Emulsfiying agent, reverse emulsions produced using this emulsifying agent and use of these emulsions in drilling wells

An emulsifying agent according to formula (I) having a molecular weight of between 3000 and 10000: ##STR1## where A represents a terminal group for polyacrylates R.sub.1 represents an oleyl group containing up to 20 mol % stearyl groups: R.sub.2 is a residue with the formula --(CH.sub.2 H.sub.4 O).sub.x (C.sub.3 H.sub.16 0).sub.y --CH.sub.3, where x is between 8 and 24 and y is between 0.75 and 2.25; l is between 1 and 3 is between 0.85 and 3.5; and n is between 0.12 and 0.5. A method of forming a reverse emulsion using this emulsifying agent. A heavy reverse emulsion, without solids comprising: 10 to 70% by volume of a dispersed brine phase having a density of between 1.20 and 2.77; 90 to 30% by volume of a continuous oil phase, and 15 to 60 g/liter of this emulsifying agent.

The present invention relates to a novel emulsifying agent. It also relates 
to reverse emulsions of water in oil, produced using this emulsifying 
agent, and the use of these reverse emulsions in mining and drilling oil 
or geothermal wells. 
The reverse emulsions conventionally used in mining and drilling generally 
contain up to 50% by volume of a brine solution of different salts or 
mixtures of salts 50% by volume of oil, one or several oil-soluble 
emulsifying agents and a solid phase. The solid phase is used to adjust 
the density of the emulsion by acting as an oil hardener; and it controls 
the filtration and regulates the viscosity of the emulsion. 
When high pressures are likely to be encountered in well drilling, 
weighting solids, such as barium sulfate or calcium carbonate are used. 
However these solids can seriously damage the reservoir. 
To overcome this problem, it has been proposed to use weighting agents not 
containing solids, such as simple or complex heavy brines based on salts 
such as CaBr.sub.2 and/or ZnBr.sub.2, CaCl.sub.2, NaCl, KCl, and K.sub.2 
CO.sub.03. 
Conventional reverse emulsion mud systems using these heavy brines are not 
temperature-stable emulsions. Attempts to make these mud systems 
temperature stable by adjusting the oil/brine ratio, by increasing the 
concentration of the emulsifying agents, and by saponifying the 
emulsifying agents with alkaline or alkaline earth salts such as 
Ca(OH).sub.2 have been unsuccessful. The temperature instability of these 
reverse emulsified muds results in precipitation of the salts; a 
segregation of the phases or coalescence; or even a fractionation of the 
oil after manufacture of the emulsion before or after aging at 150.degree. 
C. 
In addition, the use of lime or alkaline compounds can pose problems of 
compatibility with certain salts, which are only stable at a weakly acid 
pH. 
Two known reverse emulsion compositions and their respective problems are 
discussed below. 
COMPOSITION A 
This composition, has a specific mass of 1.15 g/cm.sup.3. It is composed of 
a 200 HDF oil having a density of 0.817 (sold by the TOTAL Group) and a 
mixed CaBr.sub.2 --CaCl.sub.2 brine having a density of 1.63. The H/S 
(oil/brine) ratio of this composition is 60/40. 
This emulsion further contains the agents sold under the following names: 
VERSAMOD: 20 g/liter, 
VERSAMUL: 23 g/liter, 
DOVAMUL HV: 9 g/liter, 
as well as 12 g/liter of Ca(OH).sub.2. 
When manufactured such a composition has satisfactory rheological 
characteristics (plastic viscosity VP and Yield Value YV): 
VP=20 
YV=13. 
Its electrical stability is also satisfactory (1260 volts at 60.degree. 
C.). It has a weak filtration rate (3 cm .sup.3 of oil in accordance with 
the API 13 test) but forms a very thick deposit ("cake"). 
However, after aging at 150.degree. C. for 24 hours, the emulsion of this 
composition undergoes segregation of the phases and the salts precipitate. 
