Embed Size (px)
Citation preview
Solubilities of gas hydrate forming hydrocarbon gases in aqueous
mixtures at subsea marine sediments conditions
Georgia G. Bekiari
MSc in Oil & Gas TechnologySupervisor: Prof. Nikolaos Varotsis
Purpose of this study
Τo measure the solubility of methane in the aqueous phase at constant pressure and different
temperature values. Experiments were conducted in the laboratory of PVT & Core Analysis of
the Technical University of Crete.
Study of the thermodynamic equilibrium of methane gas hydrates with pure aqueous
phase.
Study of the thermodynamic equilibrium of methane gas hydrates in aqueous solutions of
salts and at different degrees of salinity.
Study of the thermodynamic equilibrium of methane gas hydrates in aqueous solutions,
at the presence of a free gas phase.
What is gas hydrates
Gas hydrates are crystalline nonstoichiometric solid-inclusion compounds.
Clathrate
Klathronhigh
pressurelow
temperature
Sufficient
amount of
gas and
water is present
Gas hydrates form in:
Small sized molecules fit in the cages of a water molecule lattice.
Between the water molecules, which form the cell and the entrapped molecule,
Van der Waals forces are developed, which thermodynamically stabilize the
hydrate phase.
Hydrates have similar appearance and physical properties as the ice has.
1. Historical Overview
Gas hydrates are known to the international research community for almost two
centuries now. Historically, the research on natural gas hydrates can be classified
in four periods:
1st In 1800 Humphrey Davy identified chlorine as an element, in 1810 Michael Faraday was mixing
green gas with water and cooling the mixture to low temperatures. Then Villard (1888) reported
hydrates of hydrocarbons such as methane, ethane, propane, acetylene and ethylene.
2nd In 1934 E.G. Hammershmidt determined that methane hydrate was plugging NG pipelines.
3rd In 1967 Soviet geologists discovered the first major deposit in the permafrost.
4th mid 90-s from India and Japan, began self-funded gas hydrate programs in preparation for
production of methane from marine reservoirs.
Gas hydrates structures
Sloan & Koh, 2007
Formation of gas hydrates at natural environment
Natural gas, especially methane is trapped in hydrates in geological sediments. In order hydrates
to be formed:
Water
(gas, liquid, or ice)Gas
molecules
Appropriate
Pressure & Temperature
These conditions occur in nature, in permafrost and in oceanic regions. The amount of hydrates in the oceans
exceeds that in the permafrost.
The first gas production from hydrate in the permafrost is the Messoyakha field in the northern
part of the West Siberian Basin.
The Mallik 2002 well in Canada hydrates has been found at the seafloor and in deeper sediments.
Hydrates from biogenic and thermogenic gas
The two main processes that generate methane in nature are:
Biogenic from anaerobic degradation of bacterial activity. In pressure and temperature
occurring in the ocean or permafrost.
Thermogenic is generated during the thermally-activated breakdown of larger organic
molecules, are produced only under high temperature and pressure from kerogens which
are derived from organic matter.
Methane from both biogenic or thermogenic origin combines with water in sediments in order to form
hydrates.
Of the two sources of methane in natural hydrates most of the gas is biogenic.
Climate change and geohazards
Methane contributes to greenhouse effect as it is 20 times more effective than CO2 absorbing
the infrared radiation emitted from the earth.
Increasing temperatures can cause a sharp increase of methane release in atmosphere due to the
dissociation of hydrates.
Dissociation of hydrates into geological formations is an environmental risk cause disintegration
of geological strata.
The consequences are submarine landslides, destruction of underwater substructure, collapsing
boreholes and the creation of tsunamis.
Hydrates can cause plugging of oil and gas pipelines, this can have economic&
safety impacts on flow line operations.
Thermodynamic equilibrium of gas hydrates
Three phase equilibrium curve Gas-Hydrate-Aqueous phase (or ice)
Two phase equilibrium curve Ice-Aqueous phase
Experimental arrangement
Experimentation
The water ratio to methane gas is much more in order to study the
solubility in the aqueous phase so its needed to have excess water after
the formation.
Reactor temperature is set to the desired value.
Gas & water are injected under maximum flow and stirring rate (900rpm).
The mixture is maintained for one day at constant conditions while
stirring at 900rpm. Thermal and pressure stability is achieved in the
autoclave reactor.
A sample of approximately 6cc is flashed to ambient conditions. Fresh water is
injected during the sampling in order to keep the autoclave pressure constant.
