The goal of the project — being jointly performed under projects with Louisiana State University (DE-FE0028966) and the U.S. Geological Survey (DE-FE0026166) — is to provide a quantitative basis for reservoir models to account for the impact of clays and other fine-grained material (fines) on reservoir compressibility and permeability, two key factors in controlling the flow of gas and fluids toward a gas hydrate production well.

Louisiana State University (LSU), Baton Rouge, LA 
US Geological Survey (USGS), Woods Hole, MA

The quantity of methane potentially recoverable from gas hydrate is large enough to motivate federally supported production tests in several countries, which in turn motivates studies of reservoir production efficiency. Evaluating long-term production well viability involves modeling permeability evolution in the reservoir sediments around the production well because processes reducing the flow of gas into the production well also reduce the long-term economic viability of the well. Fine particles, such as clays, exist nearly ubiquitously in the permafrost and marine settings that typically host gas hydrate, and fines reacting to fluid flow by migrating and clogging pore throats can reduce flow toward the production well. Many fines are sensitive to variations in pore-fluid chemistry, swelling in reaction to in situ pore brine being displaced by fresh water liberated from hydrates during dissociation.

Additionally, fine particles tend to collect at gas/water interfaces created by the multiphase flow of gas and water. Thus, as methane and fresh water flow from the hydrate-dissociation front toward the production well, fine particles in the reservoir sands, interbedded fine-grained layers, and seal layers can be swelled and/or migrated, potentially clogging pathways and limiting flow to the production well.
 

This project is expected to result in both increases in fundamental scientific understanding of hydrate system behavior during production and site-specific impacts related to specific potential production sites offshore India by the National Gas Hydrate Program (NGHP) of India. 

Site-specific impact: Results from this research directly inform DOE-funded reservoir modeling work being undertaken for the most promising NGHP field sites. Sediment compressibility and permeability are two of the three highest-priority parameters (the third being sediment strength) for DOE’s reservoir modeling in preparation for an NGHP production test.

Ongoing work at Japan’s Advanced Industrial Science and Technology laboratory (AIST) and USGS on NGHP pressure cores is not designed to quantify how compressibility and permeability in the presence of hydrate change as the in situ pore-water brine continues to be displaced by the gas and fresh water produced during hydrate dissociation. The pressure core studies are also unable to distinguish whether the compressibility and permeability changes are due to the swelling or the migration of fines. This specific research fills that need, providing modelers at DOE and elsewhere with information on how reservoir compressibility and permeability are likely to evolve over time in the NGHP areas of interest.

Fundamental issues of global impact: to benefit production assessments of hydrate-rich systems elsewhere in the world, this research includes a systematic study of common fines and their impact on compressibility and permeability over a controlled range of concentration in sandy sediment. The matrix of experiments covering the range of common fines and fines concentration in sands will be used to build a results database that can be used to estimate property changes in hydrate-bearing sand targeted for production. The database would be most useful once a site has been cored, with X-ray powder diffraction (XRD) analysis to reveal the fines types and concentrations.

A second global scientific impact of the research will be an enhanced understanding of fracturing due to breaking clogs that form due to fines migration. Quantifying the controlling parameters for this phenomenon provides guidance on a reservoir’s permeability evolution; in addition to providing a mechanism for explaining permeability drops and rebounds that may occur during production, clog fracturing has the potential to cause long-term permeability increases as fractures remain at least partly open once formed by this process.

• Completed field sample analysis of site-specific (Krishna-Godavari Basin) dependence of compressibility and permeability on pore fluid chemistry
• Completed micromodel tests of fines migration and clogging in a 2D pore network on two high-value targets within India’s 2015 National Gas Hydrate Field Program (NGHP-02)
• Completed measurements of electrical sensitivity for NGHP-02 samples from beneath a primary gas hydrate reservoir
• Completed a suite of microfluidic model evaluations of the dependence of fines migration and clogging on both 1) physical conditions and 2) pore fluid chemistry in porous media using endmember fines in water and brine
• Acquired and completed index property analyses on conventional core material from NGHP-02
• Completed development/acquisition of equipment needed for microfluidic visualization of NGHP-02 fines
• Completed liquid and plastic limit tests (the basis for electrical sensitivity) for endmember fines using deionized water, brine, and kerosene
• Completed sedimentation tests to evaluate the impact of salinity on sediment fabric
• Completed compression tests on fines in a variety of pore fluids focused on the dependence of compressibility on pore water salinity
• Completed fabrication of 2D micromodels and associated imaging systems for use in the assessment of migration of fines, potential for clogging, and impacts on system permeability 

Project activities have been completed and a final scientific and technical report is available from the additional information section below. Key takeaways from the project include: 

• Sediment fabric controls the sediment compressibility both in terms of what initial void ratio will be and in terms of how the force chains through which the sediment supports itself are arranged.
• Pore water will freshen during production, which causes certain fines to clog more readily (i.e. kaolin) than others (i.e. bentonite). In natural systems, chemical-based variations in clogging potential are likely to be overshadowed by the increased capacity to mobilize fines as pore water freshens by the size of the mobile grains, by the presence of a mobile gas/fluid meniscus, and by the geometry of a vertical well bore.
• There is substantial importance in obtaining even small amounts of sediment from the seal, reservoir, and associated sediments when sampling hydrate systems so valuable correlations between the index properties, mineralogy, and the macroscopic response of the sediment to imposed stresses and flow conditions can be made.
• For NGHP-02, a critical outcome is that, given the large effective pressures that will need to be imposed to destabilize the gas hydrate and release the methane for production, compaction will be a primary driver in reducing the sediment porosity, and hence, permeability.
• Gas hydrate dissociation will likely occur preferentially at the interfaces between gas hydrate-bearing coarse-grained intervals and fine-grained interbeds. Resuspension of fines from the fine-grained interbeds (or the upper and lower seal interfaces) will be enhanced.
• Characterization of all components of the sediments associated with, and adjacent to, the primary gas hydrate-bearing sediment will be required to fully characterize the reservoir’s evolution during production.
• Though direct geotechnical evaluation of each sediment type in the reservoir/seal system is advisable, assessing the index properties and mineralogy of the fines can provide valuable insights into their behavior when only small amounts of in situ sediment can be recovered.

DOE Contribution: $236,127 (LSU — $180,354; USGS — $55,773)
 

$61,542

NETL – Richard Baker (richard.baker@netl.doe.gov)
LSU – Dr. Shengli Chen (shenglichen@lsu.edu) and Dr. Jongwon Jung (jjung@lsu.edu) 
USGS – Dr. William Waite (wwaite@usgs.gov)

Project Information Page

Final Project Scientific /Technical Report [PDF] July 2019
   OSTI Identifier