Date of Award

January 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Vamegh P. Rasouli

Abstract

Hydraulic Fracturing (HF) is the prime technology for enhanced production from shale plays. Due to their laminations at different scales, shales exhibit transverse isotropic (TI) properties. In most cases the lamination planes are nearly horizontal in shales setting with the symmetric axes being vertical, therefore, shales are referred to as TI vertical (TIV) rocks. This influences significantly the initiation and propagation of the induced fracture during fracturing operation. For instance, accurate estimation of the fracture initiation pressure (FIP) and fracture morphology is essential to ensure optimum hydraulic fracturing design in unconventional reservoirs. However, majority of studies of HF modelling in shales are based on the isotropic assumption to simplify the problem. While some recent studies demonstrate that ignoring the TI properties of shale results in a large error in estimation of stresses and consequently the design of the HF. The anisotropic toughness is a distinct feature of layered formations such as shale, which its understanding is vital especially when it comes to the estimation of hydraulic fracturing initiation pressure and morphology.We first used analytical models to investigate the effect of the anisotropic toughness on the FIP of two fundamental fracture geometries: transverse and axial fractures. The model I stress intensity factor was used to develop the analytical models for FIP at the tip of a fracture emanating from a horizontal wellbore drilled in the direction of minimum horizontal stress (h). The anisotropic toughness values parallel and perpendicular to the laminations were implemented to the estimation of the FIP of both transverse and axial fractures. These analytical models predict the FIP of different fracture lengths and variable local angles along the fracture based on the propagation direction and the fracture geometry (i.e. axial and transverse). The results of the analytical models showed that the anisotropic toughness has a noticeable effect on the FIP. The fracture initiation in anisotropic toughness was shown to have variable initiation pressure with direction and is controlled by the local initiation angle. Consequentely, the fracture becomes more elongated in the direction of the minimum fracture toughness and contained in the direction of the maximum fracture toughness. This study shows that toughness anisotropy has a dominant effect on whether transverse or axial fracture will initiate from an open hole lateral. The fracture with lower FIP is favorable for initiation, hence, we investigated the competition between transverse and axial fractures is in a medium with anisotropic toughness. This knowledge will help engineers to optimize the design of a small notch, which will be the point of interest along the OH section to initiate and dominate other pre-existing fractures. We then, used numerical simulations to model a number of cases of hydraulic fracturing in a medium with anisotropic toughness for variable propagation regimes. Different scenarios of radial and constant height fractures for cases of single and simultaneous propagation of multiple fractures were simulated in anisotropic toughness condition and compared to the isotropic cases. The reliability of the numerical simulator was confirmed against experimental and field data from the literature. The final step, was to simulate effect of toughness anisotropy on a field scale hydraulic fracturing model. We used the data from the Bakken Formation in the Williston Basin in North Dakota, as a field case study. The analytical results were validated using numerical simulations. Numerical simulation results showed that the overall shape of the system of fractures in case of anisotropic toughness is elongated in the direction of minimum toughness (horizontal). The results indicated that the net pressure and the fracture aperture in case of anisotropic toughness is higher compared to the isotropic case. The combined effect of stress shadow and toughness anisotropy was investigated in case of simultaneous propagation of multiple fractures. Results showed that the interaction between the fractures is high in case of toughness anisotropy. Field case hydraulic fracturing model showed to be more complex.

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