Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2093~8~
Deep-drilling cement and process for the
preparation thereof
The invention relates to a deep-drilling cement
consisting of a hydraulic binder and, if appropriate,
additives and auxiliaries and a process for the prepara-
tion thereof.
During deep drilling, slurries of water and an
agent which solidifies are pumped down through the casing
tube into the borehole, rise up again between the bore-
hole side and drilling rod and solidify there. The litre
weight of the slurry is adjusted corresponding to the
pressure and stability conditions encountered in the
borehole. In general, the slurry must be the heavier the
deeper it is used. Where, on account of the hydrostatic
pressure, the rock might possibly break up or has already
previously broken up so that slurry can penetrate, a
slurry is required to be as light as possible. This is
necessary, for example, for the cementation of the first
casing tube strings for off-shore drilling. In the case
of cementations of especially long casing tube strings
(> 1 km)~ the stressing of the tubes by the internal
pressure can become critical so that light slurries are
thçrefore necessary.
Apar~ from such technical constraints, there are
occasionally economic considerations according to which
the lead slurries used are lightweight cement slurries.
Lightweight slurries for borehole cementation
with a density of less than 1.7 kg/l, such as are used
for formations with inadequate inherent stability, are
produced by mixing bentonite, tras3, kie~elguhr, water
glass or the like with cement and much water. These have
the disadvantage that the 3trength development and non-
poro~ity are low. It is also known (compare E.B. Nelson,
WELL CEMENTING, Elsevier 1990, p. 3-14), in order to
2093~
-- 2 --
avoid a high water/cement ratio, to add materials with
low grain density, for example in the form of glas~ or
ceramic hollow beads (microspheres). However, because of
their high price, these have not prevailed and in addi-
tion have the disadvantage that they can fragment underhigh physical stressing and then no longer contribute to
reducing the litre weight of the slurry.
The object of the invention is to provide a deep-
drilling cement and a process for the preparation thereof
according to the generic clauses of Claims l and 10 which
permit a low density slurry to be prepared with which a
high compressive strength and non-porosity can be
obtained.
This ob~ect is achieved in that plastics powder
is admixed in an amount corresponding to approximately
50 to 800 kg of plastics powder per m3 of slurry, the
plastics powder consisting of hard and tough or brittle,
alkali-resistant particles with a grain size below
approximately 2 mm which are unfilled or low-filled and
~0 drastically reduce their tensile strength at least about
10C above the particular maximum static borehole
temperature.
Hard and tough and/or brittle amorphous thermo-
plastics having a glass transition temperature at least
approximately 10C above the particular maximum static
borehole temperature, and partly crystalline thermo-
plastic~ having a crystalline melting point at least
approximately 10C above the particular maximum static
borehole temperature and unfilled or low-filled thermo-
sets having a decomposition temperature at least approxi-
mately 10C above the particular maximum static borehole
temperature can be used.
Therefore, only water-insoluble plastics are
suitable which do not soften under the temperature
conditions in the borehole or a~ least not to the extent
that they adhere. In addition, unfilled or low-filled
plastics having a raw density (approximately c 1.1 g/cm3)
substantially corresponding to the specific weight of the
.
2~93~5
-- 3 --
particular pure plastic are suitable in order to be able
to keep the density of the crude slurry low. The use of
at most low-filled plastics is possible because the
slurries forming suspensions with high water content
which are produced using these are being continuously
agitated until they have arrived at the place of use and
have such a viscosity that they are pumpable and the
plastics particles do not float to the surface. In
particular the slurries can be used in the seawater area
and can be produced using seawater as mixing water.
By this means, slurries with a density less than
1.7 xg/l can be prepared, which on account of the hydrau-
lic binding provides sufficient strength and at the same
time are cost-effective, and in particular when the
plastics powder comprises comminuted waste plastics and
in particular plastics from domestic wastes whLch are not
of pure grade and are usually low-filled. The latter are
compo3ed, for example, principally of polypropylene,
polyethylene, the two generally accounting for
approximately 90~, and polystyrene. Polypropylene and
polyethylene are partly crystalline thermoplastics with
a glass transition temperature T5 ~ 0C whose service
range is between T~ and the crystallite melting point T~.
In the case of slurries with a density in the region of
2S approximately 1.40 kg/l, it is possible in general to
work with a water/solid ratio of ~ 0.5 and in the case
of slurries with a density in the region of approximately
1.20 kg/l with a water/solids ratio of 2 0.35.
The plastics powder originating from wastes is
expediently produced by comminuting molten and fused-
together plastics wastes. For melting and fusing
together, autoclaving of the plastics wastes in the form
of compact, coherent, porous pack with a high water
absorption capacity in a steam atmosphere at approxi-
mately 160 to 220C is particularly suitable. Before
melting, pre-comminuting may be expedient.
