The goal of this project is to improve the understanding of regional and local differences in gas hydrate systems from three perspectives: as an energy resource, as a geohazard, and as a long-term influence on global climate.
Performer(s)
Rice University, Houston, TX
University of Texas, Austin, TX
Oklahoma State University, Stillwater, OK
Background
Heterogeneity in the distribution of gas hydrate accumulations impacts all aspects of research into gas hydrate natural systems. The challenge is to delineate, understand, and appreciate these differences at the regional and local scales, where differences in in situ concentrations are relevant to the importance of gas hydrate as a resource, a geohazard, and a factor in the carbon cycle. Some of the key questions that remain unanswered are:
Why do regional heterogeneities in gas hydrate distribution occur?
How can we detect gas hydrate remotely?
Where are concentrated gas hydrate accumulations likely to be found?
What controls enhanced gas hydrate accumulations in some heterogeneities?
How will heterogeneities affect strategies for production of natural gas from hydrates?
How would different distributions respond to temperature perturbations?
Impact
This project will result in enhanced understanding of:
The processes controlling the behavior of the marine hydrate system.
The first-order controls on the variability in hydrate accumulations,
The potential viability of proposed methodologies for the production of natural gas from methane hydrates.
Improved safety practices during offshore drilling through a better understanding of sea-floor and wellbore stability in the presence of hydrates.
The effort may also result in advanced methodologies for remote quantification of hydrate accumulation through novel techniques for evaluation of geophysical imaging.
Accomplishments (most recent listed first)
Carbon Inputs and Outputs to Gas Hydrate Systems
Determined the amount of iodine in sediment and pore waters down boreholes at 10 locations, including three with gas hydrate from several gas hydrate systems (Blake Ridge, Peru Margin, Gulf of Mexico, Japan Sea), clearly showing that iodine accumulates in marine sediment as a function of organic carbon input over time.
Examined carbon, sulfur, and metal chemistry across the sulfate-methane transition zone (SMT) at sites in the Japan Sea and the Peru Trench to assess whether sulfate profiles can be used to determine the upward flux of methane. This assumption appears valid in the Sea of Japan, but only after all carbon fluxes are accounted for. In particular, an upward flux of bicarbonate and carbonate precipitation impact geochemical interpretations across the SMT.
Sampled pore waters associated with gas hydrate deposits in the Umitaka Spur and the Joetsu Basin, Sea of Japan (Marion Dufresne cruise MD179) using giant piston corers to see if down-hole geochemical trends previously observed in shallow sediments (Cruises UT04, KYO5, UT06) extend to sediments as deep as 40 meters below the sea floor. Samples were collected in collaboration with the University of Tokyo, and included the first samples of gas hydrate and authigenic carbonate recovered from sediment depths greater than 20 mbsf.
Numerical Models for Quantification of Hydrate and Free Gas Accumulations
Demonstrated that higher organic carbon content at the seafloor and faster temperature dependent methanogenic reaction rate affect the hydrate abundance greatly, which could be a possible explanation for gas hydrate as the carbon source for the Paleocene – Eocene Thermal Maximum (PETM) event.
Permeability anisotropy (with a lower ratio of vertical to horizontal permeability) show enhanced hydrate concentrations within high permeability conduits because anisotropy focuses more methane-charged fluid into these conduits. The Two-dimensional model results verified the anisotropy and quantify how focused fluid flow through high permeability zones affects regional and local hydrate accumulation and saturation.
Previous 1-D modeling indicated that accumulated hydrate saturation was dependent on Peclet number, Pe – the ratio of advective flux to the diffusive flux of methane. The 2-D modeling showed that the localized, focused, advective flux relative to diffusion (Pe Local ) determines the magnitude of hydrate and free gas saturation. Average local Peclet numbers and average hydrate flux (Pe1 h> ) within high permeability conduits were related, which compared favorably with the previous 1-D correlation. As a result of these efforts, the generalized model will be adapted to specific field examples (such as Walker Ridge in the Gulf of Mexico) where enhanced hydrate saturation is noted in dipping sand layers.
Lithologic structures relative to seismic blanking features were simulated using complex structures including fractures and dipping layers. The evolution of system properties (hydrate/free gas distribution, overpressure, safety factor, and acoustic properties) was evaluated.
Advanced the development of the numerical models for hydrate and free gas accumulation to increase the fundamental understanding of the accumulation phenomena.
Developed a numerical model to simulate the accumulation of hydrate and free gas over geologic time and length scales in one dimension, and initiated efforts to move to two spatial dimensions.
Initiated simulations to delineate basic modes of gas hydrate distribution in marine sediment—including systems with no gas hydrate, gas hydrate without underlying free gas, and gas hydrate with underlying free gas below the gas hydrate stability zone—for various methane sources.
Developed combinations of dimensionless variables, particularly the Peclet and Damkohler numbers, such that the dependence of average hydrate saturation on numerous parameters can be summarized using two contour maps, one for a biogenic source and one for upward flux from deeper sources (e.g., thermogenic gases). This model presents a unified picture of hydrate accumulations that can be used to understand well-characterized gas hydrate systems or to predict steady-state average hydrate saturation and distribution at locations for which seismic or core data are not available.
