| dc.description.abstract | Hydraulic Fracturing (HF) is an effective stimulation process for extracting oil and gas from
unconventional low-permeable reservoirs. The process is conducted by injecting high-pressure fluids into the ground to generate fracture networks in rock masses and stimulate natural fractures
to increase the permeability of formation and extract oil and gas. Due to the multiple and
coupled-physics involved, hydraulic fracturing is a complex engineering process.
The extent of the induced fractures and stimulated volume and reactivation of natural faults
and fractures are some of the practical issues associated with hydraulic fracturing. Acoustic
Emission (AE) monitoring and analysis are used to probe the behaviour of solid materials in
such applications. The process of elastic wave propagation induced by an abrupt local release
of stored strain energy is known as acoustic, microseismic, and seismic emission (depending on
the context and the magnitude of the event). These emissions can be triggered by material
bifurcation-instabilities like slope slipping, fault-reactivation, pore collapsing, and cracking -
processes that are all categorized as localization phenomena.
The microseismic monitoring industry attempts to relate acoustic emissions measured by
geophones to the nature of the stimulated volume created during hydraulic fracturing. This
process is full of uncertainties and researchers have not yet focused on both explicitly modeling
the process of fracture reactivation and the accurate simulation of acoustic wave propagations
resulting from the localization. The biggest gap in the modeling literature is that most of the
previous works fail to accurately simulate the process of transient acoustic wave propagation
through the fractured porous media following the elastic energy release. Instead of explicitly modeling fracturing and acoustic emission, most previous studies have aimed to relate energy
release to seismic moment.
To overcome some of the existing shortcomings in the numerical modeling of the coupled
problem of interface localization-acoustic emission, this thesis is focused on developing new computational
methods and programs for the simulation of microseismic wave emissions induced by
interface slip instability in fractured porous media. As a coupled nonlinear mixed multi-physics
problem, simulation of hydraulic stimulation involves several mathematical and computational
complexities and difficulties in terms of modeling, stability, and convergence, such as the inf-sup
stability problems that arise from mixed formulations due to the hydro-mechanical couplings
and contact conditions. In AE modeling, due to the high-frequency transient nature of the
problem, additional numerical problems emerging from the Gibbs phenomenon and artificial
period elongation and amplitude decay are also involved.
The thesis has three main objectives. The first objective is to develop a numerical model
for simulation of wave propagation in discontinuous media, which is fulfilled in Chapter 2 of
the thesis. In this chapter a new enriched finite element method is developed for simulation
of wave propagation in fractured media. The method combines the advantages of the global
Partition-of-Unity Method (PUM) with harmonic enrichment functions via the Generalized Finite
Element Method (GFEM) with the local PUM via the Phantom Node Method (PNM).
The GFEM enrichments suppress the spurious oscillations that can appear in regular Finite
Element Method (FEM) analysis of dynamic/wave propagations due to numerical dispersions
and Gibbs phenomenon. The PNM models arbitrary fractures independently of the original
mesh. Through several numerical examples it has been demonstrated that the spurious oscillations
that appear in propagation pattern of high-frequency waves in PNM simulations can
be effectively suppressed by employing the enriched model. This is observed to be especially
important in fractured media where both primary waves and the secondary reflected waves are
present.
The second objective of the thesis is to develop a mixed numerical model for simulation
of wave propagation in discontinuous porous media and interface modeling. This objective
is realized in Chapter 3 of the thesis. In this chapter, a new enriched mixed finite element
model is introduced for simulation of wave propagation in fractured porous media, based on
an extension of the developed numerical method in Chapter 2. Moreover, frictional contact at
interfaces is modeled and realized using an augmented Lagrange multiplier scheme. Through
various numerical examples, the effectiveness of the developed enriched FE model over conventional
approaches is demonstrated. Moreover, it is shown that the most accurate wave results
with the least amount of spurious oscillations are achieved when both the displacement and
pore pressure fields are enriched with appropriate trigonometric functions.
The third objective of the thesis is to develop computational models for the simulation of
acoustic emissions induced by fracture reactivation and shear slip. This objective is realized in
Chapter 4 of the thesis. In this chapter, an enriched mixed finite element model (introduced
in Chapter 3) is developed to simulate the interface slip instability and the associated induced
acoustic wave propagation processes, concurrently. Acoustic events are triggered through a
sudden release of strain energy at the fracture interfaces due to shear slip instability. The
shear slip is induced via hydraulic stimulation that switches the interface behaviour from a
stick to slip condition. The superior capability of the proposed enriched mixed finite element
model (i.e., PNM-GFEM-M) in comparison with regular finite element models in inhibiting
the spurious oscillations and numerical dispersions of acoustic signals in both velocity and
pore pressure fields is demonstrated through several numerical studies. Moreover, the effects
of different characteristics of the system, such as permeability, viscous damping, and friction
coefficient at the interface are investigated in various examples. | en |
