Note: Descriptions are shown in the official language in which they were submitted.
PR~CESS FOR DRILLING A WEIL
The invention relates to a p~ocess for drilling a well in an
underground formation i~ which process a drilling fluid is clr-
culated in said well, while drilling, characterized in that the
drilling fl~id contains a h~teropolysaccharide which has been
o~tained b~ cultivation of Pse~domonas s~. NCIB 115920
When drill mg subterranean wells such as, for exa~ple, oil
or gas wells, the rotary drilling method is ccmmonly emplcyed. The
rotary drillmg method utilizes a bit attached to a drill stem, and
a drilling fluid or "mNd" which is circulated through the drill stem
to the bottcm of the borehole where it is ejected thrcugh small
openings in the drill bit. The fluid is then returned to the surface
throu~h the annular space between the drill stem and the borehole
wall, or casing if one has been installed. Upon reaching the sur-
fa~e, the drilling fluid or "mud" is ordinarily treated to remove
cuttings obtained from the borehole, and is then recirculated.
Drilling fluids serve many functions, and should therefore
possess a number of desirable physical and rheological proFerties.
Fc,r example, the viscosity of a drilling fluid should be sufficient
to permit it to effectivel~ transport bit cuttings from the bottcm
of the bcrehole to the surface for removal. A drilling fluid should
also prevent excessive amounts of f_uid frQm flowmg from the
borehole mto surrounding formation~ by depositing on the walL of
the hole a ~hin ~ut substantially impervious filter cake. In ad-
dition,~a drilli~g fluid should be able to hold solids in sus-
pension, preventing their return to the bottom of the hole when thecirculation is reduced or temporarily interrupted. This property can
; be obtained by utili2ing additives which will impart a gel structure
to the drilling fluid to increase viscosities. The gel stN cture,
howe~er, i5 preferably such that cuttings can be removed from the
~0 drillL~g fluid by passmg the fluid through filtration equipnent
such as a shale shaker and/or sand cyclones prior to recircul~ting
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the fluid to the drill bit. A drilling fluid must also exert press-
ure on the surrounding formations, thus preventing possible collapse
of the borehole or influx of highly pressurized oil or gas in the
~ormQtion. Finally, a drilling fluid should serve as a lubricating
and cooling agent for the drill string.
Traditionally, bentoni~e or other clay solids have been utilized
to increase the viscosity of the drilling fluid. Today however,
'here is a grcwing belief that bentonite or clay suspensions have
serious limltations as a drilling flu1d base. The rheology of
bentonite-based fluids is such that the hydraulic horsepcwer
delivered to the bit at a given surface pressure is significantly
less than with drilling fluids containing certain polymers. The
lower viscosity and/or solids content of these polymer muds result
in a faster bit penetration rate which in turn decreases the drilling
costs. Those working within the industry have att~mpted to s~bsti-
tute for the clay solids o~ the older muds by various polymericma~erials including, for example: cellulose compounds such as
carboxyethyl cellulose, carboxymethyl cellulose, carboxymethyl
hydro~yethyl cellulose, hydroxya1kyl celluloses, aLkylhydroxyaIkyl
celluloses, aIkyl celluloses, and aIkylcarboxyalkyl celluloses;
polyacrylamides; Qatural galactcnannans such as guar gum, locust
b2an gum, and gums derived frcm endosperm seeds; and various other
polysaccharides.
In addition to the drilling fluids employed in drilllng sub-
terranean wells, it is known thrcughout the industry that other
fluids can also be utilized for certain specialized applications.
Fo~ e~ample, fractuL-in~ fluids, spaci~g fluids, plugging fluids,
cementing fluids, and ccmp1etian fluids may be utilized in addition
bo a drilling fluid to achieve a particular result at one stage or
another m ~he drilling operation. It is recognized by those of
ordinary skill m the art, however, that ccmpositions exhibiting
prcperties desirable for scme specialized application may no~
pexform satisfactorily where employed for another purpose. Unlike
fracturing fluids, drilling fluids are recirculated through the well
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many times. Although much of the larger particulate matter, such as
bit cut ~lgs, is remaved from the drilling fluid after each cycle
through the ~ell, the fine solids content of the fluid kecomes
progressively greater with continued circulation. Where the drilling
fluid ~cmprises a heteropolysaccharide obtained frcm Pseudomonas sp.
NCIB 11592, it has been found that it tends to encapsulate the
drilled solids suspended in the fluid. miS encapsulating charac-
teristic prevents disintegration of drilled solids and so im~roves
the effectiveness of the drilling fluid, and is considered very
desirable.
