Producing well stimulation method - combination of thermal and solvent

A method for the cyclic thermal stimulation of heavy oil adjacent producing wells to increase recovery of the oil produced therefrom by using an in-situ combustion process wherein oxygen or a fluid containing a minimum of about 75% by volume pure oxygen is injected into the well as the oxidizing medium, igniting the oil in the reservoir around the producing well so as to produce a combustion zone and to generate combustion gases consisting essentially of carbon dioxide and water in the form of steam, continuing injection of the oxygen until the combustion zone has propagated radially a distance of about 5 to 50 feet from the producing well, and thereafter recovering oil from the well. After terminating combustion, the well may be shut in for a period of time to allow the carbon dioxide and heat generated to more effectively permeate the reservoir adjacent the well prior to being returned to production status. The carbon dioxide dissolves in the oil reducing its viscosity along with the viscosity decrease resulting from the heat generated in the reservoir by combustion so that when the well is opened for production there is an improved flow of oil. The process of the invention applies to a single well or a plurality of wells spaced apart in a selected pattern with the various phases of the process cycles operated successively on the various wells in the pattern in any selected sequence.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an in-situ combustion process for the cyclic 
thermal stimulation of heavy oil around a producing well wherein oxygen or 
a fluid containing a minimum of about 75% by volume pure oxygen is used as 
the oxidant so as to react with the oil to release heat of combustion and 
to produce high concentrations of carbon dioxide. The increased 
temperature, pressure, and the dissolution of the CO.sub.2 in the 
reservoir oil reduces its viscosity and thereby increases oil production 
from the well when it is returned to production. 
2. Description of the Prior Art 
Repetitive stimulation of oil producing wells is a production practice of 
long standing. The phrase "cyclic stimulation" is often used to reflect 
anticipated production rate increases, the duration of which is relatively 
short as compared to the total life of the well. The cause of the 
production increase arises from either (1) an increase in pressure driving 
reservoir fluids toward the producing well, or (2) a decrease in 
resistance to flow of the fluids such as reduction in viscosity or removal 
of impediments to flow in the reservoir rock surrounding the well. The 
viscosity reduction may be achieved through use of a low viscosity fluid 
solvent and by increasing the temperature of the reservoir fluids and rock 
in the proximity of the reservoir. 
In wells producing heavy (viscous) oil, cyclic thermal stimulation has 
become widespread in use. Two somewhat different thermal stimulation 
techniques have been developed: (1) cyclic steam injection, and (2) cyclic 
in-situ combustion. A typical cyclic steam stimulation may include: (1) 
injection of steam into a producing interval for a period that may extend 
up to several weeks, depending on thickness of the reservoir, well 
spacing, rate of steam injection, etc.; (2) allowing a "soak" period 
(which in some circumstances is not necessary); and (3) returning the well 
to production. The heat introduced into the reservoir rock continues to be 
effective for some time in warming and reducing the viscosity of the oil, 
thus increasing the production rate. The effects of the stimulation will 
decline over a period of a few months whereupon the treatment may be 
repeated. 
Instead of using fuel-fed steam generators, cyclic in-situ combustion may 
be used to heat the reservoir. With this technique, air is injected into 
the reservoir through the producing well, which, after ignition, burns a 
small portion of the crude oil "in-situ", generating heat which is 
conveyed outward from the well into the surrounding reservoir by the flue 
gas formed and by vaporized crude oil and water. Water may be injected 
along with, intermittently, or following air injection to form steam and 
hot water which will convey the released heat of combustion farther into 
the reservoir. Although this method of stimulation may utilize fuel of 
less value than the steam process, wherein the steam is generated prior to 
injection into the reservoir, use of the latter process is generally 
favored. One major disadvantage of the combustion method is the 
requirement of compressing to injection pressure approximately four mols 
of nitrogen for every single mol of oxygen in air to support the 
combustion reaction. This increases cost and also dilutes the carbon 
dioxide concentration in the flue gas, greatly diminishing its efficacy as 
a solvent gas for reducing viscosity of the heavy oil. 
The method of this invention is a major improvement in the combustion 
stimulation technique in that it uses oxygen or a fluid containing a 
minimum of about 75% by volume pure oxygen as the oxidant injected into 
the reservoir through the production well. The cycle of the process would 
be similar to that used with air: i.e., (1) inject the oxidant, which 
after ignition causes movement of a burn front through the reservoir rock 
surrounding the well; (2) allow a "soak" period (which is optional); and 
(3) return the well to production. The latter step usually requires 
installation of a downhole pump to remove produced liquids from the well. 
