CA1143365A - Continuous borehole-logging method - Google Patents

Abstract

CONTINUOUS BOREHOLE-LOGGING METHOD
Abstract of the Disclosure A method of continuosly logging a borehole being drilled with the use of a drilling mud is disclosed.
According to the invention, the drilling mud being fed into the borehole is subjected to unipolar electric trea-tment, while measuring at the same time the value of its redox potential. This valu is maintained substantially constant at a given magnitude. Meanwhile, the value of the redox potential of the drilling mud returning from the borehole is continuously measured, and the value obtained by the measurement is compared with the given value of the redox potential of the drilling mud fed into the borehole. The difference between the two values is used to determine for a given moment the mineralogi-cal composition of the rock adjoining the bottom of the borehole, by comparing the abovementioned difference with the known in advance normal redox potentials of minerals.

Description

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 .r, , ~4 , 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-

- 2 -~ 3365 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-~ 3 --~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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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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'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 .
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 .

~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-_ g _ ;
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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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of continuously logging a borehole in the course of the drilling with the use of a drilling mud, including subjecting the drilling mud fed into the borehole to unipolar electric treatment, while measuring in the same time the value of the oxidation-reduction potential of the drilling mud, and maintaining this va-lue substantially constant at a given magnitude; conti-nuously measuring the value of the oxidation-reduction potential of the drilling mud returning from the bore-hole, comparing the last-mentioned value with the given value of the oxidation-reduction potential of the dri-lling mud fed into the borehole, and using the differen-ce between the two-values for determining for a given moment the mineralogic composition of the rock adjacent of the bottom of the hole, by comparing this difference with the known in advance normal oxidation-reduction potentials of minerals.