|Mechanisms for Methane Transport and Hydrate Accumulation in Coarse-Grained Reservoirs
||Last Reviewed 6/13/2016
The project goal is to evaluate whether the transport of methane, and the specific mechanism by which it is transported, are the primary controls on the development of persistent, massive hydrate accumulations in sediments below the seabed. The dissolved methane flux and time required to develop the accumulations observed at Walker Ridge Block 313 (WR313) in the northern Gulf of Mexico by long-distance updip migration or by short-distance local migration will be studied and defined within the scope of this research. Researchers will also determine whether there is enough methane in the dissolved phase in the fine-grained sediments to form the observed hydrate deposits or whether a gas phase is present and, if so, what the conditions are for three-phase equilibrium.
University of Texas at Austin, Austin, TX 78713-7726
Ohio State University, Columbus, OH 43210
Lamont-Doherty Earth Observatory (Columbia University), Palisades, NY 10964
Massive hydrate deposits, defined as thick (>5 feet) accumulations of high hydrate saturation (>50 percent), have been encountered in many regions worldwide. This project will focus specifically on accumulations found at WR313 during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg 2. Hydrates may be thought of broadly within a petroleum systems framework, requiring a methane source, migration mechanisms, a reservoir, and an appropriate seal. Hydrate reservoirs and seals are defined by thermodynamics rather than buoyancy as in the case of conventional oil and gas. Hydrates form most easily within coarse-grained sediments within the methane hydrate stability zone, the depth interval at which pressure and temperature favor hydrate as the stable phase. Methane sources may include microbial activity as well as thermogenic sources. The focus will be on migration mechanisms in marine hydrate reservoirs as they represent some of the least understood processes in hydrate systems, but at the same time represent a crucial link between methane generation sites and hydrate reservoirs.
Successful completion of this project will provide valuable insight into conditions necessary for the development of massive gas hydrate accumulations and the role of free gas in their persistence. This, in turn, will advance understanding of the transport and fate of methane in the subsurface, carbon cycling associated with hydrates, and the role of a free gas phase in the formation and persistence of hydrate deposits.
An existing reservoir simulator has been altered to include methanogenesis, sedimentation, salinity, and pore size effects on hydrate stability. The modified simulator has been successfully benchmarked against published 1-D and 2-D simulations and found to be consistent within 1% in time and hydrate saturation given the same input parameters. A series of 2-D simulations were conducted in a dipping sand layer at WR313 to investigate the conditions that led to the development of the shallow, hydrate saturated sand layers at WR313. Preliminary 2-D model results suggested that microbial methanogenesis may be necessary to form the observed hydrate accumulations in the shallow sands at WR313; however, long range methane migration might be necessary to form hydrate saturations in the deeper sand layers. Metabolizable organic content was found to be an important parameter in determining hydrate saturations associated with microbial methanogenesis. Additional simulations will be conducted using realistic sediment properties (such as metabolizable organic content) calculated from the 1-D reactive transport model discussed below.
In order to further constrain the sediment properties for reservoir simulations at WR313, detailed log analysis was conducted on wells in the Gulf of Mexico. In particular, data collected in the Keathley Canyon 151 research well were used to derive a method of generating synthetic pore size distributions from gamma ray and porosity logs. The method is still being refined, but preliminary results indicate that the pore size contrast between the sands and clays at WR313 may only be one order of magnitude, which might not be enough to drive sufficient diffusive flux into the sand layers. Furthermore, a journal article was published in the Marine and Petroleum Geology journal describing a new method to calculate permeability from log data using porosity, clay-sized particle mass fractions, and appropriate endmember permeabilities for clay and sand (Daigle et al., 2015). The permeability calculations have been verified against measured permeability data from three wells in the Gulf of Mexico and extrapolated to WR313 where permeability log data are unavailable. Sediment properties calculated from log data at WR313 have been applied as input parameters to the reservoir simulator and initial simulations have been completed.
