reservoirs is established.Factors affecting the fracturing fluid flowback are analyzed,and the impact of flowback rate on the production of shale gas wells is explored in conjunction with production data.The results show that with the increase of the hydraulic fracture conductivity and matrix permeability in the reservoir,the concentration of fracturing fluid decreases,the flowback pressure differential increases,and the cumulative flowback volume of gas wells will be improved to varying degrees.With the increase of fracturing fluid flowback rate,the cumulative gas production of the studied gas wells shows an increasing trend.This thesis combines laboratory experiments and numerical simulations to investigate the transport process of fracturing fluids in complex fracture networks and reveals the mechanisms of fracturing fluid imbibition and flowback.The findings can provide a theoretical basis for optimizing the fracturing transformation of shale reservoirs.