Accuracy of fully coupled and sequential approaches for modeling hydro- and geomechanical processes
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ORIGINAL PAPER
Accuracy of fully coupled and sequential approaches for modeling hydro- and geomechanical processes M. Beck1 · A. P. Rinaldi2 · B. Flemisch1 · H. Class1 Received: 27 April 2020 / Accepted: 15 June 2020 / Published online: 12 July 2020 © The Author(s) 2020
Abstract Subsurface flow and geomechanics are often modeled with sequential approaches. This can be computationally beneficial compared with fully coupled schemes, while it requires usually compromises in numerical accuracy, at least when the sequential scheme is non-iterative. We discuss the influence of the choice of scheme on the numerical accuracy and the expected computational effort based on a comparison of a fully coupled scheme, a scheme employing a one-way coupling, and an iterative scheme using a fixed-stress split for two subsurface injection scenarios. All these schemes were implemented in the numerical simulator DuMux . This study identifies conditions of problem settings where differences due to the choice of the model approach are as important as differences in geologic features. It is shown that in particular transient and multiphase flow, effects can be causing significant deviations between non-iterative and iterative sequential schemes, which might be in the same order of magnitude as geologic uncertainty. An iterated fixed-stress split has the same numerical accuracy as a fully coupled scheme but only for a certain number of iterations which might use up the computational advantage of solving two smaller systems of equations rather than a big monolithical one. Keywords Flow and geomechanics · Fully coupled and sequential schemes · Fixed-stress scheme · Numerical accuracy · Geologic uncertainty
1 Introduction Fluid injection into the subsurface is common to different scenarios, often related to the world’s energy consumption which relies heavily on the utilization of the subsurface. Coal, oil, and natural gas account for 81–85% of the world’s primary energy consumption in 2017 [13, 25]. Often, the extraction of these raw materials requires fluid injections into the subsurface: in the context of the production of shale gas through hydraulic fracturing or in form of wastewater disposal from conventional and unconventional oil and gas production. Geological gas storage or geothermal systems also involve the injection of fluids. Injections affect the field of fluid pressures H. Class
[email protected] 1
Department of Hydromechanics and Modelling of Hydrosystems, University of Stuttgart, Pfaffenwaldring 61, 70569 Stuttgart, Germany
2
Swiss Seismological Service, Swiss Federal Institute of Technology, ETH Z¨urich, Z¨urich, Switzerland
as well as the stresses and deformations of aquifers and caprocks. Caprock integrity might be compromised. Thus, the need to understand the coupling between hydraulics and geomechanics is evident. Numerical simulation of coupled hydraulic and geomechanical processes requires the choice of an appropriate coupling scheme. This can be discussed by balancing numerical accuracy and compu
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