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One-Step Acid Removal of an Invert EmulsionL. Quintero, SPE, T. Jones, SPE, and D.E. Clark, SPE, Baker Hughes Drilling Fluids

Copyright 2005, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE European Formation DamageConference held in Scheveningen, The Netherlands, 25-27 May 2005.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in a proposal submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to a proposal of not more than 300words; illustrations may not be copied. The proposal must contain conspicuous

acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abst ractRemoval of skin damage resulting from internal and external

filter cake deposition during oil well reservoir drilling withinvert emulsion drill-in fluid is desirable in order to maximize

hydrocarbon recovery. Efficient filter cake cleanup is required

for a number of open-hole completion operations, includingthe use of stand-alone and expandable sand screens, and

conventional gravel pack applications for both production and

water-injection wells.

Most of the currently used chemical methods to remove

invert emulsion filter cake and internal formation damage arebased on the use of acids, solvents and mutual solvents mixed

in clear brine. Completing a well after drill-in with emulsion-

based fluids is time-consuming and costly, and usuallyinvolves large volume multi-stage soak treatments.

Surfactant technology, when appropriately combined with

conventional acid, allows a single-stage invert emulsion

cleanup process. In this one-step cleanup method, the invertemulsion filter cake is incorporated into the organized

surfactant in solution and the acid-soluble particles are then

decomposed.

A number of injection permeability tests were performed

on Berea sandstone cores and ceramic discs using variousinvert emulsion drilling fluids for filter cake deposition and

organized surfactant in solution/acid blends for filter cakeremoval. This investigation demonstrated that, when using thistechnology to destroy the filter cake, (1) the invert emulsion is

incorporated into the surfactant solution, (2) the solids become

water-wet, (3) sludge formation between the acid and invertemulsion cake is prevented, and (4); and the majority of acid-

soluble particles are removed.

This approach appears to be a credible method for

removing formation skin damage and increasing hydrocarbon

recovery and/or water injection rates.

IntroductionMany operators are interested in improving filter cake cleanup

after drilling into reservoirs with invert emulsion drilling

fluids. More efficient filter cake cleanup is desired for anumber of open hole completions, including stand-alone and

expandable sand screens, as well as for gravel pack

applications for both production and water injection wells.

One-step invert emulsion filter cake cleanup technologyuses a single-phase microemulsion (SPME)1-3 and

conventional acid packages, in a single blend, to solubilize theoil into the SPME, reverse the wettability of the filter cakesolids, and simultaneously decompose its acid-soluble

components. Reversing the wettability of the filter cake, using

surface active chemistry, facilitates acidizing by preventing a

sludge that could form between the acid and the emulsified

cake and by making acid-soluble particles unavailable tounspent acid.

Besides the advantage of reduced skin damage, increased

hydrocarbon recovery and/or increased water injection rates, aone-step near wellbore cleanup method will save an

operator valuable rig time.

Theory of MicroemulsionIn the 1950s, Schulman and co-workers added alcohol to

surfactant-stabilized oil-in-water (o/w) emulsions to obtain

very stable homogeneous fluids that he called

microemulsions.4 These microemulsions had an average

droplet size of 10-100 nanometers (nm), much smaller thanconventional emulsions.

The first studies of microemulsions in the oil industry were

in the 1970s for enhanced oil recovery (EOR) applications.5-1

Many oil operators and universities invested considerable time

to research this topic. The research group led by Schechter and

Wade at The University of Texas at Austin made importantcontributions to the understanding of the mechanism of

increased production by microemulsions. However, theinterest dropped due to the crude oil price decrease and

because the technology was expensive, due mainly to the high

concentration of surfactant required. Since then, the oiindustry has conducted only a small amount of research on

microemulsions.

In the past 25 years, surfactant manufacturers, universitiesand research institutes have greatly increased the knowledge

of microstructures and the phase behavior of microemulsions

and have made surfactants available that are more efficient for

microemulsion formulations.12-18

A microemulsion is a thermodynamically-stable complex

fluid typically composed of a non-polar oil phase, a polar

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water phase, surfactant and an optional co-surfactant. They are

macroscopically homogeneous and, at the microscopic level,

heterogeneous, consisting of individual domains of the non-polar oil phase and polar water phase, separated by a

monolayer of surfactant (amphiphile).1, 2, 14, 15Microemulsions

are typically clear solutions, as the droplet diameter of the

organized phase is approximately 100 nm or less, and contain

two immiscible fluids, in contrast to micellarsolutions, whichare considered to be one-phase fluids and may be either water

or oil. The schematic in Figure 1 shows the differencebetween micelles and microemulsions.

