Modeling of Coupled Flow and Geomechanics for Fault Slip, Wellbore Stability, Fracture Propagation and its Field Application
Abstract
Coupling between fluid flow and geomechanical responses such as deformation and failure is essential to simulate potential damage in subsurface environments and ground surface structures. This study using coupled multiphase flow and geomechanics simulation investigates the effects of mechanical failure such as fault reactivation and slip with fluid injection in geological storage operations, wellbore stability issues stemming from depressurization in methane hydrate deposits, and fracture propagation in shale reservoirs. To solve the aforementioned coupling problems, there are two solution approaches, fully-coupled and sequential methods. In this study, we take the sequential method to make use of existing individual simulators and minimize coding efforts. We employ the fixed stress split due to its superior behavior in stability and convergence even for strongly coupled problems. The simulator packages for flow and geomechanics with fixed stress sequential scheme are validated separately with the analytical solution for the McNamee and Gibson problem for each application.
The main body of this study starts with a field application of CO2 injection into a pilot-scale site for geological storage to investigate fault reactivation and slip. We develop a simulation tool by coupling a multiphase flow simulator with an existing mechanics simulator to model the fault slip and estimate the magnitude of seismic events dynamically with injection induced pressurization and fault strength. We design a fault structure with a fault core and surrounded damage zones. Zero-thickness interface elements are employed to represent the fault core with an assumption that the fault is reactivated only along the fault core. In this study, we test several cases with different injection scenarios (e.g., in-situ stresses and reservoir pressure) to estimate the impact of pore pressure on fault reactivation and its post-failure behavior.
An attempt is also made to supplement the capabilities of coupled flow and geomechanics simulation by using it to develop a surrogate model using a U-Net based Convolutional Neural Network which can predict the distribution of displacement, pressure, and saturation fields based on permeability, injection rate, and time inputs for CO2 injection during a geological storage operation. The work presented here uses multiple realizations of permeability distributions generated using Random Gaussian distribution and varying injection rates to train a surrogate model which on validation can predict the temporal variation of dynamic properties at regions of interest with reasonable accuracy. The idea is to show how such a workflow is indeed feasible and to build a roadmap for extending the application of a similar workflow that will be capable of predicting other properties like fault reactivation and its shear displacement considering the complexities of the system.
Two-way coupled flow and geomechanics can also play a critical role in the analysis of wellbore instability during production from methane hydrate-bearing sediments. The dissociation of hydrate deposits (decrease in solid-phase saturation) directly influences the stiffness of the formation, which also leads to problematic deformation of the reservoir, especially near the wellbore. The tightly coupled sequential approach with zero-thickness interface elements (discontinuous surface) allows us to consider both confining stress and slippage phenomena. With field data from the PBU-L106 site in Alaska and UBGH2-6 site in Ulleung Basin, South Korea, axial stress profiles along the wellbore under depressurization-based gas production are investigated with sensitivity analysis with several sets of properties of interface elements between wellbore and formation to ensure safe and continuous production. The results can be used as preliminary results for the safety design of the wellbore.
Numerical methods of hydraulic fracture propagation have been actively developed to model geometry of the hydraulic fractures for forecasting the cumulative production and optimizing field development strategy. We propose a numerically stable sequential method for all-way coupled geomechanics and flow simulation in discrete fractured systems. First, we study numerical stability and accuracy for a numerically stable sequential approach between multiphase flow and hydraulic fracturing geomechanics. The proposed sequential (iterative) method is motivated by the fixed-stress sequential method used for poromechanics problems. Furthermore, we investigate the physical responses from multiphase flow on fracture propagation numerically, considering both fluid compressibility and the fracture volume change induced by poromechanics. We focus on investigating the vacuum area and the dry zone for not only a static fracture case but also hydraulic fracture propagation.
Subject
Coupled Flow and GeomechanicsGeological Storage of Carbon Dioxide
Deep Learning
Post-failure behavior
Fracture propagation
Citation
Yoon, Sangcheol (2021). Modeling of Coupled Flow and Geomechanics for Fault Slip, Wellbore Stability, Fracture Propagation and its Field Application. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /195431.