Bahman Tohidi

SUBSURFACE CARBON DIOXIDE STORAGE THROUGH CLATHRATE HYDRATE FORMATION

Rising atmospheric emissions as a result of fossil fuel consumption is a major concern for the developed and developing countries, considering the role it plays in the greenhouse effect and hence global climate change. Various schemes for underground CO2 storage (viz. geologic disposal into coal seams, depleted oil/gas reservoirs, salt caverns, and deep oceans) have already been reported in the literature. Subsurface CO2 storage through clathrate hydrate formation is a novel option for the reduction of atmospheric carbon content and permanent underground CO2 disposal over geological periods. Depths of CO2 injection, respective pressure-temperature conditions, water salinity etc. are all important factors for successful CO2 sequestration. Furthermore if CO2 is injected/stored in methane hydrate reservoirs it could be possible to produce low-carbon methane energy, thereby offsetting the cost of CO2 transportation and disposal. In this communication, we present the results of experiments carried out to understand the mechanisms of CH4 displacement in hydrate structure by injected CO2 and the formation of simple CO2 or mixed CH4-CO2 hydrates, thereby simulating the conditions of CO2 injection into CH4 hydrate reservoirs. We used two sets of experimental rigs specifically designed for studying gas hydrates in porous media. They are the Medium Pressure Glass Micromodel (80 bar) for visual observation of gas hydrate formation / dissociation and distribution in porous media, and the Ultrasonic Rig (400 bar) for studying CO2 sequestration in CH4 hydrates in synthetic porous media.

GAS SEPARATION AND STORAGE USING SEMI-CLATHRATE HYDRATES

Tetra-n-Butyl Ammonium Bromide (TBAB) forms semi-clathrate hydrates which can incorporate small gas molecules, such as methane and nitrogen at ambient temperatures and atmospheric pressure. Such favourable stability conditions, combined with ease of formation could make semi-clathrates particularly attractive for a large variety of applications. These hydrates have recently been investigated for their use in the separation of gases, and it is proposed that the same technology could potentially be used for storage and transportation of gases. To evaluate the feasibility of using TBAB hydrates for separation and storage purposes, an extensive test programme was conducted to determine: phase stability of the semi-clathrates, gas storage capacity, and composition of the stored gas. The results show that TBAB semi-clathrates have very favourable stability conditions. They can store considerable quantities of gas, and favour small molecules in their structures. These experiments suggest that semi-clathrate hydrates, such as TBAB, could have a significant potential as an alternative for industrial separation, storage, and transportation of natural gas.

Gas Hydrates in Oil Systems

Extensive studies have been conducted on hydrate formation in gaseous systems; however, little information is available on hydrate risks in oil systems. This is partly due to difficulties in measuring the hydrate stability zone in oil systems, as well as problems associated with compositional representation of such systems. Furthermore, it is reported that hydrate formation pose less risks in oil systems than gas systems, partly due to the presence of natural inhibitors, formation of water in oil emulsions, and possibly oil wet pipeline walls. However, little is known of the nature and mechanism of the natural inhibition and the response of the system to an increase in the water cut. In this communication, we present details of two experimental set-ups specifically designed to study the hydrate risks in oil systems. They include a visual high pressure kinetic rig for measuring the hydrate stability zone, crystal size, distribution and the rate of hydrate formation and dissociation, and a glass micromodel for visual observation of hydrate formation / dissociation in micro-scale. We report some of the recently obtained experimental results using the above experimental equipment. The results show that the existing techniques should be revisited for generating reliable data in oil system. We also demonstrate that proper characterization of the heavy end and taking into account the possibility of wax formation may play a role in improving the reliability of predictive techniques. The results provide better understanding and evaluation of risks associated with hydrate formation in pipelines carrying oil.

Seismic time-lapse monitoring of potential gas hydrate dissociation around boreholes - could it be feasible? A conceptual 2D study linking geomechanical and seismic FD models

Monitoring of the seafloor for gas hydrate dissociation around boreholes during hydrocarbon production is likely to involve seismic methods because of the strong sensitivity of P-wave velocity to gas in sediment pores. Here, based on geomechanical models, we apply commonly used rock physics modeling to predict the seismic response to gas hydrate dissociation with a focus on P-impedance and performed sensitivity tests. For a given initial gas hydrate saturation, the mode of gas hydrate distribution (cementation, frame-bearing, or pore-filling) has the strongest effect on P-impedance, followed by the mesoscopic distribution of gas bubbles (evenly distributed in pores or “patchy”), gas saturation, and pore pressure. Of these, the distribution of gas is likely to be most challenging to predict. Conceptual 2-D FD wave-propagation modeling shows that it could be possible to detect gas hydrate dissociation after a few days.

Thermodynamic conditions and kinetics of integrated methane recovery and carbon dioxide sequestration

Jinhai Yang, Antonin Chapoy, Bahman Tohidi, Prashant Sopanrao Jadhawar, JaeHyoung Lee, Dae Gee Huh

Gas Hydrates and Deepwater Operation: Predicting the Hydrate Free Zone

The application of extended subsea networks and transportation of unprocessed well-streams are amongst favorable options for reducing field development and operational costs. These pipelines normally convey a cocktail of multiphase fluids, including mixed electrolyte produced water and liquid and gaseous hydrocarbons and may therefore be prone to hydrate formation, which potentially can block the pipe and lead to serious operational problems. For deep water operation, even saturated saline solutions may not provide the required protection, unless combined with chemical inhibitor. The reported data on hydrate formation in mixed salt and chemical inhibitor are very limited and in some cases inconsistent. In this work, a model is introduced to predict the hydrate free zone in mixed salt and chemical inhibitor designed for offshore and deep water applications. The model is based on combination of the Valderrama modification of the Patel-Teja equation of state with non-density dependent mixing rules and a modification of a DebyeHuckel electrostatic term, which is applied to systems containing salt and chemical inhibitor by correcting the properties of the aqueous phase such as dielectric constant, density and molecular weight. A linear mixing rule is used for determining the dielectric constant of salt-free mixture by introducing an interaction parameter (in dielectric constant mixing rule), which is tuned using the freezing point data of aqueous solutions containing salt and organic inhibitor. The binary interaction parameter between salt and organic inhibitor is adjusted using water vapor pressure data in the presence of salt and organic inhibitor. The predictions are compared with experimental data and a literature model, demonstrating the reliability of the developed model.