Note: Descriptions are shown in the official language in which they were submitted.
114336S
CO~INUOUS BOREHOLE-LuGGI~G MET~OD
The invention relates to the technology of drilling
boreholes or wells, and more particularly it relates to
the method~ of continuously logging a well or borehole
being drilled.
The invention can be employed to utmost effective-
nesR in geological survey work when p~ospecting survey
wells.
The i~vention can be also effectively used in natu-
ral fuel gas and oil production for determining thelithological properties of the rock traverse~ by a pro-
duction well being drilled.
At present, the lithological composition of a rock
being drilled is determi~ed by obtaining a sample of
the rock by a core bit and subse~uently analysing the
texture and mineralogical composition of the rock~ This
technique is characterized by its time- and labour-con-
suming character, and, hence, by its high cost.
- There are also known methods of determining the
tectonics, structure and lithology of rock with aid
of electric log~ing char~s obtained by conducting elec-
tric loggi~g while drilling a borehole or well. ~he met-
hod~ are likewise characterized by their highly labour-
; co~uming character, because in order to conduct the
logging~ the drilling tool has to be pulled out of the
borehole, the appropria~e instruments h~ve to be run
into the borehole on the cable, and the data obtained
has to be subsequently interpreted and analysed.
There are further known methods of determining the
lithological composition of the rock in a borehole by
using the drilling cuttings entrained i~ the upward flow
of the drillin~ mud. The drilling cuttings are employed
according to these me~hods for determining the texture
a~d the mineralogical composition of the rock strata tra-
versed by the borehole, by using laboratory methods.
However, these la~t-mentioned known methods are
characterized by the insufficiently accurate information
the~ yield, slnce it cannot be always positively known
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114336S
to which traversed ~tratum thes~ or ot~er drilling cut-
ti~gs belong, to 8ay nothing of the considerable time
it takes to investi~ate the mineralogicdl composition
of the samples of the drilling cuttings.
It is an object of the present invention to improve
the accuracy o~ the det~rmination o~ the lithological
composition of the rock traversed by a borehole.
It is another object of the present invention to
speed up the process of the de~ermination of the litho-
lo~ical composition of the rock.
It is still a~other object of the present invention
to simplify the technique of determining the lithologi-
cal composition of rock.
These and other ob3ects are attained in a continu-
ous bor~hole-logging method i~ the course of the drilli~g
with the use of a drilling mud, which method, in accor-
dance with the invention, includes subjecting the dri-
ling mud fed into the borehole to unipolar electric
treatment, while meaæuring itS oxidation-reduction pot-
ential and maintaining this value substantiall~ permanentat a given magnitude; continuously measuring the value o~
the oxidation-reduction potential of the ~rilling mud
retur~ing from the borehole and comparing thi~ value with
the given value of the oxidation-reduction potential of
the drilling mud fed into the borehole, and using the
difference between the two values at any given moment
to detelmine the mineralogical composition of the rock
adjacent to the bottom of the borehole by comparing said
dif~erence with the known in advance normal oxidation-
-reduction potentiaIs of minerals,
The proposed method e~ables to enhance the accuracy
of the determination of the lithological compositio~ of
th~ rock, owing to the direct contact of the rock with
the drilling mud being accompanied b~ processes of io~
e~change therebetween, such processes altering the major
parameters of the drilling mud, and of its oxidation-re-
duction or redox potential, in i~rticular.
Furthermore, the proposed method enables to simpli-
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fy and speed up the determination of the lithological
composition of the rock, eliminating as it does ~uch
labour-consuming operations as taking core samples or
sampling the drilling cuttings, as well as the operations
involved in electric logging of boreholes.
The invention will be fur~her described in connec-
tion with an embodiment thereof, with reference being
made to the accompanying drawings, ~herein:
FIG, 1 shows a curve illustrating the relationship
between the respective ac~ivities "aO~" of the oxidized
form of a matter and "ared" of the reduced form thsreof;
FIG. 2 i'llustrates schematically the pattern of cir-
culating the drilling mud for performing the disclosed
method.
