Coupled Poroelastic Modeling to Characterize the 4.18-Magnitude Earthquake Due to Hydraulic Fracturing in the East Shale Basin of Western Canada

Recently, the elevated levels of seismicity activities in Western Canada have been demonstrated to be linked to hydraulic fracturing operations that developed unconventional resources. The underlying triggering mechanisms of hydraulic fracturing-induced seismicity are still uncertain. The interactions of well stimulation and geology-geomechanical-hydrological features need to be investigated comprehensively. The linear poroelasticity theory was utilized to guide coupled poroelastic modeling and to quantify the physical process during hydraulic fracturing. The integrated analysis is first conducted to characterize the mechanical features and fluid flow behavior. The finite-element simulation is then conducted by coupling Darcy's law and solid mechanics to quantify the perturbation of pore pressure and poroelastic stress in the seismogenic fault zone. Finally, the Mohr-coulomb failure criterion is utilized to determine the spatial-temporal faults activation and reveal the trigger mechanisms of induced earthquakes. The mitigation strategy was proposed accordingly to reduce the potential seismic hazards near this region. A case study of ML 4.18 earthquake in the East Shale Basin was utilized to demonstrate the applicability of the coupled modeling and numerical simulation. Results showed that one inferred fault cut through the Duvernay formation with the strike of NE20°. The fracture half-length of two wells owns an average value of 124 m. The brittleness index deriving from the velocity logging data was estimated to be a relatively higher value in the Duvernay formation, indicating a geomechanical bias of stimulated formation for the fault activation. The coupled poroelastic simulation was conducted, showing that the hydrologic connection between seismogenic faults and stimulated well was established by the end of the 38th stage completion for the east horizontal well. The simulated coulomb failure stress surrounding the fault reached a maximum of 4.15 MPa, exceeding the critical value to cause the fault slip. Hence the poroelastic effects on the inferred fault were responsible for the fault activation and triggered the subsequent ML 4.18 earthquake. It is essential to optimize the stimulation site selection near the existing faults to reduce risks of future seismic hazards near the East Shale Basin.

Junction zone stability in coaxial wells of different diameters (on the example of the Khanty-Mansi Autonomous District oil field)

The paper considers borehole wall stability in a junction zone of coaxial wells where a borehole of bigger diameter connects with a smaller one. To determine the shapes and character of rock destruction, 3D poroelastic modeling of the stressed state of the rock around the coaxial junction with account for mudcake formation was performed. The geomechanical model considers the anisotropy of the medium’s deformation properties that are characteristic for the coastal-marine reservoirs of Western Siberia. The rock failure is estimated based on the Mohr-Coulomb criterion with account for tensile destruction condition. The paper considers cases of vertical and inclined junctions of a well drilled at a depth of 2 km in sandstone productive pay with known poroelastic anisotropic properties. The stress and pore pressure analysis has been performed for a mud pressure drop range from 1 to 70 atm and coaxial junctions with different combinations of borehole diameters. The safe mud pressure window has been determined for vertical and inclined junctions. It has been found that the rock failure pattern for junction of bigger diameters is, in general, similar to that for smaller diameters with some insignificant differences in the destruction areas shapes. It has also been demonstrated that in vertical junctions, the bottom holes of smaller diameter are more stable to reduced drilling-mud pressure than the mainboreholes, while in the inclined junction it is the mainwellbore that is more stable to increased drilling-mud pressure than the bottom hole.

POROELASTIC MODELING IN STRATIFIED MEDIA ACROSS ALL FREQUENCIES

ABSTRACT. There is considered a layered heterogeneous poroelastic isotropic medium with physical parameters characterized by piecewise constant functions of the depth only. We derive a mathematical algorithm for calculating reflected/transmitted poroelastic waves across all temporal frequencies. To define the frequency effect we use the dynamic permeability expression proposed by Jonhson, Koplik and Dashen; in the time domain, this coefficient introduces order 1/2 shifted fractional time derivative involving a convolution product. The algorithm proposed is based on the formalism introduced by Ursin. The algorithm is tested numerically in a 1D-case. The numerical experiments confirm the effectiveness of the proposed algorithm in identifying the main wave events in both low frequency and high frequency regimes in the reservoir and laboratory scales.Keywords: stratified porous medium, Biot and Biot-JKD models, Ursin’s Formalism.RESUMO. Neste trabalho, consideramos um meio poroelástico estratificado, isotrópico e heterogêneo com parâmetros físicos caracterizados por funções constantes por partes em relação à profundidade. Nós derivamos um algoritmo matemático para calcular as ondas poroelásticas refletidas/transmitidas em todas as frequências temporais. Para definir o efeito da frequência, usamos a expressão de permeabilidade dinâmica proposta por Jonhson, Koplik e Dashen; no domínio do tempo, este coeficiente introduz a derivada do tempo fracionária de 1/2 de ordem de deslocamento envolvendo um produto de convolução. O algoritmo proposto é baseado no formalismo introduzido por Ursin e foi testado numericamente para o caso 1D. Os experimentos numéricos confirmaram a efetividade do algoritmo na identificação dos principais eventos de onda nos regimes de baixa frequência e alta frequência, nas escalas de reservatório e laboratórial, respectivamente.Palavras-chave: meio poroso estratificado; Modelos de Biot e Biot-JKD; Formalismo de Ursin.

Poroelasticity and Fluid Flow Modeling for the 2012 Emilia-Romagna Earthquakes: Hints from GPS and InSAR Data

The Emilia-Romagna seismic sequence in May 2012 was characterized by two mainshocks which were close in time and space. Several authors already modeled the geodetic data in terms of the mechanical interaction of the events in the seismic sequence. Liquefaction has been extensively observed, suggesting an important role of fluids in the sequence. In this work, we focus on the poroelastic effects induced by the two mainshocks. In particular, the target of this work is to model the influence of fluids and pore-pressure changes on surface displacements and on the Coulomb failure function (CFF). The fluid flow and poroelastic modeling was performed in a 3D half-space whose elastic and hydraulic parameters are depth dependent, in accordance with the geology of the Emilia-Romagna subsoil. The model provides both the poroelastic displacements and the pore-pressure changes induced coseismically by the two mainshocks at subsequent periods and their evolution over time. Modeling results are then compared with postseismic InSAR and GPS displacement time series: the InSAR data consist of two SBAS series presented in previous works, while the GPS signal was detected adopting a variational Bayesian independent component analysis (vbICA) method. Thanks to the vbICA, we are able to separate the contribution of afterslip and poroelasticity on the horizontal surface displacements recorded by the GPS stations. The poroelastic GPS component is then compared to the modeled displacements and shown to be mainly due to drainage of the shallowest layers. Our results offer an estimation of the poroelastic effect magnitude that is small but not negligible and mostly confined in the near field of the two mainshocks. We also show that accounting for a 3D fault representation with a nonuniform slip distribution and the elastic-hydraulic layering of the half-space has an important role in the simulation results.