Project Goal
The goal of NETL's gas hydrate numerical simulation studies is to obtain pertinent, high-quality information on the behavior of gas hydrates in their natural environment under either production (methane gas extraction) or climate change scenarios. This research is closely linked with NETL's experimental and field studies programs to ensure the validity of input datasets and scenarios.

Project Performers
Brian Anderson, NETL/RUA Fellow (West Virginia University)
Hema Siriwardane, NETL/RUA Fellow (West Virginia University)
Eugene Myshakin, NETL/URS

Project Locations
National Energy Technology Laboratory, Pittsburgh PA, and Morgantown WV
West Virginia University, Morgantown, WV

Background
Field-scale hydrate production tests rely heavily on reservoir-scale prediction of fluid migration, pressure profile, and gas production estimates, as well as geomechanical response of reservoir formation on the evolution of pore pressure and phase migration. The ability to predict reservoir-scale thermal, hydrological, and geomechanical processes is essential for planning and performing field-scale tests. NETL-ORD and the Regional University Alliance (RUA) conduct advanced numerical simulations to better understand the response of gas hydrate to changes in environmental conditions. NETL’s numerical modeling has included studies conducted at the molecular scale (MDS, or molecular dynamics simulations, where the forces and motions of thousands of individual molecules are computed over timescales of nanoseconds); at the pore scale using thermodynamic and kinetic equations to describe gas hydrate reservoir response around a single bore-hole; and in field-scale simulations that predict gas hydrate reservoir behavior over time-scales of decades.

A few reservoir modeling codes for gas hydrate production are available. The NETL-RUA has been conducting numerical prediction of gas production and fluid migration behavior using HydrateResSim (HRS), an NETL in-house code developed based on the Lawrence Berkley National Laboratory TOUGH+, Tough2+, and STARS codes. Increased demands on geomechanical process modelling capability have resulted in NETL-ORD’s desire to modify and upgrade the finite element/finite volume Heat and Mass transfer computer code (FEHM), which already includes hydrate modelling modules and geomechanical components. Specific objectives of this research are to (1) predict the thermal-hydrological-geomechanical (THM) responses of reservoir and adjacent formations during field-scale gas hydrate production activities, including hydrate dissociation/displacement, fluid migration, gas production, and disturbances in geomechanical integrity in reservoir and ground deformations; (2) conduct production modeling predictions using current capabilities for a potential Alaska North Slope long-term depressurization-based hydrate production test; and (3) provide analysis of the recently completed ConocoPhillips CO₂-CH₄ exchange test.

Project Description
Current numerical simulation efforts are centered on recent and potential future field-scale hydrate production testing (including depressurization and CO₂/CH₄ exchange-based production methodologies). The planning, execution, and assessment of such tests has, and will continue to, rely heavily on reservoir-scale modeling predictions of fluid migration, pressure profiles, gas production estimates, reservoir geomechanical response, pore pressure evolution, and phase migration. Research focus will include efforts in the following areas:

1. Code Development and Updates
Efforts to incorporate multi-component gas mixtures and their associated hydrates into HRS are ongoing. A version of HRS that is capable of simulating a mixture of three gas components (e.g., CH₄+CO₂+N₂ or CH₄+C₂H₆+C₃H₈) called Mix3HRS has been developed and is being used to support the analysis of the ConocoPhillips Ignik Sikumi #1 gas hydrate field trial. Additionally, this multicomponent gas hydrate reservoir simulator is being used to model laboratory experiments conducted at NETL and ConocoPhillips.

2. Modeling of the Ignik Sikumi #1 test
Simulations of the ConocoPhillips Ignik Sikumi #1 gas hydrate field trial (completed spring 2012) will be performed using the binary and ternary version of HydrateResSim (HRS). The HRS code will be enhanced as necessary to conduct the analysis. The simulations will be a collaborative effort with ConocoPhillips and, potentially, Pacific Northwest National Laboratory. The main focus of these efforts will be to simulate the exchange of carbon dioxide (CO₂) and nitrogen (N₂) with methane (CH₄) in methane hydrate reservoirs.

Parallel simulations of experimental efforts on CO₂/CH₄ exchange will continue to be performed in order to validate the HRS code development. Key parameters that will be modified are the intrinsic permeability and irreducible water and gas saturations for the rock medium. Additional variable parameters will be identified throughout the research. The injection rate and the cumulative injected fluid will be the objective functions for the best fit. The history-match simulations for the Ignik Sikumi #1 test will require iterative history matching between the injection phase and the production phase. Simulations will proceed in a sequential manner through each period of production. The results of each production period will provide the input values for subsequent periods.

3. Modeling of a long-term methane hydrate production test site
A 3-D geological model developed by the U.S. Geological Survey that includes the reservoir, faults, and nearby well paths for the Prudhoe Bay Unit (PBU) L-Pad C Sand on the Alaska North Slope will be used for numerical simulations of a long- term, depressurization-based hydrate production field test. The simulations will focus on prediction of gas and water flow rates, the extent of reservoir depressurization, and potential impingement on existing wells during the depressurization test. Varying depressurization scenarios and geostatistic realizations will be simulated.

A full three-dimensional, reservoir-scale model of a gas hydrate deposit in the hydrate-bearing sands near the PBU L-Pad will be generated. A set of geostatistical models for reservoir properties will be generated and simulations performed that explore various depressurization cases. The effects of bottom-hole pressure, gas and water production rates, and the reservoir properties on the geomechanics of the hydrate deposit will be evaluated.

4. Coupled Thermal-Hydrological-Geomechanical Modeling of Gas Hydrates
The focus of this research is on the development and application of an NETL-ORD/RUA in-house capability to predict the thermal-hydrological-geomechanical responses of reservoir and adjacent formations during field-scale gas hydrate production activities, including hydrate dissociation/displacement, fluid migration, gas production, and disturbances in geomechanical integrity in reservoir and ground deformations. Initial activity will focus on modifying an existing computer code to include water-methane hydrate capability so that coupled thermal-hydro-geomechanical responses can be investigated. Capabilities will be incorporated to enable prediction of subsidence and compaction at the reservoir and seal level, as well as at intermediate levels, which may host other transiting well bores. The code will account for subsidence and compaction that could be caused by hydrate production/displacement, as well as fluid migration, gas production, and heat transfer. The computer model modifications accomplished under these initial efforts will result in capabilities available for continued and future hydrate program use.

Accomplishments

• Publication on recent modifications to the HydResSim modeling code: Garapati, N. Anderson, B. J., 2012, “Implementation of cell potential code into HydrateResSim simulator,” Spring ACS Meeting 2012, PAPER ID: 13685.

Current Status (March 2013)
Researchers have begun 3-D simulations for a long-term, depressurization-based hydrate production test on the Alaska North Slope. Numerical simulations of the Ignik Sikumi gas hydrate field test are also underway. Simulation efforts to match results of lab-based experimental tests performed at ConocoPhillips are being performed in parallel with simulations of the Ignik Sikumi field test.

Researchers have initiated the review of the capabilities of FEHM computer code and the necessary software compilers (for modifications to the FEHM computer code or FEHM source code) were identified.

Project Schedule
Activities initiated in October 2012

Cost Information
DOE Contribution: FY12: ~$270,000

Contact Information
NETL–ORD: Yongkoo Seol (mailto:Yongkoo.Seol@netl.doe.gov or 304-285-2029)

Additional Information:
In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].