Note: Descriptions are shown in the official language in which they were submitted.
s3~
21~
CYCLICAL STEAM FLOODING METrOD
fOR VI~COUS OIL RECOVERY ~
This invention relates to a thermal method for recovering oil
from a subterranean, viscous oil-containing, permeable formation,
including tar sand deposits. Still more specifically, this method
involves a cyclical steam flood process for recovering oil from tar
sand formations penetrated by at least two wells, a fluid
communication path extending through the formation between the wells,
and employing injection of steam with specific pressurization and
drawdown cycles which are sequentially repeated.
Increasing worldwide demand for petroleum products, combined
with continuously increasing prices for petroleum and products
r~covered therefrom, has prompted a renewed interest in the sources of
hydrocarbons which are less accessible than crude oil of the Middle
East and other countries. One of the largest deposits of such sources
of hydrocarbons comprises tar sands and oil shale deposits found in
Northern Alberta, Canada9 and in the Midwest and Western states of the
United States. While the estimated deposits of hydrccarbons contained
in tar sands are enormous (eOg., the estimated total of the deposits
in Alberta9 Canada is 250 billion barrels of synthetic crude
equivalent), only a small proportion of such deposits can be recovered
by currently available mining technologies (e.g., by strip mining).
For examole9 in 1974 it was estimated that not more than about lO~ of
the then estimated 250 billion barrels of synthetic crude equivalent
of deposits in Alberta, Canada was recoverable by the then available
mining technologies. (See SYNTHETIC FUELS, March 1947, pages 3-l
through 3-14). The remaining about 90~ of the deposits must be
recovered by various in-situ techniques such as electrical resistance
heating, st~am injection and in-situ forward and reverse combustion.
Accordingly, this invention provides an improved thermal
method for effectively recovering oil from a subterranean, viscous
oil-containing, permeable formation employing a cyclical
pressurization~drawdown program.
``~`' ~
F-2161 - ~ -
Tne present invention relates to a cyclical steam flooding
method for recovering highly viscous oil from a subterranean, viscous
oil-containing, permeable formation9 including a tar sand deposit,
employing a specific prograrn of injecting steam to pressurize the
formation followed Dy a drawdown phase. The formation is penetrated
by at least one injection well and at least one spaced-apart
production well~ both wells being in fluid communication with a
substantial portion of the forrnation. Either a naturally occurring or
artificially induced fluid communication path exists oetween the
injection well and the production well. Each cycle of my process
involves two stepc. The ~irst step involves injecting steam into the
injection well at a predetermined flow rate, preferably at least 500
bbls/day, and producing fluids including oil from the formation via
the production well at a restricted flow rate equal to a value 50
percent or less than the flow rate of the steam being injected into
the injection well. Injection of steam and restricted produotion is
continued until the wellhead temperature of the produced fluids rises
to a temperature within the range of 175 to 25û~F. Thereafter the
second step is begun wherein fluid production is increased to the
maximum value and simultaneously reducing the steam injection rate to
a value about 20 percent of the original injection rateO The second
step is continued until the content of oil in the produced fluids is
unfavorable. Thereafter the steps are repeated for a plurality of
cycles until the desired oil recovery has been attained.
In the drawings appended to this specification:
Figure 1 illustrates a subterranean, viscous oil containing
foTmation penetrated by an injection well and a spaced-apart
production well, a fluid communication path located in the middle zone
of the formation such that well-to-well communication is established,
and the first phase of my process involving steam injection into the
injection well and restricted production from the production well for
pressure development in the formation.
Figure 2 illustrates the drawdown or second phase of my
process in which the injection rate o~ steam is reduced with a
simultaneous rapid production rate.
F-2161 - 3 ~
Figure 3 is a schema~ic representatlon of the method cycles
and the corresponding pressure curve.
The process of my invention a~plies to a subterranean, viscous
oil-containing formation such as a tar sand deposit with high
conductivity, areally extensive microstructures or thie~ zones~ or if
the formation does not possess naturally occurring permeability to
steam, then a suitable communication path or zone of high fluid
transmissability is formed prior ~o the aoplication of the main
portion of my invention.
Referring to Figure 1, there is shown a subterranean, viscous
oil-containing, permeable formation 10 penetrated by at least one
injection well 12 and at least one spaced-apart production well 14.
