Note: Descriptions are shown in the official language in which they were submitted.
9~ ~
This invention relates to a method for recovering oil from a
subterranean deposit.
In recovering oil from oil-bearing deposits, it is generally pos-
sible to obtain, by primary recovery methods, only a fraction of the oil
originally present, since the oil reaches the surface as a result of the natural
pressure in the deposit. In secondary oil-recovery, water is injected into one
or more injection bore holes in the formation, the oil is driven towards one
or more production bore holes, and is then raised to the surface. As a secon-
dary method, this so-called water flooding is relatively inexpensive and is
therefore frequently used. In many cases, however, only a small amount of
additional oil can be recovered from the deposit.
Effective expulsion of the oil can also be achieved by tertiary
measures. Although this is more expensive, it is urgently necessary economical-
ly in view of existing oil shortages. These tertiary measures are to be under-
stood as involving either lowering the viscosity of the oil and/or raising the
viscosity of the flood water and/or lowering the interfacial tension between
the water and the oil. Most of these processes may be classified either as
solution or mixture flooding, thermal oil-recovery processes, surfactant or
polymer flooding, or combinations of several of the above.
Thermal recovery processes involve the injection of steam or hot
water or are carried out in the form of in-situ combustion. Solution or mixture
processes involve the injection of a solvent for the petroleum in the deposit.
The solvent may be a gas and/or a liquid.
Surfactant flooding processes may be divided into surfactant-assist-
ed water flooding, conventional surfactant flooding ~"low-tension flooding"),
micellar flooding and emulsion flooding, depending upon surfactant concentration,
possibly surfactant type, and additives. These processes are based mainly upon
-- 1 --
~91114
a sharp reducti~n in interfacial ~ension bet~een the oil and the flood water.
In certain cases, ho~ever, especially ln the presence of higher surfactant
concentratiOns~ ~ater~ln-oil dispersions arise, with distinctly increased viscos-
ity as compared with oil, in which case surfactant flooding also aims at a
reduction in the mobility ratio, thus increasing the efficiency with which oil is
expelledO Pure polymer flooding i9 based upon this latter improvement in the
mobility ratio between the oil and secondary flood water.
The present invention relates to a method for recovering oil by surfac-
tant floodingO Hitherto, organic sulphonates such as alkyl-, alkylaryl- or
petroleum-sulphonates have mainly ~een descrlbed as oil-mobilizing surfactants,
but these have a very low tolerance limlt against saline deposit water. Even
salt concentrations of 1 000 ppm raise pro~lems, and the sensitivity of these
surfactants to alkaline earth metal ions is par~lcularly pronounced, the upper
critical limit being assumed to be about 500 ppm (United States Patent ~ 110 228),
~hen these surfactants are used in the presence of higher salt concentrations,
precipitation products are formed, leading to plugging of the formation. How-
ever, since the water in many deposits has substantially higher salinity~ for
example up to 250 0~0 ppm in North Germany, ways have been sought to utilize the
otherwise goo~ oil mobilizing properties of organic sulphonates for deposit
systems of higher salinityO When mixed with cosurfactants such as alcohols or
- non-ionic surfactants, organic sulphonates also proved to be less sensitive to
electrolytes~ However> a procedure such as this involves the use of a highly
complex mixture, the basic component of which, petroleum sulphonate, is itself
a multi-component mixture. If it is borne in mind that different surfactant
molecules interact absorptively in different ways with the surface of the rock,
it is safe to assume that the composition o the surfactant mixture w:ill vary
-2-
3 :~9~ ~ 4
constantl~ as it passes through the formation, and this may mean forfei~ing the
careful adaptation of the suractant com~lnation to the properties of the depo-
sit~ from the point of view of both oil mobiliæation and compatibility with the
cGntent of the deposit.
In contrast to the above-mentioned group of substances, carboxymethy-
lated alkyl- and alkylaryl-oxethylates display good compatibility, even when the
sallnity of the deposit water is extremely~high (e.g~ 2~0 000 ppm?. Up to
concentrations of about 20 000 ppm, alkaline earth metal ions have no detri~ental
effect. Since the oil mobilizing effect of these surfactan*s is good CD. BALZER
and K. KOSSWIG, Tenside detergents 16, 256 ~1979)] and since they are easy and
economical to produce, these classes of substances are very suita~le for use in
expelling oil from deposit systems Qf average and hig~ salinity (total salt
content Erom 10OO~ to 250 000 ppm).
