hartmann1988 (fotos de reboco)

Prévia do material em texto

Analysis of' Mudcake Structures Formed 
Under Simulated Borehole Conditions 
Albert Hartmann, * SPE, Technical U. of Clausthal 
Mustafa Ozerler, * * Inst. of Petroleum Research 
Claus Marx, SPE, Technical U. of Clausthal 
Hans-Joachim Neumann, Inst. of Petroleum Research 
Summary. Tiny pieces of filter cake were shock-frozen and broken while immersed in coolant. The specimens were studied with 
scanning electron microscopy (SEM) in both freeze-dried and frozen-hydrated stages. These techniques were used to show that the 
original structure of the filter cake is preserved. The investigation covered filter cakes of four different drilling fluids with respect 
to fluid loss data and thermal and chemical degradation. Honeycomb structures of bentonite containing drilling fluids were also 
found in the resultant filter cakes. The investigation showed that gel strength of the drilling fluid exerts a considerable influence on 
dynamic filter-cake structures. 
Introduction 
Both borehole stability and formation damage are functions of the 
fIltration behavior of drilling fluids. The fIltration behavior depends 
upon, among other things, the structure of the filter cake. Param-
eters affecting filter-cake structure are type and alteration of the 
drilling fluids and filtration conditions (static or dynamic). I To 
date, knowledge of fIlter-cake consistency has been based on theo-
retical considerations and conclusions from fluid loss data. 2,3 A 
single-handed evaluation from these parameters can sometimes result 
in different interpretations. The use of SEM for the study of samples 
with a high water content, such as filter cakes, requires sophisti-
cated and thoroughly proven preparation techniques. 4 The appli-
cation of shock-freezing and freeze-drying, originally used in 
biology and medicine, allows the use of SEM in drilling fluid tech-
nology.5,6 A combination of SEM study of filter-cake structures 
with the evaluation of experimentally determined fluid loss data leads 
to a better understanding of the basic principles of fIlter-cake buildup 
and formation damage. 
This paper gives an SEM-aided description of filter cakes of 
different drilling fluids with respect to fluid loss data as well as 
thermal and chemical degradation. 
Test Program 
The test program is illustrated in Fig. 1. Two freshwater- and two 
saltwater-based drilling fluids were subjected to chemical or thermal 
prestressing or both. Filter-cake buildup was then conducted stati-
cally on filter paper, in accordance with API RP 13B, dynamically 
on Bentheim sandstone (porosity = 20 to 24 %; permeability = 1.5 
to 2.5 ILm2 , average pore radius =' 15 ± 7 ILm). For the subsequent 
structural analyses with SEM, the filter cakes were treated with 
a special preparatory technique. The problem was to elucidate the 
relationships between filter-cake structure and the type of drilling 
fluid, the previous history of the drilling fluid, and the type of 
filtration. 
Drilling Fluids, Both the compositions of the drilling fluids used 
and the sequences of their preparation are shown in Table 1. The 
drilling fluids were mixed with paddle stirrers. A high-speed stirrer 
was used for the preparation of the saltwater drilling clay (SWDC) 
suspensions (preponderantly attapulgite). The clay and polymer 
components were prepared separately. After a hydration period of 
24 hours, the clay suspension was added to the polymer solution. 
The drilling fluids were then subjected to chemical stress through 
the addition of CaCl2 ' 6H20, to thermal stress in the roller oven, 
or to both. After a final mixing time of 2 hours with 6OO-rev/min 
paddle stirrers, the rheological and fIltration parameters were deter-
'Now at Eurosond. 
"Now at Research Ins!. of King Fahd U. of Petroleum & Minerals. 
Copyright 1988 Society of Petroleum Engineers 
SPE Drilling Engineering, December 1988 
mined with a Fann viscosimeter and an API filter press (Table 2). 
The corresponding parameters measured on the unstressed drilling 
fluids were used as reference values. A portion of each drilling fluid 
sample was deliberately allowed to deteriorate completely under 
intense stress. 