Using the physical properties derived from well logs, the 3-D reservoir simulator was used to model (1) the accumulation of hydrate by diffusion of microbial methane alone and (2) the accumulation of hydrate associated with advective supply of methane-saturated water without microbial methanogenesis. The 3-D reservoir simulations were conducted in a dipping sand layer with geometry based on the Orange sand at WR313 (Figure 1). Preliminary results suggest that diffusion alone may be able to form the observed hydrate deposits at WR313. However, additional simulations need to be run to analyze the effects of input parameters, mainly the rate of microbial methanogenesis. Hydrate saturations achieved in advection simulations were much lower than that of microbial methane diffusion, suggesting that advection rates would need to be much higher than originally anticipated to develop similar accumulations. Work is ongoing to test different parameters to assess the validity of the results. Reactive transport modeling is underway to constrain input parameters for additional reservoir simulations.
Figure 1: Geometry of the 3D reservoir simulations modeled after the Orange sand at WR313. Colors are contoured by depth and the dark blue layer is the surface of the methane hydrate stability zone (MHSZ).
A 1-D reactive transport model has been created to couple sedimentation and microbial methanogenesis to model hydrate accumulation in a dipping, subsiding sand layer at WR313. Because sediment porosity changes during compaction and the amount of methane dissolved in the pore fluid increases as sediment is buried, sedimentation is a key component for modeling hydrate accumulation. The reactive transport model has been fully developed and benchmarked against existing data. The model uses numerical integration to compute steady-state dissolved methane concentrations and gas hydrate or gas bubble volume fractions as a function of depth in a one-dimensional geometry. The numerical method is computationally fast and is suitable for Monte Carlo simulations. In these simulations, model outputs are computed in a large number of iterations where uncertain input parameters are allowed to vary within realistic limits. By retaining the results where the predicted gas hydrate amounts agree with observations, these simulations measure the uncertainty in the input model parameters. The reactive transport simulations were applied to quantify the uncertainty of the intensity and depth distribution of microbial methanogenesis. The results have been used to constrain the available organic carbon at the seafloor, reaction rate of microbial methanogenesis, and pore water advection rate in the 3-D, time-dependent reservoir simulator.
To test whether short migration of microbial methane can lead to the hydrate saturations observed at WR313 (up to 80%), simulations were conducted to assess hydrate accumulation from microbial methane in a series of dipping sand units based on present-day geometry while incorporating subsidence due to sedimentation. Simulation results indicated maximum hydrate saturations of 25% in the deeper sand units after 1.4 million years. Based on the organic carbon availability and methanogeneis rates at WR313, this may be the upper limit of hydrate saturation from show migration, suggesting that long migration of methane from another source is necessary to form the observed accumulations.
Current Status (June 2016)
The 3-D basin-scale reservoir simulator has been modified to include methanogenesis, sedimentation, salinity, and pore size effects on hydrate stability. The model domain was created for a realistic representation of a hydrate sand layer at WR313 and sediment properties have been modified to reflect log data calculations. Reservoir simulations are ongoing and will be further refined as input parameters are quantified in the reactive transport models. The 1-D reactive transport model has been designed and benchmarked against existing simulations. Physical properties of the sediments at WR313 have been refined based on reactive transport model results, pore size distributions, and nuclear magnetic resonance log data. Advective simulations are ongoing to investigate the effect of overpressure on driving fluid flow back up through sands and either redistributing biogenic methane or delivering methane from a deeper source to the methane hydrate stability zone. Continued work will focus on parameter testing for diffusive and advective methane supply, and simulations will continue to assess hydrate accumulation by means of short and long migration.
Project Start: October 1, 2013
Project End: September 30, 2017
Project Cost Information:
DOE Contribution: $1,679,137
Performer Contribution: $448,001
NETL – Stephen Henry (email@example.com or 304-285-2083)
University of Texas at Austin – Hugh Daigle (firstname.lastname@example.org or 512-471-3775)
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