Note: Descriptions are shown in the official language in which they were submitted.
1~35039
Background of the Invention
The present invention xelates to improved compo-
sitions and methods for drilling boreholes such as oil wells,
gas wells or the like. More particularly, the invention re-
lates to high density drilling fluids made from abrasiveweighting agents.
Drilling fluids used in the drilling of oil wells,
gas wells and similar earth boreholes are commonly water-
- based mixtures containing clays or other collodial materials
as well as certain additives which are employed depending on
borehole conditions. The drilling fluid or mud primarily
functions as a medium to carry cuttings produced by the drill
bit to the surface. Additionaly, the drilling fluid serves
as a lubricant for the bit and drill stem and prevents the
lS ingress of formation ~luids such as oil, gas and salt water
into the borehole while the drilling proceeds. The drilling
fluid serves other purposes and possesses other characteris~ics
which need not be discussed in detail here since they are
well known and described in numerous publications such as,
2Q for example, Walter F. Rogers' Composition and Properties of Oil
Well Drillin~ Fluids, 3rd Ed., Houston, Texas, 1963; Kirk-
Othmer, Encyclopedia of Chemical Technology, 2nd Ed., New York,
1965, Vol. 7, pgs. 287-307.
As noted, one of the functions of a drilling fluid
is to maintain a sufficient hydrostatic pressure in the fluid
column in the borehole to prevent ingress of formation fluids
into the borehole, and, under extreme circumstances, to pre-
vent blow out of the well. Depending upon the drilling condi-
tions encountered, sufficient hydrostatic pressure may be
3~ achieved almost with plain water. However, under normal cir-
cumstances, and usually as a safety precaution, it is necessary,
--2--
S039
for achieving the desired hydrostatic pressure to load the
mud with a suitable weighting agent to increase its density.
These weighting agents are generally in the form of finely
divided solids of a material which has high intrinsic density.
The most common weighting agent is finely ground barite,
although iron oxides, celestite, galena and other materials
have been used at various times. Barite, because of its rela-
tively high density, insolubility, chemical inertness and
heretofore ready availability has been the weighting agent of
choice. However, with the decreasing availability of barite,
other weighting agents are being sought.
Aside from the fact that barite sources are dwin-
dling, and in some areas of the world where drilling activity
is high, practically non-existent, barite is relatively soft,
being from 2.5 to 3.5 on the Mhos scale of hardness. This
relatively soft characteristic of barite effects the rheo-
logical properties of the drilling mud after the barite has
been circulated in the borehole during the drilling operation.
In the drilling operation, the entire circulating system be-
haves somewhat in the manner of a fluid mill such that thesofter mineral particles are subject to attrition and becomes
reduced in particle size. This reduction in particle size re-
sults in a direct increase in the viscosity of the drilling
mud arising from a much smaller particle size and an increase
in the total number of particles.
It has been suggested to use other weighting
materials than barite to overcome the difficulties described
above in connection with relatively soft weighting materials
such as barite which are subject to progressive comminution.
3~ British Patent 1,495,874 discloses the use of ilmenite
and/or hematite as a weighting material for drilling fluids.
--3--
039
Additionally, Columbian Patent 93~6 discloses the use of
micaceous specular hematite for use as a weighting agent
in drilling fluids.
Specular hematite is a naturally occuring mineral
widely distributed throughout the world in different forms
and different purities. For example, the micaceous specular
hematite disclosed as a drilling fluid~weighting agent in
Columbian Patent 9396 is a relatively soft, flaky, micaceous
mineral, having a distinct laminar structure. This form of
micaceous specular hematite upon grinding tends to form very
thin flat platelets and upon use in a drilling fluid these
platelets easily comminute into very small particles which
tend to increase the viscosity of the drilling fluid. This
micaceous specular hematite is substantially non-abrasive
under borehole conditions.
The specular hematite to which the present inven-
tion is addressed is a non-hydrated mineral which although
of general laminar structure, is hard and upon grinding forms
particles having irregular, abrasive edges. Such specular
hematite has a relatively high intrinsic density, i.e. SpG.
greater than about 4.3, and has a hardness on the Mhos scale
of about 5-6. Upon use as a weighting agent in a drilling
fluid, the specular hematite of the present invention does
not tend to comrninute into srnaller particles which would sub-
stantially change the rheological properties of the drillingfluid. However, because of its intrinsic hardness, the
specular hema~ite of the present invention is quite abrasive
and can result in excessive wear on the drilling equipment
particularly the drill string and the drill bit. As the search
for oil and gas results in the drilling of deeper and deeper
wells, excessive wearing or eroding of the drill string and
11;~5039
the drill bit becomes an increasingly more severe problem.