The advantages resulting from the use of oxygen or a fluid containing a 
minimum of about 75% by volume pure oxygen include: 
1. Elimination of large amounts of "inert" gas, i.e., nitrogen, which is 
costly to compress for injection. Also the presence of the inert nitrogen 
gas as a separate phase in the pores of the reservoir rock impedes the 
flow of oil toward the well. 
2. The concentration (and partial pressure) of the CO.sub.2 formed in the 
combustion reaction is increased, and correspondingly its solubility in 
the heavy oil is increased. As a result, the viscosity of the heavy oil 
containing larger amounts of solvent gas is substantially reduced, and oil 
production rate is increased accordingly. 
3. The increased CO.sub.2 content in the oil phase increases the extent to 
which the "solution gas drive" can contribute to the displacement of oil 
toward the production well. 
4. Ignition of the combustion reaction in-situ is facilitated by the higher 
oxygen concentration of the injected gas. "Auto-ignition" will occur with 
a greater number of crude oils, thus reducing the need to use downhole 
burners, electric heaters, or steam preheating to start the combustion 
reaction. (This does not preclude the use of any of these methods where 
the crude oil properties do not favor auto ignition.) 
5. Water injection along with or intermittent to the injected oxidant may 
be used as in "wet combustion" using air and water. The advantages of 
increased heat transport farther into the reservoir by the steam formed 
in-situ from heat released by the combustion reaction also apply with 
oxygen or enriched air combustion. The increased solubility of CO.sub.2 in 
the condensed water also enhances its expulsion from the reservoir to the 
producing well which also enhances the displacement of the heavy oil 
toward the producing well. 
In U.S. Pat. No. 3,174,543 to Sharp there is described a method of 
recovering oil by producing carbon dioxide in the reservoir region 
surrounding an injection well by in-situ combustion and then introducing 
water into the reservoir to drive the carbon dioxide through the reservoir 
to displace the reservoir oil toward a production well. The present 
process is an in-situ combustion stimulation process that takes place in 
the reservoir immediately surrounding the bottom of a producing well using 
oxygen or a fluid containing a minimum of about 75% by volume pure oxygen 
as the oxidizing medium which results in the formation of a combustion gas 
comprising a high concentration of carbon dioxide. The carbon dioxide 
readily dissolves in the oil and reduces its viscosity. The heat generated 
in the reservoir by combustion also reduces the viscosity of the oil phase 
thus improving its flow through the formation when production is resumed. 
By the process of this invention therefore, a more effective recovery of 
the heavy crude oil is obtained. 
Thermal oil stimulation processes using the so-called "huff-n-puff" gas 
injection techniques are disclosed in U.S. Pat. Nos. 3,332,482 to 
Trantham, 3,369,604 to Black et al. and 3,465,822 to Klein. 
U.S. Patent to Trantham, 3,332,482, discloses a process for the secondary 
recovery of viscous oil using an in-situ combustion process at the bottom 
of a producing well in which air is used as the oxidizing medium. In this 
process, air is injected into the production well and the oil surrounding 
the bottom of the well is ignited to establish a combustion zone. 
Combustion is continued until the reservoir is plugged by viscous oil 
which results in a substantial increase in pressure. Combustion is 
terminated and the well is opened for production so that the compressed 
gases within the reservoir remote from the production well and beyond the 
plugged area drive the oil into the hot burned-out area between the 
plugged area and the production well where it is heated, perhaps upgraded 
somewhat, and finally recovered through the production well. Inherent in 
this process is the production of a gas, which is normally referred to as 
flue gas, which gas is composed predominantly of nitrogen and lesser 
amounts of carbon dioxide, carbon monoxide and other gases derived from 
the crude oil. The carbon dioxide in the flue gas is diluted by the 
nitrogen and other gases and is much less soluble in the reservoir oil 
than a gas consisting of substantially pure carbon dioxide or a gas 
containing a higher concentration of carbon dioxide than the flue gas 
produced by the use of air as the oxidizing medium. The solubility in 
reservoir oil of carbon dioxide formed with air combustion, at a given 
pressure, may be five to ten times less than that formed from oxygen 
combustion. 
U.S. Pat. No. 3,369,604 discloses a method for stimulating producing wells 
using a combination of steam stimulation and in-situ combustion wherein 
air, or a mixture of air and oxygen is used as the oxidizing gas. 
U.S. Pat. No. 3,465,822 to Klein, discloses a thermal oil stimulation 
process in which in-situ combustion is initiated around a well by air 
injection followed by injection of water and injection of inert gas, 
sequentially, and thereafter opening up the well to flow of fluids, 
including oil. 