Circulation Test Facility, The investigation on dynamic filtration 
was conducted with a circulation test facility developed at the Inst. 
of Petroleum Engineering (see Fig. 2). According to the Hassler 
cell principle, a rectangular rock specimen (2) is mounted in a rubber 
sleeve (3) in an autoclave (1). The rubber sleeve is subjected to 
pressure by a hydraulic pump (4). The drilling fluid is drawn from 
the storage tank (5) with a reciprocating pump (6) and allowed to 
flow across the face of the rock specimen. The system pressure 
is imposed with a nitrogen-actuated pulsation attenuator (7). The 
resultant differential pressure provides the driving force for the dy-
namic fIltration through the Bentheim sandstone saturated with 1.5 % 
KCI solution. The fIltrate volume is measured as a function of time 
with an electronic balance (8) and transmitted through a digital-
analog converter (9) to a strip-chart recorder (10). The technical 
specifications on the facility and the experimental conditions are 
shown in Table 3. 
The advantages and disadvantages of this setup, as compared with 
those for conventional Hassler cells charged with cores, are (I) simu-
lation of the borehole conditions by upward, tangential flushing of 
the rock specimen; (2) homogeneous filter-cake buildup over the 
entire rock surface; (3) no dissolution of nitrogen in the drilling 
fluid; (4) time-consuming and expensive preparation of the rock 
plates by precise, plane-parallel sawing; and (5) no information con-
cerning the thickness of the filtration zone. 
SEM Preparation Technique 
Theoretical Basis, To study the structure of filter cakes with SEM, 
the samples, which contain more than 70% water, must be dried 
before examination. Although several methods can be used to dry 
samples with a high water content, 7 the experiments carried out 
in the framework of this research program have shown that a com-
bination of shock-freezing and freeze-drying is suitable. 8 The filter 
cakes can also be observed on SEM without freeze-drying, as so-
called frozen-hydrated specimens. This method requires some tech-
nical preconditions, such as cold-stage sample transfer from the 
coating facility to the SEM equipped with a special cold-storage 
plant. 9 ,10 This technique allows direct viewing of frozen 
specimens. 7 In relation to this technique, freeze-drying is more 
conventional. 
Freeze-drying is the removal of water from frozen material. II 
The purpose of this procedure is the transition from the solid to 
the gaseous state by sublimation without passing through the liquid 
395 
Type of drilling fluid Prestressing of drilling fluid Build-up of filter cake 
Bentoni te - fresh water- r-Uns tressed 
Chemically stressed by the addition- ~tatically 
Chalk - fresh water- of 40 g CaCl,'6 H,O to 1000 g H,O (API-filter press) 
in the drilling fluid 
- -
Attapulgite - salt water- -Thermally stressed by 16 hours'_ 
exposure to 1S0 ll e L-oynamically 
(Circulation device) 
Attapulgite-bentonite -_ I-Chemically and thermall 
salt water stressed 
Fig. 1-Test program. 
TABLE 1-COMPOSITIONS OF DRILLING FLUIDS 
Type of Drilling Fluid Composition 
Bentonite/freshwater 1,000 9 H2O+ 60 9 sodium bentonite 
5 9 sodium carboxymethylcellulose 
Chalklfreshwater 1.000 9 H2O+ 15 9 sodium bentonite 
27 9 sodium carboxymethylhydroxyethylcellulose 
15 9 potassium chloride 
150 9 chalk 
10 g'defoamer 
Attapulgite/saltwater 1,000 9 H2 0+ 55 9 attapulgite 
38 9 starch 
360 9 sodium chloride 
Attapulgite/bentonite/saltwater 1.000 9 H20 + 45 9 attapulgite 
15 9 sodium bentonite 
10 9 vinylamide/Vinylsulfonic acid 
copolymer, ammonium salt 
360 9 sodium chloride 
TABLE 2-COMPOSITIONS AND TYPES OF PREPARATION OF DRILLING FLUIDS· 
Type of 
Drilling 
Fluid 
Bentonite/ 
freshwater 
Chalkl 
freshwater 
Attapulgite/ 
saltwaterAttapulgite/ 
bentonite/ 
saltwater 
Water Na·Bent. Na·CMC CMHEC KCI Chalk SWDC VSNA 
~ ~ ~ ~ Jm. ~ ~ ~ 
9,000 600 
1.000 50 
9.600 9 sodium bentonite suspension and 1.050 9 Na·CMC solution 
2.250 150 
7,750 270 
2,400 9 sodium bentonite suspension and 8,020 9 CMHEC solution 
150 
1,500 
10,000 550 
~~ ~ 
3.000 150 
2.500 100 
4,950 9 SWDC suspension and 3.150 9 sodium bentonite suspension, 
addition of 2,600 9 VSNA solution 
"Na·Benl. = sodium bentonite. 
