|Kinetic Parameters for the Exchange of Hydrate Formers
||Last Reviewed 6/12/2015
This project will investigate the kinetics associated with producing natural gas from hydrate bearing geologic media via unconventional technologies. The principal production technology of concern for this research will be that of exchanging CO2 and N2 with clathrated CH4. The so called guest-molecule exchange technology is an attractive technology from the perspective of its potential for maintaining the geomechanical integrity of the reservoir formation, in addition to its carbon neutral potential. As with other unconventional technologies, its ability to produce natural gas will depend on our understanding the processes and our ability to exploit this understanding. The approach taken during this project will be to understand the kinetic mechanisms that control the exchange of hydrate formers using numerical simulation to interpret field-scale trials, laboratory experiments to determine kinetic parameters, and code comparison to verify mathematical models and solution schemes.
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.
Budget Period 1 - During the first budget period, the project team will investigate the kinetics of exchanging CO2 and N2 with clathrated CH4 in hydrate bearing geologic media. The project comprises two distinct components: (1) numerical investigation of the 2012 Iġnik Sikumi gas hydrate field trial, and (2) experimental investigation of kinetic exchange processes in laboratory-scale hydrate bearing unconsolidated sands. The principal objective of the numerical component will be to provide an interpretation of the data gathered at Iġnik Sikumi Well #1. The experimental component of this project is designed to provide kinetic exchange parameters needed for the numerical simulation. The principal objective of the two experiments is to provide an order of magnitude value to the kinetic exchange parameters for the field-scale simulations of the Iġnik Sikumi gas hydrate field trial.
Budget Period 2 - The project team will expand the investigations of the first budget period during the second budget period. The team will continue to run numerical simulations of the 2012 Iġnik Sikumi gas hydrate field trial in order to resolve disagreements between simulation results and field trial observations and provide a more thorough interpretation of the field results. Laboratory experiments designed to provided kinetic parameters under controlled conditions will continue, and a code comparison study focused on expanding the International Hydrate Code Comparison Study to problems involving gas hydrates of mixtures of CH4, CO2, and N2 hydrate formers were proposed. West Virginia University is preparing a suite of problems for the code comparison study, which involves hydrates of pure components and component mixtures of CH4, CO2, and N2. The laboratory experiments and code comparison study are currently unfunded.
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 (June 2015)
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 were conducted over three periods of the field trial: (1) injection, (2) soak, and (3) production. This simulation work has been documented in two papers: (1) White, M.D. and W.S. Lee. 2014. “Guest molecule exchange kinetics for the 2012 Ignik Sikumi Gas Hydrate Field Trial,” Proc., Offshore Technology Conference held in Houston, Texas, USA, 5-8 May 2014, OTC-25374-MS.; (2) Anderson, B., R. Boswell, T. S. Collett, H. Farrell, S. Ohtsuka, and M. White. 2014. “Review of the findings of the Ignik Sikumi CO2-CH4 gas hydrate exchange field trial,” Proc., 8th International Conference on Gas Hydrates (ICGH8-2014),Beijing, China, 28 July–1 August, 2014. Numerical simulation of the injection period of the Iġnik Sikumi #1 field trial revealed three important processes. The guest molecule exchange process alters the equilibrium state for the hydrate, which can lead to hydrate dissociation or secondary hydrate formation. The guest molecule exchange process alters the density of the hydrate, which can change the effective exchange process from being a one-to-one molar exchange. The concept of bound-water is critical for preventing secondary hydrate from forming and clogging the pore space. Comparison of the numerical simulations against the data collected during the injection period of the Iġnik Sikumi #1 field trial is not sufficient for uniquely determining the kinetic rate parameters for hydrate formation, dissociation, or guest-molecule exchange because agreement between the gas injection rates between the field trial and numerical simulations can be made with different kinetic parameters by moderate alterations in scaling of the reservoir intrinsic permeabilities.