COMPOSITION B 
This composition was proposed by a service company and has a density of 
1.15. Like Composition A, it contains a 200 HDF oil of a density of 0.817 
mixed with CaBr.sub.2 or CaCl.sub.2 brine having a density of 1.63. In 
addition, it contains the following additives (some of which are 
designated by their trade names): 
KENCAL (emulsifying agent): 8 cm.sup.3 /liter, 
KENOL (emulsifying agent): 24 cm.sup.3 /liter, 
F54 (emulsifying agent): 15 cm.sup.3 /liter, 
RF.sub.3 (filtrate reducer): 10 cm.sup.3 /liter, 
Ca(OH).sub.2: 5 g/liter. 
When manufactured, this mud has the correct rheological characteristics 
(VP=20, YV=11), a satisfactory electrical stability (1100 volts at 
60.degree. C.), and a weak filtration rate on the order of 5 cm.sup.3 of 
oil. 
Unfortunately, after aging at 150.degree. C. for 13 days, this emulsion is 
unstable. Moreover, it has a strong tendency to gel. 
U.S. Pat. No. 4,381,241, teaches using a polydiorganosiloxane emulsifying 
agent to produce a reverse emulsion without solids. This emulsion, 
however, has been shown to be unstable after 24 hours at a temperature of 
150.degree. C. Moreover, the complex CaBr.sub.2 --ZnBr.sub.2 brine phase 
used in the Examples of this patent is very oxidizing and degrades with a 
release of HBr, which reacts with the emulsifying agent. 
U.S. Pat. No. 4,421,656, teaches overcoming the disadvantages of U.S. Pat. 
No. 4,381,241 by combining an organopolysiloxane copolymer resin with the 
polydiorganosiloxane that acts as the base emulsifying agent. However, 
tests have shown that in emulsions produced using this composition the 
brine exhibited phase segregation after static aging for 24 hours at 
150.degree. C. 
Known emulsifying agents used for the production of heavy reverse emulsions 
which can be used in drilling wells, generally have the following 
disadvantages: 
instability of the emulsion at temperatures above 150.degree. C. (at these 
temperatures the phases segregate or coalesce and the salts precipitate); 
presence of solids which are harmful to the integrity of the reservoirs 
being drilled (the phenomena known as the appearance of "skin"). 
SUMMARY OF THE INVENTION 
One object of the present invention is to overcome the disadvantages of the 
known art with an emulsifying agent that enables reverse high density 
emulsions that are pressure- and temperature-stable, produced using a 
heavy brine and a low density oil which can be used in drillings, without 
damaging the permeability of the reservoirs being drilled. 
Another object of the invention is a reverse emulsion emulsifying agent 
that permits the formation of a reverse emulsion between two very 
different density phases and prevents the problems of segregation of the 
liquid phases, of sedimentation of heavy droplets of brine, and the 
coalescence of fine droplets of brine dispersed in the oil phase. 
A further object of the invention is the production, using the emulsifying 
agent of the invention, of heavy reverse emulsions without solids, which 
can be used in mining or drilling for oil or geothermal reserves, without 
risk of damaging the respective reservoirs being drilled. 
These and other objects of the invention are satisfied by an emulsifying 
agent according to formula (I) having a molecular weight of between 3000 
and 10000: preferably between 4000 to 8000 
##STR2## 
where A represents a terminal group for polyacrylates; 
R.sub.1 represents an oleyl group containing up to 20 mol % stearyl groups; 
R.sub.2 is a residue with the formula --(CH.sub.2 H.sub.4 O).sub.x (C.sub.3 
H.sub.16 O).sub.y --CH.sub.3, where x is between 8 and 24, preferably 
between 10 and 16, and y is between 0.75 and 2.25, preferably between 1 
and 2; 
l is between 1 and 3; 
m is between 0.85 and 3.5; 
n is between 0.12 and 0.5. 
