This project will explore the relative permeability of coarse‐grained reservoirs and the response of these reservoirs to depressurization at the macro‐ (1 m) and micro‐ (1x10‐6 m) scale. At the macro‐scale (e.g., 0.1 m to 1 m sand‐pack cores, eventually moving to natural Gulf of Mexico (GoM) cores), researchers will determine relative permeability and perform production tests (pressure dissipation). Simultaneously, they will perform micro computed tomography (micro‐CT) and micro‐Raman analysis to understand the habit and phase distribution at the micro- (pore) scale and will examine the evolution of these properties during dissipation. The project will develop constitutive relationships to describe these processes and inform reservoir simulation efforts.

University of Texas at Austin, Austin (UTA), TX 78712

Depressurization of coarse‐grained, gas‐bearing reservoirs involves multiple processes that interact at multiple length and time scales. These include, but are not limited to, relative permeability, capillary, compaction, kinetic, and thermodynamic behaviors. Two properties that are poorly understood are 1) the relative permeability behavior of these systems as the hydrate and gas saturation change, and 2) the effect of local changes in pore water chemistry as hydrate dissociation occurs. These are macro-scale behaviors that can be measured at the core scale, and they have a large impact on the production rate of methane from hydrate reservoirs. Accurate predictions of gas production from hydrates await a better understanding of these behaviors. This understanding will result from both a macro‐scale description of the behavior and a micro‐scale analysis of the underlying processes driving these macro‐scale behaviors. 

This project will explore the relative permeability of coarse‐grained reservoirs and the response of these reservoirs to depressurization at the macro‐ (1 m) and micro‐ (1x10‐6 m) scale. At the macro‐scale (e.g., 0.1 m to 1 m sand‐pack cores, eventually moving to natural Gulf of Mexico (GoM) cores), researchers will determine relative permeability and perform production tests (pressure dissipation). Simultaneously, they will perform micro-computed tomography (micro‐CT) and micro‐Raman analysis to understand the habit and phase distribution at the micro- (pore) scale and will examine the evolution of these properties during dissipation. The project will develop constitutive relationships to describe these processes and inform reservoir simulation efforts.
 

Methane hydrates within sand‐rich marine reservoirs represent a potentially enormous reservoir for methane. Previous drilling/logging in marine sand reservoirs within the GoM has verified that methane hydrate filled sand reservoirs are present and that sand reservoirs can be identified from seismic analysis. DOE is now focusing on acquiring intact samples through its project “Genesis of Methane Hydrate in Coarse‐Grained Systems: Northern Gulf of Mexico Slope,” DOE Award No.: DEFE0023919. It is anticipated that the first conventional and pressurized cores of these reservoirs will be collected under that project in spring 2017. 

Laboratory studies to determine the effect of solid phases (hydrate) on relative permeability are of the highest importance because this behavior has a large impact on gas recovery in hydrate bearing systems. Current modeling approaches are limited to relying on theoretical extensions of conventional multi‐phase flow models. It is vital now to go beyond these limitations and pursue an experimental program that will illuminate, at the core and the pore scale, the effect of methane hydrate on gas flow behavior and the process of hydrate dissociation due to perturbation. A successful testing program leading to analysis of intact cores (as is planned under this project) provides a pathway to this understanding. The learnings that result will provide a significant step forward in our ability to simulate hydrate production and make realistic estimates of the ability of the methane hydrate resource to be a viable energy source.

• Explored the relative permeability and dissipation behavior of hydrate-bearing coarse-grained sediment at the core scale
• Observed the formation and dissociation of these materials at the pore scale with micro-CT and ramen experiments. 
• Performed the first 3 phase relative permeability experiments in a hydrate-bearing medium using the steady state method (gas and water flow in the presence of hydrate). Results support a model where water is the most wetting phase, hydrate is the intermediate wetting phase, and gas is the least wetting phase. This model can be used to predict the relative permeability of gas and water in the presence of hydrate. 
• Demonstrated that hydrate dissociates at a pressure and temperature predicted for fresh water (no salinity) conditions. At the scale of a grid block, reservoir simulation models should assume that hydrate dissociates at the freshwater phase boundary.
• Observed that the chemistry and the pore habit of methane hydrates change dramatically over the timescales of experiments (hours to weeks). It takes weeks to months for hydrates to evolve from a non-stoichiometric to stoichiometric compound with 3:1 large cage vs small cage occupancy. 
• Documented hydrates forming initially in small clayey silt surfaces but gradually concentrating in large pores in sand-sized sediment.

Research activities under the project have been completed and results of the work are reported within the project’s final scientific/technical report which is available from the link in the additional information section below. Key accomplishments and findings are summarized in the accomplishments section above.

$1,199,991

$300,000

NETL – Richard Baker (richard.baker@netl.doe.gov)
UTA – Dr. Peter Flemings (pflemings@jsg.utexas.edu)

Final Scientific/Technical Report [PDF] December 2019
   OSTI Identifier: 1579563

Project Information