The surfactant molecules in these microemulsion fluids

lower the interfacial free energy to very nearly zero, which

induces the spontaneous microemulsification when thecomponents are brought together. These fluids may be oil-in-

water, water-in-oil, or a single-phase microemulsion with bi-

continuous structure. The schematic representation of phase

transition of oil/water/surfactant system according to Winsor

definition is show in Figure 2. Winsor I is formed by oil-swollen micelles in equilibrium with excess oil (i.e. two

phases, with the surfactant in the lower phase). Winsor II isformed by water-swollen reverse micelles in equilibrium with

excess water. Winsor III is a middle-phase microemulsion,

with excess water and oil. Winsor IV is a single-phase

microemulsion.1, 2, 19

For O/W microemulsion, the water diffusion is rapid,while the oil diffusion is low, and its diffusion process is

dominated by the diffusion of the oil-swollen micelles. In the

case of bicontinous microemulsions, both components are

expected to diffuse rapidly, having self-diffusion coefficientsclose to the values of the neat liquids, which suggested that

some continuity must exist between the water and the oil

domains.1, 20

Laboratory testsA concentrated SPME that contains water, a solvent, a

surfactant blend and a co-surfactant, was diluted in calcium

chloride brine and used as a soak solution to remove oil-baseddrilling fluid filter cake. The soak solution was also

formulated in an acidic media, which was designed to dissolve

the bridging particles in the filter cake.The concentration of SPME in the brine was selected by a

series screening tests whereby the criteria was to incorporate

all of the oil in the filter cake into the soak solution andchange the wettability of the solids from oil-wet to water wet.

Sandpack and Hassler Permeameters were used to evaluate

the efficiency of the soak solution to remove the filter cake

and the oil from the invert emulsion fluid filtrate. A schematicdiagram of the Sandpack and Hassler Permeameters are shownin Figures 3 and 4. The Sandpack Permeameter tests were run

using 20-micron discs (2800 mD), 140/270 sand and 40/60

gravel. The tests in the Hassler cell were run using one-inch

diameter Berea sandstone plugs with a range of permeabilities.The cleanup tests were performed using filter cakes

obtained from oil-based drilling fluids formulated either with

barite, calcium carbonate or a barite/calcium carbonate blend.The oil-base drilling fluid used was a 10 lb/gal system with a

65/35 oil/brine ratio. The properties of the fluid formulated

with a blend of barite and calcium carbonate are given in

Table 1.

The filter cake cleanup evaluation was based on water

injection permeability. Injection tests were chosen instead of

production permeability tests because injection is considered aworst case scenario, from the viewpoint of return

permeability. The test procedure begins with the measuremen

of the initial, baseline permeability to injection brine

consisting of filtered synthetic seawater. Next, a filter cake is

deposited on a ceramic disc or Berea sandstone, followed bytreatment with the soak solution for a specified period of time

The mud-off time for deposition of the filter cake ranged from1 to 4 hours. The soak time for the tests was in the range from

2 to 16 hours. The last step of the procedure is the water

injection measurement to determine the final injection

permeability.

Results and DiscussionSingle phase microemulsion evaluation

A microemulsion or other surfactant system could be alteredas the water/solvent/surfactant system changes, from Winsor

I Winsor II Winsor III Winsor IV, by changing

parameters such as salinity and water/solvent ratio. Thereforethe textures of the SPME were examined under a polarized

microscope in order to detect the existence of typical lamellar

structures that co-exist with the microemulsions in a phase

diagram of water/solvent/surfactant system. Figure 5ashows

photographs of the SPME under polarized light. Thephotographs depict a lamellar phase with associated oily

streaks.21This type of structure coexists with the bicontinuous

phases of the concentrated SPME. Dilution of the SPME in

high-salinity brine is shown by Maltese crosses dispersed in abi-refrigent matrix, indicating that the behavior of the SPME

has not shifted after the dilution. (Figure 5b)To verify that the SPME remained as a microemulsion

after the acid package addition, a compatibility test utilizing

SPME with acid was evaluated with microemulsionconcentrations up to 30% in brine and with various acids at

concentrations between 5 and 7.5%. Visual observation allows

one to determine whether or not fluid compatibilities exist. Foexample, when a microemulsion blend remains as a clear

single phase, this indicates that the fluids are compatible

Observation of phase separation and cloudiness indicates anincompatibility with acid. Figure 6 shows a photograph o

single-phase microemulsion with acid. The vial on the left side

illustrates the behavior of a microemulsion that separatesshowing a typical Winsor II behavior with water-swollen

reverse micelles in equilibrium with a water phase. The vial on

the right side illustrates the behavior of a microemulsion tha

remains stable as a SPME. This typical behavior results fromthe effect of the acid on the anionic or cationic surfactantsused in microemulsions. The use of nonionic surfactants, such

as the one used in the example of SPME 2 of Figure 6

prevents this type of acid/microemulsion incompatibility.

