Energy is everyone needed, consumpsion of energy always increase from time to time. People also make some experience to explore and find a new energy resouces. One of the potential unconventional energy that develop today is hydrate gas. In simple, Gas hydrate is ice like compounds containing methane.
So What is Hydrate Gas actually is ?
When gas molecules are trapped in a lattice of water molecules, at temperatures above 0°C and at pressures above one atmosphere, they can form a stable solid. This Solid is called Gas Hydrate (gas clathrates).
Gas hydrates (or clathrate hydrates) are solid phase in ice-like crystalline molecular complexes formed from mixtures of water and suitably sized gas molecules as the “Guest”.The water molecules, above the hydrogen bonding, form a lattice structure with some interstitial cavities. This guest gas molecules fill the lattice cavities, and when a minimum number of cavities are filled, the crystalline structure will become stable and solid gas hydrates will be formed, even at temperatures well above the melting point of water ice. When gas hydrates dissociate or melting, the crystalline lattice breaks down and becoming into liquid water (or converts to ice if conditions are below the freezing point of water) and the gas is released. This Gas is mostly Methane. Where methane gas hydrate is stable at the seafloor at water depths beneath about 500 m.
The hydrate gas stability zone extends into the seafloor sediments down to a depth where temperature exceeds gas hydrate stability, usually in some 10s to 100s of meters beneath the seafloor. At this depth, thin methane gas layers are often present causing strong reflections in seismic records. And the Reflection is followed up by line of constant temperature. The temperature in the subsurface, is the function from heat flow and depth, and make the reflection imitate the shape of the seafloor.Figure 1. Phase diagram of methane hydrate stability. The methane-water combination is a solid at low temperatures and high pressures.
How to Identify Hydrate Gas Using Geophisical Data
Using Seismic Data
Actually, identifying hydrate gas can be done by seismic data especially in offshore. It can be identified by strong acoustic impedance due to strong contrast between hydrate gas and common sedimentary layers. The depth of hydrate gas layer reflectance depends on the temperature and pressure stability of hydrate gas (see figure 1).It is commonly known as bottom simulating reflectors (BSRs).
Figure 2. Seismic Data from Gulf of Mexico. Hydrate Gas can be knowed by Present of BSR
Using Well Log
Well log data can be used to detected gas hydrates zone. In some cases, the existence of gas hydrates showed by high resistivities in electrical logs , high velocity in bore hole sonic log, and also by high mud gas.
Some logs are used to predict hydrates : Resistivity logs, density porosity logs, neutron porosity logs and borehole sonic.
Gas hydrate is an insulator that contains abundant hydrogen which is in both its methane and water. Hydrates saturation can be calculate by various methods. One of them is Density-Magnetic Resonance (DMR method) can be used to predict the estimation of hydrate saturation
Its saturation is calculated by this equation :
HIω = Hydrogen index of water = 1
HIh = Hydrogen of methane Hidrate = 0
rω = The density of water = 1.0 g/cc
rh = Then densiy of gas hydrate 0.91 g/cc
rma = The density of sand matrix
Another way to calculate Gas hydrate saturation is using Archie’s equation (1942), but in the other case Archie’s equation has to be modified due to clay content. Because it doesn’t work well for the increasing conductivity in clay. Here is the equation for homogeneous case :
Sh = Saturation of hydrate
Rt = True Formation resistivity
R ω = Formation water resistivity
R0 = Resistivity of 100% water saturated rock
n = Saturation exponent
a = Constant
m = Cementation Exponent
|Well logs (above) are taken from Green Canyon. [Track 3] High resistivity shows the indications of gas hydrates in the sand (pink shading). In otherwise, the blue shading shows the contain of gas hydrate free-zones in the washouts zone [Track 1], washout occurs mainly in the non-hydrate-bearing layers. Washout also affects on density, so its getting decrease [Track 4]. Hydrate saturation [Track 5] depends on the saturation exponent, which is n.|
|The unconfined compressive strength (blue) and the static Young’s Modulus (red) increase in the sand contains of gas hydrate, but it doesn’t affected too much whenever the gas hydrates in clays.
The well log were taken from green canyon block 955, Well H
|So from well logs, like gamma ray, resistivity, caliper, density and etc. We can predict the alternation of gas hydrates.
High resistivity values and low Sonic measurements corresponds to hydrate-rich zone. On the other way of gamma ray readings are indicating of the material of the layer for example : shale, sand or etc.
Potential of Hydrate Gas to Fullfill Future Energy Needed
Base on estimation by National Energy Technology Laboratory, global hydrate gas resources of worldwide reach 20,000 trillion cubic meters, or about 700,000 Tcf. But, development of hydrate gas exploration still limited due to technology limitation. If in the future, human can find a new technology that can explore hydrate gas economically we believe that deficiency of energy can be avoided. One of the most potential place to explore hydrate gas is Gulf of Mexico with 11,000 – 34,000 Tcf of hydrate gas or equal with 21,444 Tcf. With deep research about evolution of sedimentary basins that can be produced hydrate gas and locating place that potential to reserve hydrate gas it can be very useful for accelerating hydrate gas exploration in future decade.
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Majumder Mandira, 2009, “Identification of gas hydrates using well log data – A review, GEOHORIZONS, 39-48, 2009.
National Energy Technology Laboratory. 2011. Energy Resources Potential of Methane Hydrates. Morgantown : U.S. Department of Energy.
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