|Kinetic Parameters for the Exchange of Hydrate Formers
||Last Reviewed 11/30/2015
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.
Budget Period 3 – The project team will change the focus of the investigations from the kinetics of exchanging CO2 and N2 with clathrated CH4 in hydrate bearing geologic media to geomechanical processes associated with producing natural gas hydrates. The results of the Nankai Trough experiment indicate the critical importance of understanding the geomechanical processes in producing natural gas hydrates from suboceanic deposits. During the third budget period the work will be focused on developing fully coupled capabilities for simulating the deformation of the reservoir and overlying strata with changes in effective stress from changes in pressure and temperature. The geomechanical modeling capabilities will be limited to linear elasticity, but will include the ability to predict mechanical failure through a Mohr-Coulomb criterion. Other operational modes of the STOMP simulator have realized geomechanical capabilities via coupling with the Abaqus simulator, but this approach creates two immediate limitations: 1) the geomechanical component can not be directly altered by the STOMP development team, and 2) the coupled codes can not be converted to a full parallel implementation for execution on distributed memory computers.
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. 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.
Support from DOE NETL and KIGAM have yielded simulation capabilities in the STOMP-HYDT-KE simulator that allow for the modeling of fully coupled multifluid hydrologic, heat transfer, hydrate thermodynamics, and geochemistry. Moreover the simulator is formulated to model the exchange of hydrate formers, hydrate dissociation, and hydrate formation as kinetic processes for a ternary hydrate former system N2, CO2, and CH4. The missing element in this suite of capabilities is the coupling with geomechanics; where, changes in pore pressure and temperature yield changes in effective stress, resulting in rock deformation or failure. These deformations or changes in stresses in turn yield changes in porosity and intrinsic permeability, which directly impact the hydrologic system. The proposed work will allow for the coupling to be integrated into a single simulator with capabilities for execution on sequential, shared-memory parallel, and distributed-memory parallel computers. Kinetic hydrate simulations are computationally expensive and coupling geomechanics adds to that expense, which makes parallel computing a necessity to realize problem solutions to real-world problems at reservoir scales.
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.
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.
Current Status (November 2015)
In 2012 the U.S. DOE/NETL, ConocoPhillips Company, and Japan Oil, Gas and Metals National Corporation jointly sponsored the first field trial of injecting a mixture of N2-CO2 into a CH4-hydrate bearing formation beneath the permafrost on the Alaska North Slope. Known as the Ignik Sikumi #1 Gas Hydrate Field Trial, this experiment involved three stages: 1) the injection of a N2-CO2 mixture into a targeted hydrate-bearing layer, 2) a 4-day pressurized soaking period, and 3) a sustained depressurization and fluid production period. Data collected during the three stages of the field trial were made available after a thorough quality check. The Ignik Sikumi #1 data set is extensive, but contains no direct evidence of the guest-molecule exchange process. This study uses numerical simulation to provide an interpretation of the CH4/CO2/N2 guest molecule exchange process that occurred at Ignik Sikumi #1. Simulations were further informed by experimental observations. The goal of the scoping experiments was to understand kinetic exchange rates and develop parameters for use in Iġnik Sikumi history match simulations. The experimental procedure involves two main stages: 1) the formation of CH4 hydrate in a consolidated sand column at 750 psi and 2°C and 2) flow-through of a 77.5/22.5 N2/CO2 molar ratio gas mixture across the column. Experiments were run both above and below the hydrate stability zone in order to observe exchange behavior across varying conditions. The numerical simulator, STOMP-HYDT-KE, was then used to match experimental results, specifically fitting kinetic behavior. Once this behavior is understood, it can be applied to field scale models based on Ignik Sikumi #1. A poster documenting this work will be presented at the 2015 AGU Fall Meeting, Ruprecht, C.M., J.A. Horner, and M.D. White. 2015. “Experimental and Numerical Investigation of Guest Molecule Exchange Kinetics based on the 2012 Ignik Sikumi Gas Hydrate Field Trial." Abstract submitted to AGU Fall Meeting, San Francisco, CA. PNNL-SA-112587.
Hydrate exchange experiments are being conducted to observe the kinetic behavior of CO2/N2/CH4 mixtures. 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. Three experimental iterations have been run this quarter, each time improving the uniformity of the initial hydrate distribution across the sand packed column, a crucial initial condition.
To observe the hydrate distribution in the core, improvements have been made to the experimental apparatus. Eight new thermocouple sensors have been added that span the column and allow us to observe the temperature distribution. This addition led to the implementation of custom-built foam cooler which insulates the experimental column. A pressure transducer has also been added to the outlet in order to allow for more accurate measurements of the pressure conditions across the core. While the procedure remains largely the same, the method to form the initial methane hydrate has evolved. Current experiments allow time for the methane to fully saturate the aqueous phase prior to lowering the temperature and raising the pressure. Additionally, the sand packed column is now horizontally aligned, in order to reduce the effects of gravity on flow through the column. The fine-grained silica sand (20-40 mesh; 0.4-0.5 mm) that has been used for all experiments will be replaced with a finer-grained “Accusand”, due to non-uniform initial aqueous saturations.
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
FY15 – DOE Share - $50,000
Total Funding to Date: $220,000
NETL – John Terneus (John.Terneus@netl.doe.gov or 304-285-4254)
PNNL – Mark White (firstname.lastname@example.org or 509-372-6070)
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