Those copetent'in the art know that a drilling mud
in its general form is a heterogeneous fluid system
alwayS containing the particles of solid phase, low-mo-
lecular ions and polyelectrolytes - polymers of which
the molecules contain groups capable of ionization in a
solution. The presence in the drilling mud of these
components is a prerequisite of ensuring the most essen-
tial properties of the drilling muds from the well-dri-
lling point of view. Among these properties, in the first
place, is the'capability of the mud to exhibit the mini-
mized pbysical and chemical action on the rock makingup the borehole walls, to cIeate the mi~i~ized disturban-
ce of the thermodynamic, chemical and physical processes
taking pl~ce at the borehole-formation interface. This
major condition re~uires maintaining the stability of the
drillin~ mud i.e. mal~taining stable its major characte-
ristics throughout the drilling time, notwith~tanding the
action of aggressive salt~, i.e. the salt resistance of
the mud.
Thé polgelectrol~tes, i.e~ the chemical agents em-
ployed f'or stabiliza~ion of the properties of drillingmuds provide protection to the particles a~d phases of
the dispersion system which is the drilling mud against
the adverse action of the environment to which the disper-
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~1~336S
sion syste~ is exposed in the course of the drilling.
~ urthermore, they are adsorbed on the surfaces of
the solid phase particles, modifyi~g t~lis surface, for-
mi~g a solvate shell, and thu~ enabling to consider the
properties of the sur~ace of the solids as that of a
single two-dimensional macromolecule of a polyelectrolyte.
On the other hand, the lyophilic portions of the
solid phase particles, which have not absorbed polyelec-
trolyte molecules~ are altogether similar to the latter
by their physical and chemical porperties. Like true
molecules of higher polymers, they have groups capable
of ionization in a solution (the silanol groups), and
they also are capable of retaining at their surface low-
-molecùlar ions and hydratiorl water molecules.
Therefore, one of the essential conditions of deter-
mining the stability of drilling muds is the evaluation of
the physical arld chemical equilibrium of the polyelectro-
lyte~ in the drilling mud concerned.
~he size of the molecules of the polyelectrolytes,
same as their other properties, may vary within a range
far broader than that associated with-common macromolecu-
les. ~he polyelectrolytes can be classified or divided
into po b acids, polybases a~d polyampholYtes or polyam-
phibolytes (i.e. copolymers containi~g both basic a~d
acidic groups).
Among the chemicals or chemical agents emplyed for
treating drilling muds there are the representatives of
all the abovelisted classes or group~. The majority of
the polyelectrolytes include weak acidic or basic groups
(e.g. carbox~ groups such as caboxymethyl cellulose, or
amino groups). Therefore, they can be ioniz~d only in
the presence of a strong alkali ~in case of polyacid~),
or of a stro~g acid (in case of polybases). Irl this case
the chain of the polyelectrolyte has bound thereto charged
groups, while the surrounding medium contain~ low-molecu-
lar ions of the opposite sign, i.e. the cou~ter-ions.
The properties of molecule~ of polyelectrolgtes in a
~olution are defined by the presence of electrostatic
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1143365
interactio~ o~ the electrically charg~d groups of a
chaln with one another and with the low-molecular ions
of the solution which latter~ as a whole, i8 usually
electrically neutral.
Experimentally obtai~ed data (particularb those
obtained from experiments with transfer of labelled
or traced Na and from measuring the activity coefficien-
ts of low-molecular ions) have proved that the molecule
of a polyacid or polybase usually attracts and retain~
adjacent thereto a considerable number of solvated (or
hydrated) counter-ions, this number more often than not
being more than one half of the number of the chargcd
groups of the chain.
The elec~rostatic repelling of the similarly char-
ged group~ in polyacids and polyb~ses results i~ a
sharp alteration of the confoI~ity properties of the
macromolecules, particularly, in the selling of their
size in a solution, which i8 usually a major asset fr~m
the point of view of th~ quality of drilling muds.