Experimentation
The amount of produced water is measured gravimetrically at ambient conditions.
The amount of released gas is measured volumetrically at ambient conditions.
The dissociation of hydrates was achieved through a step-wise depressurization
from the hydrate equilibrium pressure under continuous stirring, that brings
hydrates outside the thermodynamic stability envelope. Dissociation produces
gas-saturated water and water-saturated gas.
With the pressure stabilized we get one more sample at the three phase
equilibrium (aqueous phase + hydrate + vapor) following the same
procedure.
Continue to the next set of temperature and pressure conditions
Measurement of Methane Solubility
Moles of the gas are calculated with the ideal gas equation of state.
Humidity is calculated from water pressure saturation data and it is
excluded from the measured volume of the gas.
Moles of liquid water are calculated by the formula:
The solubility of methane gas is calculated:
Thermodynamic equilibrium of methane gas hydrates & methane
solubility in zero salinity aqueous phase
The pressure was kept constant at 174bar & the temperature varied between 278 K and 289 K.
During the experimental measurements we:
Monitored the equilibrium pressure.
Monitored the equilibrium temperature.
Measured the concentration of the methane dissolved in the aqueous phase.
The concentration of methane that is dissolved in water is determined as molar
concentration in the aqueous phase.
Thermodynamic equilibrium of methane gas hydrates and methane
solubility in aqueous phase of different salinity
In the two-phase equilibrium (GH + aqueous phase), the addition of salts or other organic
compounds, such as methanol, reduces the water chemical potential in the aqueous phase.
As a result, there is diffusion of water molecules from the hydrate to the liquid phase. This
destabilizes the crystalline lattice and shifts the hydrate formation curve, to lower temperatures
and higher pressures.
The concentration of dissolved methane in the aqueous phase was measured as a
function of the NaCl concentration, under constant pressure (174 bar) and for a
temperature range of 278 K -289K. These conditions were selected in order to
simulate in a seabed area in which the aqueous and the hydrate phase coexist
(diphasic region).
Experimental Results
The literature curve is from Sloan, 1998, the simulation curve was produced using one of the most
commonly used simulation programs (Sloan's CSMGem), and the red one is constructed for the
measured data.
The methane hydrate formation curves
Experimental Results
Methane solubility in a zero salinity aqueous phase at 174bar and different temperatures.
The conditions correspond to the two-phase thermodynamic equilibrium of GH and liquid
water.
Experimental Results
Methane solubility in a zero salinity aqueous phase (after hydrate dissociation). The conditions
correspond to the three-phase thermodynamic equilibrium.
Experimental Results for the aqueous phase of salinity 3%
Methane solubility in 3% salinity aqueous phase at 174bar and different temperatures. The conditions
correspond to the two-phase thermodynamic equilibrium.
Experimental Results for the aqueous phase of salinity 3%
Methane solubility in 3% salinity aqueous phase (after hydrate dissociation). The conditions correspond to the
three-phase thermodynamic equilibrium.
The three phase equilibrium (aqueous phase+hydrate+vapor), is starts from the black dot line. Higher temperature
values correspond to conditions inside the three-phase envelope.
Experimental Results for the aqueous phase of salinity 6%
Methane solubility in 6% salinity aqueous phase at 174bar and different temperatures. The
conditions correspond to the two-phase thermodynamic equilibrium.
Experimental Results for the aqueous phase of salinity 6%
Methane solubility in 6% salinity aqueous phase (after hydrate dissociation). The conditions
correspond to the three-phase thermodynamic equilibrium.
% NaCl in aqueous phase
Methane solubility in aqueous solutions at 174bar for different temperatures and salt
concentrations. The conditions correspond to the two-phase thermodynamic equilibrium.
Observations - Conclusions
Dissociation of hydrates was required before measuring at a new formation temperature.
The minimum time required to homogenize the hydrate crystals in the reactor was 1 day,
conditions were kept constant and the stirring rate was the maximum.
The stirring rate in the reactor, should be maximum, 900 RPM, for achieving rapid
homogenization of the newly formed hydrate crystals.
The experimental results of methane solubility are consistent with the results of the simulation
program CSMGem and the literature, although some significant exemptions were observed.
The thermodynamic behavior of hydrates was affected by the salinity of the aqueous
phase. Higher salinity resulted in the reduction of the chemical potential of water
molecules which shifted the thermodynamic equilibrium to lower temperatures.
Methane solubility in pure aqueous phase increases with temperature for
the two phase equilibrium conditions.