In the case of numerous inves~igations of residual oil mobilization in
model formations, ho~ever, using carboxy~ethy~Lated oxethylates as surfactants, it
was observed that the passage of the stratum o oil through the formation is
accompanied b~ a sharp increase in pressureO Thus even in the case of relatively
highly permeable artificial formations, pressure gradients of up to about 40
bars~m ~ere observed. When transerred to the field, ~hese lead to pressures
~0 far above the petrostatic pressure and would thus exclude the use of these
suractants in tertiary petroleu~ recovery. Pressure gradients of a similar
order of magnitude are also mentioned in the relevant literature (C. MARX,
H~ MURTADA, M. BURKOVSKY, "Erdoel Erdgas" 93, 303 (1977)). The authors explain
that the high pressure differences are due to the formation of emulsion zones
which are supposed, however, to be restricted to the region of the front of the
flood. However, our experiments showed no local restriction of pressure g adi-
~3-
ent. ~oreo~er, since crude oil emulsions5 stabi1ized ~ith carboxymethylatedoxethylates, are structuTall~ viscous, the la-rge pressure differences can also
not be reduced at will b~ reducing the velocity of flooding. In the case of
surfactant flooding in field tests, using carboxymethylated oxethylates,
u~controllably high pressure gradients ~ould have to be expected.
It was therefore desired to discover, for carboxy!ethylated oxethylates
with good oil mobilizing proper~ies3 a surfactant flooding procedure which does
not lead to high pressure gradients. Lowering of the pressure gradient is
possible with a sharply delayed surfactant breakthrough obtained by suitable
adaptation of the volume of surfactant to the deposit. However, this procedure
assumes homogeneous formations and, while these are possible with artificial beds
of sand, the~ are scarcel~ to be found in actual deposits. It would therefore be
scarcely possible to solve the problem in this ~ay.
The problem was initiall~ solved by the use of carboxymethrlated alkyl-
and alkylaryl-oxethylates having a degree of carboxymethylation of from 10 to
90% as the surfactants, and by a procedure in which the surfactant is selected
for the deposit in such a manner that the phase-inversion temperature (PIT) of
the original crude oil/deposit ~ater/surfactant/possible additives system is from
0 to 10C above the temperature of the deposit CPatent Application P 30 33 926.5
~24).
It has no~ been found, surprisingly enough, that surfactant flooding,
using carboxymethylated oxethylates having a 90 to 100% degree of carboxymethy-
lation, produces considerably improved oil extraction results, in that the
amount o~ surfactant used to extract the oil may be substantially less than that
required ~ith products having a degree of carboxymethylation of between 60 and
~0%, also recognizable by distinctly less surfactant molecule retention.
~ ~9~
Accordingly, the invention provides a method for recover-
ing largely emulsion-free oil from a subterranean deposit of
average or high salinity, which method comprises injecting a 1 to
20% solution or dispersion, in formation water, of a carboxy-
methylated oxethylate surfactant of the formula
( C 2 2)n C 2
wherein R signifies a linear or branched aliphatic radical with 6
to 20 carbon atoms or an alkyl aromatic radical with 3 to 16 carbon
atoms in the alkyl group, _ signfies a number from 3 to 30, and M
represents an alkali or alkaline earth metal ion or ammonium, and
the degree of carhoxymethylation is incomplete, into an injection
bore-hole, the surfactant being selected in such a manner that the
phase-inversion-temperature of the crude oil/formation water/
surfactant system, with possible additives, is from 0 to 10C
above the temperature of the deposi-t, the degree of carboxymethyl-
ation of the surfactant, i.e. the anionic part, amounting to from
90 to 100% of the total.
Thus the use according to the invention of surfactants
having a 90 to 100% degree of carboxymethylation is of consider-
able advantage economically. This high degree of carboxymethyl-
ation greatly favours molecular uniformity, so that chromatographic
separation of ~e surfactant mixture into its constituents, and
thus uncontrollable variations in phase conditions during the
passage of the surfactant slug from the injection zone to the
production zone is extremely unlikely~
In order to avoid high pressure gradients, the
surfactant is adapted to deposit conditions. A criterion of this
adaptation is the phase-inversion-temperature (PIT) of the deposit-
5 -
::
specific emulsion consisting of crude oil, formation water,
surfactant and possible additives, which should be from 0 to 10,
preferably from l to 5C, above the temperature of the deposit.