Demonstrated that continuous change of hydrate saturation and acoustic velocity over a long spatial distance (~300 m) is possible, indicating that a gradual change of acoustic properties may induce weak Bottom Simulating Reflector (BSR) responses or even no BSR response.
Determined through numerical simulations that the ratio of absolute sediment permeability to the sedimentation rate is the key dimensionless group controlling overpressure generation. The effect of overpressure, in turn, limits the amount (thickness) of free gas that can accumulate below the GHSZ.
Developed numerical and analytical models for inferring gas hydrate saturation in marine sediments from pore water sulfate profiles. Results from these models are in agreement with gas hydrate saturations estimated from resistivity logs/chloride data at several sites along Cascadia Margin.
In addition to methane and sulfate profiles, modeled bicarbonate (DIC), calcium, carbon isotope profiles to interpret upward methane flux from deeper hydrate bearing sediments. The geochemical profiles across the SMT are interpreted to be a result of sulfate reduction by the anaerobic oxidation of methane (AOM) reaction at the SMT. This enables the depth to the SMT to act as an indicator for upward methane flux, which in turn can be related to the average hydrate saturation.
A dimensionless, two-dimensional (2-D) model was developed to simulate gas hydrate and free gas accumulation in marine sediments over geologic timescales. Focused fluid flow through a vertical fracture network and/or high permeability sand layers affecting regional and local hydrate accumulation and saturation can be elucidated with the help of this 2-D model. These 2-D simulations have shown that the permeability contrast between the confining unit (e.g., silt, clay) and the reservoir (e.g., sand or permeable fracture) is crucial to hydrate and free gas saturation and distribution. Simulations are being extended with specified fluid flux and methane input from deeper sources, thus allowing comparison of local methanogenesis and deeper methane sources on flow pathways and hydrate/free gas accumulation. Increased fluid flux from deeper sources is expected to result in enhanced concentrations of hydrate and free gas.
ANALYSIS OF PRODUCTION STRATEGY
A thermal, compositional, kinetic simulator for CO2 flooding of the methane-hydrate bearing sediments has been developed. The 1-D CO2 injection simulations indicated that the fluid saturation remains low due to the adverse mobility ratio of the displacement of water with gas/liquid CO2. A specific fraction of CO2 should be present in the fluid phase to form CO2 hydrate; however, the fluid phase is continuously decreasing due to the methane-hydrate dissociation. The CO2-hydrate energy of formation is utilized in the methane-hydrate dissociation which maintains the temperature of the core. The methane-hydrate dissociation front moves at a slow rate of approximately 1/40 cm/hr for the kinetics assumed in the base case. To dissociate methane-hydrate by CO2 injection, the CO2 mole fraction needs to be very high in the fluid phase or operate at a relatively lower pressure (shallower reservoirs of methane-hydrates).
Successfully completed initial evaluation of a production simulator by evaluating the scenarios established under the DOE-NETL code comparison study.
Initiated evaluation for the potential use of warm water aquifer injection for the purpose of hydrate production. Researchers also determined that for warm water injection, production well pressure and injection temperature and pressure play an important role in the production of gas from hydrate deposits.
Methane production was simulated for different injection pressures, injection temperatures, and production pressures for 3000 days and total production of gas was compared for these parameters. Testing has shown that depressurization alone is effective in dipping unconfined reservoirs, but the gas production rate is much slower than that for warm water injection. As the injection point of the warm water moves down the reservoir, the high gas recovery phase gets delayed, but the time for completion of gas recovery becomes shorter. The cost of wells and warm water must be optimized along with the gas production to determine the optimal strategy for producing hydrate reservoirs.
Vertical wells for unconfined reservoirs are less efficient than the horizontal wells. Horizontal wells increase the area of sweep of hydrate bearing sediments, leading to more production.
The injection well should be at the contact point between the hydrate and water zones; for high production rates, the production well should be on the top of the hydrate bearing layer. The distance between the production well and the injection well should be optimized based on the hydrate saturation and the aquifer size.
SEA-FLOOR AND BOREHOLE STABILITY
Methane migration and fracture generation were modeled at southern Hydrate Ridge with a 1-D model that incorporates fluid flow, methane hydrate formation, and fracturing behavior. The model inputs were based on geophysical observations from two seismic surveys acquired 8 years apart showing changes in the methane gas reservoir feeding active methane venting near the southern Hydrate Ridge summit. Fractures were shown to form within a few years of the onset of pressure buildup. Once fractures form, gas is able to move upwards through the RHSZ by formation of hydrate in the fractures (40-80% of the fracture volume), reducing the local salinity to the conditions required for RHSZ gas formation. Gas reaches the seafloor by this process after 70-80 days. The methane reservoir becomes depleted through fracture system venting to the seafloor after an additional 30-50 years. The results show that the observed activity at southern Hydrate Ridge is part of a highly transient process involving methane gas migration, fracture genesis, and seafloor venting with variations on time scales of years to decades, illustrating the dynamic nature of hydrate deposits and the potential transience of many observed features.
