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OpenGeoSysCitation for published version:Kolditz, O, Bauer, S, Bilke, L, Bottcher, N, Delfs, JO, Fischer, T, Gorke, UJ, Kalbacher, T, Kosakowski, G,Mcdermott, CI, Park, CH, Radu, F, Rink, K, Shao, H, Shao, HB, Sun, F, Sun, YY, Singh, AK, Taron, J,Walther, M, Wang, W, Watanabe, N, Wu, Y, Xie, M, Xu, W & Zehner, B 2012, 'OpenGeoSys: an open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes inporous media', Environmental Earth Sciences, vol. 67, no. 2, pp. 589-599. https://doi.org/10.1007/s12665-012-1546-x

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OpenGeoSys: An open source initiative for numericalsimulation of thermo-hydro-mechanical/chemical (THM/C)processes in porous media

O. Kolditz · S. Bauer, L. Bilke, N. Bottcher, J.O. Delfs, T. Fischer,U.J. Gorke, T. Kalbacher, G. Kosakowski, C.I. McDermott, C.H. Park,F. Radu, K. Rink, H. Shao, H.B. Shao, F. Sun, Y.Y. Sun, A.K. Singh,J. Taron, M. Walther, W. Wang, N. Watanabe, Y. Wu, M. Xie, B. Zehner

Abstract In this paper we describe the OpenGeoSys(OGS) project, which is a scientific open source initia-tive for numerical simulation of thermo-hydro-mecha-nical-chemical (THMC) processes in porous media.The basic concept is to provide a flexible numeri-cal framework (using primarily the Finite ElementMethod (FEM)) for solving multi-field problems inporous and fractured media for applications in geo-science and hydrology. To this purpose OGS is basedon an object-oriented FEM concept including a broadspectrum of interfaces for pre- and post-processing.The OGS idea has been in development since themid eighties. We provide a short historical note aboutthe continuous process of concept and software de-velopment having evolved through Fortran, C, andC++ implementations. The idea behind OGS is toprovide an open platform to the community, outfittedwith professional software engineering tools such asplatform-independent compiling and automated bench-marking. A comprehensive benchmarking book hasbeen prepared for publication. Benchmarking has beenproven to be a valuable tool for cooperation betweendifferent developer teams, e.g. for code comparisonand validation purposes (DEVOVALEX and CO2BENCH projects). On one hand, object-orientation(OO) provides a suitable framework for distributedcode development; however the parallelization of OOcodes still lacks efficiency. High-performance-computin(HPC) efficiency of OO codes is subject to future re-search.

O. Kolditz(B)Helmholtz Centre for Environmental Research – UFZ / TUDresdenPermoserstrasse 15, D-04318 Leipzig, GermanyE-mail: olaf.kolditz@ufz.de

Keywords porous media · thermo-hydro-mechanical/chemical · open source software · OpenGeoSys · car-bon dioxide storage

1 INTRODUCTION

1.1 Background

Coupled process modeling has been considered in var-ious engineering problems and geo-scientific applica-tions since the computation method was introducedfor problems of soil consolidation, dam constructionand oil/gas field exploration in early 1970. However,substantial progress in experimental and theoreticalstudies regarding the fully coupled effects of tempera-ture, hydraulics and mechanics, as well as chemistry,in fractured porous media was just made in the lasttwo decades due mainly to demands from the per-formance and safety assessment of high-level nuclearwaste repositories. Numerical methods and computercodes have been developed successfully within the in-ternational DECOVALEX project (www.decovalex.com).Meanwhile a wider range of applications associatedwith THMC coupled problems such as geothermalreservoir engineering, CO2 and energy storage, con-struction of underground repositories etc. can be foundin different international conferences, e.g. GeoProc(www.mech.uwa.edu.au/research/geoproc), ComGeo(www.com-geo.org/).