The injection well 12 and production well 14 are in fluid
communication with a substantial portion of the formation by means of
perforations 16. While recovery of the t~pe contemplated by the
present invention may be carried out by employing only two wells, it
is to be understood that the invention is not limited to any
particular number of wells. The invention may be practical using a
variety of well patterns as is well known in the art of oil recovery,
such as an inverted five spot pattern in which an injection well is
surrounded with four production wells, or in a line drive arrangement
in which a series of aligned injection wells and a series of aligned
production wells are utili~ed. Any number of wells which may be
arranged according to any pattern may be applied in using the preCent
method as illustrated in U.S. Patent No. 37927~716 to Burdyn et al9
issued December 23, 1975. m e
injection well 12 and production well 14 may be spaced-apart a
predetermined distance depending upon the particular well pattern
selected which may vary from about 200 to about 1000 feet. A naturally
occurring or artificially induced fluid ccmmunication path 18 exists
between the injection well 12 and the production well 1~. As
illustrated in Figure 1, this fluid communication path 18 consists of
at least one horizontal micro-structure or fracture such that
well-to-well fluid communication is established. Fluid com~unication
can be induced by forming at least one horizontal fracture in the
3~
F 2161 - ~ ~
s~
formation by the use of hydraulic fracturing fluids -thereby
establishing fluid communication channels through the formation
between the injection well and the production well and the fracture
can be expanded by subsequent treatment such as by injecting solvents
into one or both of the wells to enter the -thief or communication
channels in a repetitive fashion until adequate communication between
the wells is established.
The process of my invention comprises a series of cycles,
each cycle consisting of two parts. The first step of the cycle is a
pressurization phase wherein steam is injec-ted into the formation 10
via the injection well 12 at a predetermined flow rate, preferably at
least 500 bbls/day9 and fluids including oil are produced from the
formation via the production well 14 with restriction of the flow rate
in order to increase the pressure in the communication path 18 and the
portion of the formation adjacent thereto. The rate of flow of fluids
from the production well 14 during this step is restricted to 50
percent or less of the steam injection rateO The injected steam is
saturated and the preferred steam quality is from 50 to about 95
percent. As illustrated in Figure 1, the injected steam migrates as
shown by the arrows both horizontally and immediately above and below
the communication path 18 to form a steam-swept zone 20 which
mobilizes viscous oil by reducing its viscosity and displaces the
mobilized oil through the formation 10 toward the production well 14
for recovery. During the pressurization phase, the temperature of the
fluids recovered from the production well 14 at the wellhead is
monitored. Injection of steam and restricted production of fluids
including oil is continued until the wellhead temperature of produced
fluids is within the range of 175 to 250F. Once the produced fluids
reach this temperature, the first step is concluded.
Thereafter, the second step of the cycle is initiated wherein
the fluid production rate is increased to the maximum value and
simultaneously reducing the injection rate of steam to a value about
20 percent of the original injection rate at which steam was injected
into the injection well 12. For example, if the original steam
injection rate was 500 bbls/day, the steam injection rate is reduced
F-2161 - 5 -
to about 100 bbls/day. During this second drawdown phase, the steam
from the steam-swept zone 20 migrates toward the communication path 18
as shown by the arrows in figure 2 and the formation pressure is
significantly decreased. The drawdown phase is continued until the
produced fluids from the production well 14 contain an unfavorable
amount of oil. The oil recovery process is continued with a plurality
of cycles comprising pressurization and drawdown.
The pressure response behavior of the formation during the
pressurization-drawdown cycles is shown graphically in Figure 3
wherein curve 1 represents the pressurization phase with restricted
flow rate of fluids from the production well and curve 2 represents
the drawdown phase of the cycle wherein steam injection is restricted
and production is rapid. During the pressurization phase as shown by
curve 1, the formation is pressurized to a value above the formation
pressure and during the withdrawal phase as shown by curve 2 the
formation pressure is significantly decreased. During the injection
phase the steam is forced above and below the steamed zone and a small
pressure gradient exists between the steamed and unsteamed zones as
well as the normal gradient near the wells. lhese gradients are
reversed during the drawdown phase of the process. This reversal of
the pressure gradients forces the oil into the communication path and
hence into the production well for recovery.
From the foregoing specification one skilled in the art can
readily ascertain the essential features of this invention and without
departing from the spirit and scope thereof can adapt it to various
diverse applications. It is my intention and desire that my invention
be limited only by those restrictions or limitations as are contained
in the claims appended immediately hereinafter below.
