title: Coupled Thermal-Hydraulic-Mechanical (THM) modelling of underground gas storage – A case study from the Molasse Basin, South Germany
Creators: name: Zain-Ul-Abedin, Muhammad
Publisher: name: UNSPECIFIED
Year: 2022

description: Thermal-hydraulic-mechanical (THM) models of gas storage in porous media provide valuable information for various applications. The range of these applications varies from prediction of ground surface displacements, determination of stress path changes, and maximum reservoir pressure to storage capacity for maintaining fault stability and overburden integrity. The study, conducted in collaboration with research institutes and storage companies in Germany, addresses the numerical modelling of geomechanical effects caused by the storage of methane in a depleted gas field. The geomechanical assessment focuses on a former gas reservoir in the Bavarian Molasse Basin east of Munich, for which a hypothetical conversion into underground gas storage (UGS) is considered. The target reservoir is of Late Oligocene age, i.e., the Chattian Hauptsand with three gas bearing layers having a total thickness of 85 m. The reservoir formation is highly porous with an average porosity of 23% and permeability is in the range between 20 mD and 80 mD. The reservoir has produced natural gas from 1958 till 1978 and has been in a shut-in phase ever since. The storage operations require precise understanding of reservoir mechanics and stresses; therefore, the selected methodology helps to analyze these issues in detail. The geomechanical analysis is performed with the help of a state-of-the-art THM model with the following objectives: (1) analyze the variation of principal stress field induced by the field activities (2) analyze the effective stress changes with changing pore pressure in short-term as well as long-term using hypothetical injection-production schedule cases (3) prediction of ground surface displacements over the field, (4) analyze the possible reactivation of faults and fractures as well as the safe storage capacity of the reservoir; and (5) thermal stress changes with injection of colder foreign gas in underground reservoir. The methodology comprises 1D mechanical earth modelling (MEM) to calculate elastic properties as well as a first estimate for the vertical and horizontal stresses at well locations by using log data. This modelling phase provide complete analyses of log, core and laboratory data which leads to detailed 1D MEM of all the wells available for case study reservoir. This information is then used to populate a 3D finite element MEM) which has been built from seismic data and comprises not only the reservoir but the entire overburden up to the earth’s surface as well as part of the underburden. The size of this model is 30 × 24 × 5 km3 and 3D property modelling has been done by applying geostatistical approach for property inter-/extrapolation. The behavior of pore pressure in the field has been derived from dynamic fluid flow simulation through history matching for the production and subsequent shut-down phases of the field. Subsequently, changes in the pore pressure field during injection-production and subsequent shut-down phases are analyzed for weekly and seasonal loading and unloading scenario cases. The resulting pore pressure changes are coupled with 3D geomechanical model in order to have complete understanding of stress changes during these operations. In two scenario cases, the surplus electricity in Germany from renewable energy sources such as solar and wind from the year 2017 is considered. It results that the German surplus electricity can be stored in underground gas storage facilities with a Power-to-Gas (PtG) concept and that the stored gas can be reused again. Additionally, fault reactivation and thermal stress analyses are also performed on THM model in order to evaluate maximum threshold (injection) pressure as well as safe storage capacity of the reservoir. The fault reactivation already occurs at 1.25 times the initial reservoir pressure which provides a safe storage rate of 100,000-150,000 m3/day in the case study reservoir. The validated THM model is ready to be used for analyzing new wells for future field development and testing further arbitrary injection-production schedules, among others. The methodology can be applied on to any UGS facility not only in German Molasse Basin but anywhere in the world.