PERMAFROST AND GAS HYDRATE BASICS
Water in pore space freezes when the temperature of the rock is below zero degrees Celsius (32 degrees F), A phenomenon called freezing point depression (FPD) causes the actual freezing temperature to be somewhat lower than 0 C. FPD is a function of pressure, salinity, and pore size, and is usually about minus 1 to minus 2 degrees Celsius in clean coarse grained sandstones. It can be as much as minus 8 C in very fine grained silts and shales.
View from the North Pole showing areas of continuous (blue) and discontinuous permafrost (grey)
Clay bound water in shale does not freeze, so shale properties change only slightly, depending on the amount of free water in the effective porosity of the shale. Silty shales have more porosity than pure shale and are more strongly affected by freezing.
Studies designed to locate the base of permafrost were sponsored by the Geological Survey of Canada in the 1960's, 1970's, and again in the early 1980's as more log data became available. Permanent temperature recording systems using thermistors equally spaced along a cable were installed in numerous observation wells throughout the Arctic. These surveys form the basis for static temperature data that is still relied upon today.
In a sandstone interval, the base of permafrost is easy to pick on the resistivity log. In shales, it is much more difficult. The base of permafrost is often picked at the base of a frozen sandstone; this depth is called the base of ice-bearing permafrost (IBPF).
Long spaced sonic log (left) and deep resistivity log (right) are used to identify the base of frozen rocks, around 1800 feet in this example, based on reduction in resistivity and increase in sonic travel time. Frozen rock may contain water-ice (permafrost) or gas hydrates (solid gas) or both.
Example of long and short spaced sonic logs in a permafrost section. The long spaced curve shows the frozen rock velocity (as travel time). This curve cannot be used for porosity calculations. The short spaced curve shows the thawed travel time, which can be used for porosity calculations after shale corrections are applied. In shaly zones, the two curves give similar values because the clay bound water does not freeze, although any free water in the silt fraction of the shale does freeze.
Macro photo of a gas hydrate sample from a core (GSC Bulletin 585)
Gas hydrates are often found in or below permafrost zones on land, or in deep water along continental margins. They can extend below the base of permafrost, even though formation temperature is above 0 C. Hydrates have been discovered or inferred along the coastlines of all continents, even at temperate latitudes, and in deep water trenches in the Pacific.
The quantity of gas in a hydrate does not depend on the depth, pressure, or temperature of the reservoir, as is normally the case for natural gas. Hydrates can contain far more gas at shallow depths than a conventional reservoir at the same depth. This can result in unexpectedly high pressure in the wellbore as the hydrate thaws, with all the dangers of blowouts and damaged equipment that this suggests.
A good description of gas hydrates is contained in "Naturally Occurring Gas Hydrates in the Mackenzie Delta", C. Bily and J. W. L. Dick, CSPG Bull., 1974.
crystal with water molecules (red)
The various phases of water, ice, hydrates. and free gas are determined by a phase diagram, which depends on the specific gravity of the gas, salinity of the water, temperature, and pressure. The latter two factors are functions of depth, so the phase diagrams are often plotted versus depth, using assumed pressure and temperature gradients. Schematic examples are shown below.
An increase in temperature increases the pressure required to form hydrates, while small percentages of ethane or propane lower the hydrating pressure considerably. Hydrogen sulfide and carbon dioxide also decrease the required pressure.
Thawing of gas hydrates generates gas at pressures well above those expected at these shallow depths. It may be impossible to raise mud weight sufficiently to prevent a blowout, so chilled mud and a quick casing job are indicated. If a well is cased and cemented, that gas pressure may cause wormholes in the cement, leading to a permanent leak to surface as long as borehole temperature is higher than the hydrate stabilization temperature. High quality cement jobs in large, cold boreholes are notoriously difficult.
Continued thawing may cause casing collapse or rock subsidence, with loss of wellbore integrity. Successive thaw-freeze cycles aggravate these conditions and may cause vertical expansion of the rocks during refreezing..
The typical production scheme proposed for gas hydrate wells is by depressurization. During production, wellhead control is difficult due to the high pressure and cold temperatures. Hydrates may reform in the plumbing. Produced water must be lifted and disposed of before it refreezes. Thawing allows fines migration which annoys pumping and compression equipment. Some experimental production methods involve the addition of heat or methanol to release the gas. Heat may affect non-gas bearing intervals in the permafrost, leading to casing collapse or movement.