Na'(;MC ~ sodium carboxymethylcellulose. 
CMHEC m carboxymethylhydroxyethylcellulose. 
SWDC = saltwater drilling clay (preponderantly attapulgite). 
VSNA = vinylsulfonatelvinylamide. 
Starch NaCI 
~ ~ 
380 
3,600 
3.600 
Mixing 
Time 
(hrs) 
2 
2 
2 
2 
2 
2 
1 
2 
0.5 
2 
1 
0.5 
2 
2 
1 
1 
1 
Note: The mixing was done with a paddle stirrer at 600 revlmin (Exception: The attapulgite suspension was prepared by a Hamilton Beach high-speed stirrer 
with 16.000 rev/min). 
Period of 
Hydration 
(hrs) 
24 
24 
24 
24 
24 
24 
396 SPE Drilling Engineering, December 1988 
1, Autoclave 
2, Rock sarrole 
J , Rubber s I feve 
" PlJT1l un I t 5, Drilling fluid tank 
with heating devi ce 
6, Piston DlJI'I) 
7 , Pulsation attenuator 
8, ElectronIc balance 
9, 0lgltal-anal09 converter 8 
10 , Strip-chart recorder 
Fig. 2-Clrculatlon lost facility. 
TABLE 3-TECHNICAL SPECIFICATIONS OF THE 
CIRCULATION DEVICE AND TEST CONDITIONS 
Maximal 
Temperature, ac 120 
Operating pressure, MPa 10 
Closure pressure, MPa 15 
Backpressure, MPa 10 
Annular velocity, mls 00 
During the Tests 
20 
0.7 
2 
o 
0.6* 
Dimensions of the rock sample: 15)( 120x35 mm. 
Annular cross-sectional area: 15)( 10 mm.· 
'getor. It>e txJildup oIlhe fi lt'" caka. 
state. Ice sublimation is a function of temperature and vacuum-
the lower the temperature, the higher the vacuum required for the 
water to leave the sample. Freeze-drying is usually carried out at 
about 200 K [- 100"F] and at a pressure of 20 to 100 MPa [2900 
to 14,500 psij .12 Lower temperatures diminish ice recrystallization 
but require much higher vacuum and/or longer drying times. The 
n«:cssary time for freeze-drying depends on the propenies of the 
samples-such as size, shape, the amounts of free and bound water, 
and the ratio of fresh to dry weight. l3 Allowing the specimen to 
reach ambient temperature gradually under vacuum before it is re-
moved from the freeze-dryi ng device is imponant for the procedure. 
Several authors show that shrinkage does bke place during the 
freeze-drying process on medical and biological specimens. l . iO• t2 
Therefore, filter cakes were investigated after freeze-drying and 
on frozen-hydrated specimens (Figs. 3 and 4). The characteristic 
honeycomb structures formed by the bentonite platelets 14. t~ are 
clearly evident. The results of both tests have shown that there are 
no significant differences in filter-c.ake structure. This can be at-
tributed to the fonn sbbility of aqueous clay/polymer systems. For 
the structural analysis with the help of SEM , both techniques are 
suitable. 