For a long-term performance and safety assess-ment of a nuclear waste repository in a deep geologicalformation, an important issue is to guarantee the iso-lation of an underground repository. To answer thisquestion, solute transport processes under the cou-pled conditions involving mechanical stability, ther-

Accepted for publication in Environmental Earth Sciences and published by Springer Verlag (2012). Cite as: Kolditz, O, McDermott, C & Bilke, L 2012, 'OpenGeoSys: an open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes in porous media' Environmental Earth Sciences. DOI: 10.1007/s12665-012-1546-x

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mal loading from the high-level waste, and chemistryin the groundwater should be predicted numerically.Also, for construction planning of such a complex andthe implementation of experimental data gained fromin situ tests, a multiple process coupled code is re-quired.

Through the rapid development of computer tech-nology, complicated geoscientific problems can be an-alyzed in a coupled manner using modern numeri-cal codes. However, the understanding of the com-plicated coupled processes based on the experimentaldata available and implementation of the developedalgorithm into the numerical codes are major chal-lenge for scientists, which require interdisciplinary andinteractive cooperation.

Quality management is nowadays a standard toolfor production and development to ensure the highquality of a produced result. A numerical code dealingwith the coupled THMC process is a highly compli-cated software product since the different processeshave different characteristic features, e.g. time andspatial scales, nonlinearities, and interaction degreeetc. To maintain a high quality of the developed code,benchmark testing is therefore necessary, especially inthe case that scientists from different disciplines andorganizations are working on the same code. There-fore, code verification and validation of selected testcases are documented during the code development,and finally a benchmarking book for the code devel-opment is produced and quality ensured (1).

1.2 Historical note

Considerable efforts have been made in the past forporous media code development to address above men-tioned problems in geosciences and hydrology, e.g.TOUGH ((2), (3)), STOMP (4), HydroGeoSphere (5),FEFLOW (6), SUTRA (7) (8), DUMUX (9; 10), MIN3P(11), MT3D (12) or in particle hydrodynamics (13). Inthe abstract we describe the continuous developmentof OGS beginning in the eighties.

RockFlow/FEFLOW-F:

In the mid eighties there was a request by the Fed-eral Institute of Geosciences (BGR) to the Institute ofHydromechanics (University of Hannover) concerningthe development of a simulation program for fracturedrock. The idea of RockFlow (RF) was then born andthe development of a computer code based on multi-dimensional FEM in order to represent flow processesin complex geological structures. At the same time atthe Academy of Sciences (Chemnitz) the FEFLOW

code was being developed for density-dependent flowprocesses in porous media (14) (15). The pioneeringwork of RF-1 was done in a series of doctoral disserta-tions (16), (17), (18), (19). Both codes FEFLOW andRockFlow were implemented with FORTRAN at thattime.

The next stage in the early nineties was related tothe coupling of the individual RF-1 modules throughfile interfaces and the improvement of computationalefficiency, e.g. by introducing an iterative equationsolver. RF-2 was successfully used in several appli-cation projects in the fields of waste deposition andgeothermal energy (20),(21). A scientific ”market” forRF in Applied Geoscience was opened.

RockFlow-C:

It was soon determined that the coupling of the dif-ferent RF modules via file interface was inefficient.Moreover, for the use of grid-adaptive methods, dy-namic data structures were necessary. Consequently,in the late nineties a complete re-organization of RFwas started. The implementation of RF-3 was startedin C (22), (23). Major research topics of the RF groupwere multi-phase flow (24), grid adaptation (25), re-active transport (26), and deformation processes (27).Besides the numerical parts, geometric modeling andmeshing methods became more and more importantfor real-world applications (28), (29).

GeoSys/RockFlow-C++:

Due to the increasing functionality, the RF code be-came more and more sophisticated and difficult tohandle. Consequently, the introduction of object-orientedmethods was necessary. RF-4 or now GeoSys was (again)completely re-designed and rewritten in C++ (30),(31). Several doctoral theses have been completed inthe fields of geotechnical simulation (DECOVALEXproject, (32), (33), (34)), contaminant hydrology (Vir-tual Aquifer project, (35), (36), (37)), geothermal reser-voir modelling (Urach Spa project, (38)). Alongsidecomputational mechanics, progress had been made inthe pre-processing for numerical analysis (39), (40),(41). First GeoSys/RockFlow habilitations had beencompleted (42), (43), (44). As mentioned in the begin-ning, it is impossible to cite everything. Other impor-tant works during the Tuebingen time are e.g. (45),(46), (47) concerning particle tracking, coupled hy-drosystems, and reactive transport simulation.

