Note: Descriptions are shown in the official language in which they were submitted.
~~~8~
5499
A WELLBORE HEATING PROCESS FOR INITIATION OF
BEL~nT FRACTURE PRESSURE STEAM SI~IATION
FROM A HORI?pNTAL WELL LpCATED IN AN
TNITPTAT.T ~( ~$jj~,
Field of the Invention
This invention is directed to a method for the recovery of
viscous hydrocarbonaceous fluids frarn a formation. More
specifically, it is directed to the removal of said fluids from a
formation containing heavy viscous hydrocarbons or tar sands by the
application of steam heat.
BACKGROUND OF THE INVENTION
The use of horizontal wells in oil reservoirs is currently of
high interest within the oil industry. Horizontal wells allow more
reservoir surface area to be contacted and thereby reduce inflow
pressure gradients for reasonable oil production rates.
Alternatively, for typical pressure gradients within the wellbore
region, the productivity of a horizontal well is greater than that in
a vertical well.
She possible benefits of horizontal wells are c~rently being
exploited in the Canadian tar sands. Reservoirs in Canada that may
be categorized as immobile under reservoir conditions include the
Gold Lake and Athabasca deposits.
Current practices for producing the above immobile tar sands
include mining and steam stimulation by formation fracturing.
However, minirig is not practical below very shallow depths.
Furthermore, steam stimulation by formation fracturing is not
feasible in those reservoirs underlain by water aquifers. In
general, fracturing in zones underlain by water aquifers results in
large amounts of water production and nonuni.form development of the
2
steam zone. tame water influx is due to penetration of the fracture
into the water aquifer.
Steam stimulation belay fracture pressure in vertical wells is
not practical due to the very low injectivity of the formation to
steam and the very small area of reservoir contact. IrKxeased area
of contact can be achieved by the use of long horizontal wells (1,000
to 3,000 ft as oarnpared to 30 to 100 ft for a vertical well). 'Ibis
increased area of contact allcyws more of the reservoir's area to be
heated by steam injection. This results in more oil production due to
the increased volume of the heated zone. Unfortunately, for immobile
tar sands, even when heated, injectivity may remain very law.
Injection of a large steam slug into a horizontal well underlain by a
water aquifer may result in a fracture into the aquifer.
Therefore, what is needed is a method for removing
hydrocarbonaceous fluids fraan immobile tar sands or viscous fluids
via a horizontal wellbore which will allow steam injectivity therein
before a substantially large steam slug is injected so as to avoid
fracturing into an aquifer.
St~RY OF THE INVF3~1TION
Zhis invention is directed to a process for obtaining initial
steam injectivity at below fracture pressure in initially immobile
tar sands or other very viscous hydrocarbonaoeous fluid containing
formation, where at least one horizontal wellbore is utilized. A
horizontal well is drilled into the formation or immobile tar sand
reservoir. Steam is then injected into the formation at a previously
determined rate and at a pressure slic~tly above the formation or
reservoir pressure. The steam is continuously circulated for a
pre-determined time which causes the horizontal well to act as a heat
conducting rod in the formation. Steam, however, does not enter the
formation. Upon reaching the pre-determined time, steam injection is
discontinued and a hydirocarbonaoeot~.s fluid mixture is produced to the
surface via said well.
.. 226483
3
Once the hydrocarbonaceous fluids have been removed from the
portion of the formation or reservoir heated by the steam, a voided
area is created. (hereafter, a substantially large steam slug is
injected into the voided area of the formation or reservoir and a
steam stimulation enhanced oil recovery (DAR) is ooamnenoed.
It is therefore an object of this invention to heat a portion of
the formation penetrated by a horizontal well where initial steam
injectivity is substantially small or none estistent.
It is another object of this invention to steam heat a desired
portion of the formation so as to subsequently remove
hydrocarbonaceous fluids while keeping the formation from being
fractured.