The objects of the invention are also satisfied by a method of forming a 
reverse emulsion using the emulsifying agent of formula (I), above. 
Further objects of the invention are satisfied by a heavy reverse emulsion 
without solids comprising: 10 to 70% by volume of a dispersed brine phase 
having a density of between 1.20 and 2.77; 90 to 30% by volume of a 
continuous oil phase; and 15 to 60 g/liter of the emulsifying agent 
according to formula (I), above. 
Still further objects of the invention are satisfied by a method of 
drilling wells using this heavy reverse emulsion.

DESCRIPTION OF THE INVENTION 
The optimum concentration of emulsifying agent depends on the oil and brine 
used, as well as the oil/brine ratio by volume. 
The brine used in the present invention can be a simple or complex brine 
composed of various salts or mixtures of salts such as NaCl, CaCl.sub.2, 
MgCl.sub.2, CaBr.sub.2, KCl, CaBr.sub.2, or ZnBr.sub.2. 
In the emulsifying agent of the invention according to formula (I) x is 
preferably between 10 and 16; y is preferably between 1 and 2; and A 
preferably represents a dodecylmercaptan group. In addition, in the most 
preferred emulsifying agent of the present invention according to formula 
(I), l is 2.0; m is 1.75; n is 0.25; x is 16; and y is 1.5. 
The emulsions formulated according to the present invention are 
particularly advantageous since they enable the following specific 
problems to be overcome as indicated: 
Problems of Clogging: The emulsifying agent is oil-soluble and enables the 
production of a reverse emulsion of brine in an oil phase; no clogging due 
to solids is therefore to be feared in potential reservoirs to be drilled. 
Problems of High Pressure: The specific mass desired for the emulsion is 
added using a simple or complex brine phase, and it is therefore easy to 
modify the mass by adjusting the density of the brine or by modifying the 
ratio of the oil/brine volumes. 
Problems of Temperature: The emulsions of the invention are stable up to at 
least 150.degree. C., as indicated in the Examples, below, in the static 
and dynamic modes. 
When using the emulsions of the invention for drilling wells, in particular 
for drilling oil wells, additives such as the following can be 
additionally used, either alone or in combination: completion fluids; so 
called "work over" fluids; packer fluids; so-called "spacer" intermediate 
fluids; drilling fluids; and viscous plugs. 
The invention is further described with reference to the following 
non-limiting Examples. In these Examples, all ratios and percentages are 
by weight unless otherwise indicated. 
In all these examples, the emulsifying agent used is represented by formula 
(I), above, in which l was 2.0, m was 1.75, n was 0.25, x was 16, and y 
was 1.5. This emulsifying agent is manufactured as TEGOPREN LE 1733, by 
the German company T.H. GOLDSCHMIDT A.G. 
EXAMPLE 1 
This Example relates to the preparation and the physical characteristics of 
various emulsions in accordance with the invention. 
1. Preparation of Emulsions 
Emulsions having an oil/brine ratio of 50/50 by volume and final density of 
1.56 were prepared using the following materials: 
so-called "ecotoxic" (TR2) type oil, having a density of 0.826, 
CaBr.sub.2 --ZnBr.sub.2 brine having a density of 2.31, 
TEGOPREN LE 1733 emulsifying agent: at a concentration between 15 and 60 
g/liter, 
anticorrosive: 0.4% by weight of brine. 
These emulsions were prepared by dissolving the emulsifying agent in the 
oil phase, using magnetic stirring for 5 minutes and at ambient 
temperature. Using a pump (JABSCO mark model 2182000), the brine was 
slowly incorporated into the oil phase. The solution was then maintained 
with stirring for a variable period of time depending on the amount of 
emulsion manufactured. For a volume of one liter of emulsion for example 
the minimum stirring time was one hour. 
The temperature of the mixture was regulated below 40.degree. C. After 
total dispersion of the brine in the oil the anticorrosive was 
incorporated by mixing using the JABSCO pump. 
