Note: Descriptions are shown in the official language in which they were submitted.
~3 [l~
6340(2)
RECO_ RY OF HEAVY OIL
This invention relates to a method for the recovery of heavy
oil, especially bitumen from tar sands.
As reserves of conventional crude oils (approximately 15 to
30 API) decline, increasing importance will be attached to
efficient methods for recovering heavy crude oils (8-12 API) and
the even heavier bitumens (less than 8 API). Most bitumens are
associated with minerals such as clays and quartz, and are known as
tar sands.
The ~lberta tar sands are among the largest deposits of their
kind in the world and are estimated to contain about one trillion
barrels of bitumen in place. The Athabasca region alone has
reserves of 250 billion barrels. About 0.7 million acres of the
Athabasca deposit is overlain by 150 ft, or less, of overburden and
is potentially capable of being mined from the surface. The
remaining 16.6 million acres are at such depths that the bitumen can
only be recovered by in-situ methods.
The crude bitumen occurs in beds of sand and clay, usually
partly connected together, and in porous carbonate rocks.
In high grade tar sand the pore space is filled with bitumen
(typically 15-20X weight) and water.
In low grade tar sands, i.e. containing less than lOX by weight
bitumen, clusters of small particles exist within the framework
formed by the coarse inorganic grains. These particles, known as
fines, are saturated with water. Thus the amount of connate water
in the tar sand increases with increasing fines.
~3~0~
content.
The bitumen typically has an API gravity of 7 and is denser
than water at room temperature but becomes less dense than water at
elevated temperatures.
Ln the case of deposits near the surface the overburden may be
removed and the tar sand recovered by open cast mining.
Mined tar sands are refined by the aforementioned hot water
process. A dsscription of this process is given in US~ ~ 474 616.
In broad summary, this process comprises first conditioning the
tar sand, to make it amenable to flotation separation of the bitumen
from the solids. Conditioning involves feeding mined tar sands, hot
water (80C), an alkaline process aid (usually NaOH), and steam into
a rotating horizontal drum wherein the ingredients are agitated
together under relatively hlgh shear conditions.
During conditioning, the mined tar sand (in which the bitumen,
connate water and solids are tightly bound together) becomes an
aqueous slurry of porridge-like consistency, wherein the components
are in loose association.
The slurry leaving the drum is screened to remove oversize
material and then flooded or diluted with additional hot water.
The bitumen is then recovered by primary and secondary froth
flotation.
This process suffers from the disadvantages that bitumen/water
emulsions are formed and the separated water contains colloidal
dispersions of clay, fines and oil which are extremely stable and
present serious problems in their disposal.
For deposits at a greater depth, the technigue of jet leaching
can be employed. Jet leaching is a known technique for the
extraction of tar sands which comprises drilling and fixing casing
until the pay æone is reached. The ore is then fragmented by
directing high velocity jets of water onto it and the bitumen is
pumped to the surface, leaving most of the solid particles downhole.
In a conventional single stage jet leaching process alkaline
water (pH> 11) at high temperatures (i 80C) is used to mobilise the
bitumen. The water is generally deaerated and stripped of
~3~
divalent metal ions.
Alternative approaches for deep deposits are the use of cyclic
steam stimulation or steam drive to recover the bitumen.
Cyclic steam stimulation is otherwise known as "huff and puff".
In this process, steam is injected and the bitumen produced through
the same well. The steam is injected down the well for several
weeks. On the termination of steam injection, bitumen flows freely
up the well for about one week, after which it has to be pumped to
the surface. Pumping can usually be continued for several months
before more steam must be injected.
In steam drive, the steam acts to heat the deposit and drive
the bitumen from an injection well to a production well.
In all these methods, oil recovery is assisted when the sand is
water wet and hindered when it is oil wet.
In the case of tar sands, most of the sand is water wet.
It is an ob~ect of the present invention to condition
chemically the material in situ by in~ecting an aqueous solution of
an alkaline compound into the reservoir.
Thus according to the present invention there is provided a
a two stage method for the recovery of heavy crude oil or bitumen
associated with a solid inorganic substance and water from an
underground deposit, which method comprises chemically conditioning
the deposit ~n situ with an aqueous solution of an alkaline compound
and subsequently treating the conditioned deposit to recover heavy
crude oil or bitumen from it.