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problems requires the development of interfaces foradvanced pre- and post-processing purposes, e.g. forgeometrical modeling, meshing and visualization (sec.2.2).

2.1 Object-Oriented (OO) FEM

Fig. 1 shows the mathematical framework to be solvedwhen dealing with THMC processes in porous media.

– T process: Heat transport in multiphase systemsincluding phase changes (e.g. evaporation, conden-sation, latent heat),

– H process: Non-isothermal multiphase flow of liq-uids and gases as well as supercritical fluids,

– M process: Non-isothermal elastic and in-elasticdeformations,

– C Process: Multi-componental mass transport aswell as bio/geochemical reactions.

The resulting balance equations for mass, momentumand energy conservation have to be completed by cor-responding equations of state (EoS) and constitutivelaws (e.g. plasticity and creep of clay and salt rock, re-spectively). The tetrahedron in Fig. 1 illustrates thestrong degree of coupling between the different pro-cesses which requires adequate numerical methods forcoupling partial differential equations.

The general idea behind object-orientation of pro-cesses is that the basic steps of the solution proce-dure: calculation of element contributions, assemblyof equation system (including treatment of boundaryconditions and source terms), solution of the equa-tion systems, linearization methods and calculationof secondary variables, are independent of the spe-cific problem (e.g. flow, transport, deformation pro-cesses). The process (PCS) class provides basic meth-ods in order to solve a PDE in a very general way.Fig. 2 depicts the concept of object-orientation forthe finite element method (OO FEM) implementedin OGS (63). The PCS object is designed to managethe complete solution algorithm in order to build theglobal equation system (EQS). In fact, the PCS object’only’ administrates references to geometric (GEO)objects (points, polylines, surfaces, volumes); MSHobjects (mesh nodes, elements and mesh topology),node-related data such as initial (IC) and boundary(BC) conditions as well as source terms (ST); mate-rial data of porous media (fluid (MFP), solid (MSP),medium (MMP) and chemical (MCP) properties); pa-rameters of the different numerical methods (NUM).

2.2 Pre- and Post

Professional pre- and post-processing methods andtools have become more and more important for nu-merical analysis as the complexity of models, as well asthe availability of high-resolution data, has increasedsignificantly. OGS is trying to address these challengesin different ways.– Development of an OGS data explorer for visual

data processing (Fig. 3),– Providing interfaces to a large variety of special-

ized software and common interfaces (Fig. 4),– Utilizing scientific visualization for model devel-

opment, computational steering and outreachingresearch results (Fig. 12),

– Using platform independent software-engineeringtechnologies for distributed code development andmaintenance,

– Fostering open source activities - which is a basicphilosophy behind the OGS project.The user Interface of OGS Data Explorer (OGS-

DE) is depicting the integration of a typical data setfor hydrological modeling in Fig. 3. A raster file andthe boundary of the model region have been importedinto the software along with boreholes and precipita-tion data. Also shown are detailed views of a timeseries curve and a borehole stratigraphy from the in-vestigation site in the Middle East. In addition to typ-ical GIS tools, based on data integration OGS-DE cangenerate models or numerical analysis (e.g. groundwa-ter models) and represent the computational results inthe geographical / geological context. Graphical meth-ods are based on VTK and are suitable for 3D visual-ization.