Cooling the specimens as rapidly as possible is neccliS8ry to avoid 
ice-crystal-growth damage during tne shock-freezing process. The 
specimens should be cooled down to 133 K [ - 220"F] within a few 
millis«:onds to obtain ice crystals only in microcrystalline or in 
glassy fonn (vitrification) . For optimal shock-freezing, the 
specimens must be as small as possible. The thermal contact with 
the coolants has to be good and the temperature gradient high. 
SPE Drilling Engineering. December 1988 
Fig. 3- Freeze-drled tIIter cake of unstressed bentonitefCMC 
drilling fluid. 
397 
~se I 
~ .. ~ 
, OW .... ~"hO< 00' . 
Fig. 5-Scl'lematlc presentation 01 rshock-lreezlng and freeze-
drying technlquel. 
- 20 
~ 
> -~ 
§ 
u 
~n o Itr, .. 
<!)o;h • • . Hre .. 
I!Ither • . Itr ... 1 ,h •• . • no trIer •. 
ttre •• 
5 10 IS 
Time t 
20 25 30 
In min 
Fig. 6-Stltlc IIItrlte volumes 01 bentonltelfreshWlter drill-
Ing fluid VS. time with respect to prestressing. 
Fig. 7-Statlc filter cake of unstressed bentonite/freshwater 
drilling lIuld. 
Several coolants with low melting points-such as propane ( 133 
K [ - 220°F I) . isopentane ( 11 3 K [-256°F]) . Freon 12 (123 K 
[ - 238°FJ). or 2' methyll)utane ( 11 3 K [ -256°F))--can he used for 
shock. rreezing.4 These coolants must be cooled with the help or 
liquid nitrogen. Liquid nitrogen is not suitable for direct plunging 
ortne specimens. With a liquid·nitrogen coolant bath. tne fonnation 
of an isolating gas mantle impairs the thennal contact around the 
specimen (Lcidenfrost"s phenomenon). Liquid nitrogen can be used 
liS coolant only if subcoolcd to 63 K [ ~346°F]. where solid nitrogen 
noats in liquid nitrogen. 
398 
Fig. 9-8tatlc IlIlar clke 01 unatressed chalk/freshwater drill-
Ing fluid. 
~ 
" ~c s trl •• C 'lim . ' tr ' lI ;; 160 I!Itl'ler," . t tt ... X en' .. . '~CI t~. r," . 
~ 120 
I trlsl 
-' 
~ 
I~ ~ 80 ~ --- '0 --~ > --~ 5 10 IS 20 25 30 -
i! Time t in !!li n 
u 
Fig. 10-Statlc filtrete volumes of chalkl1relhwater drilling 
lIuld VI. time with respect to prestressing. 
ApplK-lition of Method. Thc filter cakes were broken or cut inlo 
liny pieces and plunged into ~ubcooled liquid nit rogen or in 2-
melhylbutane cooled by liquid nitrogen (Fig. 5). After shock-
rreezing. the samples were broken inside the coolant bath again 
to have an undisturbed surface for the SEM study. In the next step, 
the specimens were transferred while still covered with liquid 
nitrogen or 2-mcthylbutane to the precooled cold stage of the freeze-
drying device. Freeze-drying occurred in IWO phases: (I) ini tial 
drying phase with sublimation at the ~ urface at 200 K r - IOO"F] 
and (2) final drying phase at a temperaturc of about 253 K [- 4 °F1. 
SPE Drilling Engineering. December 1988 
FIg. 11-Statlc tIIter cake 01 thermally streased chalk/fresh· 
water drillIng fluid . 
Freeze-drying time varied from a few hours to several days de· 
pending on the specimen size and watcr content. The freeze-dried 
filte r-cake specimens were finally COOted with an electrical con-
ducting layer and suJ(!ied with SEM. 
R.sults .nd Discuaalon 
Static Filter Cakes or Prestressed Drilling Fluids. Befl-
tQn jteIFresh""ater DriJling Flujd. The filtration volume is plotted 
as a function of time for bentonite/freshwater drilling fluid in Fig. 
