Work conducted under this field work proposal (FWP) includes two distinct phases. Ongoing Phase 2 work is discussed directly below.

Phase 2 Project Information 
Characterization of Natural Hydrate Bearing Core Samples

The overarching goal of this project is to gain an improved understanding of the dynamic processes of gas hydrate accumulations in geologic media by combining laboratory studies, numerical simulation, and analysis of shipboard infrared imaging of hydrate core samples. This project comprises four principal components: (1) fundamental laboratory investigations, (2) numerical simulator development and verification, (3) hydrate core characterization and analysis, and (4) applied laboratory and numerical investigations.

Pacific Northwest National Laboratory (PNNL), Richland, Washington

Fundamental Laboratory Studies: Reliable thermodynamic, kinetic, and physical property data for gas hydrates and physicochemical properties of the hydrate-bearing sediments are required for detailed analyses of hydrate systems. Laboratory studies will be conducted on natural hydrate-bearing core samples from Mallik, Hydrate Ridge, the Gulf of Mexico, and India, as well as on synthesized gas hydrate samples. Measurements will include high-pressure x-ray diffraction (HXRD) in association with the resonant ultrasound spectroscopy (RUS) technique to examine hydrate formation/dissociation processes. For determining methane abundance and location on a grain-to-grain scale, a completely new method of characterizing gas hydrate will be developed that utilizes focused ion beam (FIB) environmental scanning electron microscopy (ESEM) that is capable of collecting Electron Backscatter Diffraction (EBSD) images.

Numerical Simulator Development and Verification: Numerical simulation provides scientists and engineers with analytical tools for investigating different strategies for producing natural gas hydrate-bearing geologic formations. A fundamental objective for this project is to develop numerical simulators with capabilities for investigating a broad spectrum of coupled processes and production technologies. Corollary to this objective is the validation of the developed numerical tools against experimental observations at the laboratory and field scales. This project has supported the development of two numerical simulators: an equilibrium-based simulator and a kinetic-based simulator. The equilibrium-based simulator (STOMP-HYD) was used during the DOE-NETL sponsored international code comparison activities. The kinetic-based simulator (STOMP-HYDT-KE) will be used to model ternary hydrate systems involving a mixture of CH₄, CO₂, and N₂ guest molecules.

Hydrate Core Characterization and Analysis: A key issue in analyzing hydrate samples retrieved from the field is pressure loss during handling and the unavoidable warming of the sampler as it rises to the surface that will induce varying amounts of hydrate decomposition. The effects of partial hydrate decomposition on the measured properties of core samples are at present very poorly understood. A much better fundamental understanding of the effects of hydrate saturation and thermal and pressure cycling on the properties of hydrate-bearing sediments is needed. Infrared data collected on hydrate core samples during the Ocean Drilling Program (ODP) Leg 204 expedition (Hydrate Ridge); the Integrated Ocean Drilling Program (IODP), Expedition 311 (Cascadia Margin); and the Indian National Gas Hydrate Research Program (NGHP), Expedition 01 (east and west coasts of the Indian subcontinent and the Andaman Islands) will be analyzed to quantitatively assess hydrate abundance and compare hydrate occurrence as determined from well logs and other hydrate proxies.

Applied Laboratory and Numerical Investigations: Experimental research has demonstrated the value of using N₂together with CO₂ as the injectant in the guest-molecule-exchange technology for producing natural gas hydrates from both sub-oceanic and arctic deposits. The benefit of using N₂ with CO₂ comes from the preference for N₂ to displace CH₄ from the small cages in the sI hydrate structure,; whereas, CO₂ preferentially displaces CH₄ from the large cages. Pure CO₂ sI hydrates naturally form with CO₂ occupying both the small and large cages. However, during the guest-molecule-exchange process, the thermodynamic preference is for CH₄ to remain in the small cages. The additional benefit of combining N₂ and CO₂ is that the N₂ shifts the hydrate equilibrium curve, which provides production operators with a system control. Injecting pure CO₂ into a natural gas hydrate-bearing formation can only be controlled by the injection pressure and fluid temperature, where pressure is the primary control mechanism. By including N₂ in the injectant mixture, the concentration of N₂ can be used as an additional controlling mechanism. KIGAM has previously conducted and is currently conducting laboratory-scale experiments involving the injection of gaseous N₂-CO₂ mixtures into pure CH₄ hydrate-bearing cores and sand columns.

