Description: Injection of CO2 into CH4-hydrate bearing sediments, and the resulting in-situ replacement of CH4-hydrate by CO2-hydrate, has been proposed as a technique for the emission-free production of natural gas from gas hydrates. While the hydrate conversion is thermodynamically feasible, many studies conclude that the overall process suffers from mass transfer limitations and CH4 production is limited after short time. To improve CH4 production various technical concepts have been considered, including the injection of heated supercritical CO2 combining chemical activation and thermalstimulation. While the feasibility of the concept was demonstrated in high-pressure flow-through experiments and high CH4 production efficiencies were observed, it was evident that overall yields and efficiencies were influenced by a variety of processes which could not be disclosed through bulk mass and volume analysis. Here we present different numerical simulation strategies which were developed and tested as tools to better understand the importance of mass and heat transport relative to reaction and phase transition kinetics for CH4 release and production, or for CO2 retention, respectively. The modeling approaches are discussed with respect to applicability for experimental design, process development or prediction of CH4 production from natural gas hydrate reservoirs on larger scales.

Description: Water permeability in gas hydrate bearing sediments is a crucial parameter for the prediction of gas production scenarios. So far, the commonly used permeability models are backed by very few experimental data. Furthermore, detailed knowledge of the exact formation mechanism leads to severe uncertainties in the interpretation of the experimental data. We formed CH4 hydrates from a methane saturated water solution and used Magnetic Resonance Imaging (MRI) to measure time resolved maps of the three-dimensional gas hydrate saturation. These maps were used for 3D Finite Elements Method (FEM) simulations. The simulation results enabled us to optimize existing models for permeabilities as function of gas hydrate saturation.

Description: The Alaska North Slope comprises an area of about 400,000 km2 including prominent gas and oil fields. Gas hydrates occur widely at the Alaska North Slope. A recent assessment by the USGS estimates 0.7-4.47 x 1012 m3 of technically recoverable gas hydrates based on well data and drilled hydrate accumulations. In spring 2012 a production field trial, testing CO2/N2 injection and depressurization, was conducted by USDOE/JOGMEC/ConocoPhillips at the Ignik Sikumi site. The 3D geological model of the Alaska North Slope developed by the USGS and Schlumberger is used to test the new gas hydrate module in the petroleum systems modeling software PetroMod®. Model results of the present extent of the gas hydrate stability zone (GHSZ) are in good agreement with results from well data. The model simulations reveal that the evolution of the GHSZ over time is primarily controlled by the climatic conditions regulating the extent of the permafrost during the last 1 Myr. Preliminary model runs predict the highest gas hydrate saturations near the major faults and at the bottom of the GHSZ, where thermogenic methane gas accumulates after migration through the most permeable stratigraphic layers (e.g. Sag River Sandstone Fm, Ivishak Fm). Gas hydrate saturations predicted for the Mount Elbert Stratigraphic Test Well and the Ignik Sikumi sites are basically controlled by the alternation of layers with different permeability and the fault properties (time of opening, permeability, etc). Further results including a total gas hydrate assessment for the Alaska North Slope will be presented during the conference.

Description: Due to their favorable P-T conditions and organic-rich deposits, sub-seafloor sediments in the northern Gulf of Mexico are known to have a large potential for gas hydrate accumulations. The presence of gas hydrates within sediments of the Green Canyon block has been proven by various methods, incl. seismic imaging, geochemical analysis, and drilling conducted mainly as a part of Joint Industry Project (JIP) Phase II. Gas hydrates reported therein usually occur as tens up to hundreds of meters thick sections with moderate to high concentrations within a range of 50 – 70 vol. % of pore space, and hence, seem to offer a considerable natural deposit of methane gas. The main focus of this study was to explore the complex effects of a set of control- parameters responsible for hydrocarbon migration and storage within the Gas Hydrate Stability Zone (GHSZ) on the accumulation of gas hydrates. To investigate the processes of basin formation and its subsidence history, source rock maturation, hydrocarbon migration and expulsion, and to quantify the gas hydrate accumulation potential, 3-D numerical study has been conducted using PetroMod. The area of interest extends over ~14 km x 33 km and covers the edge of the Sigsbee Escarpment representing the main salt mobility front in the region. The simulation contains full depositional history of the Green Canyon block, incl. salt deposition and re-mobilization as well as its further implications for temperature field, fluids migration and sedimentary layers distribution. Methane generation has been resolved by in-situ POC degradation and deep thermogenic mobilization from two distinct hydrocarbon sources. As a result, we present a number of likely scenarios of gas hydrate formation and accumulation in the study area that have been calibrated against available data.

Description: The injection of CO2 into CH4-hydrate-bearing sediments has the potential to drive natural gas production and simultaneously sequester CO2 by hydrate conversion. Currently, process conditions under which this goal can be achieved efficiently are largely unknown. While the recent Ignik Sikumi field test suggests that a combination of N2/CO2 injection with depressurization yields effective CH4 production, in a previous study (Deusner et al., 2012) we showed that a combination of CO2 injection and thermal stimulation eliminates mass transfer limitations observed at cold reservoir temperatures. These high-pressure flow-through studies revealed that the injection of supercritical CO2 at 95 °C triggers dissociation of CH4-hydrates and counters rapid CO2-hydrate formation in the near-injection region. We also observed a strong effect of reservoir temperature on CH4 production and CO2 retention. The efficiency and yield of CH4 production was highest at a sediment temperature of 8 °C compared to 2 °C and 10 °C. At 2 °C CO2 hydrate formation was rapid and clogged the sediment at the injection spot. Outside the CO2-hydrate stability region, at 10 °C, we observed fast CO2 breakthrough and a comparably low CH4 production. Experiments comparing discontinuous and continuous CO2 injection showed that alternating periods of equilibration and CO2 injection improved the overall CH4 production. We hypothesize that slow formation of secondary CO2-rich hydrate improves the accessibility of the CH4-hydrate distributed in the sediment by locally changing permeability and fluid flow patterns. In situ measurements showed dynamic changes of local p-/T-gradients due to gas hydrate dissociation or dissolution and secondary gas hydrate formation. In addition, continued reconfiguration of guest molecules in transiently formed mixed hydrates maintain elevated gas exchange kinetics. Online effluent fluid analysis under in-situ pressure conditions indicated that CH4 released from CH4-hydrates is largely dissolved in liquid CO2.. It is a current objective of our studies to further elucidate rheological properties and gas exchange efficiencies of CO2-CH4 mixed fluids that approach equilibrium with gas hydrates and to study the effect of in situ CH4-CO2-hydrate conversion and secondary gas hydrate formation on sediment geomechanical parameters.