6. Because the protective colloid used. carboxymethylcellulose 
(CMC), is not stable toward divalent clItions. chemical stress that 
results from the addition of CaCl2 '6H 20 causes a substantial in-
crease in the filtrate volume. This is reflected in the structural 
analyses perfonned on the filter cakes by SEM. In the unstressed 
case. the gel structures present in the drilling fluid are decisive for 
the texture (Fig. 7). The applied differential pressure of O. 7 MPa 
[101 .S psi] is not sufficient to disrupt the electrostatically mduced 
edge-to-face allachment between the bentonite platelets. The 
chemical prestressing of the drilling fluid resulted in complete dis-
ruption of the characteristic honeycomb structure (Fig. 8). This 
process is associated with partial dehydration and coagulation of 
the bentonite particles (fact·to-face ~lIachment). 16 This resulted in 
enlargement of the effective-flow cross section within the filter cake 
SPE Drilling Engineering. December 1988 
Fig. 13-Stallc IIIter cake 01 un.tressed attapulgltelsaltwatar 
drilling fluid (no saluratlon with NaCI). 
FIg. 14-Statlc filte r cake 01 unatreased al tapulgite/sallwater 
dril ling fluid . 
~ e 
u 
c 
- 160 
0 
~120 
0 
> 
~ 80 • , -
" -• > -• 5 
§ 
u 
10 15 
~n lJ .tress 
<?cl'le •. stress 
[!Jtl'lerll. s tres s 
X c~ .... ~nd tl'ler •. 
,tress 
20 25 JO 
TIme t In min 
Fig. tS-Static IUlrale volumes 01 attapulglte/saltwater drill. 
Ing Iluid vs. t ime with respect to prestressing. 
399 
Fig. 17-Statlc !liter cake 01 unstressed attepulglte/bentonltel 
saltwater drilling fluid (no saturation with NaCI).and thus in an increase in filtrate volume. t7.18 A further deterio-
ration of the filtralion p,r0perties is attributed to the conversion of 
Na-CMC to Ca-CMC. 9 The laner no longer exerts a sealing effect 
within the filter-eake struelure . 
Chalkl FreshwaJer Drilling fluid. The filter cake formed from 
chalk/freshwater drilling fluid presents an entirely different picture 
(Fig. 9). A characteristic feature is the presence of partially an-
gular and partially rounded chalk particles of various siJ:cs, which 
cause a decrease in filtrate volume. in contrast to bentonite/fresh-
water drilling fluid (compare Figs. IO and 6). Examination at high 
magnifications has indicated that the chalk particlcs are frequently 
coated with a polymer film, lose their sharp edges and fracture 
surfaces, and are progressively more strongly bonded together. The 
Structure-fonning bentonite platelets, whose honeycomb structures 
are in part still discernible, provide additional sealing. 
The chalk product used contains numerous skeletal remains of 
coccoliths, which lived in the sea as plankton. Thennal prestfCSsing 
of the drilling fluid does not seriously impair the rIlter--cake structure, 
as can be seen in Fig. I I; locaiiJ:ed honeycomb structures are still 
present even after subjection to thermal stress. This indicates the 
thermal stability of carboxymcthylhydroxyethylcellulose (CMHEC), 
the protcctive colloid used. A comparison of the filtration curves 
(Fig. 10) indicates only a slight increase in filtratc volume during 
thermal prestressing of the drilling fluid . 
AIWpulgite!Saltwalu Drifting Fluid. For the structural analysis 
of filter cake, it is often advisable first to visuaiiJ:e the individual 
Fig. 18-Statie fitter cake of unstressed attapulgitefbentonltei 
saltwater drilling fluid . 
Fig. 19-5talic filter cake 01 chemically and thermally stressed 
attapulgltelbentonlte/aaltweter drilling fluid. 
components. A 6% attapulgite/freshwater suspension is shown at 
high magnification in Fig. 12. The individual attapulgitc needles 
are clearly recognizable. A characteristic featu re is the pronounced 
matting of the needles to form bundles.2() The filter cake formed 
by the attapulgitelsaltwater drilling fluid beforc the addition of salt 
is illustrated in Fig. 13. Besides the attapulgite agglomeration, the 
structure of the filter cake is affected by the crosslinking action of 
thc starch used as a filtration control agent. The filter cake formed 
from the attapulgite/saltwater drilling fluid salted with NaCI all the 
way to saturation appears dense and compact (Fig. 14). The ab-
sence of honeycomb structures is because of the weak tendency of 
the anapulgite needles to form gel SlfUctufCS.2l The salt crystals 
discernible at high magnification should be regarded as artifacts 
lhat arise upon recrystallization during freeze-drying. The lhermal 
degradation of starch causes an cxtremely high filtrate volume (Fig. 