Numerical Simulator Development and Verification: This project has supported the development of two numerical simulators for the production of natural gas hydrates from geologic deposits: STOMP-HYD and STOMP-HYDT-KE. The STOMP-HYD simulator assumes equilibrium conditions and has simulation capabilities for four classes of production technologies: (1) thermal stimulation, (2) depressurization, (3) inhibitor injection, and (4) guest molecule exchange. For the guest-molecule exchange technology, STOMP-HYD is limited to binary hydrate systems with CH₄ and CO₂ guest molecules. The STOMP-HYDT-KE allows for non-equilibrium conditions with a kinetic transition from non-equilibrium to equilibrium conditions. STOMP-HYDT-KE has capabilities for the four classes of gas hydrate production technologies, but extends the hydrate system to a ternary capability with CH₄, CO₂, and N₂guest molecules. Numerical simulators with proven capabilities for modeling the production of gas hydrates from geologic accumulations provide scientists and engineers with needed tools for assessing reservoir potentials and developing production strategies. Producing gas hydrates from geologic reservoirs involves thermal, multi-fluid hydrological, and geomechanical processes, with a potential for geochemical considerations. Numerical simulation allows for consideration of these processes in a coupled manner.

Hydrate Core Characterization and Analysis: A fundamental objective for this component of the project is to develop techniques for correlating hydrate abundance to a wide range of physical, chemical, and stratigraphic properties that will enable accurate prediction of hydrate occurrence based on expected or known values of the correlated variables. During the course of this project, characterization techniques and procedures have been developed and applied during hydrate-core-recovery expeditions at Hydrate Ridge, Northern Cascadia Margin, and the India Ocean. Sampling approaches—for which there is close spatial coupling among infrared anomalies, pore-water analyses, and grain-size analyses—are enabling the development and verification of multivariate approaches for cross-correlating all three measurements.

Applied Laboratory and Numerical Investigations: The guest-molecule-exchange production technology for natural gas hydrates conceptually involves the injection of sI hydrate formers to replace those occupying the cages of a natural gas hydrate deposit in geologic formations (primarily sub-oceanic and arctic). The technology holds promise in that there is potential for effecting natural gas production without significantly altering the existing hydrate structure, thus avoiding changes in the mechanical stress of the system. A secondary benefit is that the process yields sequestration of CO₂ in stable hydrate form. The challenge of the technology is devising production approaches that can be controlled and yield high exchange efficiencies. Numerical simulation is an invaluable tool for understanding complex physical processes such as those involved in the guest-molecule-exchange (e.g., multi-fluid flow, heat transfer, phase transitions, thermodynamic equilibrium, kinetics, geologic heterogeneity). Without experimental validation, however, numerical simulation becomes somewhat speculative in that the code developer is responsible for defining and implementing the important processes.

Hydrate Core Characterization and Analysis: During Leg 204 (2002) of the Ocean Drilling Program, extensive infrared (IR) images were obtained shipboard on cores from Hydrate Ridge, off the Oregon shore. The challenge has been to extract quantitative information on hydrate abundance from the IR data and to analyze that information by correlating it with stratigraphic information, geochemistry, physical property data, and other proxies for hydrate occurrence. A major step forward occurred during the Integrated Ocean Drilling Program’s Expedition 311 (2005) when gas hydrate IR anomalies were closely linked with pore water (chlorinity and salinity) anomalies and sediment grain-size. This was possible because of the enhanced IR data collection and display system developed for Expedition 311.

A comparable approach was used in the Indian National Gas Hydrate Program (NGHP) Expedition 01 (2006) using a similar data collection and display system. Infrared imaging was performed on all cores (except pressure cores), immediately after retrieval. A research-grade, automated IR imaging system was used to collect quantitative IR images on each 20 cm of core. Cores were imaged while still in the butyrate core liner. Handheld IR images were also taken of split core faces. Gas hydrate-bearing intervals are cooler than non-hydrate bearing zones due the endothermic nature of dissociation of gas hydrate to CH₄ gas and liquid H₂O. Temperature data were extracted from the IR images and used to define a ΔT or the extent of cooling produced by gas hydrate dissociation. Numerous variables control the IR temperature of the core liner surface, but the ΔT is readily measured for each gas hydrate zone in the core and thus provides a comprehensive and spatially detailed record of gas hydrate occurrence in marine sediments. In conjunction with PWC, the ΔT values are converted to % pore space occupied by gas hydrate, and then the gas hydrate occurrences are summed to estimate the total amount of gas hydrate in the GHOZ at the site.

Preliminary simulations of the recovery of hydrate-bearing cores from sub-oceanic deposits from the NGHP Expedition 01, Site 17, were conducted with STOMP-HYD to assess the ability to inverse model the hydrate saturation distribution in the core from the external infrared signal and core lithology. The core sample was modeled using a two-dimensional cylindrical domain as a layered system hydrate-bearing volcanic ash (3 cm) with hydrate-free marine sediments above and below. The numerical simulations were able to predict the infrared signature on the core surface, demonstrating the potential for in situ hydrate assessments using numerical simulator reconstruction. Color-scaled images generated from the numerical simulations with STOMP-HYD show the hydrate saturation and temperature after 50 minutes.