Also, in a Society of Petroleum Engineer of AIME article, SPE 9228, 
presented on Sept. 23-26, 1979, in Las Vegas, Nev., entitled "A Parametric 
Study of the CO.sub.2 HUF-n-PUF Process" there is disclosed the results of 
Mathematical Model studies of the use of carbon dioxide as a solvent gas 
in cyclic well stimulation. The carbon dioxide is not prepared in the well 
by in-situ combustion as in the present process and offers no advantages 
associated with the heat generated by oxygen combustion of the reservoir 
oil. 
None of the prior art discloses the improved method of recovering oil 
around a well using in-situ combustion stimulation wherein the oxidizing 
medium is oxygen or a fluid containing a minimum of about 75% by volume 
pure oxygen so as to produce increased concentrations (and partial 
pressures) of carbon dioxide in the combustion gases. The carbon dioxide 
dissolves in the reservoir oil reducing its viscosity, thereby 
facilitating its flow to the production. The viscosity of the reservoir 
oil is further reduced by the heat generated in the reservoir by 
combustion. 
SUMMARY OF THE INVENTION 
This invention is directed toward a method for the cyclic thermal 
stimulation of heavy oil producing wells by in-situ combustion around the 
producing well using oxygen or a fluid containing a minimum of about 75% 
by volume pure oxygen as the oxidizing medium which results in improved 
recovery of the oil from the reservoir. The oxygen is produced from air on 
the earth's surface near the producing wells by means of a cryogenic unit. 
The use of such an oxidizing medium comprising oxygen or at least 75 vol. 
% oxygen produces a combustion gas comprising high concentrations of 
carbon dioxide and water in the form of steam. The steam aids in carrying 
heat farther into the reservoir, and the carbon dioxide is an effective 
solvent in that it dissolves in the heavy oil at even greater distances in 
the reservoir beyond the combustion zone and steam heated zone thereby 
reducing its viscosity. During combustion, the heat generated is absorbed 
by the reservoir which extends radially from the production well resulting 
in further reduction of the viscosity of the heated heavy oil as it 
subsequently flows toward the producing well. Combustion may be continued 
until the combustion zone travels a radial distance in the range of about 
5 to 50 feet from the production well, after which in-situ combustion is 
terminated and the well is opened to production whereby fluids including 
oil are recovered from the reservoir. In addition, when combustion has 
been carried out in the stated portion of the reservoir, the production 
well may be shut-in for a predetermined interval of time to enhance the 
solvent effect of the carbon dioxide and the thermal effect of combustion. 
The length of this soak period will depend upon the field characteristics 
of the producing well. Water may also be mixed with the oxidant to enhance 
the transport of heat farther into the reservoir thereby increasing the 
effectiveness of the thermal effects. The various steps of the process may 
be repeated for a plurality of cycles until the recovery of oil is 
unfavorable. 
The various phases of the process cycle may be operated successively on a 
plurality of spaced-apart wells in any selected position in any sequence. 
When the first well is on in-situ combustion, one (or more) of the 
adjacent wells is prepared for ignition so that it is ready for the 
in-situ combustion phase of the process when the first well is put on 
production following termination of the in-situ combustion phase. The 
various phases of the process may be repeated in each well for a plurality 
of cycles.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the present invention, oil is recovered from a reservoir 
by cyclic thermal stimulation of one or more producing wells using an 
in-situ combustion process wherein oxygen is used as the oxidizing fluid 
instead of air. Although the preferred oxidizing fluid is pure oxygen, 
some sacrifice in the actual performance of the process may be needed to 
make it more practical and economically feasible and therefore the 
oxidizing fluid may contain a minimum of about 75% by volume pure oxygen. 
The oxygen, upon reacting with the hydrocarbons in the reservoir, yields 
principally gaseous carbon dioxide and water as follows: 
##EQU1## 
The carbon dioxide acts as a solvent since it will dissolve in the 
reservoir oils and therefore appreciably lower the viscosity of the oil 
even in the absence of the thermal effects. The amount of dissolution will 
depend on the local reservoir pressure and temperature, but will be 
substantially greater than that experienced if air is used because of the 
higher concentration of carbon dioxide. The water formed will be initially 
in the form of steam which will aid in conveying the heat of combustion 
farther into the reservoir, enhancing the effect of the heat released. 
By using oxygen or a fluid containing a minimum of about 75% by volume pure 
oxygen as the oxidizing medium, the large amount of nitrogen introduced 
into the well when air is used would be eliminated, along with the 
deleterious effect of gas phase nitrogen on the permeability of the liquid 
oil phase. Water injection along with the oxidizing medium after 
combustion is initiated may be used to moderate the high temperature 
generated and to obtain greater distances of penetration into the 
reservoir for more effective heat distribution. It would not necessarily 
add gaseous products to be subsequently produced. 