Notwithstandin~ the advances made in drilling fluid technology
in recent years~ there remains a need for a suitable fluid that will
significantly improve bit penetration (and hence the drilling rate),
will remain shear stable even after being recirculated through the
borehole many times, will reduce the pressure losses due to friction
in the drill pipe, will p m mote better lifting and therefore less
grir~ing of cuttings at the bottom of the borehole, will mamtain
borehole cuttings in a suspended state during drilling interruptions,
and will encapsulate the drilled solids suspended in the drillirlg
fluid.
The drilling fluid used according to the Lnvention has these
beneficial properties.
The invention resides in a process for drilling a borehole
utiliz mg a rotating drill bit, wherein a drilling fluid is cir-
culated past the surface of the drill bit, ccmprising employing
as the drilling fluid the aqueous liquid of a heteropolysaccharide
obtained from Pseudamona~ sp. N~IB 115g2. The drilling fluid is made
by admixing the heteroFolysaccharide with water, circulating the
admixture thus formed for a period effective to permit hydration
of the p~lymer. ~he hetercpolysaccharlde is disclosed by EuroFean
pat~nt application No. 81200479.4 (P~lication No. 0040445). The
composition disclosed herein will exhibit extremely desirable
physical and rheological prcperties, including improved flow
characteristics as well as increased shear stability over a
prolonged period of use.
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The drilling fluid of the subject invention is preferably made
at the use site by slowly adding a h~teropolysaccharide solution or
e~llsion to water circulating through con~lentional blendin~ equip-
mnt such as a jet hopper, a~d into the mud pit. The water employed
is sui~ably as fresh as is reasonably obtainable, although brines
may equally well be employed and may be preferable where shale
sloughing or formations with a high clay content are expected to be
encountered. m is versatility is another advantage of the drilling
fluid of the present invention. When all the polymer has been added,
the fluid can be applied withcut extra circulation since hydration
of the hetercpolysaccharide was already substantially complete. An
effective amount of a suitable buffer may be added if desired to
prcm~te hydration, as will be understood by those sXilled in the
art.
The unweighted drilling fluid thus formed preferably ccmprises
15 from 0.1 to 10 Xg polymer per m3 of fluid. Most preferably the
unweighted drilling fluid ccmprises fro~ 1 to 5 kg polymer per m3.
If desired, the viscosity of the drilling fluid may be increased at
a point in time subsequent to the Lmtial preparation of the dril-
ling fluid by gradual addition of more polymer and/or crosslinker.
Biopolymers are widely used in drilling, ccmpletion and work~
over fluids. AquRous solutions containing the biopolymers have
pronGunced pseudcplastic rheological characteri~tics which give them
high carrying capacities at low shearJcirculation rates. Unfortunate-
ly, they often cann~t be effectively acid-degraded and are, there-
~ore a potential source of formation Lmpairment. It has now been
~und that the hetercpolysaccharide obtained from Pseudo~onas sp.
NCIB 11592 is very well acid-degradable e.g. it degrades within an
hour in the presence of a 10 ~ HCl solution.
Rheological measurements on solutions of the present hetero-
polysaccharide show that it is an effective viscoslfier up to 55 C
in fresh water and up to at least 70 C in concentxated brines. me
plot of viscosity agaLnst t.emperature is reversible in the sense
that identicaL plots are obtained by mcreasing or decreasing
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~?erature. Moreover this heteropolysaccharide is a very pawerful
viscosifier and at low shear rate, it has an excellent carrying
capQcity.
~ ot-rolling tests are conducted to measure the shear and
temperatur~ degrada~ion of the pres2nt heteropolysaccharide. A
~odium chloride brine viscosified by this heteropolysaccharide is
hot rolled for 18 hours at 80 C, and its viscosity is measured at
21 C bsfore and after rolling. ~o reduction in viscosity is found.
mis confirms the high shear stability of the present heteropoly-
saccharide. A calcium chlorids based drilling fluid containing HEC
IhYdroxYethyl cellulose), the heteropolysaccharide obtained from
Pseudcmonas sp. NCIB 11592 and calcium carbonate were formulated and
used to drill the production hole of an oil well in the Norwegian
Troll field. No mud engineeLing problems occurred and production
tests showed the well to be highly unimpaired.
In addition the productivity of the well far exceeded the
productivities of the pre~ious ten wells drilled in ~he field using
other mud syst~ms.
As mentioned hereinbefore drilling fluids may contain many
different substances. The drilling fluid to be used in the process
according to the invention preferably contains in addition to the
heteropolyaccharide abtained from Pseudomonas sp NC~B 11592, at
least one fluld loss additive, at least one thinner, at least one
weightlng agent and/or at least one salt.
Especially preferred examples of these materials are bentonite,
baryte, one or m~re other polymers, one or more starches, one or
more lignosulphonates, gypsum, dolcmite and/or lye. The content of
the het~ropolysaccharide obtained frcm Pseudcmanas sp. NCIB 11592 in
the drill mg fluid to be used m the present process ~ay vary within
wide ranges. Preferably this drilling fluid contains from 0.1 to lO
kg of the hetercpolysaccharide per 1000 kg of fluid.
