Date of Award

January 2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Vamegh Rasouli

Abstract

Hydrocarbon production from unconventional plays, including shale oil and gas, has begun in the USA since nearly two decades ago and experienced a rapid growth. Horizontal drilling and multistage hydraulic fracturing (HF) are the prime technologies in development and stimulation of unconventional reservoirs. Open hole (OH) and cased hole (CH) are the primary completion methods used in unconventional reservoirs. OH compared to cased hole (CH) completion is more desired as it requires less operation cost and time while it results in more production due to larger area of wellbore exposure to the formation. However, OH completion is susceptible to near wellbore issues such as wellbore stability, tortuosity and proppant screen out. Both completion methods have been practiced in field applications. This research is focused on studying the HF initiation and near wellbore propagation in OH condition. In OH fracturing, the natural fractures intersecting the wellbore wall are with different orientations and sizes and may be closed or open with different apertures. After fluid injection into the wellbore, these cracks will compete against each other to receive the fluid and act as the initiation point of fracture. Therefore, we may observe transverse fractures which are perpendicular to the wellbore axis, or axial (longitudinal) fractures parallel to the wellbore axis or a combination of both. The weakest fracture will be the initial point of fracture initiation and propagation. In this study we review different analytical models to determine which parameters affect fracture initiation and near wellbore propagation and which one may have the main impact. We use data from the Bakken shale formation in North Dakota for modelling and simulation. This knowledge will be used to design a small crack or fracture, which is known as notch, at the point of interest along the OH section, to dominate other existing fractures xiii and be the point of fracture initiation. We also use lattice numerical simulations, which is a particle based model to simulate a number of cases and compare the results with analytical solutions. The results of this study indicated that stress anisotropy and notch orientation and dimension are the most important parameters that dominate the fracture initiation point and type of fractures propagating (i.e. transverse or axial). The next parameters include formation properties as well as fluid injection rate and viscosity. When the notch size is small, usually axial fractures are dominant, however, beyond a certain notch size, transverse notch will initiate and propagate. The notch size at the cross over point is a strong function of stress anisotropy and moves to the larger notch sizes when stress anisotropy reduces; to the extent that in isotropic stress condition regardless of the size of the notch, no transverse fracture will initiate. The simulation results showed how, in case of axial notch, the base length of the notch along the wellbore axis, plays an important role in fracture initiation pressure. The results indicated that the larger the notch size the lower the initiation pressure and easier for fracture to propagate. If the notch is not along the preferred plane of propagation, after moving away from near wellbore drilling induced zone, the fracture tends to reorient itself to align to the preferred direction. Simulation of multiple axial and transverse fractures and random fractures, similar to real field cases, showed that the initiation pressure increases as the number of fractures increases and that following the knowledge obtained from this study we can determine the most likely fractures that will serve as the fracture initiation point. This conclusion suggest that the near wellbore fractures should be picked up accurately using the image logs and other tools in order to analyze them using the workflow presented in this research to design the geometry of the notch that will dominate existing fractures for multi-stage HF operation.

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