In particular, mixed-grain-plastics powder is
used which preferably has a grain distribution in which
2~3~83
-- 4 --
23448-194
at least 90% of the particles are smaller than 1 mm. Since
during comminution of the plastics practically no fines usually
result, it may be expedient to introduce mineral dusts and/or
flours, in particular quartz flour or fly ash, as additives in
an amount of up to approximately 20% by weight of the plastics
powder. In general the amount of the plastics powder is at
least about 80% of the additives.
The slurry produced can contain approximately 200 to
1,200 kg/m3 of hydraulic binder and approximately 50 to 800
kg/m3 of plastics powder.
For the hydraulic binder, Portland cement, slag cement,
high-alumina cement or similar cements, or cement substitutes
such as, for example granulated blast furnace slag, if desired
as a mixture of at least two of these cement types/cement
substitutes, are suitable.
- Normally a proportion of fibre in the plastics powder
is not desired because of the increased water requirements which
this usually causes. These increased water requirements can be
explained by a felting effected by the fibres resulting in
interstices retaining excess water. Surprisingly, despite the
high water/cement ratios resulting from this (see following
Examples l and 2), sufficient strengths are attained for the
field of application. Nevertheless, the proportion of fibre in
the plastics powder, in order to produce slurry litre weights of
less than or equal to 1.3 kg/l, should be as far as possible less
than lQ% by weight, in particular < 5% by weight.
For particular applications, however, it may be
advantageous to have larger proportional amounts of fibres, in
2093~5
- 4a -
23448-194
particular up to 25~ by weight, in the plastics powder, for
example for slurries for sealing such rents and lines of
stratification which effect losses during the cementation of
the annular space.
To reduce the water requirements of the hydraulic
binder, a thinning agent can be added. The addition of an
agent for increasing the wettability of the plastics powder by
water, for example in the form of surfactants
2~93~a
-- 5 --
with hydrophilic and hydrophobic groups, is also advan-
tageous in order to obtain a suspension which is as
homogeneous as possible and thus water requirements which
are as uniform as possible.
Further additiveq such as setting-regulating
agents, agents for reducing the gas permeability (~uch as
latex suspensions), agents for regulating the water
retention capacity etc. can be added.
The plastics powderjcan form a dry mixture with
the hydraulic binder and any additives and auxiliaries;
however, it can also be admixed during the preparation
of the ~lurry.
Exam~le 1
A light slurry for obtaining a high early
strength on the North Sea floor is prepared in an amount
of 1 m3 and with a density of 1.6 kg/l from:
800 kg of Portland cement PZ 55 according to
DIN 1164
440 kg of seawater
214 kg of plastics powder (45% of polypropylene, 45%
of polyethylene, 10% of polystyrene, grain
distribution < 2 mm with 90% ~ 1 mm, water
requirements 150%)
3 kg of thinning agent
0.3 kg of foaming inhibitor
At 10C an early ~trength of approximately 2 MPa results
after 12 h and eventually a final strength of approxi-
mately 25 NPa.
(The water requirements were determined here by
weighing out 10.0 g of substance to an accuracy of 0.1 g
and transferring it quantitatively into a wet folded
filter. 100 ml of desalinated water are poured over the
sample. The excess water run~ off through the filter and
i~ caught in a 100 ml measuring cylinder. In the case of
inert substances, the measurement is completed when,
after the filtrate has been allowed to pass through
several times, there is no change in the amount of
filtrate which has run through. The difference between
2093~
-- 6 --
100 ml and the volume of filtrate read off i~ multiplied
by 10 and provides the water requirements in per cent.)
Examnle 2
A light slurry is prepared in an amount of 1 m3
with a density of 1.3 kg/l from:
500 kg of cement API 10 Class G
313 kg of plastics powder according to Example ?
500 kg of fresh water
1.3 kg of thinning agent
0.25 kg of foaming inhibitor
This results in a strength of approximately 7 NPa in thé
temperature range of up to lOO-C.
In the accompanying diagram the uniaxial com-
pressive strength in NPa is plotted on the ordinate and
the density of the slurry on the abscissa. The diagram
includes curves regarding the final strengths at tempera-
tures below 100C, curve A relating to slurries reduced
in density by the addition of 25~ trass, curve B to
slurries reduced in density by the addition of glass
hollow beads and curve C to slurries according to the
invention reduced in density by the addition of plastics
powder. The increase in strength in the density range
investigated with re~pect to known deep-drilling cements
is clearly recognisable.