In the first stage the preferred alkaline compound is sodium
hydro~ide.
This is preferably added to render the pH of the conditioning
solution in the range 11 to 12.5.
The alkaline solution is left in a quiescent state so that
physico-chemical action (rather than mechanical) detaches the
bitumen from the inorganic solid making it more amenable to
recovery.
In the second stage, a number of options are open to recover
the bitumen. These include steam drive, cyclic steam simulation, a
'~i;, ,~"
r6~
130~ 0
-- 4
modified cyclic steam simulation in which steam and alkali
are injected together, cold or hot water drive and jet
leaching, preferably the latter, where tar sand is
concerned.
It is believed, although applicants do not wish to be
bound by this theory, that the presence of the alkali
causes the water films surrounding the inorganic grains to
swell and thereby promote the separation of bitumen from
the tar sand matrix.
In all cases, in situ conditioning will increase
yields and, in the case of cyclic steam stimulation, will
extend the periods between injection of steam.
The invention is illustrated with reference to the
following Examples and the accompanying drawings.
In the accompanying drawings:
Fig. 1 is a graph of bitumen recovery against initial
pH of sodium hydroxide conditioning agent;
Fig. 2 is a graph plotting bitumen recovery against
contact time of the conditioning agent illustrating the
e~fects of contact time and temperature; and
Fig. 3 is a graph of bitumen recovery against
temperature of leachant illustrating the effect of varying
the temperature of the leachant in the extraction step
following the conditioning process.
A medium grade (11.6 wt ~ bitumen) Athabascan tar
sand sample was used. The tar sand had poor processing
characteristics and as such was considered to be suitable
for investigating experimental procedures aimed at
improving processability.
Example l
Static Tests
A series of experiments were conducted in which
samples of tar sand (10 g) were conditioned in a sample
jar by contacting with sodium hydroxide solution (10 ml;
over pH range 9 - 13), initially at a known temperature
(20 - 80C), and allowed to cool naturally for a known
time period (0.25 - 24 h).
After this conditioning stage, a quantity o~
~a
o
- 4a -
deionised water (35 ml) was added to the jar and bitumen
removal effected by heat treatment in a water bath (0.5 h
at 80C) followed by inversion of the jar to promote
bitumen separation and flotation.
Example 2
Dynamic Jet Leaching Tests
A series of experiments were carried out in parallel
with the static jar tests using a laboratory scale jet
leaching test system as described in GB 2176224A.
Samples of tar sand (125 g) were conditioned by
contacting with sodium hydroxide solution (50 ml over pH
range 9 - 13), initially at a known temperature
(20 - 80C), and allowed to cool naturally
~ 3~
over a known time period (0.25 - 24 h).
After conditioning, deionised water (1350 ml) was added to the
closed jet leaching loop and bitumen recovered by jetting with
leachant at known temperature (30 - 80C), for a constant time
period (0.67 h) and jet velocity (ca 3 m s~l).
For both static and jet leaching tests, the extent of
bitumen recovery was determined gravimetrically by solvent
(toluene) extraction of the cleaned sand following bitumen
extraction.
Pigure 1 illustrates the bitumen recovery from the tar sand for
both the static and jet leaching tests, as a function of the initial
pH of the sodium hydroxide conditioning agent. For the two
different experimental procedures, there i9 a critical pH (ca 11)
below which the conditioning process i5 less effective. Increasing
this pH resulted in tar sand conditioning, characterised by rapid
disintegration of the tar sand matrix, the formation of a stable
clay dispersion in the supernatant conditioning fluid, and the
adoption of a sandy rather than oily tar sand appearance. At higher
pH (> ca 12.5) the characteristic physical transformation of the tar
sand matrix was not observed. Here the bitumen recovery can be
ascribed primarily to an emulsification type extraction mechanism,
as was evident by the appearance of the leachant following a typical
jet leaching experiment. For all conditioned samples subsequent
bitumen recoveries in a second stage extraction step were much
higher than those observed in a single stage extraction without
prior tar sand conditioning where typically < 5% bitumen was
recovered. The apparent limit in bitumen recovery (ca 85%) is
thought to arise from the relatively moderate shear conditlons (low
jet velocity, short jetting times) imposed on the tar sand during a
typical experiment.
