Boring or penetrating the earth – Processes – Sampling of earth formations
Patent
1993-03-23
1994-08-16
Dang, Hoang C.
Boring or penetrating the earth
Processes
Sampling of earth formations
73155, 166264, 175309, E21B 4908
Patent
active
053378385
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention concerns a method of taking and analysing level determined pore gas/liquid samples, e.g. water from a subterranean formation, comprising drilling a hole in said formation and passing samples at a desired depth into a cavity in the drill string used for the drilling, following which the samples are analysed.
BACKGROUND OF THE INVENTION
Contamination from chemical depots, agriculture and other sources of contamination has led to an increasing need of rapidly and exactly mapping the chemical conditions in a subterranean formation and e.g. ascertaining whether the water in its water deposits can be used as drinking water. Over time various methods and devices have been developed for this purpose, which, however, are either slow or cumbersome to use or give analysis results which are not sufficiently reliable.
Thus, the U.S. Pat. No. 4,363,366 proposes the use of a hollow auger bit having in one or more of the sections of the drill string a plurality of about 0.3 mm narrow slots, through which pore gas/liquid from the formation can enter the cavity of the string and be pumped or sucked via this up to the surface of the ground where the samples are analysed. However, before this takes place the samples have been in direct contact with the atmospheric air in the open cavity of the string, whereby i.a. the oxygen of the air affects the samples, which are moreover subjected to a pressure drop causing their content of carbon dioxide to degas. To this should be added that the samples are also easily contaminated by oil and packer material from the joints of the drill string, and these factors in combination will change the chemistry of the samples to such a degree that the achieved analysis results will not give a true picture of the chemical conditions in the formation concerned. Another problem is that very narrow slots must necessarily be used to prevent the drill string from being filled with mud, but this entails in turn that the slots will easily clog so that the process ceases operating when the auger bit has reached a distance down into the formation. The bit must therefore frequently be pulled up for cleaning of the slots, and this greatly delays and impedes the work and adds to the costs involved.
The U.S. Pat. No. 4,669,554 discloses another method which is so designed that it is possible to convey undisturbed pore gas/liquid samples to the surface of the ground to obtain true analysis results. In this case a ram with a sample chamber is driven down into the ground, and then a pore gas/liquid sample is collected in the sample chamber, which is subsequently sealed with respect to the surroundings, and the sample is conveyed up to the surface of the ground, the ram with the sample chamber being pulled out of the formation. The samples are hereby representative of the pore gas/liqud of the formation whose chemical conditions can therefore be determined correctly. However, only one sample at a time can be taken in this manner, and the process is therefore both slow and expensive and is in reality inapplicable when the chemical conditions of a subterranean formation is to be plotted with a greater vertical resolution within a reasonable period of time.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method of the type stated in the opening paragraph which can take level determined pore gas/liquid samples from a subterranean formation at short intervals and more rapidly and easily than known before to give true analysis results by means of these samples, and which can moreover currently monitor the formation at a desired depth.
This is achieved in that the method of the invention characterized by dry drilling the borehole by means of a hollow earth bit, e.g. a hollow auger bit, isolating the drill string in the vicinity of the tip of the drill a cavity section in sealed relationship with the rest of the cavity of the string, said cavity section having a sample chamber which communicates with the isolated cavity section e.g. via a filter, providing
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