2. Tests Carried Out on the Emulsions 
The rheological and electrical stability characteristics of the emulsions 
were measured using API norms. Both static and dynamic aging tests were 
carried out at 150.degree. C. 
The graph of FIG. 1 shows the variation in electrical stability as a 
function of the concentration of the emulsifying agent, after manufacture 
and after maintenance at 150.degree. C. for 96 hours. FIG. 1 shows that 
there exists a minimum threshold of concentration in emulsifying agent of 
15 g/liter below which the emulsion was unstable. An improvement in 
electrical stability was noted after aging, with this improvement being 
less substantial above a concentration of emulsifying agent of 
approximately 45 g/liter. The electrical stability was maintained after 
aging above a concentration of 35 to 40 g/liter of emulsifying agent. 
The graphs of FIG. 2 illustrate the variations in the plastic viscosity VP 
and in the Yield Value YV as a function of the concentration of 
emulsifying agent after manufacture (plots VP(1) or YV(1) and after aging 
at 150.degree. C. for 96 hours (plots VP(2) and YV(2)). These rheological 
characteristics were even greater for concentrations of emulsifying agent 
between 15 and 30 g/liter. The VP and YV values varied inversely to the 
concentration of emulsifying agent, after aging at 150.degree. C. The high 
increase in the concentration of emulsifying agent between 35 to 60 
g/liter had only a slight influence on the rheological characteristics, 
both before and after aging. 
FIG. 3 illustrates the variation in the electrical stability VB in the 
dynamic mode as a function of the aging time, when the concentration of 
the emulsifying agent was 45 g/liter. A very clear increase was noted in 
the electrical stability of the emulsion as a function of the aging time. 
This evolution was less noticeable between 60 and 96 hours. 
FIG. 4 represents the evolution of the plastic viscosity VP and of the 
Yield Value YV as a function of the aging time in the dynamic mode when 
the concentration of the emulsifying agent was 45 g/liter. The rheological 
characteristics of the emulsion vary little with the aging, and the aging 
time therefore had little influence on these characteristics. It was 
apparent that the system can have phase separation after aging in the 
static mode at 150.degree. C., after 48 hours, while it remained stable 
after aging at 150.degree. C. in the dynamic mode. Below 40 g/liter of 
emulsifying agent, the system was unstable after aging for 48 hours at 
150.degree. C. in the static mode. 
FIG. 5 shows the evolution of the electrical stability VB in the static 
mode and in the dynamic mode, as a function of time, when the 
concentration of the emulsifying agent was 45 g/liter. It can be noted 
that this electrical stability was weaker in the static mode than in the 
dynamic mode. 
FIG. 6 represents the evolution of the plastic viscosity VP and of the 
Yield Value YV as a function of the aging time for a concentration of 
emulsifying agent of 45 g/liter. It can be noted that the plastic 
viscosity was stable in dynamic aging, while it had a tendency to fall 
slightly in static aging. The Yield Value remained stable and was not 
affected by the method of aging. 
The influence of the H/S (oil/brine) volumetric ratio on the electrical 
stability of the emulsion and on its rheological characteristics was also 
studied. 
FIG. 7 represents the evolution of the electrical stability VB as a 
function of the H/S ratio immediately after manufacture of the emulsion, 
after aging for 48 hours, and after aging for 162 hours respectively in 
the dynamic mode at 150.degree. C. 
FIG. 8 shows the evolution of the electrical stability as a function of the 
aging for different H/S ratios when the concentration of the emulsifying 
agent was 45 g/liter. It can be noted that it was only for an H/S ratio of 
50/50 that the aging had a favorable influence on the stability. 
FIG. 9 illustrates the evolution of the plastic viscosity VP and of the 
Yield Value YV as a function of the H/S ratio after manufacture of the 
emulsion (graphs VP(1) and YV(1)) and after 162 hours of aging in the 
dynamic mode at 150.degree. C. It can be noted that an increase in the 
concentration of brine increased the rheological characteristics of the 
emulsion. For an H/S ratio of 40/60, the plastic viscosity increased as a 
function of the aging time. 
