Note: Descriptions are shown in the official language in which they were submitted.
CA 02449869 2003-11-18
OIL BASED DRILLING FLUID
Field of the Invention
The present invention relates to drilling fluids and more particularly to oil
based drilling
fluids used in the drilling of wells in nonconventional oil bearing
formations.
Background of the Invention
Large hydrocarbon reserves exist in the form of nonconventional deposits,
particularly
oil sands of the type found in the Fort McMurray arE:a of northern Alberta.
These
deposits are non-conventional in the sense that, urslike conventional
hydrocarbon
deposits, the hydrocarbon exists in the form of a highly viscous bitumen that
adheres to
a particulate substrate, typically sand. As such, the bitumen will not flow to
the well bore
under formation pressure, nor can the bitumen be pumped from the well to the
surface.
To date, the principle means of extraction has been to mine the oil sands, and
to then
treat the mined product to separate the bitumen from the substrate. The
separation
techniques are numerous and increasing.
Amongst operators in the oil sands, a methodology attracting more interest due
to
improved economics is referred to as steam assisted gravity drainage (SAGD).
This
technique involves the drilling of large diameter (200-3'I 1 mm) horizontal
holes through
the oil sand formation to form producer well bores. Additional adjacent wells,
which may
also have diameters in the 311 mm range; are then drilled far the injection of
steam.
The steam injection is required in order to allow the insitu bitumen to
mobilize and be
produced from the producer well. The produced bitumen is then refined to
recover its
usable hydrocarbon component.
A major problem when drilling into oil sands is the accretion of highly
viscous and tacky
bitumen onto the metal surfaces of the drilling tools and pipe, be it jointed
pipe or coiled
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CA 02449869 2003-11-18
tubing. The bitumen accretion can become so severe 'that pipe and tools can
become
stuck in the well bore, preventing their extraction. The accretions result in
additional
operational challenges well known in the industry including plugging and
increased
energy consumption required to overcome the resistance of the bitumen against
formation walls and other component and tool surfaces.
To date, the most prevalent method of preventing accretion has been to
physically cool
the drilling mud used when drilling the well bores. 'fhe cooled mud preserves
the
bitumen's inherent viscosity, which prevents it from mobilizing due to the
frictional heat
generated by drilling, and thereby limiting its accretion onto metal surfaces.
However,
cooling of the drilling mud is far from completely effective, and adds
considerably to
drilling costs. Cooled drilling muds are themselves more viscous, increasing
horsepower
requirements for their circulation.
Another primary concern with SAGD is waste management. The muds used to drill
the
production and injection wells are water based which after use incorporate two
liquid
phases and a solids phase. The first liquid phase is of course the water which
can be
recycled but which must be significantly processed prior to recycling in order
to remove
drilled solids and liquid hydrocarbon contaminants. This is time consuming and
costly.
Moreover, since water is required for steam injection, use of water in
drilling muds
reduces the supply available for steam generation. This is a not insignificant
consideration in the Fort McMurray area where water deposits are limited.
Summay of the Invention
It is an object of the present invention to provide a drilling mud which
obviates and
mitigates from the disadvantages of the prior art.
It is a further object of the present invention to provide an oil based
drilling mud for use
in the drilling of well bores in nonconventional oil sands deposits. Drilling
with oil will
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CA 02449869 2003-11-18
reduce the waste stream to only solids and oil. By eliminating water, extra
time normally
required for separation of oil and drilled solids from water based fluids is
not required
when recycling the recoverable and reusable oil component of the drilling mud.
If the oil
sands are drilled with an oil based mud, the problem of accretion and
subsequent
differential sticking can be reduced or perhaps even eliminated. The use of a
low
aromatic base oil such as a linear alpha olefin, because of the absence or
near absence
of aromatics, will help prevent dissolution of the bitumen being drilled to
limit accretion.
If the bitumen is not being dissolved, an ancillary benefit should be greater
bore hole
integrity.
Detailed Description of the Preferred Embodiment
The mud system proposed by the present invention consists of gelling a low
aromatic
hydrocarbon base oil with a phosphate esterlmetal complex. The pH of this
system
would be slightly acidic. This fluid would have the necessary rheological
properties for
drilling purposes. At the completion of drilling, after the production liner
has been set,
the system's viscosity can be broken by increasing the alkalinity of the fluid
to raise its
pH.
Suitable low or no aromatic base oils can include the following: AmodriIIT""
1410, Amodrill
1500 (linear alpha olefins), or Amodrill 2110 (poly alpha olefin) available
from BP, Pure
DriIIT"" HT-40N, HT-30N (paraffin base oils) or 1A-35 (paraffin base oil)
available from
PetroCanada, or Enviro-DriIITM, DriIIsoIT"" or LVT-200T'~. The base oils can
be gelled
with ChemoilT"' Gel or ChemoiITM Link, for example.
The gelled system will have a pH of about 4. If the pH is increased to about 5
or above,
the micelles will begin to degrade and will be substantially or completely
gone at a pH
of about 7.
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CA 02449869 2003-11-18
Once the system's viscosity is broken, drilled solids can be rapidly separated
from the
oil based fluid by settling andlor centrifuging. Any bitumen that may have
been
incorporated into the drilling fluid can also be separated such as by
centrifuging utilizing
its higher specific gravity compared to the low aromatic base oil being used
for the
drilling fluid. The low aromatic hydrocarbon base oil can then be reused for
drilling, the
solids disposed of and any bitumen separated out can be directed to the
refining facility
to be blended with produced bitumen.
The present oil based fluid described above does not require oil wetfiing
surfactants or
emulsifiers of the sort normally used in oil based systems. As such, there is
no concern
with wettability alterations to the reservoir. A hydrocarbon based drilling
fluid will also
prevent the dispersion of clay minerals which could otherwise leach into the
formation
to diminish its permeability and impair production.
The above-described embodiments of the present invention are meant to be
illustrative
of preferred embodiments and are not intended to limit the scope of the
present
invention. Various modifications, which would be readily apparent to one
skilled in the
art, are intended to be within the scope of the present invention. The only
limitations to
the scope of the present invention are set forth in the following claims
appended hereto.
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