Summary of the Invention
It is therefore an object of the present
invention to provide a new and improved drilling fluid
which can utilize relatively hard, abrasive weighting .
agents.
The present invention relates to a water based
drilling fluid, comprising an abrasive weighting agent
which is a non-hydrated mineral having a hardness of at
least about 4.5 on the Mhos scale, and a specific gravity
of at least 4.4, said weighting agent having a particle
size such that at least 85% by weight passes through a 325
mesh screen, and about 98% by weight passes a 200 mesh
screen, said weighting agent being present in an amount
sufficient to substantially increase the density of the
drilling fluid and, a water dispersible anti-wear agent
present in said drilling fluid in an amount sufficient to
substantially reduce the abrasiveness of said drilling
fluid.
In another embodiment, the present invention
provides an improved earth borehole drilling method
characterized in that the novel drilling fluid described
above is circulated in the borehole during the drilling
operation.
rief Description of the Drawings
Fig. 1 is a graph showing the abrasive weight
loss caused by various drilling fluid compositions.
i~3S039
Fig. 2 is a graph showing the abrasive wear
characteristics of various drilling fluid compositions.
Description of the Preferred_Embodiment
The weighting agents useful in the drilling fluids
of the present invention are hard, non-hydrated minerals
having a specific gravity of about 4.3 or greater, as
exemplified by specular hematite. Specular hematite is a
commonly occuring mineral which chemically is ferric oxide
(Fe2O3). It is described, for example, in Ivan Kostov
Mineralogy, (Oliver ~ Boyd, Edinburg and London) 1st Eng.
Ed. (1968). Typically, specular hematite contains 69.94%
Fe and 30.06% O. Some Ti may be present. Specular
hematite is often found mixed with hydrated iron oxides,
iron carbonate, and magnitite as well as iron silicates.
However, deposits of non~hydrated specular hematite are
available, to remove water of hydration is not required.
The weighting agents useful in the present
invention will have a density of at least 4.3, preferably
at least 4.5. When necessary, the weighting agent can be
upgraded by well known metallurgical procedures, such as
flotation, to achieve the desired minimum density. The
weighting agent will have a hardness, on the Mhos scale of
hardness, of at least 4.3. Indeed, many specular hematites
have a hardness of 5-~. For use in the present invention,
the weighting agents will have a particle size such that at
least 85% by weight passes through a 325 mesh screen and at
least about 98~ passes a 200 mesh screen. More preferably,
the particle size should
il;~SV39
be such that 90-92% passes a 325 mesh screen. The desired
particle size of the weighting agent can be accomplished by
milling procedures well known to those in the art.
The amount of weighting material present in the
drilling fluid will vaxy widely depending upon the desired
density of the dxilling mud. The latter, of course, is
dependent upon the drilling conditions encountered or expec-
ted. In any event, the ~uantity of barite necessary to a-
chieve a desired mud density is well known and tables showing
this relation are readily available. From a comparison of
the intrinsic density of the weighting agents herein with
that of barite, such tables may be used to determine the
quantity of the weighting agen~ necessary to provide drilling
muds or fluids of any desired density.
As previously indicated, the weighting agents of
the present invention are materials which are relatively hard
and hence less susceptible to progressive comminution. Con-
comitantly, the weighting agents exhibit relatively high
abrasion characteristics which can result in excessive erosion
of drilling equipment such as pumps, drill pipe and drill bits
during the drilling operation. It has now been discovered
that if suitable water dispersible anti-wear agents are added
to the drilling fluids made with the relatively hard weighting
agents o the present invention, the abrasiveness of the
drilling fluid can be reduced to the point where virtually no
more wear is observed than that which occurs with drilling
fluids of similar density employing the much softer barite
as the weighting agent. The anti-wear agent used in the
drilling fluid of the present invention must be dispersible
in water to the extent that it forms a colloid, emulsion,
or other such heterogeneous system in which the anti-wear
--7--
S039
agent is generally uniformly dispersed or distributed in
the water phase. The anti-wear agent should be of a type
which will permit the desired rheological properties of the
drilling fluid to be achieved. Preferably, the anti-wear
S agent will be a material which, in sufficient amounts, will
produce a useful drilling fluid using the hard weighting
agents of the present invention which will exhibit wear and/or
abrasion characteristics generally similar or substantially
the same as those obtained with a drilling fluid of substan-
tially the same density or weight using barite as a weightingagent. One group of anti-wear agents that has been found to
be particularly effective in fresh water based drilling
fluids of the pxesent invention are those which contain fatty
acids such as oleic acid, stearic acid, linolic acid, palmitic
acid, and the like. Such fatty acids are commonly found in
vegetable oils such as cottonseed oil, soy bean oil, corn
oil, castor oil, linseed oil, etc. The fatty acids, either
in pure form or in the form of the vegetable oils mentioned
above are admixed with small amounts of surfactants or other
agents so as to be rendered water dispersible. Such surfac-
tants can include, for example, long chain alcohols, alkanol
amines, sulfonated vegetable oil derivatives such as sulfonated
castor oil, etc. A particularly effective anti-wear agent is
a mixture of soy bean oil, long chain alcohols, and sulfonated
castor oil. This composition and similar materials make ideal
anti-wear agents since they need only be added in relatively
small amounts to the drilling fluid in order to reduce the
abrasiveness to an acceptable level. Additionally, since
only relatively small amounts are needed only minor to negli-
gible impairment of rheological properties occurs.