It is yet another object of this invention to cause a radial
space to be created longitudinally in a reservoir along a horizontal
wellbore by steam heating through conduction in said ~llbore.
BRIEF DF~S~'ION OF T~ DRAWINGS
Fig. 1 is a schematic representation of a horizontal wellbore in
a formation while steam is being injected therein.
Fig. 2 is a graphical illustration depicting heat front movement
by conduction in a reservoir via a horizontal. wellbore.
Fig. 3 is a graphical illustration shaving various rates of heat
transfer fr~an a horizontal w~ellbare into a reservoir.
Fig. 4 illustrates graphically optitrnun steam injection rates
sufficient to satisfy conduction requirements during the initial
stages of operation.
Fig. 5 depicts graphically oil viscosity distribution around a
horizontal wellbore in space and time.
2026483
4
DESCRIPTION OF 'Iii PRg'E~Rm ~pp~VTS
In the practice of this invention as shown in Fig. 1, steam is
circulated through a horizontal wellbore 10 through over~en 12 and
into an oil rich zone 14. Oil rich zone 14 can ~ri.se immobile tar
sands, asphalt, or asphaltic materials for example. The steam which
is circulated into wellbore 10 has a pressure slightly higher than
the reservoir pressure but at a pre lower than the reserve~ir~s
fracture pressure. For this reason, a reservoir fracture 18 is not
created so as to avid coa~u~amicating with water aquifer 16.
The steam is allowed to remain in wellbore 10 while avoiding the
injection of steam into oil rich zone 14. It is necessary for the
wellbore 10 pressure to be slightly higher than the reservoir
pressure to obtain steam penetration into wellbore 10. Thus steam
does not enter the reservoir as it passes wellbore 10.
Although not shown, wellbore 10 contains perforations at desired
intervals along its length. Maintainir~ the steam injection rate as
above causes wellbore 10 to act as a co~luctir~g rod within oil rich
zone 14. Heat ~r~ductance away from wellbare 10 is used to preheat
the area of the reservoir adjacent to wellbore 10. After the oil
rich zone 14 has been heated for the amount of time desired, immobile
viscous hydrocarbons in oil rich zone 14 are warmed sufficiently to
beg mobile. Mrobile hydrocarbons flora into wellbore 10 where they
are produced to the surface.
Once the warmed hydrocarbons have been rte, a void is
created in oil rich zone 14 adjacent to wellbore 10. This wided
area is then used as an area in which to initiate steam stimulation
of oil rich zone 14. A steam stinailation method which can be used to
fill the wid created by the removal. of warmed hydrocarbons is
discussed in U.S. Patent No. 3,434,544 which issued to Craig et al.
on March 25, 1969.
Steam stimulation is continued for a time
sufficient to remove desired hydrocarbons from oilrich zone 14.
Steam passing horizontal well 10 may be repented by
the following partial differential equation:
2~D~6~.8~
2
aT r t - ~ at(r.t2 + 1 ~T r.t ( )
1
at ar r ar
with boundary conditions
T(r,t) -~ finite as r -~ ~ (2)
and
T(r,t) = Ts at r = w (3)
amd an initial condition
T(r,t) = o at t = 0 (4)
where vCis the thermal diffusivity of the formation, T(r,t) is the
temperature at any time and radial distance from the wellbore, Ts is
the steam temperature, T is the initial reservoir temp~xature, r is
o.
the radial distance frown the wellbore, w is the radius of the
wellbore, and t is the time.
The above set of equations have an analytical solution which has
been presented by H. S. Carslaw and J. C. Jaeger in a publication
entitled Conduction of Heat in Solids, Second Edition, Oxford Press,
(1959) pp. 334-338. This resultant temperature profile, as a
function of time, detxxmines how long the well must be operated in
conduction mode.
Heat transfer rates away from the wellbore are given by R. B.
Eird, W. E. Stewart ~.ncl F. LT.. L;ic~tfrn~ ire a ra~h~ ~!~.ati~~ ~~~itl~
Transport Phenomena, published by John Wiley & Sons, New York, N. Y.
