Characteristics And Influence Rules Of Fracture Propagation For Radial Borehole Fracturing In Tight Sandstone Formations

Radial borehole fracturing combines ultra-short radius radial boreholes with hydraulic fracturing and is expected to effectively improve the stimulation of tight oil and gas reservoirs.The radial borehole fracturing design needs to clarify the main controlling factors that affect the fracture initiation and propagation of radial borehole fracturing.Through laboratory experiments,numerical simulation,and theoretical analysis,this paper investigates the fracture propagation of radial borehole fracturing in tight sandstone reservoirs and the influence of the fracture initiation method on fracture propagation.The main work and results are as follows:1.The laboratory experiments of fracturing radial boreholes simultaneously were carried out,and the influence of radial borehole parameters(azimuth,diameter,layout,and vertical distances between adjacent layers of radial boreholes),horizontal principal stress difference,and fracturing fluid injection rate on the fracture geometries and breakdown pressures were studied.The results show that fracturing multi-layer radial boreholes can induce fractures to extend parallel to the radial borehole axes.The special phenomena appear that fractures turn to the directions of minimum horizontal principal stresses gradually after deviating from radial boreholes.The vertical distances between adjacent layers of radial boreholes and azimuths of radial boreholes are the key factors that affect fracture propagation perpendicular to the radial borehole axis.When the azimuths are 30°,increasing the vertical distances to 24 times the diameters of radial boreholes can develop fractures cutting through two adjacent layers of radial boreholes in the rock samples.2.Considering the fluid-solid coupling,the finite element-mesh free method was used to simulate the fracture propagation of fracturing two layers of radial boreholes under reservoir conditions.The influences of azimuths,horizontal principal stress differences,and rock matrix permeabilities on fracture geometries were revealed.The results show that there exists extrusion between the hydraulic fractures of two radial boreholes in the same layer.The extrusion is induced by the radial boreholes and hydraulic fractures.The fracture geometries are controlled by the rectification of a single radial borehole,the extrusion between radial boreholes in the same layer,and the deflection of the maximum horizontal principal stresses.Reducing the horizontal principal stress differences strengthens the inducing of radial boreholes to fractures.3.A method of fracturing multi-layer radial boreholes step by step is proposed,and laboratory experiments of fracturing radial boreholes step by step are carried out.The results show that this method can fracture two sets of radial boreholes distributed in different vertical planes.The results demonstrate that this method is feasible and can solve the problem that not all the radial boreholes are fractured when fracturing fluids are simultaneously injected into them.Besides,the azimuth is the key factor affecting the fracture geometries.When the azimuths are 0° and 30°,the fractures can cut through radial boreholes in different layers within the rock samples.If the radial boreholes with larger azimuths are fractured,it will generate higher breakdown pressures.Fracturing radial boreholes with smaller azimuths first and then radial boreholes with larger azimuths should be selected.4.Numerical simulation of fracture propagation for fracturing two layers of radial boreholes step by step was carried out.The influences of azimuths,horizontal principal stress differences,and rock matrix permeabilities on fracture geometries were analyzed.The fracture geometries of fracturing radial boreholes simultaneously and fracturing radial boreholes step by step were compared.The results show that the hydraulic fractures of radial boreholes in the same layer will attract each other.The fracture geometries are controlled by the rectification,attraction,and deflection.The fracture geometries of fracturing radial boreholes simultaneously or fracturing radial boreholes step by step change with the layouts of radial boreholes.The essence of such changes is that the direction of the extrusion or attraction changes relative to the direction of the deflection.The principle of the adaptability of fracturing radial boreholes simultaneously and fracturing radial boreholes step by step is to take advantage of the extrusion or attraction between radial boreholes to reduce the deflection caused by the maximum horizontal principal stresses.The research can provide theoretical bases for the radial borehole fracturing design when this technology is applied to exploit tight sandstone reservoirs.