Numerical Modeling of Fracture Complexity With Application to Production Stimulation

Abstract

The commercial exploitation of shale resources requires the design of hydraulic fracture programs to optimize the stimulated reservoir volume. Challenges include the engineering of staging, sequencing, injection volume, and proppant selection. This paper describes the development of a 3-D finite-element model to simulate growth of fracture complexes as functions of rock stress, brittleness properties, and injection conditions. The fracture complex is modeled as a major fracture, which consists of a cohesive crack and microcracks that form with variable continuum mechanical damage. Variations of permeability and porosity affect the continuum mechanical damage, which is used to investigate the fracturing sequence of three stages of injections in a horizontal well and generates three sets of fracture complexes. Inputs include a geomodel, the initial geostress, and pore pressure. Elastoplastic deformation coupled with both porous and gap flows are simulated. The model is also used to investigate the influence of rock brittleness on the fracture complexity. Material brittleness is the product of the elasticity modulus and Poisson’s ratio. Three sets of values of material brittleness were used in the calculation for comparison. The same loads and initial conditions of the first example were used in the model. The results include two sets of distributions of fracture propagation and damage evolution for two different injection sequences, pore pressure distribution after fluid injection stimulation, three sets of numerical results of distribution of fracture, and mechanical damage corresponding to different values of brittleness. Numerical results indicate that adjusting the sequence of multi-stage fracturing significantly influences the stimulated formation volume fractured. The model can be used to design more optimal fracture programs.

Numerical Investigation of Hydraulic Fracture Propagation Based on Cohesive Zone Model in Naturally Fractured Formations

“…Owing to a modification of the CZM, the latest efforts to model hydraulic fracturing based on ABAQUS have introduced the pore pressure node in cohesive elements since ABAQUS 6.12 [14][15][16][17]20,21]. Further, the presence of joint pore pressure nodes among intersection pore pressure cohesive elements makes it possible to model the intersection of fractures.…”

“…A representative numerical method for modeling hydraulic fractures is the finite element method based on the CZM, which considers the effects of the fracture tip process zone and softening. Owing to the advantages of the CZM, a finite model based on the CZM was used to simulate full coupling between the incompressible viscous fluid and rock matrix, which was demonstrated to be relatively accurate in hydraulic fracturing under plain strain conditions [13][14][15][16]. For the strong non-linear coupling in the hydraulic fracturing process, empirical methods and the linear elastic method, which ignore the effects of the plastic zone and softening, still occupy an important position for design and optimization of hydraulic fracture schemes.…”

Comparison of Different Hydraulic Fracturing Scenarios in Horizontal Wells Using XFEM Based on the Cohesive Zone Method

“…When the spacing is increased, a longstanding opening of the fracture mouth can be maintained, which is useful to the injection and placement of proppant for simultaneous fracturing. Unlike the common assumption in previous studies that the trajectories of fractures will propagate in the direction of maximum horizontal principle stress [25,37], with simultaneous fracturing, only the middle fracture follows a direction that is perpendicular to the minimum horizontal principle stress. The side fractures tend to deviate, which is due to the stress interference induced by the middle fracture.…”

Uncertainty Analysis of Factors Influencing Stimulated Fracture Volume in Layered Formation

“…The cohesive zone model (CZM), which overcomes the stress singularity before the crack tip, is widely used in the nucleation and growth of hydro-fractures in porous elastic formations [37,38]. The constitutive relation in the bond zone reflects the separation and slip of traction and fault surface, and CZM can be adopted using the finite element method (FEM) to investigate hydraulic fracture propagation [39][40][41], including the fracture tip material softening.…”