The Yield Value was slightly sensitive to the aging time. It remained high 
for H/S ratios of 40/60 but fell very rapidly as soon as the H/S ratio 
reaches 70/30, and even after 50/50. 
EXAMPLE 2 
This example relates to emulsions having a final density of 1.15. It was 
produced with a single salt brine (CaBr.sub.2) (having a density of 1.49 
when the H/S ratio was 50/50 by volume), and a density of 1.66 when the 
H/S ratio was 60/40 by volume). a 200 HDF oil having a density of 0.817, 
and the emulsifying agent concentration was varied from 25 to 45 g/liter. 
FIGS. 10 and 11 represent variations in the electrical stability VB as a 
function of the concentration of the emulsifying agent, after manufacture 
of the emulsion and after aging in the dynamic mode at 150.degree. C. for 
114 hours. It can be noted that immediately after manufacture, whatever 
the H/S ratio under consideration, the increase in the concentration of 
emulsifying agent was translated into an increase in electrical stability. 
This evolution was much more noticeable for the H/S ratio 60/40. . As a 
general rule, after aging at 150.degree. C. for 114 hours, these emulsions 
were stable, and the electrical stability remained higher when the H/S 
ratio was 60/40 than when it was 50/50. Aging did not increase the 
electrical stabilities. Above the concentration of 35 g/liter of 
emulsifying agent, there was no increase in the stability for the H/S 
ratios of 50/50 or 60/40. 
FIGS. 12 and 13 each represent variations of plastic viscosity VP and of 
the Yield Value YV as a function of the concentration of emulsifying 
agent, for various H/S ratios, after manufacture of the emulsion and its 
aging in the dynamic mode at 150.degree. C. for 114 hours. It can be noted 
that, after manufacture, the plastic viscosity was greater for a 50/50 
ratio than for a 60/40 ratio. For an H/S ratio of 50/50the evolution of 
the plastic viscosity was no longer substantial after a concentration of 
35 g/liter of emulsifying agent. 
After aging in dynamic mode at 150.degree. C. for 114 hours the following 
observations were made: 
At the H/S ratio of 60/40, there was no substantial increase of VP and YV. 
At the H/S ratio of 50/50, VP increased substantially as the concentration 
of emulsifying agent increased from 25 to 35 g/liter. VP then stabilized 
at a concentration of from 35 g/liter up to 45 g/liter. YV increased 
substantially as the concentration of the emulsifying agent increased from 
25 to 30 g/liter, then had a tendency to decrease as soon as the 
concentration was greater than 30 g/liter. 
In addition, after aging the emulsion the Yield Value was lower at 
concentrations of 40-45 g/liter of emulsifying agent, than at manufacture. 
When the H/S ratio was 60/40 or 50/50 and the concentration of the 
emulsifying agent was fixed at 35 g/liter, a sedimentation of the heavy 
dispersed phase was observed after aging for 70 hours at 150.degree. C. in 
the static mode. This sedimentation does not result in phase separation of 
the emulsion because slight stirring was sufficient to rehomogenize it. 
EXAMPLE 3 
Emulsions having a density of 1.15 were prepared with 200 HDF oil having a 
density of 0.817; brines having a density varying between 1.33 and 1.41 
and containing as salt(s) either CaBr.sub.2 ; ZnBr.sub.2, or a CaBr.sub.2 
ZnBr.sub.2 mixture; with an H/S ratio by volume of 44/56; and with the 
emulsifying agent at a concentration of 35 g/liter. 
FIGS. 14 and 15 represent the evolution of the electrical stability of the 
emulsions as a function of the H/S ratio, respectively after manufacture 
of the emulsions and after their aging in the static mode at 150.degree. 