PIT's are determined with the aid of measurements of
electrical conductivity. To this end, an emulsion is produced
consisting of the crude oil (possibly "live" oil) and formation
water from the relevant deposit (phase-ratio 1 : 1), the
surfactant (2 to 5% based on the aqueous phase), and possible
additives. The electrical conductivity of the emulsion is then
measured as a function of temperature. Upon reaching the PIT, an
o/w emulsion becomes a w/o emulsion and vice-versa, with an abrupt
increase or decrease in electrical conductivity. More particularly
this is a temperature range of a few degrees C. The temperature at
which electrical conductivity reaches an average value between the
upper (o/w) and the lower (w/o) level is recorded as the PIT.
The present invention relates to the use of carboxy-
methylated oxethylates with a 90 to 100% degree of carboxymethyl-
ation as oil mobilizing surfactants. According to German Patent
; 2418444, these compounds may be obtained by reacting oxethylates
of the formula R-(O-CH2-CH2)nOH with excess chloroacetic acid and
alkali metal hydroxide or alkaline earth metal hydroxide. In this
connection, R signifies a saturated or unsaturated, straight-chain
or branched alkyl radical with 6 to 20, preferably 8 to 18 C-atoms,
or an alkylaryl radical with 3 to 16, preferably 6 to 14 C-atoms in
the alkyl radical. _ may assume values of from 1 to 30, preferably
from 1 to 20. The cation may be sodium, potassium, lithium,
ammonium, calcium or magnesium. The following may be used as
alcohols upon which the oxethylates of the carboxymethylates are
- 6 -
. ` '~,~7g'1~
based; for example: hexyl alcohol, octyl alcohol, nonyl alcohol,
decyl-, undecyl-, lauryl-, tridecyl-, myristil-, palmityl- and
stearyl alcohol, but also unsaturated alcohols such as oleyl
alcohol, for example. The alkyl chain may be straight or
branched. Commercial mixtures of these alcohols may be used with
particular advantage, preferably with chain lengths differing by
no more than 4 C-numbers.
i
- 6a -
, ~ j ..
~ ~g3Ll~
The follo~ing may be used as al~yl phenQls, for example: ~utyl phenol, hexyl
phenol, octyl phenol~ nonyl phenol, decyl p~enol, undecyl phenol, dodecyl phenol,
tridec~l phenol, tetradecyl phenol. Also suitable are phenols having a plurality
of alkyl groups, for example dlalkyl phenols. The alkyl chain may be straight or
branchedO In particular, commercially available mixtures of such alkyl phenols
may be used, preferably with chain lengths differing by not more than ~ C-num-
bers.
Oxethylation may be carried out in the presence of catalytic quantities
of alkali metal hydroxide, with 1 to 30, preerably 1 to 20 moles of ethylene
oxide per mole of hydroxyl compound. The resulting mixtures have an almost
Poisson distribution of homologues. ~ependlng upon the mathod of production, the
carboxymethylated oxethylates thus obtained generall~ contain a certain amount of
unreacted oxethylate, but according to the invenkion this should be small. Thus
the formula R'~OCH2 CH~)n-OC112-COOM usually signifies a mixture having different
amounts of unreacted oxethylaten Thls makes it possible to define a degree of
carboxymethylation. Especially efective are mixtures having degrees of carboxy-
methylation of from 90 to 100%, preferabl~ _ 95%, percentages in this case being
by weight.
The above mixtures of anionic and partly non-ionic surfactants, known
20 as carboxymethylated oxethylates, are generally soluble, or at least easily
dispersible, in conventional deposit water, and no precipitation is observed.
; In selecting surfactants with the proviso that the phase-inversion
-; temperature of the crude oil/formation water/surfactant system shall be between
0 and 10C above the temperature of the deposit, the procedure is as follows:
~ith a knowledge of the temperature of the deposit, or possibl~ of a temperature
range, an investigative measurement of the PlT is made from crude oil, formation
--7--
l :~79~
~ater and possibly gas rom the deposit and a carboxymethylated oxethylate of
the above-mentioned formula whlch a~pears to be suitable. This measurement is
repeated, if neces-sary, ~ith further surfactants of this class and possibly
~dditives .