Fig. 4 shows the implemented interfaces for data-input (top row), data-output (bottom row) and na-tive OGS files. Native files are currently in the pro-cess of being converted to XML to ensure easy val-idation of files and conversion to other file formats.OGS supports many standard file formats such asGeoTiff andimage formats for raster data, as well asESRI shape files for vector data. In addition, a numberof hydrological and geological software formats canbe imported (e.g. GMS, Petrel, GOCAD) as well asthe scientific open source data format netCDF whichmight be very useful for code comparison purposesand for 3D data visualization. The native file formatsof our software are largely compliant with XML speci-fication. While taking into account existing standardssuch as the GML-standard by the Open GeospatialConsortium or Google’s KML, the file format used byOGS is much more compact for geographical informa-tion. It also includes additional information needed

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Fig. 3: Outline of the OGS graphical user interface for data exploration and integration (OGS-DE)

Fig. 4: Overview of OGS interfaces for data import and 3D visualization

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Fig. 5: THM modeling in a tunnel system (Visualiza-tion by B. Zehner)

would take - in the worst case - until contaminantscould appear in the biosphere. Modeling is an impor-tant tool for risk assessment.

Fig. 6 depicts the application area - carbon captureand storage (CCS). The question is how to safely de-posit CO2 from power plants by liquefying and inject-ing it into the subsurface for long-term storage. Twobasic concepts for geological storage are under discus-sion: depleted gas reservoirs and deep saline aquifers.After many years of operation numerous former gasreservoirs are depleted. These reservoirs are in an under-pressurized status and can take up large volumes offluids. Keeping the reservoir under-pressurized, im-pervious cap rocks and borehole sealing are impor-tant considerations for storage. THM/C modeling isrequired in order to calculate possible fluid storagecapacity, to assess risks and to better understand thehighly coupled processes in the CO2 injection area aswell as their consequences for optimal storage con-cepts (65).

Fig. 7 shows the application area geothermal en-ergy, which is one of the alternative future energyresources under consideration. So-called shallow anddeep geothermal systems can be exploited. Shallowsystems are already commercially used, e.g. for heat-ing purposes. Deep geothermal reservoirs can be usedfor electric power production as high temperatures upto 200řC can be produced. THM/C modeling is re-quired to design these geothermal power plants, e.g. inorder to optimize production efficiency and reservoirlifetime. The significant cooling of the reservoir dueto fluid reinjection gives rise to thermo-mechanical ef-fects which need to be controlled in order to avoidreservoir damage (52).

Fig. 6: Subsurface reservoir for CO2 storage

Fig. 7: Simulated temperature field of water reinjec-tion into a geothermal reservoir

3.2 Hydrology

The second application area for coupled process sim-ulation is hydrology (66). River basins or catchmentsare also subject to THM/C coupled processes, but in-clude however a completely different range of ther-modynamic conditions than deep geological systems.Hydrological processes are very complex to describeas they vary highly in time and space. The evaluationof groundwater recharge is vital to a sustainable waterresources management (so called safe yield). To thispurpose, i.e. the understanding of small scale phenom-ena such as root / soil water interaction is of tremen-dous significance (53). Typically groundwater mod-

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– Continuous contributions to the repository,– Software-engineering concept, coding conventions,– Participating in code debugging, providing new

benchmarks for code testing,– Organizing of training courses, teaching activities,– Joint research activities, joint publications (scien-

tific impact is most important),– ... unexpected ideas to make the OGS initiative

more attractive.

Benchmarking has been recognized as an efficienttool for scientific collaboration. Recently, PNNL, LBNL,UFZ and others have begun establishing workshop se-ries to foster the benchmarking idea: setting up testcases with increasing complexity for method devel-opment and code comparison (Fig. 11). In additionto representing model complexity, one of the key ef-forts is to develop codes which are suitable for mod-ern HPC platforms such as PetaFlop supercomputers.It has been realized that those challenges are beyondsingle team capabilities. Some of these on-going initia-tives are e.g. HydroBench (for hydrological concepts),CO2BENCH and CO2FRAME (for CO2 storage).

Fig. 11: International benchmarking initiative

Acknowledgements

We acknowledge the support of the OpenGeoSys projectby following institutions:– Academy of Sciences (former GDR)– BGR Hannover– University of Hannover– Deutsche Forschungsgemeinschaft (DFG)– Federal Ministry of Education and Science (BMBF)– University of Tubingen– Helmholtz Center for Environmental Research (UFZ)– Technische Universitat Dresden

The authors are very grateful to Alissa Hafele forlanguage editing this paper.

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