|Kinetic Parameters for the Exchange of Hydrate Formers
||Last Reviewed 12/16/2013
The overarching goal of this project is to gain an improved understanding of the dynamic processes of gas hydrate accumulations in geologic media by combining laboratory studies, numerical simulation, and analysis of shipboard infrared imaging of hydrate core samples. This project comprises four principal components: (1) fundamental laboratory investigations, (2) numerical simulator development and verification, (3) hydrate core characterization and analysis, and (4) applied laboratory and numerical investigations.
Pacific Northwest National Laboratory (PNNL), Richland, Washington
A new simulator in the STOMP simulator series for the production of natural gas hydrates from geologic accumulations has been developed and is currently being benchmarked. This new simulator, STOMP-HYDT-KE, is capable of modeling the production of natural gas hydrates via depressurization, thermal stimulation, inhibitor injection, and guest molecule exchange. The “HYDT” in the simulator name indicates the ternary hydrate system CH4-CO2-N2, and the “KE” indicates that the exchange of guest molecules and formation/dissociation of gas hydrates is modeled as a kinetic process. A major difficulty in developing STOMP-HYDT-KE was devising an equation of state that was reasonably accurate, computationally efficient, and free of convergence failures. The CH4-CO2-N2 system is particularly difficult to resolve in the hydrate stability region, as the mixture is often near its critical point. A hybrid tabular-cubic equation of state was developed that overcame convergence issues associated with pure cubic equations of state near the mixture critical point. The hybrid scheme uses tabular data and an innovative interpolation algorithm to establish the existing phases, gas molar fractions, and phase compositions. The cubic equation of state is then used to calculate phase densities and fugacity coefficients. STOMP-HYDT-KE is currently being benchmarked and has been successfully applied to an experiment conducted at the Korea Institute for Geoscience and Mineral Resources involving the injection of a CO2-N2 mixture into a CH4 hydrate-bearing, unsaturated column of sand.
Important issues regarding reservoir stimulation techniques, safety, and cost must be addressed before large scale commercial recovery of natural gas from hydrates can be attempted. Reservoir modeling is an important tool for addressing these issues; however, applying this modeling tool requires access to reliable thermodynamic, kinetic, and physical property data for gas hydrates and physicochemical properties of the hydrate-bearing sediments themselves. Laboratory studies to characterize gas chemistries of synthetic gas hydrate sands have been conducted using a residual gas analyzer. The proposed experiments will leverage the results of previous experiments where the gas composition within a synthetically rich methane hydrate core was successfully monitored, allowing the use of acquired hydrate dissociation kinetics. Furthermore, proposed measurements using the pressurized x-ray diffraction technique are unique and will be some of the first reported. This technique was successfully used to track mineral dissolution, carbonation reactions, and mineral volume changes. The types of structural information gained from this technique are believed to improve fundamental understanding of mechanisms that occur during the gas swapping process.
The conventional technologies for producing natural gas hydrates from geologic repositories—especially those with pore-filling type hydrates—are reasonably well understood, and numerical simulations have been compared against field trials (Kurihara et al. 2008). In contrast, the guest-molecule-exchange approach for natural gas hydrate production is emerging unconventional technology. Laboratory-scale experiments by ConocoPhillips and researchers at the University of Bergen, Norway have demonstrated the exchange of CO2 with clathrated CH4, but there have only been a limited number of numerical simulation investigations of the technology. This project provides an opportunity for a recently developed numerical simulator, STOMP-HYDT-KE, to be used to aid in the interpretation of data collected from the 2012 Ignik Sikumi gas hydrate field trial. The ultimate objective of this field of research is to develop numerical simulation tools capable of predicting the performance of the guest-molecule-exchange technology at the reservoir scale, including the geomechanical stability of the process. The work represents a first step in validating a numerical simulator capable of modeling the kinetic exchange of hydrate guest molecules. A credible interpretation of the Ignik Sikumi gas hydrate field trial, realized through numerical simulation, will greatly increase understanding of the fundamental exchange processes for hydrate formers.
The goal of this experimental work is to conduct measurements of methane hydrate dissociation and structural stability in hydrate-bearing sediments using a high-pressure cell and state-of-the-art analytical techniques. The kinetic exchange rates obtained on the ternary gas system will be utilized to validate numerical codes and the structural data will further support the concept of continuous stability of gas hydrate structures during gas swapping.
At the start of this project, the STOMP-HYDT-KE simulator was nearing the end of its development stage and had been demonstrated against laboratory-scale experiments conducted at the Korea Institute for Geosciences and Mineral Resources. The simulator, however, had not been applied beyond that verification exercise. Project personnel initially reviewed the numerical solution scheme and algorithms of the STOMP-HYDT-KE simulator. This review prompted a moderate redesign of the phase conditions, flash algorithms, boundary conditions, initial conditions and sources. The phase conditions were collapsed to four core conditions with options within each core condition: (1) aqueous saturated without hydrate, (2) aqueous unsaturated without hydrate, (3) aqueous unsaturated with hydrate, and (4) aqueous saturated with hydrate. The boundary conditions were collapsed to energy and flow types, and the number of initial condition variable options was reduced. All of the elements of the code redesign have been implemented and verified for proper execution.
Current Status (December 2013)
The STOMP-HYDT-KE simulator is currently being applied to the Ignik Sikumi field trial with the objective of using numerical simulation to interpret the collected data from the field trial. Simulations to define the injection period of the field trial are being conducted. The first simulation of the injection period used a simple radial domain starting at the well casing and extending radially to 2000 ft (610 m). The formation was initialized at reservoir conditions of 41°F (5°C) and 1080 psia (7.446 MPa), with a porosity of 0.4, and hydrate saturation of 0.75. The hydrate was assumed to be composed of pure CH4. The intrinsic permeability of the reservoir rock, the high permeability clastic sandstones of the Sagavanirktok Formation, was assumed to be 1 D, and the effective permeability of the hydrate filled rock was assumed to be 1 mD. For simplicity, the 306-hour injection period was modeled as being at the following constant temperature, pressure, and composition conditions: 41°F (5°C), 1420 psia (9.791 MPa), mole fraction of N2 = 0.775, mole fraction of CO2 = 0.225. This simple simulation yielded total injection amounts of N2 and CO2 of 142.5 and 41.0 Mscf, respectively, which compare reasonably well with the experimental values of 167.3 and 48.6 Mscf, respectively. This simple simulation, however, predicts a stable injection rate, unlike the improving injectivity observed in the experiment.
The experimental tasks have been delayed to permit the transfer of laboratory equipment between two buildings on the PNNL campus (PNNL vacated the building holding the hydrate laboratory equipment at the end of FY13). The hydrate laboratory equipment was moved and installed at its new location in the beginning of December. The specific equipment needed for the CH4-CO2-N2 exchange and pressurized x-ray diffraction studies have been configured. These two experiments are now proceeding.
Project Start: June 1, 2013
Project End: December 31, 2014
Project Cost Information:
All DOE Funding
FY13 - DOE Share - $90,000
Total Funding to Date: $90,000
NETL – John Terneus (John.Terneus@netl.doe.gov or 304-285-4254)
PNNL – Mark White (email@example.com or 509-372-6070)