A numerical model to simulate fracture network induced by hydraulic fracturing for 3D shale gas reservoir with geo-stress interference

Abstract EN

In the two-dimensional simulated reservoir volume (SRV) model, the position of natural fractures is determined only by the approaching angle, and it is only required to consider the influence of horizontal principal stress in its failure criterion.

In the three-dimensional shale reservoir, the position of natural fracture needs to be determined by the dip angle and approaching angle while considering the influence of vertical and horizontal principal stress in its failure criterion.

In addition, the simultaneous opening of several hydraulic cracks will lead to the change of geo-stress, thus causing the change of the conditions of the natural fracture and shear failure by the induced tress.

in order to analyze the generation of three-dimensional fracture network, this paper establishes a mathematical model based on elastic mechanics, three-dimensional rock failure criterion, full permeability tensor, and material conservation equation.

firstly, the finite element method model of geological stress caused by fracturing is established based on the crack propagation model, and the finite difference method module based on three-dimensional fluid diffusion control equation and full permeability tensor is used to solve the reservoir pressure distribution.

Then, the three-dimensional rock tensile and shear failure criteria are used to determine whether any grid units are destroyed.

once a grid has occurred in any form of rock failure criterion, the permeability of the corresponding grid unit will also change due to the change of natural fracture opening.

finally, the stimulated reservoir volume is represented by the region of increased permeability.

this paper presents the sensitivity analysis of the multi-factor after using the micro-seismic data to validate the numerical model, including the influence of injected fluid volume, natural fracture approaching angle, dip angle and horizontal principal stress difference on the size of SRV (shape, size, border width, length, etc.).

results show that, compared to without the induced stress, the tensile failure volume decreases and the shear failure volume increases, while the total SRV increases due to the induced stress.

when the vertical principal stress in the three-principal stress is maximum, the length of the SRV increases with the increase of natural fracture approaching angle, dip angle and horizontal principal stress difference, but its width decreases.

Its width and length increase at the same time only with the increase volume of injected fluid; the size of SRV increases with the increase of the injection fluid volume and the natural fracture dip, but decreases with the increase of the natural fracture approaching angle and horizontal principal stress difference.