Freezing Effects on Core Samples
Freezing of core is a topic of conflicting viewpoints, comprising data from a variety of sources and disciplines. As such the subject does not lend itself to an evaluation in a general form. The points below are taken from various technical papers and are intended as a quick guide to the problems associated with freezing. Core is frozen using three basic methods, the rate of freezing being related to the operating temperature of the system being used.
A freezer operating at around -30C
Dry ice at -78C
Liquid Nitrogen at -196C
Rate of Freezing is influenced by the method of freezing, the local conditions, and the sleeving or packaging of the core.
Slow Freezing can result in damage to grain boundaries and may influence measurements of resistivity, sonic velocity and permeability. There will be a marked fluid migration influencing saturation determination and precluding the use of chemical tracers.
Faster freezing limits damage to grain boundaries and there is less fluid migration. However the large thermal shocks can cause fracturing along thin bed boundaries.
The effect of freezing is a function of brine saturation and salinity. If the saturation profile changes along the core length the freezing rate would also change when using a fixed method of freezing. Variable rates of freezing would make corrections difficult.
Freezing results in the migration of salt from the outer sections of the core to the inner. The increase in salinity at the centre of the core will depress the freezing point and take longer to freeze. This results in an error in Sw determination and precludes the use of chemical tracers.
Freezing of cores is inefficient in gas reservoirs and where Sw is low.
A potential source of damage to the core is its transfer and stacking in the freezer unit. At this stage the core is unsupported and is consequently easily damaged, subsequent freezing will be providing mechanical stability to cores that no longer warrant the need for the process.
A freeze thaw cycle will damage the core. Therefore once the core is frozen it must remain frozen.
Summary
Given the above points, it is clear that freezing the core is not the best method of handling the core. The logistical problems of freezing before damage, maintaining frozen to the laboratory, and throughout are enormous. It may be necessary to freeze the core for petrophysical sampling, but it is clear that freezing of core should always be the last option taken. Selective or localised freezing can be undertaken for plugging and as a last resort if necessary the core can be frozen for slabbing.
Lithotarge foam Injection is recommended to stabilise the cores, as this will fill all voids and maintain a small overburden pressure on the core samples. The adaptability of Lithotarge ensures that whatever the well site scenario, a solution can be forthcoming.
References:
The Effect of freezing of slightly consolidated Cores, O. Torsæter et al SPE 14300 1985
Reservoir Properties of unconsolidated rocks, P. Worthington et al SCA 1987.
Major advance in sampling and preserving unconsolidated core, K. Sincock et al Eurocas 1990.
The effects of freezing on tracer distribution in cores, J. Gidman et al Eurocas 1992
The mechanics of frozen ground, N Tsytovich, 1975.
