Methane Hydrate Production From Alaskan Permafrost Progress Report Page: 70 of 225
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Pore volume is an uncertain percentage larger than stressed pore volume, so the only check is
qualitative. Fines can also be lost during saturation, which further confuses the situation.
Saturation state of thawed samples is an even more difficult problem. These samples started
as some mixture of water, ice and rock grains. In the case where the sample is completely
grain supported, saturation state can be calculated assuming the frozen pore space was
completely filled with ice or water. In this case, total pore volume Vp is given by
Vp = Vice + Vwi
where Vice and Vwi are initial ice and water volumes.
After thawing, final water volume, Vw, is given by
Vw = Vwi + Vwt
where Vwt is the volume of water from the ice.
iwt Vice
Pw
Densities of ice and water are given by Pice and pw.
Water saturation in a thawed sample, Sw, is
Sw = VW/Vp
So that
V .i P V. p.
Sw = -"i + ce= Swi + ice SIce = Swi + 0.917SIce
Vp Pw Vp Pw
Swi is water saturation in the frozen state, Sice ice saturation and 0.917 the density ratio -ice
Pw
Measurements on frozen state samples indicated about 25% unfrozen water. This would
produce a water saturation of 94% in the thawed samples. Ice provides some frame support in
the frozen samples so that grains may fill a larger part of the volume in thawed samples than in
frozen. That is, calculated Sw is a minimum value.
A final concern in measuring NMR pore volume is the hydrogen index of the pore fluid.
Amplitude of the NMR signal is proportional to the number of hydrogen atoms in a fluid state in
the instrument's measurement volume. This amplitude is converted to a fluid volume by taking
its ratio to a calibration sample of known volume and hydrogen index HI. As long as saturation
fluid and calibration fluid are the same, HI cancels out. In measurements on permafrost
sections, the calibration fluid was a 5% by weight KCI solution. This was the same solution
used to resaturate samples.
HI of brine in frozen and thawed samples is not known. Analysis of logs in adjacent wells and a
water sample recovered from the core at 1265 ft just below the base of the frozen zone show
DE-FC26-01NT41331 - 64- Maurer Technology Inc.
Core and Fluid Analysis Anadarko Petroleum Corp.
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Sigal, Richard; Newsham, Kent; Williams, Thomas; Freifeld, Barry; Kneafsey, Timothy; Sondergeld, Carl et al. Methane Hydrate Production From Alaskan Permafrost Progress Report, report, February 1, 2005; United States. (https://digital.library.unt.edu/ark:/67531/metadc786910/m1/70/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.