~he mean size of the molecules of polyelectrolyte~
are able, with a~ increased degree of ionization, to
swell fivefold or even more. The increased concentration
of lowmolecular ions i~ a solution, i.e. the ion strength
of the.solution affects the interaction of the charged
groups and brings the sizes and other properties of the
molecules of polyelectrolytes closer to those of common
macromolecules, whereby the acticity of the molecules of
the polyelectrolytes significatly decreases.
~ - It can be.seen from the a`bovesaid that the variation
of the co~centration of low-molecular ions in a solution
(i.e. of inorganic acids, acids and bases) displays the
same mechanism of affecting the polymer el~ctrolytes and
the lyophilic portions of.the solid phase particles,
which i8 exhibited in the variation of the intensity and
character of interaction of the char~ed groups of macro-
molecul~s with the surrounding medium. This variation is
closely associated with the oxidation-reduction equilib-
rium of the molecules of a polyelectrolyte in a Solution,
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~1 ~336~
determining ~s it does the possibili~y of electron
exchange between the macromolecule and the low-molecular
ions, atoms and molecules surrounding it and constitu-
ting the dispersio~ medium of the drilling mud. ~hus,
the ratio of the oxidized and reduced fo~s of the
matter in the drilling mud play~ a decisive role as far
stability of the polyelectrolyte moleculesg andl hence,
of the drilling mud as a whole, and its salt resistance
are concerned.
Quite naturally, every reaction altering the activi-
ty of organic substances in the drilling muds results
in the varying degree or state of the axidation of the
macromolecules of polyelectrolytes, which i8 the case,
for instance, with thermal-oxidation destruction of car-
15- boxymethyi cellulose.
Oxidation-reduction reactions are those involving
mutual o~idation or reduction of various substances.
Oxidation of a substance involves removal of electrons
from its components, while reduction involves addition
of electrons. When an electrode made of an inert metal
is immersed in a liquid, there is creaded at the electrode-
-solution inter~ace a voltage differenc~ which i8 the oxi-
dation-reductio~ pote~tial, also referred to in the pre-
sent dislosure as the redox potential.
The value of the rsdox potential of a system is a
measure of the intensity of oxidation-reduction processes
taki~g place in the system, depending on the ratio the-
rein of the concentrations of the oxidized and reduced
forms of the ions elther maki~g up the system or i~tro-
duced thereinto.
~ here~ore, to evaluate the stability of drilling
muds, it is possible to resort to the measurement of the
02idatin~-reduction potential of the system, which is
representative of the ratio of the oxidized and reduced
components in the drilli~g mud.
The redox potential " ~" of a solution with the oxi-
dizing activity "aO~" and reducing activity "ared" is de-
termined by the ~ernst equation:
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- 114336S
'Y ~ Yc + ~; ~ ax
where ~0 is the normal potential of an inert (platinum
or gold) electrode, mV;
R is the uni~er~al ~as coL~tant;
T is the temperature, ~;
z is the number of the electrodes taking part in
the reaction,
F is the ~araday constant, and
ln i8 the symbol of natural logarithm.
This formula enables to plot the curve (curve "A't
in ~IG. 1) of the relationship between the redox poten-
tial ~y " and the ratio of the activities "aO~" and
"a~ed'l, respective-ly, of the oxidized and reduced forms
of the matter, which ha~ the general shape illustrated
i~ ~IG. 1.
Under statio~ary conditions~ i.e. those characteri-
zed ~y very slow variation with time of the conditions
of energy exchange with the neutral environment, the re-
do~ potential of a drilling mud usually acquires theequilibrium value corresponding to the ratio aOX:ared=
0.5:1. Such an important indicator of the chemical acti-
vity of the system as the pH number acquires under these
conditions the ~eutral value of pH=7.
Any A~y variàtion of these two variables ~rom the posi-
tive equilibrium mea~s that the system becomes u~stable
energy-wise and is capable of oxidation-reduction reac-
tions with the environment, e.g. the rock of the borehole
wall ~, as well as within the system itself, i.e. betwee~ -
its particles and phases.
