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Numerical Modeling of Hydraulic Fracturing in Weakly Consolidated Sandstones
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- Author / Creator
- Taghipoor, Siavash
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Hydraulic fracturing is a technique to enhance hydrocarbon production by inducing fracture(s) into reservoir rock. A fracture is induced by injecting fluid into the reservoir at pressures greater than the formation breakdown pressure. The fracture(s) geometry, mode, initiation and propagation pressure, and other characteristics, may vary depending on geomechanical conditions such as in situ stresses and the rock’s physical and mechanical properties. Hydraulic fracturing was originally used to stimulate wellbores drilled into brittle hard rocks. These rocks typically behaved like linear elastic material and exhibited low permeability. Recently, there has been interest in stimulating unconsolidated and poorly consolidated formations which possess low shear strength and high permeability. In these cases, the assumption of linear elastic fracture mechanics (LEFM) and small leak-off from fracture walls may not be valid. Laboratory experiments have shown that hydraulic fracturing of weakly/unconsolidated sandstones can occur in the form of shear failure/fractures, (a) tensile fracture(s) or a combination of the two. The tensile fracture’s conductivity is a nonlinear function of the fracture width. Shear failure/fracturing results in dilative deformation, which enhances rock permeability. Shear dilation increases the local stresses and, consequently, increases the fracturing pressure. Most of the current continuum-based numerical models require a predetermined hydraulic fracture direction. Some recent continuum models have been adopted to capture fractures in a general direction, but they either lack a proper tensile fracture-flow law, or do not simulate the development of shear failure/fracture and the interaction between the shear and tensile fractures. Beside continuum-based models, models have been developed based on the discrete element method. These models do not impose the limitations of continuum-based models, but are computationally costly and impractical for large-scale field problems. The main objective of this research is to develop a hydraulic fracture model for weakly/unconsolidated sandstones and combine it with field observations to study the main mechanisms involved and features required for modeling hydraulic fracturing. These include the fracturing direction, fracture modes and their interaction, and fracturing pressure and its variation over time. This proposed numerical model can simulate poroelasticity effects, rock shear failure/fracturing, tensile fracturing, leak-off, and shear-induced permeability variation. This thesis presents a method to implement the cubic law to describe the flow inside a tensile fracture in a continuum-smeared tensile fracture model. Touhidi-Bahgini’s shear permeability model, which describes the shear-induced permeability enhancement of oil sands, is implemented to simulate shear failure. The smeared shear and tensile fracture schemes (including both geomechanical and flow aspects) are implemented to develop the smeared hydraulic fracture model. The model is validated by simulating a series of well tests in oil sands during cold water injection. According to the simulation results of the well tests, at injection pressures below the vertical stress, shear failure governs the reservoir response resulting in a breakdown pressure (to induce a tensile fracture) larger than the maximum in situ stress. Sensitivity analysis illustrate the high sensitivity of the fracturing pressure and length to the minimum and maximum in situ stresses, the mechanical properties of the reservoir sand such as the elastic modulus and cohesion, and the physical properties of the reservoir sand such as the absolute permeability. Propagation pressure is shown to be directly and fracture length is shown to be inversely proportional to the magnitude of the maximum principal stress as larger deviatoric stress would induce more intense shearing and larger dilation around the wellbore and the tensile fracture. Results also show lower propagation pressure and longer tensile fractures for sandstones with higher cohesion. A smaller elastic modulus is found to result in a shorter fracture and lower breakdown pressure but higher propagation pressure. It is also shown that absolute permeability of a reservoir has little influence on its breakdown pressure. However, lower permeabilities tend to lower the propagation pressure and increase the length of the tensile fracture.
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- Graduation date
- Fall 2017
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.