The amount of anti-wear agent in the drilling fluid
113503g
can vary widely but generally will be in the range of from
about 0.1 to a~out 5% by weight. In general, however, the
anti-wear agent need be present only in an amount which will
effect substantial reduction in the abrasiveness of the drilling
fluid.
Preferably the drilling fluid will include thickening
agents. Such thickening agents may consist of clay bearing
~ormations such as shale or argillaceous sands which are en-
- countered in the drilling operation and which are picked up
by the drilling fluid or mud and become a part thereof.
Alternately or additionally, clays may be added to the mud.
Typical non-limiting examples of highly collodial clays in-
clude smectites, particularly bentonite, attapulgite, and
sepiolite, bentonite being the clay of preferance. A wide
variety of other water dispersible materials, particularly
organic colloids, may serve as thickening agents including
starch derivatives, cellulose derivatives, synthetic polymers
as sodium polyacrylate, natural gums such as guar gum, gum
kiraya, bacterial gums such as that elaborated by certain
Xanthomonas species, and the like. Mixtures of such suitable
thickening agents may be employed if desired. The technology
of using such thickening agents in drilling muds is well de-
veloped and well known to those skilled in the art. The
various thickening agents have a wide diversity in weight
effectiveness in terms of the number of pounds thereof which
must be incorporated in a barrel of drilling fluid or mud
of a given type in order to achieve desired consistency.
In addition to the thickening agents, the drilling
mud may optionally contain fluid loss control agents, vis-
30 cosity control agents, anti-foaming additives, freezing poin~
depressants, and the like.
5039
It will be appreciated that in the method of the present
invention employing the drilling fluids herein and while the
borehole is being drilled during the drilling operation,
additional amounts of the weighting agent and/or anti-wear
agent can be added to maintain or alter the drilling mud
density and abrasiveness, as desired.
To more fully illustrate the present invention, the
following non-limiting examples are presented.
The specular hematites employed were obtained from U.S.
Steel Corporation, were non-hydrated and had a specific
gravity range of 4.9 to 5.1.
Example 1
A base drilling fluid or mud was prepared by stirring 25
grams of AQUAGEL (Trade mark of a bentonite clay marketed by
NL Baroid, Houston, Texas) and 50 grams of Glen Rose shale
into 350 ml. of water for 20 minutes using a high speed dis-
persator. To the mud was then added 5 grams of Q-BROXIN
(Trade mark of a lignosulfonate marketed by NL Baroid,
Houston, Texas), 5 grams of CARBONOX (Trade mark of an organic
humic acid material marketed by NL Baroid, Houston, Texas),
1.5 grams of caustic soda and 4 grams of AKTAFLO S (Trade mark
of a drilling fluid surfactant marketed by NL Baroid, Houston,
Texas) while stirring for 10 minutes on the dispersator. The
mud thus prepared was allowed to set overnight.
Example 2
Using the base mud prepared in Example 1, various
weighted drilling muds were prepared using BAROID (Trade mark
of a barite marketed by NL Baroid, Houston, Texas), and spec-
ular hematite. Evaluations of the weighted drilling muds thus
produced were obtained for a 13 pound per gallon (ppg) mud
(Table I), a 16 ppg mud (Table II) and an 18 ppg mud (Table
III).
--10--
5039
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1~35039
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--13--
1135039
As can be seen comparing the results in Tables I, II and
III above, drilling fluids prepared with specular hematite
compare favorably with a conventional drilling mud prepared
using barite as a weighting agent.
Example 3
To test the comparative attrition rates under high
shear of various weighting agents in drilling muds, the 16
-ppg drilling muds prepared according to Examples l and 2 were
used. As seen ~rom the data in Table IV below, the attrition
rate of specular hematite is considerably less than that of
barite in comparable drilling muds. This resistance to
attrition is beneficia~ in assuring that the rheological prop-
erties of the drilling fluid will not be as susceptible to
change during drilling operations in which the mud is being
constantly circulated in the bore hole.
