No abstract

The study reports on the differences between theoretically expected and effectively obtained volume fractions of THF hydrate depending on the THF-H2O ratio in the initial solution against the background of using it as a substitute for natural hydrate in laboratory simulations. Besides the stoichiometric solution, initial solutions with either H2O or THF as excess phase were prepared to define the wanted volume of hydrate in advance. In order to achieve a chemical equilibrium a complete conversion of H2O and THF into THF hydrate and the presence of a pure excess phase is impossible. Based on the specific enthalpy of hydrate-and ice melting gained from calorimetric measurements, considerably lower than expected hydrate volumes are concluded. For the stoichiometric solution, containing 19.1 Wt% THF, enthalpy recalculations and the occurrence of an ice melting endotherm indicate incomplete conversion with a residual of 4.3 Vol% unconverted THF-H2O solution. The deviations from expectations increase with decreasing amount of aspired THF hydrate saturation and are stronger when formed from H2O excess solutions with up to 25 Vol% less hydrate than projected for full conversion. THF-rich solutions form hydrate with melting enthalpies that recalculate for up to 15 Vol% hydrate less than theoretical assumptions.In samples with initial THF concentrations below 5 Wt% and above 82.7 Wt% no hydrate formation was evident.Based on the results we propose corrections to the initial solutions when defined THF hydrate volumes are required. Furthermore, THF excess and temperatures below zero assure stable conditions for hydrate-liquid setting at atmospheric pressure.2

In 2012 the production of CH 4 from hydrate-bearing sediments via CO 2 injection was conducted in the framework of the Iġnik Sikumi Field Trial in Alaska, USA. In order to preserve the injectivity by avoiding a formation of CO 2 hydrate in the near-well region, a mixture containing 77 mol% N 2 and 23 mol% CO 2 was chosen. The interpretation of the complex test results was difficult, and the nature of the interaction between the N 2 -CO 2 mixture and the initial CH 4 hydrate could not be clarified. In this study we present the results of our experimental investigations simulating the Iġnik Sikumi Field Trial at different scales. We conducted (1) in situ Raman spectroscopic investigations to study the exchange process of the guest molecules in the hydrate phase on a molecular level in a flow-through pressure cell with a volume of 0.393 ml, (2) batch experiments with pure hydrates and hydrate-bearing sediments in pressure cells with volumes of 420 ml, and (3) the injection of a CO 2 -N 2 mixture into a hydrate-bearing sediment in a large-scale reservoir simulator with a total volume of 425 L. The results indicate a dissociation of the initial CH 4 hydrate rather than an exchange reaction. The formation of a secondary mixed hydrate phase may occur, but this process strongly depends on the local composition of the gas phase and the pressure at given temperature.

The “guest exchange” of methane (CH4) by carbon dioxide (CO2) in naturally occurring gas hydrates is seen as a possibility to concurrently produce CH4 and sequester CO2. Presently, process evaluation is based on CH4–CO2 exchange yields of small- or medium-scale laboratory experiments, mostly neglecting mass and heat transfer processes. This work investigates process efficiencies in two large-scale experiments (210 L sample volume) using fully water-saturated, natural reservoir conditions and a gas hydrate saturation of 50%. After injecting 50 kg of heated CO2 discontinuously (E1) and continuously (E2) and a subsequent soaking period, the reservoir was depressurized discontinuously. It was monitored using electrical resistivity, temperature and pressure sensors, and fluid flow and gas composition measurements. Phase and component inventories were analyzed based on mass and volume balances. The total CH4 production during CO2 injection was only 5% of the initial CH4 inventory. Prior to CO2 breakthrough, the produced CH4 roughly equaled dissolved CH4 in the produced pore water, which balanced the volume of the injected CO2. After CO2 breakthrough, CH4 ratios in the released CO2 quickly dropped to 2.0–0.5 vol %. The total CO2 retention was the highest just before the CO2 breakthrough and higher in E1 where discontinuous injection improved the distribution of injected CO2 and subsequent mixed hydrate formation. The processes were improved by the succession of CO2 injection by controlled degassing at stability limits below that of the pure CH4 hydrate, particularly in experiment E2. Here, a more heterogeneous distribution of liquid CO2 and larger availability of free water led to smaller initial degassing of liquid CO2. This allowed for quick re-formation of mixed gas hydrates and CH4 ratios of 50% in the produced gases. The experiments demonstrate the importance of fluid migration patterns, heat transport, sample inhomogeneity, and secondary gas hydrate formation in water-saturated sediments.

Karst systems are found in many regions around the world. In the order of 10% of the continental surface is karst (Ford & Williams, 2007;Mangin, 1975). Karst is incredibly complex and manifold, and the processes that dominate karstification strongly depend on the hydrological and geomorphological properties of the karstic systems, which are subject to constant change while karstification is ongoing. Essentially, karstification happens in soluble rocks in contact with water, typically at the earth's surface or close to it. Karst research has evident relations to the disciplines and sub-disciplines of hydrology, geology, speleology, geomorphology, hydrogeology, etc. Karstic rocks are typically carbonate rocks made of Calcium and Magnesium minerals, where limestone (CaCO 3 ) and dolomite ( 𝐴𝐴 CaMg[CO3]2 ) are the most important subtypes. During karstification, these rocks are eroded mechanically, and, more importantly, corroded chemically. The corrosion of calcite and dolomite is driven by the availability of dissolved CO 2 in the water.