The invention will ncw be further elucida~ed by mean5 of the
follcwing ~xample to which it is ~y no means restric~d.
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1 :XAME~E
Both cutting encapsul~ting and trlaxlal shale tests were
carried ou~ on the relatively water sensitive Pierre shale. In the
cutting encapsulation tests, a set amount of Pierre shale cuttings
of a specific size distribution (1.7-3.4 mm~ was added to a base mud
containing the test polymer. Mud and shale cuttings ~ere hot rolled
for either 2 or 16 hc~lrs at 60 C. and the percentage of shale
cuttings still hav m g the original size distribution measured. This
percentage is termed the "recovery". Composition and properties of
the base mud are given in Table 1.
In the standard tri~xial shale test, a 50 mm diameter 25 mm
long cylindrical reconstituted shale sample with a 6 mm diameter
axial hole through it is put under a confin mg stress and mud is
circulated throu~h the hole at a specific rate. When the sample has
collapsed due to erosion or the axial hole has closed due to swelling
the test is stopped. The time and m~de of failure are recorded and
erosion and water content of the samples measured. m e ~d ccmposi-
tions, mud properties and test conditions pertaining to these tests
are given in Table 2. In essence, the tests ccmpared the performance
of a mud cont~inLng hydrolysed polyacrylamide (PA~) and a m~d in
which the hydrolysed polyacrylamlde (PAA) was replaced ky the
heteropolysaccharlde abtained from Pseudcmonas sp. NCIB 11592.
The res~lts of the cutting encapsulation tests are given in
Table 3. They show that the encapsulating properties of the hetero-
polysaccharide cbtained from Pseudamonas sp. ~CIB 11592 are
excellent. It ls wor~h noti~g that these biopolymers achieved good
~ncapsulating properties without Lncurring excessive plastic
; viscosities.
The results of the triaxial tests are gi~en in Table 4. At
first sight, the performance of tne neteropolysaccharide obtained
from Pseudc~onas sp WCIB 11592 is far superior to that of the PAA,
h~wever, this requlres scme qualification. m e mode of failure is
cf. Darley, H.C.H., "A laboratory Investigation of borehole
stability". J. Pat. Tech., July 1969, 883-893, TransO AIME, 246.
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dif~erent for the two polymers. In the case of the PAA mud, no
erosion takes place and approximately 5~ water is embibed by the
shale resultmg in hole closl~e by swelling. PA~ invades the swollen
shale and glues the potentially erodable shale surface particles
together and to the shale substrate.
In the case of containLng t~le heteropolysaccharide obtained
frcm Pseudomonas sp NCIB 11592, 4% water uptake and 20% erosion
akes place prior to failure by collapse.
A possible explanation for the differing performances of the
two products lies in their structure. Both are long chain molecules
but the heteropolysaccharide abtained frcm Pseud onas sp ~CIB 11592
(which consists of sugar units) is much bulkier due to its branched
structure. ~oth m~lecules appear to have the required configuration
and charge distribution ~o bind clay platelets but, presumably, the
size of the heteropolysaccharide obtained from Pseudcmonas sp NCIB
11592 molecule prevents it invading the shale surface layer.
This uniq~le prcperty opens up new possibilities in the ormu-
lation of muds designed to drill swelling shale formations. Tight
holes are a ccwmon prablem ~hen drilling with PAA-containin~ mud due
to the fact that PAA reduces erosion of swelling shale formations:
PAA is primarily added to the m~d for cutting encapsulation. The
possibility now exists of using the heteropolysaccharide obtained
fram Pseudcmonas sp NCTR 11592 which will allow controlled erosion
of the swellmg shales and prevent shale cutting disintegration.
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¦Fresh water
8 kg/m3 carboxymethyl plastic viscosity = 15 cP (PV)
~ellulose
10 kg/m3 stabilose LU yield point = 2 Pa (YP)
1 kg/m3 Farafonmaldehyde
able 1 - Composition and properties of the base mud used for the
cutting encapsulation tests.
7resh water _ _ _ .
30 kg/m3 bentonite Confining pressure : 225 bar
2.15 kg/m3 polymer pump speed : 6 l/mun
~e~fective concentration) _
100 kg/m3 XCl plastic viscosity - 10 cP
yieLd F int = 2 5 Pa
able 2 - ~ud ccmposition and properties and ~est conditions in the
triaxial shale test.
polymer is either hydnolysed polyacrylamide (PAA) or the
heteropolysaccharide obtaln~d from Pseudomonas sp NCIB 11592.
**
Plastic viscosity is defined o~ page 735 of the book "Drilling
and drilling fluids" by G.V. Chilingarian and P. Varabutr
(El~evier Scientific Publishing Co. 1981).
Yield point is def;ned on page 740 of the above bcok.
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