15) because the sealing action of the starch is then absent. t9 The 
filtcr cake is friable and highly permeable (Fig. 16). 
AttapufgilelBI!ntonitelSaftll'ater Drifting fluid. The lack of 
sufficent gel strength with the attapulgite/saltwater drilling fluid is 
because of a mixture of attapulgite and bentonitc in a ratio of 3: I . 
To ensure better thermal stability, a completely synthetic product 
was used as a protective colloid. The filter cake obtained from the 
unstressed at tapulgitefbentonite/saltwater drilling fluid before the 
addition of salt is depicted in Fig. 17. Despite the low bentonite 
content. the bentonite platelets affect the structure and constitute 
the macrostructure by edge-to-face attachment. The considerably 
SPE Drilling Engineering. December t988 
-ro 
~no 5 tr ••• 
<!>chell. stre" 
I!lthe ...... s tr ••• X ehe • . and tner •. 
stress 
5 10 15 20 
TIme t In min 
25 30 
Fig. 20-51811c filtrate volume. ol attapulglte/bentonlte/aalt-
water drilling fluid VI. tlma with r.epecl to prestrening. 
Fig. 21-Oynamlc IIIter cake of chemically and thermally 
atrnsed anapulgltelbentonlteJultwlteT drilling fluid (no satu-
ration with NaCI). 
smaller attapufgile needles form a microstructure al the walls of 
the honeycomb. Salting the drilling fluid with NeCl ali lhe way to 
saturation causes apparent th ickening of the honeycomb walls in 
the filler cake (Fig. 18), The cause of this behavior is the same 
as that described for the attapulgitefsaltwater drilling nuid . TIle com-
bined prestressing causes disruption of the gel structures formed 
by the bentonite (Fig . 19) and thus a more pronounced increase 
in filtrate volume (Fig. 20). A possible explanation for pure pro-
tective action by the thermally stable polymer is the low concen-
tration in the initial drilling fluid . The polymer filament molecules 
become attached predominantly to the attapulgilc needles; conse-
quent ly, the major pon ion of bentoni te remains unprotected during 
exposure to elevated temperature. The results are coagulation and 
dehydration of the bentonite and thus an increase in the effective-
flow cross section. 
Dynamic Filter Cakes. Dynamic filter cakes from unst ressed 
drilling fluids of high gel strength containing bentonite do not ex-
SPE Drilling Engineering. December 1988 
Fig. 22-Dynamic filter cake 01 un,tre,sed attapulglte/salt· 
water drilling fluid . 
Fig. 23-Dynamlc !liter cake of bentonite suspension treat-
ed with thinner. 
hibit appreciable structural differences in comparison with the cor-
responding static filter cakes. A structural orientation. which might 
pos.~ibly be expected to occur in the direction of flow for the drilling 
fluid. depends on (I) shear stress in drilling fluid, (2) differential 
pressure gradient , (3) extent of hydration of bentonite panicles. and 
(4) intensity of gel structures in drilling fluid . 
During the investigation, the gel strength of the drilling fluids 
was impaired by chemical or tncrnlal prestressing or both. The filter 
cake fonned from 3ttapulgitelbcntonitelsal!water drilling fluid before 
salting with NaCI and after the combined prestressing is shown in 
Fig. 21. A structural orientation within the fillCrcake corn:sponding 
with the morphology of the rock surface is clearly evmnt. Also. 
for fil ter cakes of unstressed drilling fluids. an orientation of the 
solids in the drilling fluid can occur in the direction of flow if the 
drilling flu id exhibits a very low gelstrength---e.g., in the case of 
an attapulgi te/saltwater drilling fluid (Fig. 22). Borst and SheilS 
studied the effect of adding thinner to drilling fluids with SEM. 
