Modelling Near-Wellbore Hydraulic Fracture Branching, Complexity and Tortuosity: A Case Study Based on a Fully Coupled Geomechanical Modelling Approach
Authors: Ferguson W. , Richards G. , Bere A. , Mutlu U. , Paw F. 
Published: January 2018

This paper presents a 3D advanced near-wellbore coupled flow-geomechanical model that simulates all phases of the wellbore construction and stimulation in order to quantify near-wellbore hydraulic fracture branching, complexity and tortuosity. The model integrates wellbore drilling, completion system installation, perforation and stimulation stages in a single simulation.

The approach is based on the finite element methodology that allows coupling between the injection fluid flow and the geomechanical behaviour. The rock formation is represented by a poro-elasto-plastic model that can honour mixed mode fracturing through the utilization of an advanced constitutive material model. This includes a unified compaction, tensile and shear failure envelope that is calibrated against supplied laboratory data. Fluid flow coupling is single phase where the flow path is governed by localised damage and associated conductivity changes. Hydraulic fracture itinerary is controlled entirely by the material and stress state and can follow any arbitrary path with potential branching as dictated by the combined material and stress states. Simultaneous fracture propagation from multiple perforations with stress shadowing can therefore be achieved.

Prior to stimulation, drilling of the wellbore and subsequent creation of the perforations leads to a complex near-wellbore stress state due to the redistribution and reorientation of stress. Upon stimulation, models demonstrate significant fracture branching complexity, due to factors such as near-wellbore stress perturbation, mixed-mode shear and tensile damage, in-situ stress anisotropy and cement bond strength as the fractures propagate from individual perforations. For multiple perforation cases and during the early stages of stimulation, stress shadowing suppresses tensile stresses at the inner perforations, leading to preferential propagation from the outer perforations. In particular, dominant, transverse, elliptical fractures initially propagate from the outer perforations and some curving/tortuosity of the fractures is observed. Longitudinal fractures, oriented perpendicular to the outer fractures, propagate from the inner perforations and coalesce, forming an "H-shaped morphology". Fractures also spread from the base of the perforations and travel along the wellbore axis further contributing to the complexity. Sensitivity simulations demonstrate that near-wellbore fracture complexity and tortuosity increases with both a near-isotropic in-situ stress state and with a weak cement bond.

The novelty of this fully coupled advanced hydraulic fracture model is in the ability to (i) incorporate all stages of wellbore construction (from drilling to perforation to cementing), (ii) take into account mixed-mode fracturing (iii) include near-wellbore and perforation stress perturbations within the framework of an advanced poro-elasto-plastic constitutive material model to simulate fracture coalescence, complexity, tortuosity and branching from multiple perforations.

The simulation presented can contribute to a fuller understanding of the pressure drop between well and advancing fracture, and also the shape, size and orientation of the initial propagating fracture. In terms of operational benefits, this technology allows operators to design optimum perforation patterns based on the stress and material state to ensure pump pressures are kept to a minimum and fractures propagate along the desired path.