The role of high-pressure flow-through experiments for evaluating the mechanical behaviour of gas hydrate-bearing soils

The role of high-pressure flow-through experiments for evaluating the mechanical behaviour of gas hydrate-bearing soil

GaMin’11 – an International Inter-laboratory Comparison for Geochemical CO2 - Saline Fluid - Mineral Interaction Experiments

Due to the strong interest in geochemical CO2-fluid-rock interaction in the context of geological storage of CO2 a growing number of research groups have used a variety of different experimental ways to identify important geochemical dissolution or precipitation reactions and – if possible – quantify the rates and extent of mineral or rock alteration. In this inter-laboratory comparison the gas-fluid-mineral reactions of three samples of rock-forming minerals have been investigated by 11 experimental labs. The reported results point to robust identification of the major processes in the experiments by most groups. The dissolution rates derived from the changes in composition of the aqueous phase are consistent overall, but the variation could be reduced by using similar corrections for changing parameters in the reaction cells over time. The comparison of experimental setups and procedures as well as of data corrections identified potential improvements for future gas-fluid-rock studies

Testing a thermo-chemo-hydro-geomechanical model for gas hydrate-bearing sediments using triaxial compression laboratory experiments

Natural gas hydrates are considered a potential resource for gas production on industrial scales. Gas hydrates contribute to the strength and stiffness of the hydrate-bearing sediments. During gas production, the geomechanical stability of the sediment is compromised. Due to the potential geotechnical risks and process management issues, the mechanical behavior of the gas hydrate-bearing sediments needs to be carefully considered. In this study, we describe a coupling concept that simplifies the mathematical description of the complex interactions occurring during gas production by isolating the effects of sediment deformation and hydrate phase changes. Central to this coupling concept is the assumption that the soil grains form the load-bearing solid skeleton, while the gas hydrate enhances the mechanical properties of this skeleton. We focus on testing this coupling concept in capturing the overall impact of geomechanics on gas production behavior though numerical simulation of a high-pressure isotropic compression experiment combined with methane hydrate formation and dissociation. We consider a linear-elastic stress-strain relationship because it is uniquely defined and easy to calibrate. Since, in reality, the geomechanical response of the hydrate-bearing sediment is typically inelastic and is characterized by a significant shear-volumetric coupling, we control the experiment very carefully in order to keep the sample deformations small and well within the assumptions of poroelasticity. The closely coordinated experimental and numerical procedures enable us to validate the proposed simplified geomechanics-to-flow coupling, and set an important precursor toward enhancing our coupled hydro-geomechanical hydrate reservoir simulator with more suitable elastoplastic constitutive models

High-Pressure Systems for Gas-Phase Free Continuous Incubation of Enriched Marine Microbial Communities Performing Anaerobic Oxidation of Methane

Novel high-pressure biotechnical systems that were developed and applied for the study of anaerobic oxidation of methane (AOM) are described. The systems, referred to as high-pressure continuous incubation system (HP-CI system) and high-pressure manifold-incubation system (HP-MI system), allow for batch, fed-batch, and continuous gas-phase free incubation at high concentrations of dissolved methane and were designed to meet specific demands for studying environmental regulation and kinetics as well as for enriching microbial biomass in long-term incubation. Anoxic medium is saturated with methane in the first technical stage, and the saturated medium is supplied for biomass incubation in the second stage. Methane can be provided in continuous operation up to 20 MPa and the incubation systems can be operated during constant supply of gas-enriched medium at a hydrostatic pressure up to 45 MPa. To validate the suitability of the high-pressure systems, we present data from continuous and fed-batch incubation of highly active samples prepared from microbial mats from the Black Sea collected at a water depth of 213 m. In continuous operation in the HP-CI system initial methane-dependent sulfide production was enhanced 10- to 15-fold after increasing the methane partial pressure from near ambient pressure of 0.2 to 10.0 MPa at a hydrostatic pressure of 16.0 MPa in the incubation stage. With a hydraulic retention time of 14 h a stable effluent sulfide concentration was reached within less than 3 days and a continuing increase of the volumetric AOM rate from 1.2 to 1.7 mmol L(-1) day(-1) was observed over 14 days. In fed-batch incubation the AOM rate increased from 1.5 to 2.7 and 3.6 mmol L(-1) day(-1) when the concentration of aqueous methane was stepwise increased from 5 to 15 mmol L(-1) and 45 mmol L(-1). A methane partial pressure of 6 MPa and a hydrostatic pressure of 12 MPa in manifold fed-batch incubation in the HP-MI system yielded a sixfold increase in the volumetric AOM rate. Over subsequent incubation periods AOM rates increased from 0.6 to 1.2 mmol L(-1) day(-1) within 26 days of incubation. No inhibition of biomass activity was observed in all continuous and fed-batch incubation experiments. The organisms were able to tolerate high sulfide concentrations and extended starvation periods

The role of high-pressure flow-through experiments for evaluating the mechanical behaviour of gas hydrate-bearing soils

Results from two recent field trials, onshore in the Alaska permafrost and in the Nankai Trough offshore Japan, suggest that natural gas could be produced from marine gas hydrate reservoirs at compatible yields and rates. However, both field trials were accompanied by different technical issues, the most striking problems resulting from un-predicted geomechanical behaviour, sediment destabilization and catastrophic sand production. So far, there is a lack of experimental data which could help to understand relevant mechanisms and triggers for potential soil failure in gas hydrate production, to guide model development for simulation of soil behaviour in large-scale production, and to identify processes which drive or, further, mitigate sand production. We use high-pressure flow-through systems in combination with different online and in situ monitoring tools (e.g. Raman microscopy, MRI) to simulate relevant gas hydrate production scenarios. Key components for soil mechanical studies are triaxial systems with ERT (Electric resistivity tomography) and high-resolution localstrain analysis. Sand production control and management is studied in a novel hollow-cylinder-type triaxial setup with a miniaturized borehole which allows fluid and particle transport at different fluid injection and flow conditions. We further apply a novel large-scale high-pressure flow-through triaxial test system equipped with μ-CT to evaluate soil failure modes and triggers relevant to gas hydrate production and slope stability. The presentation will emphasize an in-depth evaluation of our experimental approach, and it is our concern to discuss important issues of translating laboratory results to gas hydrate reservoirs in nature. We will present results from high-pressure flow-through experiments which are designed to systematically compare soil mechanical behaviour of gas hydrate-bearing sediments in relevant production scenarios focusing on depressurization and CO2 injection. Experimental datasets are analyzed based on numerical models which are able to simulate coupled process dynamics during gas hydrate formation and gas production