Cleanup of filter cake with SPME in brine

The first part of the filter cake cleanup study included a series

of screening tests with the SPME in brine and no acid in theformulation. The injection permeability tests were performed

on ceramic discs in a Sandpack permeameter. A cake

deposition time of one-hour at 150F and 500 psi was used to

obtain a filter cake with a reasonable thickness to break in

http://surfactants.net/micelle.htmhttp://surfactants.net/micelle.htm

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order to verify the SPME efficacy. The soak solution,

containing 10% SPME mixed with a 10 lb/gal CaCl2 brine,

was allowed to soak at 150F and 200 psi for up to 2 hours.Upon contact of the soak solution with the filter cake, the soak

destabilized the invert emulsion and penetrates the filter cake,

as shown in Figures 7a, 7b, and 7c. The results shown in

Figure 7bdemonstrate that the 10% SPME soak solution in

10 lb/gal CaCl2 works to completely destroy the emulsioncontained in a 1-hr filter cake. The remaining filter cake

residue was altered to a water-wet condition to the point that itcould be easily decanted from the test cell after soaking and

before water injection. Figure 7cis a photo of the filter cake

remains after a soak treatment, decantation and water

injection. In the case of an invert emulsion system formulatedwith barite only and treated with the SPME, the filter cake

particles became water-wet, slurrified and very porous as

shown in Figure 8.

Table 2 shows the results of the injection return

permeability tests. In the case of a vertical cell set-up, thealtered filter cake remained in place, as it cannot be displaced

due to equipment limitation. However, the filter cake was veryporous and loosely consolidated and, as a result, it was

possible to obtain a high percentage of water injection

(97.7%). In the case of horizontal cell setup, the disturbed

filter cake was easily displaced, and the percentage water

injection was 106%.

One-step cleanup of filter cake: SPME with acid in brine

The second part of the filter cake cleanup study was the

evaluation of the one-step soak formulation, an SPME inconjunction with an acid package in brine. A number of

laboratory tests were performed using SPME together with

various types of acid treatments on invert emulsion filtercakes.

Injection permeability tests in the Sandpack Permeameter

Injection permeability tests were performed on ceramic discsin a Sandpack permeameter.

The first phase of the tests consisted of filter cake

deposition on a 20 m ceramic disc. Next, the filter cake wasexposed to a SPME containing acid for 4 hours.

Figure 9shows the injection permeability results after 2-hrmud-offs and 4-hr soak times using 10 lb/gal OBM and a soak

solution formulated with 30% SPME and 7.5% HCl in 9 lb/gal

CaCl2 brine. The injection return permeability was 103.6%

and, after the test, the ceramic disc with the filter cake shows

that the acid-soluble particles were removed and only a small

amount of barite is observed. Figure 10Table 3shows the result of tests where the invert emulsion

filter cake, formulated either with calcium carbonate orbarite/calcium carbonate exposed to a blend of the SPME and

acid in 10 lb/gal CaCl2 brine. The result indicates that the

combination of SPME and acid, either acetic acid or

hydrochloric acid, gave water injection return permeabilityresults greater than 100%. It is believed that the >100%

results are due to the reduced interfacial tension of the

injection fluid arising from (1) the additives surfactants in the

acid package, and (2) the single phase microemulsion.

Injection permeability tests in the Hassler Permeameter

Tests were also conducted in a Hassler Permeameter using

Berea Sandstone. The mud-off time of these tests was 16 hrsand the filter cake was flushed with base oil at 200 psi, to

simulate a wellbore displacement to remove the excess oi

from the external filter cake. The SPME concentration wa

15% in 10 lb/gal CaCl2 containing 10% Acetic Acid and the

soak time was 2 hours. Table 4 shows the results of thesereturn injection permeability tests. The first test was only a

simple injection test. Test 2, was divided into two parts. ParI (Test 2a) was an injection test performed using the exact

procedures used in the first test. Part II (Test 2b) incorporated

a water injection step, but in the production direction

followed by a second injection in the original direction. Theseresults demonstrate that there is value in producing the filter

cake first and then re-injecting. However, the origina

injection results, without production flow, also gave very good

results.

The effects of mud-off time and calcium carbonate loading

It has been demonstrated in previous laboratory testing thalong mud-off times have significant (negative) effects on

permeability, especially if a static filter cake is allowed to

form over long periods of time. If a static filter cake i

allowed to build during field applications, high rate

displacements are pumped to reduce the static filter cakethickness.