On the North Slope of Alaska, the gas hydrate issue is further complicated by the presence of oil and free gas with hydrates in various combinations.
Deepwater offshore hydrate production has its own issues, and although a huge resource is postulated, I am not aware of any intentional attempts to produce it. No wonder shale gas is so popular!
Short spaced sonic logs reading the thawed zone can be used to calculate porosity, but compaction and gas corrections will be required. Long spaced sonic logs reading the frozen zone are difficult to analyze for porosity due to an unknown amount of excess (unfrozen) water along with the ice or hydrate.
Logging while drilling is recommended as there is less borehole rugosity and less thawing. Resitivity, shear and compressional sonic, density, neutron, and gamma ray are the usual logs required. If chilled invert mud is used, open hole logging may be successful.
Freezing of water causes salt rejection, leaving some excess unfrozen water with moderately high salinity. Higher salinity water tends to increase the SP deflection but the higher resistivity of the ice tends to reduce SP deflection. The net result is low SP deflection in frozen intervals. SP in the unfrozen intervals behaves normally.
log analysis of permafrost -
Resistivity will read high values in both water ice and gas hydrate in sand sequences; shales will show typically low resistivity with moderate gamma ray values. Clay bound water does not freeze, so shale resistivity does not incresase much when frozen.
A long spaced sonic will read the frozen rock velocity (or travel time) but short spaced sonics will see the thawed zone velocity. Many hydrate zones are poorly consolidated, so caliper logs may show large washouts as the rock thaws. In large or rough boreholes, both density and sonic logs may show large spikes or noise.
Neither resistivity nor porosity logs are very helpful in distinguishing gas hydrates from water ice. The best indicator is the gas mud log because large quantities of disassociated gas are released as the hydrate is thawed. No significant gas is released from water ice. Free gas and even oil are also possible and gas mud logs will show less than in a hydrate zone.
Quantitative log analysis is complicated by the inability of standard models to differentiate between water ice and gas hydrates. Free gas and gas hydrate (if thawed deeply enough) can be distinguished by gas crossover in cleaner sands.
treating ice and hydrates as if they were hydrocarbons, standard
porosity and Archie-type saturation models can give an estimate of ice plus hydrate content (black shading in
Track 3) and free water (white shading). In this model:
Standard deterministic or probabilistic multi-mineral models using
quartz, clay, ice (water ice or hydrates) and free water will also
work. In these models:
nuclear magnetic resonance log is run, the effective porosity from
this log is the water filled porosity. Ice and hydrates are not
The base of permafrost is chosen by a nearby permanent temperature log (around 650 feet in this example). Black shading below this depth down to 1000feet is gas hydrate and there may be gas hydrates in the permafrost zone. Since salt rejection increases water salinity in the excess water, the water resistivity is unknown and variable, so the quantities of ice, hydrate, and excess water are not very accurate. The mud gas log is vital.
In older wells, the sonic log was often very noisy and seismic reference surveys were used to determine acoustic velocity. The beginning of low velocity would indicate the base of permafrost or base of gas hydrates, or shales.
These surveys were superseded by crystal cable surveys, the forerunner of the vertical seismic profile, which would be used today for this purpose. VSP's and their predecessors can be run in cased holes provided the shot point is far enough from the wellbore, otherwise the velocity derived from the survey will be that of the casing. A good description of the use of this tool is "Permafrost Investigation by Crystal Cable Surveys, Mackenzie Deltas", I. H. Wallace and A. J. Stuart, CSEG, 1975, from which the following two illustrations were taken.
Crystal cable survey time vs depth plot with interpreted velocity values. Base of permafrost could be as shallow as 1400 feet or as deep as 2000 feet.
VOLUME IN PLACE
The volume of hydrocarbon in a gas hydrate is a function of the hydrocarbon type only. Water saturation is meaningless. The ratio of gas to water would range from 433 scf/bbl for propane to 1230 scf/bbl for methane.
This is equivalent to 170 cubic feet of methane per cubic foot of pore space (or 170 m3 per cubic meter of pore space) at standard temperature and pressure and 60 cubic feet of propane per cubic foot of pore space, regardless of depth of burial.
place is derived by converting pore volume to gas volume:
Gas Hydrate EXAMPLES
The 2002 Production Research Project drilled additional wells near Mallik L-38 to test various production schemes and evaluation techniques. The logs and a computed result are shown below.
illustration is from "FORMATION
EVALUATION OF GAS HYDRATE BEARING
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