The concept of a two stage chemical conditioning/extraction
type process also resulted in further benefits such as improved
quality of both recovered bitumen and leachant, compared to bitumen
recovered by a single stage alkali extraction process, as summarised
in Table 1.
~;. '
~3~:300~0
Effect of Contact Time/Temperature
Th~se aspects ar~ illustrated in Figure 2 in which bitumen
recovery ls seen to increase with increasing contact time and
conditioning temperature, approaching a plateau in recovery at
around 24 h contact time. At low contact times, the effect of
varying the temperatura of the conditioning agent i~ more importan~,
whereas for longer times (> 24 h), the bitumen recovery ~
independent of the initial conditioning temperature. For example,
tar sand conditioned at 20~C for 24 hours results in an equivalent
bitumen recovery to that conditioned initially at 80C for 24
hours. This suggests that the predominant mechanism in the
conditioning process and, in particular, the migration of bitumen
within the tar sand matrix is chemically rather than temperature
controlled.
Effect of Leachant Temp_rature FollowinR Conditionin~
Figure 3 shows the effect of varying the temperature of the
leachant in the extraction step following the conditionLng
process. After tar sand conditioning at 20C (24 h), a leachant
temperature of only 45C results in high (ca 80%) bitumen recovery.
Higher leachant temperature only marginally improves the extent of
rscovery, although the kinetics or rate of bitumen recovery is
observed to increase significantly.
Such a profile is in sharp contrast to the strong temperature
dependence exhibited for an unconditioned single stage bitumen
extract~on process in which a hiBher total concentration of sodium
hydroxide than for the two stage proces~ of conditioning and
recovery (5.6 x 10-3 moles NaOH cf to 7.9 x 10-4 moles) was present
in the leachant.
The data in Table 1 reveal that the total alkali consumption in
both the two stage process (conditioning + recovery) and the single
stage alkali extraction procedure are similar, although the mode of
operation o the sodium hydroxide differs greatly.
In the two stage process the alkali serves to reiect un-wanted
clay particles from the tar sand matrix and cause~ migration of
bitumen droplets away from sand grains ~n readiness for detachment.
~lL3 (~ 1)0
Reduction of pH in the second stage extraction step (to ca pH 9)
promotes clay settling and discourages bitumen emulsification. By
comparison, in the single stage alkali process, a similar chemical
driving force is not present to the same extent, such that bitumen
removal is more dependent on a viscosity reduction mechanism with
the requirement of high leachant temperatures. Furthermore, the
relatively high final alkalinity of the leachant (ca pH 11.2)
promotes stabilisation of the clay dispersion and emulsification of
the bitumen.
Thus a two stage process (chemical conditioning followed by jet
leaching) not only improves the degree of bitumen recovery but also
produces a hi&her quality bitumen with lower solids and water
content. A further advantage is gained in that stable fines
suspensions are not produced thereby reducing the need for effluent
treatment and water processing prior to recycling.
--~ 8 __ _
~`0
a O In
C~ ~ h _
E~ ~ o _u~ ~ O X
H H O . . . O
C~, .C ~ I~ ~ O ~ I~
'~ ~ C~
P _ g
l~_ 8 h
l ~ . X
eO ~ ~ P
h C~ O~ ~ ~ O ~q
b O O J 1~
U~ ^ O 4~ R
h c~l ~ ~d td
C E-l _l ~ ~ h
-- R ~ ~1 S '-- --I .C
te~ 5~ g ~cZ; ,CJ,~q
~ O .~ H Ll
'1 H .C El --
a t) z .,~ _ PX
a P ~ ~o u~ h 1~
8 ~ 0~ o ~ a ~a
:~ R O 1~ ~ ~ l R R
~ ~a ~ ~ . . o Il~ .,~
m ~ v (o 1~ o ~ ~ v v
a ~ . ~
_ .
I ~ ~
l ~ ~ z
~ o ~0 ~ ~
0 ~ ~ ~ .r~ ~q
O ~ Ll
~J ~ ~ ~ ~ ~ a) ~
~r; g 3 ~ :~ cr .Y
t) o ~ ~ ~o
~ 3 ~1 ~ a ~ ~a
~ ~ ~ ~ ~ ~ o o
. ~ _
~ 8
~,