-14-
iiL3SI~)39
TABLE IV
Wet Screen Analysis % Absolute
Les5 Than 325 Mesh Increase In
Fines
Initial Final
M~d (15 min) Mud (8 hr)Final Minus Im tial
R~ROID 93.2 97.0 3.8
Specular H~matite 92.0 93.0 1.0
~35039
Example 4
A water-based drilling mud was prepared using 15
grams of AQUAGEL and 60 grams of Glen Rose shale hydrated
into 350 ml. of water followed by the addition of 300
grams of the particular weighting agent to be evaluated.
Muds made with specular hematite were thinned with 0.1 ppb
BARAFOS (Trade mark of a polyphosphate compound marketed
by NL Baroid, Houston, Texas)~ To test the abrasive or
wear characteristics of the muds produced, the effect of
adding anti-war agents, two methods were employed. One
method, Method A, took into account the phenomena of wear
caused by the ~ovement of particles within a fluid against
a metal surface. In this method, the over-all loss of
weight from the entire surface was measured.
Method A
A Hamilton Beach mixer, Model 936, operating
at 15,000 rpm was used to produce laboratory simulated
abrasion effects and particle breakdown. For each
drilling mud, a new weighted, mixer blade was used as a
reference site for abrasive activity. Periodically, the
blade was examined for weight loss after various stirring
periods. The muds were immersed in a cooling bath during
the mixing to avoid overheating and evaporation. Thick-
ening of the weighted muds during stirring was minimized
by the addition of BARAFOS. The results of the wear rate
tests are shown in Table V below.
-16-
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i~S039
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--17--
~i35039
A second wear o~ abrasion test method, Method B,
was employed which took into account the movement of metal
surfaces against each other while exposed to a flowing fluid.
In Method B, the object was to measure weight loss from the
area e~posed to loads.
Method B
. _
The test block used for measuring extreme pressure
(EP) properties was modified by machining so that a constant
wear surface would be exposed to the test string during
tests. A ring speed of 432 rpm was selected since this is
approximately equivalent to drill pipe rotating at 130 rpm
and is fairly typical in well drilling methods. Various
loads of torque were applied to the ring and block at this
speed for intervals of 15 minutes. The init.ial weight and
final weight of the test block were measured and a weight
loss of the block determined in each mud. The wear rate was
then determined and calculated for one hour. The results
are shown in Tables VI-VIII below.
~ -18-
ll;~S039
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11;~5039
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~3S039
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-- 21 --
5039
As can be seen from the data in Tables V-VIII
above, the specular hematite, when compared with barite
shows a much higher abrasiveness. With reference to Table V,
it can be seen that the addition of an anti-wear agent
greatly reduces the abrasive wear of a drilling mud weighted
with specular hematite. The results are graphically demon-
strated in Fig. 1 in which a drilling mud weighted with
specular hematite and containing no anti-wear agent tcurve
A) is compared with a similarly weighted mud containing
about 4 ppb of an anti-wear agent, (Curve B~, and a typical
barite weighted drilling mud (Curve C).
With reference to Tables VI-VIII and Fig. 2, it
can also be seen that the wear rate of muds made with
specular hematite is much greater than that of a conventional
mud weighted with barite. However, as the data, plotted in
Fig. 2, shows, the addition of relatively small amounts of
anti-wear agent greatly reduces the wear rates. Note for
example, that the wear rate of a specular hematite weighted
mud (Curve A), which is quite abrasive without the addition
of anit-wear agent, exhibits an abrasiveness, when combined
with about ppb of anti-wear agent, (Curve D), similar or
substantially the same as that of a conventional drilling
mud weighted with barite (Curve C). Even at relatively low
levels of anti-wear agent, i.e. 4 ppb, (Curve B), the
abrasiveness is greatly reduced.
It can thus be seen that vastly improved drilling
muds, exhibiting excellent rheological properties, can be
made with hard or relatively hard weighting agents without
excessive wear and abr~sion when the drilling mud contains,
in addition, an anti-wear agent such as described above.
The invention may be embodied in other speci~ic
` li35039
forms without departing from the spirit or essential charac-
teristics thereof. The present embodiments are therefor
to be considered in all respects as illustrative and not
restrictive, the scope of the invention being indicated by
the appended claims rather than by the foregoing description,
and all changes which come within the meaning and range of
equivalence of the claims are therefore intended to be em-
braced therein.
-23-