A series of guest-molecule exchange experiments have been conducted involving the replacement of mixtures of N2 and CO2 with clathrated CH4 under different temperature and pressure conditions. The target pressure and temperature conditions varied between being within and outside the stability zone for the N2 and CO2 mixture, but always within the stability zone for pure CH4 hydrate. The first study targeted under this task was the development of a standardized procedure to perform in situ monitoring of pore gas chemistry during the replacement of methane in a CH4 hydrate bearing porous sand with CO2 through the titration of a gaseous mixtures consisting of different ratios of N2/CO2. The continuous monitoring of pore gas chemistry would provide clear evidence of the rates associated with the exchange of CH4 with CO2. The experimental procedure involves three main stages: 1) the formation of CH4 hydrate in a porous sandstone, 2) replacement of the core gas with a N2/CO2 gas mixture, and 3) the monitoring of the core gas chemistry over time during the exchange process. The goal of these scoping experiments is to develop kinetic exchange rates and parameters for use in the simulations conducted under the Iġnik Sikumi History Match task.
For the initial test, fine-grained silica sand (20-40 mesh; 0.4-0.5 mm) was moistened with de-ionized water to 9% saturation and compacted to 0.42 porosity in a cylindrical, semi-transparent HYDEX column (1.9 cm inner diameter; 20.4 cm length) equipped with a thermocouple inserted in the center of the column. The column was sealed, wrapped with copper tubing, and cooled to +2°C by circulating chilled fluid though the tubing. The column was then pressurized to 1,000 psi with CH4 and cooled to -2°C, and subsequently cycled between +2° and -2°C for several days to promote hydrate growth. Initially hydrate formation was confirmed through changes in temperature and over time, through visual observations of a white fine grained material appearing in the pore spaces.
The exchange experiment was conducted while the column was maintained at 2°C and ~1000 psi. Using the ISCO syringe pump, a gas mixture consisting of 90% N2 and 10% CO2 was titrated into the bottom of the column and allowed to flow out through the top at 0.5 mL/min. Constant flow was maintained by utilizing a finely tuned pressure relief valve at the outlet (top of column) as well as a by-pass value. A continuous gas sample was collected from the outlet and passed directly into the RGA where partial pressures for masses corresponding to CH4 (16), CO2 (44), and N2 (28) were monitored. Initially, before the start of the titration, CH4 has the highest partial pressure. However, shortly after introducing the N2/CO2 gas mixture, the N2 partial pressure began a steady increase and the CH4 partial pressure declined. This is an expected trend and indicates the changing concentrations of CH4 and N2 in the column. Concentrations of CO2, monitored through observing the partial pressure of mass 44 (CO2), remain constant for the first 2.2 hours before showing signs of an increase. Increasing CO2 concentrations occurred until about 6 hours in to the titration, after which they remained relatively constant. After 20 hours, the column was isolated and the temperature increased above the hydrate stability zone (~18°C) to allow all the existing hydrate to disassociate. The final gas analysis showed ~ 3 times the amount of CO2 to CH4, which indicates a significant amount of CO2 hydrate existed in the column. Staff will repeat this experiment with the addition of a flushing step to remove any free CH4 in the column prior to disassociating the hydrate. Funding for the x-ray diffraction experiments was deverted to further the guest-molecule exchange experiments because of the value of the collected kinetic data.
Project Start: June 1, 2013
Project End: July 31, 2016
Project Cost Information:
All DOE Funding
FY13 DOE Share: $90,000
FY14 DOE Share: $80,000
Total Funding to Date: $170,000
NETL – John Terneus (John.Terneus@netl.doe.gov or 304-285-4254)
PNNL – Mark White (firstname.lastname@example.org or 509-372-6070)
Quarterly Research Progress Report [PDF-315KB] October - December, 2014
Quarterly Research Progress Report [PDF-186KB] July - September, 2014
Quarterly Research Progress Report [PDF-156KB] January - March, 2014
Quarterly Research Progress Report [PDF-418KB] October - December, 2013
Quarterly Research Progress Report [PDF-210KB] July - September, 2013