Integrated sediment properties work, geologic hydrate accumulation work, hydrate production work, and DOE-sponsored Joint Industry Project (JIP) hydrate work in the Gulf of Mexico to develop forward models of hydrate accumulation to test the JIP predictions and provide accurate and realistic sediment models for hydrate production models.
Measured permeability of samples to evaluate new techniques for determining permeability anisotropy and for retrieving robust permeability data from logging measurements. Published a manuscript on nuclear magnetic resonance-based permeability estimation in fine grained, hydrate bearing sediments.
Developed a safe drilling program that will maximize the understanding of hydrate in the Gulf of Mexico and provide data for modeling these accumulations.
Measured permeability to evaluate new techniques for estimating permeability anisotropy and retrieving robust permeability data from logging measurements. This includes assessment of fabric for flow anisotropy, linking observations and new theory.
Assessed sediment stability in hydrate systems through two different approaches. In the first, infinite slope stability analysis is being used in the geologic accumulation models. This is the first step in trying to address the evolution of geohazards related to hydrate systems. This technique is computationally inexpensive, applicable in geologic and reservoir models, and provides a quick look at stability to identify locations for detailed stability analysis. The second stability analysis evaluates fracture genesis in fine-grained sediments to assess the condition for failure (fracture) and how it relates to fracture-hosted hydrate.
Developed a fracture genesis model. This model provides complementary data for other DOE studies looking at geomechanical properties and fractures in hydrate systems and how they relate to the presence of free gas within the hydrate stability region.
The fracture genesis model has been applied to four study regions (Blake Ridge, Hydrate Ridge, Keathley Canyon, and KG Basin) to understand the time-scales for fracture genesis and implications for when overpressures generate fractures or when hydrate heave generates fractures. This has implications for understanding hydrate distribution in fine-grained systems and how gas may migrate through fine-grained layers en route to accumulating in coarser-grained intervals.
New modeling includes free gas migration into and through the regional gas hydrate stability zone. The migration of free gas into the stability zone alters the hydrate saturation, the ability to vent free gas at the seafloor, and the timing for fracture genesis. This model is now being used to look at specific hydrate systems where free gas appears to exist within the regional gas hydrate stability zone.
The regional slope failure models have been extended from 1-D (infinite slope analysis) to 2-D using the method of slices to investigate two distinct slope failures offshore Cascadia that may be related to hydrate. Preliminary stability calculations suggesting that gas pressures are necessary to drive the failure. Ongoing research is testing how the gas pressures evolved in this gas hydrate setting due to free gas accumulation and/or gas hydrate dissociation during sea-level drop.
Beginning to isolate the causes for small, local failures (e.g., fractures) versus large, regional failures (submarine landslides) via numerical models being generated and field data that exist with the collaborators.
GEOPHYSICAL IMAGING OF GAS HYDRATE AND FREE GAS ACCUMULATIONS
Identified and initiated processing of a limited seismic dataset from the India hydrate expedition (NGHP1). The identified seismic line has three inline wells, all of which were drilled in 2001. The drilling was based on BSR signatures that appear to be similar at the well locations, but the recovered hydrate concentration was found to be varying.
Initiated collaboration with National Institute of Oceanography (NIO), India to demonstrate geophysical imaging with multichannel seismic data from the Krishna-Godavari (K-G) basin off the Indian east coast.
Demonstrated the ability to generate synthetic seismic response, both in 1-D (successfully) and in 2-D (tested for simple cases). Ricker wavelet, a type of widely applied source signal in seismic simulation, was used in the synthetic seismic response.
Demonstrated the ability to evaluate visco-acoustic properties in marine sediments, with consideration of hydrate/free gas distribution. This has implications for understanding hydrate variations within short distances.
Modified existing frequency domain waveform inversion code for application with conventional multi-channel seismic data from the KG Basin.
Developed a rock physics model for elastic wave modeling in the presence of fractures. This model will provide complementary data for other DOE studies looking at fracture porosity and connectivity pattern in unconsolidated, hydrate-bearing sediment.
Applied the fracture rock model to well data from the NGHP-01 Expedition to understand the relative hydrate distribution in background matrix and fractures. This has implications for the hydrate storage capacity of basins with high fracture density.
Up-scaled the model from log to seismic and demonstrated a method for hydrate quantification in fine-grained sediments using conventional multi-channel seismic data from the KG Basin.
Current Status
(May 2012)
The project has been completed. The final report is available below under “Additional Information”.
Project Start
Project End
DOE Contribution
Phase 1 - $3,082
Phase 2 - $295,415
Phase 3 - $249,125
Phase 4 - $354,098
Phase 5 - $259,335
Planned Total Funding (if project continues through all project phases):
DOE Contribution: $1,161,055
NETL – John Terneus (John.Terneus@netl.doe.gov or 304-285-4254)
Rice University – Dr. George Hirasaki (gjh@rice.edu or 713-348-5416)
If you are unable to reach the above personnel, please contact the content manager.
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].
