Study On Fracturing Fluid Leakoff And Fracture Extension Mechanism Of Unconsolidated Sandstone Reservoir

There are rich reserves of unconsolidated sandstone,the exploitation of which has received more and more attention in China.Unconsolidated sandstone with shallow burial has poor cementation,low strength,complicated mechanical properties,and complicated failure patterns during hydraulic fracturing.Since existing studies could not fully explain the fracture initiation and propagation mechanism of unconsolidated sandstone fracturing,therefore,this paper conducted physical simulation experiments and numerical simulation experiments of unconsolidated sandstone fracturing in the laboratory for a more extensive and comprehensive analysis of fracture initiation and propagation mechanism.Fracturing fluids commonly used at the field are linear gel fracturing fluid and crosslinked fracturing fluid.The former one without wall-forming property has constant filtration loss rate with time going while the latter one forms filter cake during the filtration loss,reducing the loss rate as time increases.The results of fracturing fluid filtration experiments show that the cumulative filtration loss is proportional to the square root of time,and the filtration loss coefficient of the crosslinked fracturing fluid is directly proportional to the power index of the fluid pressure,and proportional to the inverse of the square root of the polymer concentration.Two kinds of fracturing fluids were used for laboratory hydraulic fracturing experiments,and the results of the experiments show that the filtration loss of the linear gel fracturing fluid is large,forming more complex fractures and even no fracture with the fracturing fluid completely filtered in the sandstone.The overall filtration loss of the crosslinked fracturing fluid is greatly reduced,and the smaller the fracturing fluid filtration coefficient is,the smaller the filtration loss is.Increasing the injection rate can reduce the fracturing fluid filtration loss,which is beneficial to fracture formation.Simple and smooth tensile fractures can be formed in reservoirs with different permeability by using crosslinked fracturing fluid.Based on the ABAQUS finite element software,a three-dimensional fluid-solid coupling mechanical model was established in which the variation law of fracturing fluid filtration coefficient obtained from the experiment was taken into consideration to perform numerical simulation of unconsolidated sandstone fracturing.Simulation results show that the average efficiency of linear fracturing fluid is 20.92%,and that of the crosslinked fracturing fluid is 60.90%.A large amount of linear fracturing fluid loss entered the reservoir,resulting in a small internal and external pressure difference.Besides small scale and complex patterns,the fracture is accompanied by significant shear fracture,increasing the risk of height containment failure.The crosslinked fracturing fluid has low filtration loss,causing a large pressure difference between the inside and outside of the fracture,which is beneficial for initiation and propagation of planar tensile fractures.The fracture,simple and flat,is hardly accompanied by shear fracture.