The carboxymethrlated oxethylate is tailored to measure from the re^
sults of these measurements, and its oil mobilizing effectiveness for the parti-
cular deposit-system may be verified by one or more preliminary tests in a bed
of sand used as a model formation or in original drill cores themselves.
Sodium salts of carboxyme~hylated oxethylates, obtained by reacting
oxethylates ~ith chloroacetic acid in the presence of caustic soda offer many
po~nts of departure for "molecular architecture" with a view to setting up a
desired phase-inversion temperature in a specific system:
R - ~OCH2CH~)n - OH ~ ClCH2C~O~I NaCl }I O>
~ 2 2)n 2
R n Reaction
variable variable ~variable within limits)
The salts of carboxymethylated oxe~:hylates consist mainly of three
variable constituents: the hydrophobic radical R, the oxethylate chain, and the
carboxymethyl groupg the proportions of which, in the product mixture obtained,
may be varied within broad limits by controlling the reaction betueen the initial
oxethylate and chloroacetic acidO However, as discussed hereinbefore, the degree
of carboxymethylation must amount as far as possible to betueen 95 and 100%.
Thus ~he actual variables are only the hydrophobic radical R and the degree of
oxethylation.
An impression of the effect of the alkyl radical on phase-inversion
temperature in a specific system may be gathered from Table 1.
8-
',`
91~.4
The relatio~ship between the P~T and the length of the alkyl radicalin the sodium salts oi carboxymeth~lated fatt~ alcohol oxetbyla~es with 4.4 EO
~degree of oxethylatiQn) and a degree of carboxymethylation (CM) of 65%; crude
oil A ~see D. BAL~ER and Ko KOSS~IG, locOcit.)~ formation water A ~see D. BALZER
and K. K~SSWIG, loc~cito), phase ratio 1 ; 1, 2~ surfactant.
Table l
C-Number m Alkyl group PIT ~ C)
12 74
13*) 53
14 41
_ 37
*)
1 : 1 Mixture of C12 and C14.
The relationship between the orude oil emulsion PIT ~as in Table 1)
and the degree of oxethylation (EO~ in the case of carboxymethylated nonylphenol
;~ Qxethylates ~CM about 98%~ in tbe EQ range between about 5 and 6.5 is linear,
with about ~30C/EO the coefflcient is unusuall~ strongly pronounced. In con-
trast to this, the effect o tbe degree o carboxym~thylation (C~ with only
abaut ~OO5OC/% CM is relatlvel~ sligh~o These relatlons~ips demonstrate the
variability offered b~ the class of carboxymethylated oxethylates as regards
adaptation to a deposit.
The volume of the surfactant slug to be injected, ~he surfactant
content thereof, possible additives, and the type and amount of polymer solution
controlling mobility, may be optimized by model flooding tests.
After these preliminar~ tests have been carried out, the surfactant is
introduced into the deposit, and dissolved or dispersed in formation water, with
.~_
~ 9 ~
the aid of lnjection pumps It has been found that carboxymethyl-
ated oxethylates, even in the presence of very high salinity~ are
very good emulsifiers for crude oil and other hydrocarbons, so
that the surfactant can also be introduced into the deposit in the
form of an emulsion produced above ground. The surfactant
solution, or emulsion, may be injected either continuously or the
form of a slug, i.e. a closely restricted volume of 0.02 to 2 PV
(= multiple of the pore volume of the deposit). The size of the
surfactant slug is governed mainly by surfactant concentration and
economicsO The surfactant concentration generally amounts to 0.2
to 30%, preferably 1 to 20%.
Surfactan-t flooding is preferablv preceded by water
flooding, with produced formation water being used as the flooding
liquid. The size of the water slug i9 between 0.01 and 4 PV,
preferably between 0.05 and 1.0 PV. After the surfactant slug, a
polymer slug may be injected into the deposit, both for control of
mobility and for protec-tion of the surfactant solution from
penetrating formation water. To this end, a polymer or mixture of
polymers is dissolved in the formation water in a concentration
such that the viscosity is from 2 to 6 times as high as that of
the oil. In the case of deposits of average or higher salinity
(total salt con ent between 3 and 28%), use is preferably made of
biopolymers such as polysaccharides or cellulose derivatives which,
in the presence of increased salt concentration still have ade~uate
viscosity and produce no preciptation.