C. for 166 hours. It can be noted that at manufacture the variation of the 
H/S ratio did not involve substantial modifications in the electrical 
stability for brines based on CaBr.sub.2. On the other hand, there was a 
drop in electrical stability with the brine based on ZnBr.sub.2. Thus, the 
level of stability depended on the type of salt used in the emulsion. 
After aging at 150.degree. C. in the static mode, the electrical stability 
increased for the CaBr.sub.2 brine. In addition, if the H/S ratio 
decreased (H/S&lt;44/56), the electrical stability dropped, which confirms a 
destabilization of the emulsion when the H/S ratio is 35/65. 
FIGS. 16, 17, 18, 19, 20, and 21 represent the evolution of the plastic 
viscosity VP and of the Yield Value of the emulsions as a function of the 
H/S ratio for emulsions based on CaBr.sub.2 --ZnBr.sub.2, ZnBr.sub.2, and 
CaBr.sub.2 respectively, after manufacture of the emulsions and after 
aging in the static mode at 150.degree. C. for 166 hours. It can be noted 
that after manufacture the decrease in the H/S ratio resulted in a strong 
decrease in the viscosity of the emulsion, no matter what type of brine 
was used. 
After aging at 150.degree. C. in the static mode, a decrease of the H/S 
ratio had the following consequences: 
a drop in VP and YV regardless of the type of brine; 
a decrease in YV, which is more substantial for the ZnBr.sub.2 brine than 
for the CaBr.sub.2 and CaBr.sub.2 --ZnBr.sub.2 brines; 
a non-continuous evolution of VP for the CaBr.sub.2 brine, before and after 
aging, except when the H/S ratio was 40/60; 
a tendency for the plastic viscosity to drop for the brines based on 
CaBr.sub.2 --ZnBr.sub.2 and ZnBr.sub.2, both before and after aging. 
EXAMPLE 4 
This example describes the effect of contamination of a stable emulsion 
prepared in accordance with the invention with an FGN clay or soft water. 
An emulsion having a final density of 1.15 and an H/S ratio by volume of 
40/60 was prepared using a 100 HDF oil having a density of 0.817, a brine 
based on CaBr.sub.2 having a density of 1.376, and 35 g/liter of the 
emulsifying agent. 
Contamination tests with clay FGN as a filler have been carried out with 
clay rates of 25, 50, 75, 100, 125 and 150 g/liter of emulsion. No phase 
separation of the emulsion was noted, even after an addition of 150 
g/liter of clay. 
The effects of contaminating the emulsion with clay are illustrated by FIG. 
22, which shows the variations in the electrical stability VB as a 
function of the amount of clay added, expressed in g/liter of emulsion, 
and by FIG. 23, which shows the variations in plastic viscosity VP and 
Yield Value YV as a function of the amount of clay added. In both cases 
the tests were carried out after aging of the emulsion at 150.degree. C. 
for 72 hours in the dynamic mode. 
As soon as the concentration of clay was greater than 50 g/liter, a very 
high drop in electrical stability was noted. This decrease in stability 
did not create a phase separation of the emulsion. No sedimentation was 
noted after 24 hours of rest. After aging for 24 hours at 150.degree. C. 
in the dynamic mode, the emulsion remained stable. 
VP and YV remained stable after contamination with clay up to 
concentrations of 100 g/liter. Above that concentration, the contamination 
was too great and very high values for YV and VP were observed. 
Using the same emulsion, contamination tests were carried out with soft 
water using 0, 2, 4, 6, 8 and 10% of soft water to the total volume of the 
emulsion. This produced H/S ratios varying from 40/60 to 36/64. FIG. 24 
shows the variations in electrical stability as a function of water 
contamination expressed in % by volume after aging of the emulsion in the 
dynamic mode for 72 hours at 150.degree. C. 
No particular variation in electrical stability was noted up to a 
contamination of 10% soft water by volume. 
