Research On Mechanisms Of Retention And Flowback Of Fracturing Fluids In Artificial Fracture Network

Large-scale Stimulated Reservoir Volume(SRV)fracturing is one of the key techniques for the successful development of shale reservoir.A large amount of fracturing fluids are injected into the reservoir.The data of the field shows that the flowback rate of the fracturing fluids after fracturing is generally low.Some Wells have low flowback rate and high production rate,which is inconsistent with the production of conventional reservoirs.The retention of fracturing fluids are closely related to reservoir damage and gas production.Deep understanding of the retention mechanism of the fracturing fluids has been a guide to understand the pattern of the flowback and the impact on the production.Based on shale reservoir characteristics,this study introduces the retention mechanism of fracturing fluids in detail.The flowback process of fracturing fluids is analyzed from the perspective of material and energy balance.The mechanisms of fracturing fluids trapped in the tight reservoir fractures include surface bound water film,and ‘locking' in fractures,the capillary force effects,imbibition of fracture surface,the impact of gravity and viscous trapping.Hydraulic fracturing fluids mechanical energy,reservoir gas pressure energy and matrix pore infiltration energy are the main sources of fracturing fluids flowback energy.In this paper,we built an experimental system of fluids' retention in fractures by using the evaluation apparatus of flow conductivity,carried out an experiment of fluids' retention in a fracture with the Silurian shale outcrop of the southeast of Chongqing.The research shows that the fluids' retention lessens with the increase of the aperture,and it gets more retention with more roughness,larger displacement pressure difference and more imbibition near fracture surface.The effect of gravity is the key to the difference of fracturing fluids retention in different fracture dip angles.We analyze the flowback rate under different fracture dip angles and put forward the empirical formula of the flowback rate contribution ratio and fracture dip angles.In this paper,we use numerical simulation software to simulate the retention of fracturing fluids in fracture networks.The research results show that the initial reservoir water saturation values increase will reduce the retention of fracturing fluids,low matrix permeability and high permeability will aggravate the retention of the fracturing fluids.The relative permeability curve with narrow phase permeability blind zone is more suitable for shale reservoirs.The fracturing fluids gets more retention with more wet and complex fracture network structure.Natural fractures are the main place where fracturing fluids are retained.As the shut-in time increases,the retention of fracturing fluids will gradually increase.The purpose of this paper is to understand the retention mechanism and rules of fracturing fluids in shale reservoirs,and to provide a theoretical basis for formulating a reasonable fracturing fluids flowback system.