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A radial well has a unique wellbore configuration. Fracture propagation in radial well deflagration fracturing is studied rarely. The mechanism of interaction between deflagration fractures, natural fractures, and micro-fractures is still unknown. Based on continuum mechanics, damage mechanics, and variational principles, a numerical model of fracture propagation in deflagration fracturing is established with the Hamilton principle and phase-field fracture theory. The effects of horizontal principal stress difference, natural fracture distribution, and micro-fractures around the wellbore on fracture propagation in deflagration fracturing are studied. First, when no natural fractures are developed around the radial well, fractures are initiated at both ends of the radial well. Second, when there are three natural fractures around the radial well, the created fractures have the morphology of shorter fractures in the middle and longer fractures on both sides under stress interference mechanisms. Third, a larger density of natural fractures causes obvious stress superposition, changes the initiation points of radial wells and fracture morphology, and increases fracture width and reservoir stimulation volume. Fourth, as the micro-fractures increase, their interference and induction effects on deflagration fractures are enhanced gradually, and the deflection angle of fractures increases by 38.7%. The study provides a reference for optimizing deflagration fracturing in a radial well. [ABSTRACT FROM AUTHOR]

The technology of radial-well-assisted hydraulic fracturing is applied in the stimulation of low-permeability hydrocarbon reservoirs where commercial production cannot be achieved by the conventional fracturing method. Here, a study on the reservoir stimulation effect and the fracture propagation pattern of radial-well-assisted deflagration fracturing was carried out. Based on smooth particle hydrodynamics (SPH), rock mechanics theory, and finite element theory, a numerical model of radial-well-assisted deflagration fracturing was established by integrating the JWL state equation. Research on the effects of the deflagration position, radial well azimuth and horizontal principal stress difference on the fracture propagation was carried out. The results show that the deflagration position, radial well azimuth and horizontal principal stress difference have significant effects on the fracture area in deflagration fracturing. The closer distance from the deflagration position is, the larger the radial well azimuth and the smaller the horizontal stress difference are, leading to a larger fracture area, which is conducive to reservoir stimulation. During fracturing, both shear fractures and tensile fractures are formed. The formation and conversion of shear fractures and tensile fractures are related to the deflagration position, radial well azimuth, horizontal principal stress difference, etc. [ABSTRACT FROM AUTHOR]

There are some major problems in the study of fracture network evolution during multicluster fracturing, such as the simple assumption that the reservoir is homogeneous, without considering the heterogeneity of rock mineral particles. In addition, different viscosities used in fracturing fluid and fluid injection methods can affect fracture propagation. As reservoir stimulation enters the 2.0 era, it is important to study the evolution of multicluster fracture networks based on reservoir heterogeneity to improve the stimulated reservoir volume. For this purpose, a 2D, coupled stress-seepage-damage field multicluster fracturing numerical model was developed in this study, and a globally embedded 0-thickness cohesive element was used to simulate the random propagation of hydraulic fractures. Moreover, the elastic modulus, tensile strength and mixed fracture energy of the cohesive element mesh were randomly assigned using the Weibull distribution probability density function to simulate the reservoir heterogeneity. In addition, the dynamic distribution of the injected fluid rate during multicluster fracturing was implemented based on the Bernoulli equation. The capability of the model was validated with analytical solutions of the Khristianovich-Geertsma-de Klerk (KGD) problem and similar numerical modeling criteria of multicluster fracturing and was used to investigate the geological and engineering factors that influence the evolution of the fracture network. The results show that the fracture network formed by hydraulic fracturing is more complex when the reservoir has significant heterogeneity and the injection pressure has a larger extreme value. As the reservoir becomes increasingly homogeneous, using low-viscosity fracturing fluid does not significantly contribute to fracture network complexity or effectively reduce the extreme value of the injection pressure. Shortening the pad fluid injection time can significantly increase the fracture network complexity, but it increases the extreme value of the injection pressure, which requires a higher pressure tolerance for fracturing equipment. The results of this study have important significance for the parameter optimization of multicluster fracturing. [ABSTRACT FROM AUTHOR]

• Topology structure and occurrence strike of natural fractures are determined based on geo-statistics. • Fracture propagation model in transversely isotropic shale reservoir is established under seepage-stress coupling. • Effect of natural fractures with different topology structures on hydraulic fracture propagation is studied quantitatively. Natural fractures tend to further promote fracture network which is initiated by hydraulic fracturing. However, how natural fractures with various topology structures impact hydraulic fracture propagation in continental shale reservoir is still unclear. With Fullbore Microscan Imager (FMI) and geo-statistics of Chang 7 reservoir in Yanchang Formation, typical natural fractures are classified into three topology structures, which are natural fractures with type I, type II and type III nodes respectively, and the average occurrence strikes of natural fractures with type I, type II and type III nodes are SW260°, SW255° and SW255°. Coupling with seepage field and stress field, a model simulating fracture initiation and propagation is established considering the transversely isotropic property of continental shale reservoir. On this basis, the role of typical natural fractures with topology structure for hydraulic fracture propagation is studied qualitatively and quantitatively by Finite Element Method (FEM) with globally embedded cohesive element. Natural fractures with type II nodes have largest effect on inducing hydraulic fracture propagation, hydraulic fractures can communicate most easily with them, and also the length and velocity of hydraulic fracture propagation are the longest and fastest, so fracture network is the most complex after fracturing in naturally fractured continental shale reservoir with type II nodes, which follows by the effect of hydraulic fracture propagation in naturally fractured continental shale reservoir with type III nodes. In comparison, natural fractures with type I nodes have insignificant impacts on hydraulic fracture propagation. The research results are conductive to fracturing design in continental shale reservoir. [ABSTRACT FROM AUTHOR]

In different geological and engineering conditions, it is important to study the stress interference of multiple fractures and interaction of natural fractures in "well factory" fracturing to improve the complexity of fracture networks. For this purpose, a 2D, coupled stress-seepage-damage field cohesive model with random propagation of hydraulic fracture was developed, and a random function was used to simulate the natural fractures distribution in a real reservoir. Orthogonal test was designed to investigate the geological and engineering parameters impacting the evolution of zipper fracturing and modified zipper fracture networks. The results show that in naturally fractured reservoirs, hydraulic fracture can open natural fractures and promote the formation of complex fracture networks by combined, induced manner. Higher stress differences lead to simpler fracture networks and straighter hydraulic fracture propagation paths; When pre-existing natural fracture is longer, the propagation direction of hydraulic fracture is more likely to be paralleled with natural fracture; However, the stress shadow effect of multiple fractures may potentially lead to failure of some hydraulic fracture initiation. In addition, the fracture initiation sequence of multiple fractures has a significant effect on the evolution of the fracture networks. The results of the study have important significance for parameter optimization of well factory fracturing in naturally fractured reservoirs. [ABSTRACT FROM AUTHOR]