(1960) p. 319. These rates are presented by the following equation.
q = -k aT(r.t) (5)
ar
r~
w
202648
6
where q is the heat flux fr~n the wellbore and k is the thermal
conductivity of the formation. This flux is the amount of heat input
to the well, in terms of steam injection, required to exactly satisfy
the heat conduction away from the well. Zhe integral has been
presented graphically in Carslaw and Jaeger mentioned above.
An example of calculations for time and steam injection rates
during the conduction heating phase follow. For these calculations,
a reservoir pressure of 3,000 KPa has been ass~mved. A reasonable
wellbore pressure to achieve heat conduction in the absence of either
connective heat transfer away from the wellbore or a formation
fracture is assumed to be 4,000 KPa.
Figure 2 depicts the thermal history of the well. The abscissa
presents radial distances from the center of the wellbore. Table 1
lists all physical parameters used in the calculation.
TABLE 1
Oil Density = 60 lxm/ft3
Water Density = 62.4 ll~n/ft3
Rock Density = 165 lxm/ft3
Formation Density = 133 ltxn/ft3
Oil Saturation = 0.77
Water Saturation = 0.23
Oil Heat Capacity = 0.5 Btu/lbin F
Water Heat Capacity = 1.0 Btu/ltm F
Rock Heat Capacity = 0.24 Btu/lbn F
Formation Heat Capacity = 0.352 Btu/lbtn F
Formation Thermal Conductivity = 38.2 Btu/ft day F
Formation Thermal Diffusivity = 0.81 ft2/day
Representative Length of Well = 1,000 ft.
2o~e~s~
7
TABLE 2
Oil Viscosity as Function of Temperature
Temperature (F) Viscosity (cp)
50 26,725
100 1,508
150 223
200 59
250 23
300 11
350 6.3
400 4.0
450 2.8
500 2.0
550 1.5
The representative horizontal wellbore was taken as 9 inches and
its length 1,000 feet (these are typical Values of horizontal wells).
Calculation of the heat capacity, thermal conductivity, and density
of the overall formation proceeded as described by Pmts (1982).
Implicit in these results is the assumption that the wellbore is
raised essentially instantaneously to steam temperature at time zero.
After 0.5 day a radial isotherm of 200°F exists to a radial
distance
of 1.0 ft frcen the well center. After 35 days, this isotherm has
moved to slightly less than 3.5 ft.
Figure 3 presents calculated rates of heat transfer fr~n the
wellbore. Clearly, the heat transfer rate has declined by a
significant amount after 35 days. These heat transfer rates are
converted to steam injection rates in Figure 4. These steam
injection rates are the optimum rates necessary to operate the well
in conduction mode. After 35 days the steam injection rates drop and
plateau.
202483
8
Frcfln these calculations, the optimum time to heat the horizontal
well in conduction mode is 35 days. The optimum steam injection
rates range frarn about 200 BBL/day to slightly less than 100 BBL/day,
cold water equivalent (GWE).
The oil viscosity distribution, based on the oil viscosity data
in Table 2, is shown in Figure 5. This distribution is a result of
the heat conduction frown the wellbore. After 35 days, the oil
viscosity is reduced to less than 100 cp for all distances less than
3.5 ft. Since the horizontal well is so long this represents a
significant total heated volume of 38,000 ft3. Ass~ning that 30% of
the heated fluid can be recovered upon well flawback then this heated
zone can be drained of 3,420 ft3 of fluid.
Thus, a steam slug consisting of 3,420 ft3 of fluid can be
injected into the zone that was initially immobile as a result of
using the long horizontal well as a conductor.
Obviously, many other variations and modifications of this
invention as previously set forth may be made without departing from
the spirit and scope of this invention as those skilled in the art
readily understand. Such variations and modifications are considered
part of this invention and within the purview and scope of the
apper~d claims.