For examining the e ffect of the gel strength on the filter-cake 
'01 
TABLE 4-PROPERTIES OF 6%* BENTONITE SUSPENSION 
Plastic Bingham 10-Second Gel 10-Minute Gel API Fluid 
Viscosity Yield Point Strength Strength Loss 
(mPa·s) (dPa) 
Untreated 8 115 
Treated with 5 g 
thinner per liter 14 14 
of suspension 
• Mass content. 
structure during dynamic fIltration, a 6% bentonite suspension was 
deflocculated by conditioning with modified polyacrylate. The rheo-
logical and fIltration parameters are given in Table 4. Examination 
of the SEM photographs yields the following results (Fig. 23). 
.1. Extremely thin filter cake. 
2. Orientation of the solids in the drilling fluid in the direction 
of flow. 
3. Fluidized structure in the proximity of the rock. 
4. Good sealing action of the fIlter cake (compare fluid loss before 
and after the addition of the thinner in Table 4). 
The importance of the gel strength of the drilling fluid for the 
filter-cake structure has been confirmed by this experiment. 
Conclusions 
Based on the investigations conducted for visualizing and inter-
preting filter-cake structures, the following conclusions have been 
reached. 
1. Frozen-hydrating and shock-freezing with subsequent freeze-
drying are processes well suited for preparing filter cake appropri-
ately for SEM. 
2. Using bentonite gives rise to a honeycomb structure within 
the filter cake.This is induced by the edge-to-face attachment of 
the bentonite platelets. 
3. During cherr.ical and/or thermal overstressing of the drilling 
fluid, the bentonite platelets lose their structure-forming property 
through coagulation. 
4. With the use of SWDC, the acicular clays form bundles within 
the filter cake. 
5. A structural orientation in the direction of flow for the drilling 
fluid within the filter cake can be observed only if the gel strength 
has diminished within the drilling fluid. 
6. The application of SEM in drilling fluid engineering does not 
replace but rather supplements fluid loss analyses. 
Acknowledgment 
The investigations have been performed within the framework of 
SFB 134 "Petroleum Technology-Petroleum Chemistry" at the 
Technical U. of Clausthal with funds provided by the German Re-
search Assn. 
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119 
0 
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0 12 8.9 
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19. Thomas, D.C.: "Thermal Stability of Starch- and Carboxymethyl 
Cellulose-Based Polymers Used in Drilling Fluids," SPFJ (April 1982) 
171-80. 
20. Giiven, N., Leroy, L., and Lee, L.J.: "Reactions of Attapulgite and 
Sepiolite in High Temperature Drilling Fluids," Proc., Inti. Conference 
on Geothermal Drilling and Completion Technology, Albuquerque, NM 
(1981) 26/1-26/22. 
21. Bulian, W. and Djahanschahi, H.: "Uber die Temperaturbestiindigkeit 
von salzgesiittigten Bohrspiilungen, " Erdal-Erdgas-Zeitschrift (1969) 
85, 5, 168-89. 
51 Metric Conversion Factors 
cp x 1.0* E+oo mPa's 
ft3/(min-ft) x 5.08* E-03 mls 
OF (OF-32)/1.8 °C 
OF (OF+459.67)/1.8 K 
in. x 2.54* E+Ol mm 
Ibf/loo ft2 x 4.788026 E-Ol Pa 
mL x 1.0* E+oo cm3 
psi x 6.894757 E+oo kPa 
*Conversion factor is exact. SPEDE 
Original SPE manuscript received for review Oct. 5, 1986. Paper accepted for publication 
Nov. t 7, 1987. Revised manuscript received June 8, 1988. Paper (SPE t 5413) first presented 
althe t986 SPE Annual Technical Conference and Exhibition held in New Orleans. Oct. 5-8. 
SPE Drilling Engineering, December 1988