A series of static tests were performed on a Sandpack

Permeameter using different calcium carbonate loadings. Fo

each test, the cake deposition time, prior to the second waterinjection permeability measurement, was 2 to 3 hours. The

results presented in Table 5show that an increase in mud-offtime resulted in a reduction of injection permeability from

84.3% to 48.2% for an OBM system with high concentration

of calcium carbonate (Tests 1 and 2). However, the increase insoak time from 2 to 16 hours generated a high percentage

increase in water injection (from 67.3% to 90.3%).

Conclusion1. One-step invert emulsion filter cake cleanup wa

efficiently achieved using SPME and acid blends in brine.2. The injection permeability test results prove that the

SPME cleanup technique partially removes OBM and

OBM filter cakes, water-wets the solid particles andavoids sludge formation.

3. The invert emulsion system with barite treated with the

single-phase microemulsion resulted in a loose, porous

filter cake that may be easily displaced.4. The high injection permeability tests results with inver

emulsion systems formulated with barite, calcium

carbonate or combinations of barite and calcium

carbonate proves that the one-step filter cake cleanuptechnique is effective for variations in fluid formulations.

5. A 15-30% single-phase microemulsion, with an acid in

brine, incorporated the oil phase of an OBM filter cake

into the aqueous soak solution and acidized the calciumcarbonate in one step.

6. Because invert emulsion systems are the drill-in fluids o

choice to drill pressure depleted reservoirs in many globa

operations, the SPME technology presented in this paper

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Table 2 Return inject ion permeability us ing 30% SPME in 9 lb/gal CaCl2brine as cleanup soak

PermeabilityMud-off time/Soak time, hrs

Testset-up

Drilling fluid

Initial,mD

Final,mD

Injectionpermeability, %

2/4 Vertical 10 lb/gal OBM withbarite

82.5 80.4 97.7

2/4 Horizontal 10 lb/gal OBM withbarite

80 84.8 106

Table 3 Return injection permeability using one-step cleanup soak in the Sandpack Permeameter

Test set-up: Vertical Cell Position

PermeabilityMud-off time/

Soak time, hrs

10 lb/gal Soak

solution

10 lb/gal

Drilling fluid Initial,

mD

Final,

mD

Injection

perm, %

2/4 15% SPME and 10%acetic acid

OBM withCaCO3

180.8 194.9 107.8

2/4 30% SPME and 10%acetic acid

OBM withCaCO3

304.1 588 193.3

2/4 30% SPME and 7.5%hydrochloric acid

OBM with barite+ CaCO3

696.7 722 103.7

Table 4 Return injection permeability using one-step cleanup soak in the Hassler Permeameter 16-hr mud off

Soak solution: 15% SPME and 10% acetic acid in 10% CaCl2

PermeabilityTest Drilling fluid

Initial,mD Final, mD % Injectionpermeability

1 10 lb/gal OBM with CaCO3 2364 2017 mD 85.3 %

2a 10 lb/gal OBM with CaCO3 1323 mD 1150.9 mD 86.9 %

2b* 10 lb/gal OBM with CaCO3 1323 mD 1326 mD 100.3 %

*: Test 2b included a water flow in the production direction after the first injection test (Test 2a) andbefore re-injecting water in the injection direction. No additional soaking was done.

Table 5 Effect of time on injection permeability using one-step cleanup soak in the Sandpack Permeameter

Soak solution: 15% SPME and 10% acetic acid in 10% CaCl2

PermeabilityTest Mud-off time/Soak time, hrs

CaCO3inOBM, lb/bbl Initial,

mDFinal,mD

Injectionpermeability, %

1 2/2 128 230.5 194.3 84.3

2 3/2 128 95.3 45.9 48.2

3 3/2 50 179.8 121.0 67.3

4 3/16 50 239.7 216.4 90.3

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Figures

Figure 1 Schematic representation of micelles and microemulsions

Figure 2 Phase transition of oil /water/surfactant system according to Winsor definition

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Figure 3 Sandpack Permeameter

Figure 4 Hassler Permeameter

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Figure 5 Optical textures observed in the single phase microemulsion

Figure 6 Compatibility o f SPME with acid

Figure 7 Invert emulsion filter cake cleanup w ith SPME in CaCl2brine

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Figure 8 Barite filter cake cleanup wi th SPME in CaCl2brine

Init. Inj Perm

696.7 mD Final Inj Perm722 mD

% Injection Permeability = 103.6

Figure 9 Injection return permeabilit y after one-step filter cake cleanup

Figure 10 Invert emulsion filter cake cleanup wit h SPME and hydrochloric acid in CaCl2brine