In the case of formation water relatively low in al~aline
earth metal ions, it may be desirable to add to the surfactant
so]ution, and to the pre- and post-flooded formation water, soluble
,~ - 10 -
, ~
~ ~9 ~ ~ 4
alkaline earth metal salts, for example CaC12, MgC12, Ca(N03)
or Mg(N03)2. These additives must be taken into account in
adapting the surfactant to the deposit, i.e. in measuring the PIT.
The
~ lOa -
, ,. -)
~ ~9~
injection of the polymer solution iS preferably followed by normal ~ater
floodlng, and this is cont~nued until oil can be recovered economically.
The following Examples are intended to illustrate the me~hod of the
inventionO
E~a~mple 1.
An artificial formation wa$ produced by means of a thermos~at-equipped
high yressure tube 70 cm in length and 5 cm in diameter, fitted with a tempera-
ture measuring means and a pressure gauge, the tube being closed at each end
with threaded caps, and having a capillary inlet and an outlet with a pressure-
maintaining valve, the said tu~e bçing filled with quartz sand with roundededgesO The sand was then saturated with formation water A by means of a high
~ressure metering pump and ~he desired te~era~ure ~was set by means of a th~rmo-
stat. The permeability of the bed of sand was determined with the aid of a
pressure transmitter. This was followed by saturation ~ith crude oil A, the
content of adhering water being measured simultaneousl~. Water flooding was
then applied at a flooding velocity of about 2.5 m/d. After about 1.5 PV of
formation water had been introduced ~1 PV - 750 ml)l producing between 98 and
lQ0% dllution, the surfactant was injected as a slug. This was followed by a
slug consisting of a polymer solution ~0.4 PV) and about 3.~ PV of formation
water. Surfactant flooding, polymer flooding, and subsequent after flooding with
formation water were carried out at a flooding velocity of about 1 m/d. The
temp0rature of the formation was 49C, the pressure 60 bars, the porosity of the
artificial formatlon about 45%, the permeability about 1 200 mD, and the content
of adhering water about 26%.
Formation water A contained about 20% of NaCl, 0.1% of KCl, 1.2% of
CaC12, and 0.4% of MgCl2. The polymer used was hydroxyethyl cellulose tO.25%
- 1~79~4
dissolved in formation ~ater, vis:cosi~y at 25C about 60 mPa.s~. Crude oil A
~as a paraffin-~ase oil with n20 ~ 1~486~ dz~ ~ 0~863 g~cm3, and ~20 = 19 mPa.s.
The surfactant solution consisted of 0.15 PV of a 60 6% dispersion of
carboxymethylated nonylphenol oxethylate with 5O4 moles of ethylene oxide/mole
~6.5 g) in formatlon water. The degree of carboxymethylation ~as a~out 98% and
the PIT of the relevant crude oil emulsion was 52C. ~ater flooding produced
75% oil extraction. It was found possible to increase this by 25% to 100% total
oil extraction after a further 102 PV after the start of the surfactant flooding.
At this time, dilution dropped t~ about 50%. During transportatio~ of the oil
bed produced by the surfactant, an average pressure gradient of P.S bar/m of oil
bed was measured. 3~3 g of surfactant, distributed relatively homogeneously in
the packed sand ~about 2.4 kg~, ~as detected b~ analysisO
Example 2 (comparison example),
A formation as in Example l was produced, ~as wetted ~ith formation
water A and saturated with crude oil A ~porosity 48%, permeability l 200 mD,
content of adhering water 24%, temperature 53~C)o The water flooding produced a
maximum of 77% of oil extraction. The introduction of 002 PV of surfactant
solution ~10.3 g of carboxymethylated nonylphenol oxethylate with about 6 moles
of ethylene oxide/m~le, a carboxymethylation C~ degree of about 75%, dispersed in
formation water A, PIT of crude oil emulsion 56C), followed by the polymer
solution and formation water A as in Example 1, made it possible to increase the
oil extraction, after a further 1.4 P~ after the start of surfactant flooding,
by 20% to 97%. During transportation of the oil bed produced by the surfac*ant
in the formation, an average pressure gradient of 0.7 har/m was measured at a
flooding velocity of 1.2 m/dO ~fter the flooding test, 8.5 g of surfactant were
detected in the artificial formation, much more than in Examples 1 and 3 accor-
-12-
9 1 1 '1
ding to the invention.