Depending on it~ lithological properties, the rockmaking up the walls o~ a borehole may contai~ a great
number o~ various minerals which, whe~ contacted by an
electrolyte, i.e. the drilling mud, alter the redox pote~-
~5 tial o~ the mud, æuch alteration being possible withina i~ m ficantly broad range.
Given as an illustration in Table 1 hereinbelow are
the normal o~idation-reductio~ potentials ~c correspon-
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114~365
ding to eventual electrochemical reactions of interac-
tlon of various ions con~ained in the crystal lattice
of rock-forming minerals with a liquid, e.g. water or
drilling mud.
Table 1
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X e a c t i o n s ~ 0,V
Cr3 + e - 0.41
Sn2+ ~ 2e + 0.153
Cu + + e + 0.167
~e3 + e~ ~ 0.771
Mn3+ + e~ + 1.51
As it can be see~ from the ~able, differe~t minerals
display significantly di~ferent normal redox potentials.
Therefore, when in the course of drilling a borehole or
well the drilling mud which initially has had the equi-
librium state contacts the rock of the hole bot~om, and
we know in advance the values of the normal redo~ pote~-
tials of all the kinds o~ rock making up the formationsof a given field, the change in the value and the sign
of the redox potenlial of the mud returning from the bo-
reholé following its con~act wlth the rock can be used
to determine the lithological composition of the fo~ma-
tion being drilled.
~et us consider a concrete example.
To determine the lithological compoqition of the rocktraversed by a borehole, there is prepared an initial dril-
ling mud, whereafter it is sub~ected to unipolar electr-
ic treatment, while measuring at the same time i~ theelectrolizer 1 (~IG. 2~ the value of its redox potential.
B~ varying the ~oltage at the electrodes 2 and 3 and
the current, the value of the redox potential is brought
to the magnitude corresponding to the oxidation-reduction
equilibrium (in case o~ drilling muds this magnitude is
within the range o~ 1.6 V to ~ 1.8 V.
In practical ca~e~, when drilling muds are prepared,
depending on the type of the clay used and on the type and
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~143365
ratio of the chemical agents introduced into the mud,there may prevail therein either the oxidation poten~
tial or the reduction one. To stabilize it, the mud is to
to be treated either i~ the zo~e of the negative electro-
de 2 (in this case the chemical reactions in the mudwould have the reduction character), or in the zo~e of
the positive electrode 3 (then the chemical reactions
would have the oxidation character). The electrodes are
supplied from a direct-current source 4.
With the drilling mud thus treated, the value o~
the redox potential is maintai~ed permanent at the mag-
nitude predetermined by the adopted technology.
The st~bilized drilling mud is pumped into the bore-
hole 5 by a pump 6. The stabilized redo~ pote~tial of
the treated mud is monitored by the sensor 7 responsive
to the redox potentlal, e.g. a ~yukov's calomel-electro-
de sensor connected to a secondary instrument or indica-
tor 8.
When the rock 9 is drilled, there t~ke place in the
zone of the contact o~ the drilling mud with the rock
oxidatio~-reduction reactio~s due to the ion egchange
between the drillin~ mud and the minerals making up the
rock.
The d~illing mud returning from the borehole 5 is
directed into a vessel 10 where a sensor 11 a~d a regis-
tering device 12 connected thereto are operated to measu-
re continuously the redox potential of the drilling mud
after the latter's contact with the rock being drilled.
Then the value of the redox potential of the mud returning
from the borehole is compared with the preset value of the
redox potential of the mud fed into the well.
~ y comparing the known in advance normal redox poten-
tials of the minerals making up the formations with the
value of the alteration of the redox potential of the mud
returning from the well, it is relatively easy to deter-
mine the mineralogical and lithological composition of the
rock being drilled at a given moment.
~he employment of the disclosed method enables to con-
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43~65
siderably cut the time required for determining the lit-
hological composition of various types of rock with suf-
ficient accuracy, and also to save mo~ey, energy and men-
-hours.
The disclosed continuous borehole logging method
can yield the ma~imum effect in the natural fuel gas
and oil production industIy, when prospecting and produc-
tion wells are drilled.
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