Numerical Simulator Development and Verification: The National Energy Technology Laboratory (NETL) and the U.S. Geological Survey (USGS) are guiding a collaborative, international effort to compare methane hydrate reservoir simulators. The goals of the effort are (1) to exchange information regarding gas hydrate dissociation and physical properties, enabling improvements in reservoir modeling; (2) build confidence in all the leading simulators through exchange of ideas and cross-validation of simulator results on common datasets of escalating complexity; and (3) establish a repository of gas hydrate-related experiment/production scenarios with the associated predictions of these established simulators that can be used for comparison purposes. PNNL has contributed to these code comparison activities by helping to define test problems, executing its STOMP-HYD simulator on the test problems, reporting simulation results, and participating in the analyses of the simulation predictions. Personnel conducting the code comparison study have completed six test problems and are currently working on the final suite of hydrate production problems for a range of hydrate accumulations on the Alaska North Slope.

A new simulator has been added to the series of STOMP simulators for modeling natural gas hydrate production from geologic accumulations. The first simulator in the series, STOMP-HYD, was capable of simulating four production technologies: (1) depressurization, (2) thermal stimulation, (3) inhibitor injection and (4) CO₂ exchange. This simulator assumed equilibrium conditions between the mobile and hydrate components of the hydrate formers, CH₄ and CO₂. Experiments conducted at the Korea Institute of Geoscience and Mineral Resources (KIGAM), however, demonstrated that guest molecule exchange was a kinetic process, with respect to the time scales for flow through geologic media. The second simulator in the series, STOMP-HYD-KE, extended the capabilities of STOMP-HYD, by solving separate conservation equations for the mobile and hydrate components of the hydrate formers, CH₄ and CO₂. Hydrate formers transitioned between mobile and hydrate forms via hydrate formation, dissociation, and exchange, where all three mechanisms were controlled via kinetic rates. The STOMP-HYDT-KE simulator extends the capabilities of its predecessor by including a third hydrate former, N₂. As with the two other hydrate formers, CH₄ and CO₂, the mobile and hydrate components of N₂ are solved separately. In its full capability configuration, the STOMP-HYDT-KE solves nine conservation equations at each grid cell: (1) energy, (2) water mass, (3) mobile CH₄ mass, (4) hydrate CH₄ mass, (5) mobile CO₂ mass, (6) hydrate CO₂ mass, (7) mobile N₂ mass, (8) hydrate N₂ mass, and (9) inhibitor mass. The modular design of the simulator allows for one or two of the hydrate formers and/or the inhibitor to be eliminated from the solution. The transition between STOMP-HYD-KE and STOMP-HYDT-KE involved two significant changes in the code: (1) equation of state module and (2) ternary hydrate equilibria.

Applied Laboratory and Numerical Investigations: Experiments conducted at KIGAM on the exchange of carbon dioxide and nitrogen gas mixtures with methane hydrate in unsaturated, sand-filled columns, showed carbon dioxide and nitrogen in the effluent stream early in the production process, indicating the guest-molecule exchange was occurring as a kinetic process. The first attempt at modeling these experiments was conducted using the STOMP-HYD simulator, an equilibrium and binary hydrate simulator. The equilibrium assumption in STOMP-HYD yields full exchange of the injected carbon dioxide with the methane in the gas hydrate, yielding no carbon dioxide in the effluent stream, as observed. The binary-hydrate limitation in the simulator required ignoring the injected nitrogen. To resolve these limitations in the STOMP-HYD simulator, two joint projects were created: a kinetic-exchange project funded by KIGAM and a ternary-hydrate project funded by NETL. The kinetic-exchange project was completed with the development and application of the STOMP-HYD-KE project. The ternary-hydrate code is currently under development. In addition to the application of numerical simulation to guest-molecule exchange experiments, a more general short course on the application of the STOMP simulator to problems involving the coupled subsurface processes of multi-fluid flow and transport, heat transfer, geochemistry, and geomechanics was held at the International School for Geoscience Resources (IS-Geo) at KIGAM.

(December 2013)
This project has been completed.

All DOE Funding *
FY01 - DOE Share - $100,000
FY02 - DOE Share - $40,000
FY03 - DOE Share - $200,000
FY04 - DOE Share - $210,000
FY05 - DOE Share - $228,000
FY06 - DOE Share - $450,000
FY07 - DOE Share - $250,000
FY08 - DOE Share - $94,000
FY09 - DOE Share - $260,000
FY10 - DOE Share - $130,000
Total Funding to Date - $1,962,000
* Includes Phase I Funding

NETL – John Terneus (john.terneus@netl.doe.gov or 304-285-4254)
PNNL – Mark White (mark.white@pnnl.gov or 509-372-6070)