Example 3
The formation used was as in Example l (formation waterA; crude oil A, porosity 45%, permeability 1 100 mD, adhering
water 28%, temperature 57C). After water flooding, which
produced a 74% oil extraction, 0.2 PV of a surfactant solution
(10.3 g of carboxymethylated nonylphenol oxethylate with about
5.8 moles of ethylene oxide/mole, carboxymethylation degree CM
97% dispersed in formation water A, PIT 61 C~) was injected into
the formation and, after a further 1.3 PV after the start of
surfactant flooding, a total oil extraction of 99% was obtained.
After the flooding test, 4.5 g of surfactant were detected in the
pores by analysis. This is a definitely smaller amount of
retained surfactant than that observed in comparable tests carried
out with carboxymethylated oxethylates with a degree of carboxy-
methylation of between 60 and 80%. This again demonstrates the
superiority of almost pure anionic products.
Example 4
A tightly packed artificial formation was produced as in
Example l; it was wetted with formation water B and saturated with
crude oil B (porosity 39%, permeability 120 mD, adhering water
content 26%, temperature 41C). After flooding with water, which
produced a 72% oil extraction, 0.13 PV of surfactant solution
(5.6 g of carboxymethylated nonylphenol oxethylate wi-th about 5
moles of ethylene oxide/mole, degree of carboxymethylation CM 97%,
PIT of crude oil emulsion B 43 C), dispersed in formation water B,
was injected, followed, as in the previous examples, by 0.4 PV of
- 13 -
polymer solution and about 3 PV of formation water~ In ~his case
the flooding velocity was 006 m/d. After about 1.6 PV after the
start of surfactant flooding, the surfactant produced a 97% oil
extraction. An average pressure gradient of about 5.5 bars/m was
observed during transporta-
- ~ - 13a -
~i 7 7 9 ~
-
tion of the oll bed through the formation. T~is relatively high value is
attributable mainly to the low permeability of the formation.
A surfactant content of 3.4 g was detected in the pores by analysis
after the flooding test.
Formation ~ater B contains approximately 1~% of NaCl, 2.2% of CaC12~
0.5% of MgCl2, and small a~ounts of KCl and SrCl20 Crude oil B has a paraffin
base, n2~0 = 1~480~ density g20 ~ 0.86, viscosity n20 = 9 mPa.s.
Example 5
A formation similar to that in Example 1 ~porosity 44% ~ permeability
10 740 mD) was wetted with formation water A and saturated with crude oil A ~adher-
ing water content 25%~ test-temperature 54C~o Water flooding produced a 71%
oil extraction. By injecting 0.17 PV of surfactant solution ~80 3 g of car-
boxymethylated alfol-1214-oxethylate with 4.5 moles of ethylene oxide/mole, CM
94%, dispersed in formation water A, PIT 56C~, followed by polymer solution and
ormation water A as in Example 1, oil extraction was increased by 21% to 93%
after a further 1.3 PV0 During oil bed transportation in the formation, an
average pressure gradient of 106 barslm was measured at a flooding velocity of
1.5 m/d, After the flooding test, 4.6 g of surfactant were detected by analysis
in the packed sand.
Example 6.
A formation similar to that in Example 1 ~porosity 43%, permeability
560 mD) was wetted with formation water A, saturated with crude oil A ~adhering
water 23%?, at a temperature 61C. After flooding with water, which produced a
70% oil extraction ~dilution 95 to lOQ%~, 0.2 PY of surfactant solution ~9.4 g
of carboxymethylated alfol~l6-oxethylate with 7 moles of ethylene axide/mole, C~
91%, PIT of the crude oil emulsion 65C) was injected~ followed by 0.4 PY of
~14Y
... ~ , ~. ~ ~ 9 ~
polymer solutlon and about 3 PV of formation water A. After l.l PV after the
start of surfactant injection, a further 17% of oil was obtained, the dilution
dropping to about 60%. During transportation of the oil bed in the formation,
an average pressure gradient of about l bar/m was measured at a flooding velo-
city of 1.2 m/d. Analysis of the pore space gave a surfactant content of 4.0 g.
: - 15 ~
