Model of Hydraulic Fracture Initiation from the Notched Open Hole

Abstract

In stimulating tight carbonate formations, the propagation of multiple transverse fractures is highly desirable to contact as much matrix as possible. The application of this method to openhole well environments is challenged by the dominating impact of hoop stresses in the near-wellbore vicinity rather than far-field stress in the producing layer. As a result, even if the open hole is drilled in the direction of minimal horizontal far-field stress, there is a high probability that hydraulic fractures initiate longitudinally and then turn to the preferred fracture plane, creating undesired tortuosity.One of the approaches towards controlling both the position and direction of fracture initiation is to cut notches in the wellbore wall at specified positions. As pressure increases during fracturing, those notches can locally eliminate the influence of the wellbore hoop stress and develop high tensile stress concentrations initiating transverse hydraulic fractures at lower pressures.A theoretical model is proposed herein that aims to predict the position, orientation, and pressure at which a fracture initiates. In the model, the 3D stress state around wellbore and notch(es) is analyzed using the brittle fracture criteria. In the numerical implementation, the stresses are efficiently resolved using the boundary element method. The model is used to interpret published laboratory data on fracture initiation including those from hydraulic fracturing block tests. It is shown that the conventional maximum tensile stress (MTS) criterion fails to reproduce the observed trends in initiation pressure and fracture orientation. The nonlocal modification of the MTS criterion based on the stress averaging technique (SAMTS), reveals a good match with initiation pressures in simplified tests. When applied to hydraulic fracturing block test data, SAMTS captures the observed fracture orientations while overestimating the absolute pressure values. The discussion of possible reasons for that overestimate and the way forward concludes the paper.

Simulating fully 3D non-planar evolution of hydraulic fractures

“…Therefore, in the problems of elasticity the BEM has gained popularity because of its boundary-only discretization that reduces the dimensionality of the problem (Rizzo 1967). The Conventional BEM (Rizzo 1967) can be implemented for the problems of fracture initiation from the cavity, which is confined by some surface S * without any fractures (Alekseenko et al 2013;Aidagulov et al 2015;Briner et al 2015a, b). However, using the conventional BEM to collocate the coincident points on the opposite crack surfaces produces a singular system of algebraic equations.…”

“…The key point of the experimental investigation (Chang et al 2014) was to obtain the conditions of two possible scenarios: one longitudinal fracture propagation or multiple transversal fracture propagation. The simulation of fracture initiation is described in Aidagulov et al (2015) and here the fracture propagation is simulated. The key point of the numerical investigation was to show that the model is able to predict the fracture propagation direction correctly.…”

“…It consists of the wellbore, two circular notches and two incipient fractures obtained by solving the initiation problem with the original computational domain (Fig. 38a) (Aidagulov et al 2015). The parameters of the problem are: E = 20 GPa, ν = 0.2, σ ∞ x = 20.7 MPa, σ ∞ y = 15.5 MPa, σ ∞ z = 24.1 MPa, σ c = 5.2 MPa, K I c = 1 MPa √ m, Q in = 0.5 cm 3 /s, notch depth is equal to the wellbore diameter.…”

“…The solution of 3D fracture initiation for the different cases is presented in Esipov et al (2011a), Alekseenko et al (2013), Aidagulov et al (2015). It is shown that the fracture can initiate from a wellbore, an intersection between the wellbore and a perforation, and from the perforation itself.…”

First Application for a Sequenced Fracturing Technique to Divert Fractures in a Vertical Open Hole Completion: Case Study from Saudi Arabia

“…It was further demonstrated that two independent transverse fractures had initiated from two notches placed in a single open hole horizontal wellbore. Furthermore, Aidagulov et al (2015) developed a model that predicts the orientation, position, and pressure at which fracture will initiate from the notched open hole, which built the confidence needed to perform a hydraulic fracture treatment in a high stress area with pressure limitations due to the completion.…”