Study On Stress Shadow Characteristics And Fracture Propagation Law In Multi-cluster Perforation Hydraulic Fracturing

The application of hydraulic fracturing technology in unconventional reservoirs will change the stress field and seepage field of surrounding rock.There are risks of geological and environmental disasters such as groundwater pollution,fault activation and seismic activity.The law of hydraulic fracture propagation is one of the keys to study the above problems.With the application of multi-cluster perforation fracturing technology in horizontal Wells,multi-cluster fracturing leads to competitive propagation among fractures.And the law of its evolution is more complex.Therefore,mathematical and numerical models of multi-cluster perforation hydraulic fracturing are established in this paper through theoretical modeling and numerical simulation.Using these models,the spatial distribution and evolution characteristics of hydraulic fracturing stress shadow were studied,and the spatial propagation law of multi-cluster hydraulic fracturing fractures and its influencing factors were explored.Ultimately,this will drive the safe development of unconventional oil and gas.The main research results of this paper are as follows:(1)The numerical model of multi-cluster perforation hydraulic fracturing and the numerical solution method are established.A fracture-pore flow model coupled with wellbore flow was developed and implemented in a three-dimensional numerical model to simulate multi-perforating hydraulic fracture propagation.Using step by step iteration coupled solution strategy,to ensure that the calculation accuracy and significantly reduce the difficulty of solving.The effectiveness of the model in simulating hydraulic fracture propagation,fracture reorientation,and transient fluid leak-off was verified.The new numerical model also conforms to the conservation of mass in the process of flow distribution.(2)The spatial distribution and evolution characteristics of hydraulic fracturing stress shadow are revealed.The research shows that the stress shadow of hydraulic fracturing will cause the change of the magnitude and direction of stress field.In the region perpendicular to the fracture plane,the minimum horizontal stress increases.On the contrary,it decreases at the front end of fracture propagation.The area away from the crack,the smaller the stress deflection angle.The X and Y coordinates of the maximum stress deflection point are related to fracture propagation and fracture volume change,respectively.The stress shadow varies continuously during the fracturing and after shut-in.The degree and range of stress shadow increased with the fracturing operation and decreased after shut-in.There is a superposition effect of hydraulic fracturing stress shadow,which increases with the increase of fluid injection rate or the decrease of fracture spacing.The stress shadow superposition effect can cause the fracture to propagate in direction.The stronger the superposition effect,the greater the degree of steering.(3)The spatial propagation law of multi-cluster perforation hydraulic fracturing and its influencing factors are revealed.The research shows that the multi-cluster perforation hydraulic fracturing fractures may appear uneven and asymmetrical propagation patterns.The propagation is influenced by many factors,such as fluid leak off,injection rate,perforation spacing,perforation friction,horizontal well inclination,layered heterogeneity,reservoir heterogeneity,and strata inclination.Compatibility of the stress shadow,fluid leak-off,and operational parameters(injection rate,fluid viscosity,and perforation spacing)is the pre-condition for generating an evenly propagated fracture pattern.The increase of horizontal well inclination will enhance the stress shadow effect,resulting in uneven fracture propagation.Layered heterogeneity affects the vertical propagation of hydraulic fracturing fractures and the stress distribution of cap rock.Reservoir heterogeneity and strata inclination have significant influences on the symmetry and equality of the cluster fracture pattern.The uneven propagation of hydraulic fracturing fractures may lead to the failure of cap rock,and there is a good correlation between them.Increasing the spacing of perforations,decreasing the number of perforations and appropriate injection rate can promote the uniform propagation of hydraulic fracturing fractures and reduce the possibility of cap rock failure.