FIG. 25 shows the variations in plastic viscosity VP and Yield Value YV of 
the emulsion as a function of the amount of soft water contamination, 
expressed in % of water by volume after aging of the emulsion in the 
dynamic mode for 72 hours at 150.degree. C. There was noted a continuous 
increase in rheological characteristics (VP and YV) as soon as the 
emulsion was contaminated with water. This variation became very large 
after 4% water contamination. 
EXAMPLE 5 
This Example illustrates the usefulness of reverse emulsion fluids without 
solids according to the invention used as completion fluids in oil 
drillings. 
Tests were carried out on a Hassler type permeability measuring cell. 
The Hassler assembly consists of a soil sample (core) in a flexible rubber 
tube. At each end of the core are extended steel heads that permit the 
injection or recovery of fluids at either end as well as the measurement 
of the pressure. Sealing is provided by inflating the rubber tube with a 
liquid under pressure. 
The calculation of the permeability is carried out using Darcy's Law. Under 
Darcy's Law, for a given core of fixed geometric characteristics, at 
constant temperature, the permeability is a function of the differential 
pressure and of the flow of fluid in the core. Using a pump with a 
constant flow, the measurement of permeability comes from measuring the 
variations of differential pressure in the core. The emulsion used had a 
density of 1.57 and an H/S ratio of 50/50 it was composed of a TR3 oil, a 
brine based on CaBr.sub.2 --ZnBr.sub.2, with an H/S ratio of 50/50, and 30 
g/liter of the emulsifying agent. The emulsion had undergone aging for 72 
hours at 150.degree. C. in the dynamic mode. 
The standardized tests were carried out under the following conditions: 
Preparation of the Cores (Ground Samples) 
Coring (diameter 40 mm--length 60 mm). 
Drying for 16 hours at 100.degree. C. 
Cleaning and washing the cores with toluene. 
Drying for 16 hours at 100.degree. C. 
Saturation with Kerdane (registered mark) under a vacuum for 24 hours. 
Initial Permeability in Both Directions (A.rarw..fwdarw.B) 
Measurement of the initial permeability (Ki) with a distillation cut 
(185.degree. C.-235.degree. C.) of the Kerdane type. 
Measurement of the permeability in both directions from top (A) to bottom 
(B), and from bottom (B) to top (A) at a stabilized temperature of 
65.degree. C. and at a constant flow; the pressure, temperature and the 
flow were measured and recorded. 
Clogging in Direction A Towards B (A.fwdarw.B) 
The fluid being tested (reverse emulsion) was injected under a pressure of 
25.10.sup.5 Pa, for 1.5 hours and at a temperature of 65.degree. C., until 
the filtrate appeared. 
Unclogging in the Direction (B.fwdarw.A) 
The fluid remaining in the circuit was purged and Kerdane was injected, 
under a pressure of 35.10.sup.5 Pa, and at a temperature of 65.degree. C. 
for 10 minutes. 
Return Permeability In Both Directions (B.THETA..thrfore.A) 
The measurement of the permeability was carried out at a stabilized 
temperature of 65.degree. C., under a constant flow and constant pressure, 
under injection of Kerdane. The results obtained appear in the following 
Table. 
TABLE 
______________________________________ 
Permeability 
First core 
Second core 
______________________________________ 
Ki 331 mda 50 mda 
B .fwdarw. A 
Kr 287 mda 27 mda 
B .fwdarw. A 
##STR3## 86.7% 54% 
Ki.sub.A.fwdarw.B 
411 mda 50 mda 
Kr.sub.A.fwdarw.B 
140 mda 28.5 mda 
% invasion 66% 43% 
##STR4## 
______________________________________ 
These results show that this emulsion was slightly clogging for cores 
having an initial permeability of 400 mda (Fontaine bleau stone). 
While the invention has been described with reference to the above specific 
embodiments, it Will be apparent to one skilled in the art that various 
changes and modification can be made in these embodiments without 
departing from the spirit and scope of the invention.