For the purpose of simplicity in describing the invention, reference 
sometimes will be made herein to only one production well in my in-situ 
combination stimulation process. However, it will be recognized that in 
practical application of the invention, a plurality of such wells may be 
used and in most cases will be utilized. 
In carrying out this invention, an oxidizing gas comprising oxygen or a 
fluid containing a minimum of about 75% by volume pure oxygen is injected 
into a producing well and combustion is initiated in any suitable 
conventional manner such as by locating an electrical or gas-fired heater 
within the well so as to initiate a combustion zone around the well and 
generate combustion gases consisting principally of carbon dioxide and 
water in the form of steam. Continued injection of oxygen moves the 
resulting combustion zone outward into the reservoir and the carbon 
dioxide in the combustion gases dissolves in the reservoir oil reducing 
its viscosity. The heat generated by combustion also lowers the viscosity 
of the reservoir oil surrounding the production well and the steam aids in 
conveying the heat of combustion farther into the reservoir. 
Combustion is continued through the reservoir around the production well 
until the combustion zone advances a radial distance of about 5 to 50 feet 
from the production well. Combustion is then terminated and the production 
well is returned to a producing operation. 
An alternative method of carrying out the invention is to shut in the 
production well after the combustion zone has moved a radial distance of 
about 5 to 50 feet from the production well to allow a soak period in 
which heat generated in the reservoir will distribute itself and also 
allow the carbon dioxide to more effectively dissolve in the heavy oil at 
greater distances from the well thereby lowering its viscosity. For 
optimum results, the length of the soak period will vary depending upon 
the characteristics of the producing well such as depth, rate of 
production, frequency of stimulation periods and size of stimulate 
treatment. After the soak period is terminated, the well is then returned 
to producing operation. 
Another embodiment of this process is to inject water continuously or 
periodically with the oxidizing fluid in the production well after 
combustion is initiated which serves to obtain greater distance of 
penetration of combustion heat into the reservoir for more effective heat 
distribution. The water serves to recuperate the heat stored in the 
burned-out reservoir, which would otherwise be tested. This heat is then 
used to evaporate water. The steam thus formed condenses downstream of the 
combustion zone, where it contributes to further heating of the reservoir. 
This technique is known as wet combustion. As another variation, a 
predetermined amount of water may be injected after oxygen injection has 
been terminated whereupon the water is converted into steam by scavenging 
heat from the high temperature zone created by combustion thereby 
extending the distance into the reservoir that is benefited by the heat of 
combustion. 
The substantial concentration of carbon dioxide produced in the reservoir 
in-situ acts as a local pressurizing agent, a solvent in the oil phase 
lowering the viscosity of the oil, and together with the thermal effects 
of combustion stimulates the reservoir and significantly increases the 
production rate of the oil. 
The oxygen used may be obtained from any type of separation plant capable 
of providing the desired purity. A highly expedient approach is to inject 
oxygen into the production well that may be supplied from cryogenic units 
from which the oxygen in liquid phase is pumped at any desired pressure 
level and thereafter passes through a heat exchanger to vaporize the high 
pressure liquid oxygen. This eliminates the need for compressor and 
attendant equipment. The cryogenic units may be portable and operated at 
the well site. Equally effective is use of oxygen available in the gaseous 
phase which may be compressed with gas compression equipment to the 
pressure level desired for injection into a well. 
The sequence of the process steps including in-situ combustion, reservoir 
soak period, injection of water to propagate heat further into the 
reservoir followed by production may be repeated in each well for a 
plurality of cycles until further recovery of oil is unfavorable. 
The process of my invention may be best understood by referring to FIG. 1, 
in which an oil-containing reservoir 10 is penetrated by a production well 
12 in fluid communication with the entire thickness of the reservoir by 
means of perforations. On the surface, a cryogenic unit 14 for producing 
liquid oxygen from air is positioned near the production well 12. Air is 
introduced into the cryogenic unit 14 through line 16 and the cryogenic 
unit is operated to produce substantially pure liquid oxygen. A suitable 
cryogenic unit is the one disclosed in an article by K. B. Wilson entitled 
"Nitrogen Use In EOR Requires Attention to Potential Hazards", Oil & Gas 
Journal, Vol. 80, No. 42, pp. 105-109, 1982, the disclosure of which is 
hereby incorporated by reference. Liquid oxygen produced by cryogenic unit 
14 flows through line 18 and is pumped by cryogenic pump 20 through a heat 
exchanger 22 via line 24 to vaporize the liquid oxygen. The need to use a 
compressor conventionally used in an in-situ combustion operation is 
eliminated thereby reducing the hazards associated with large scale 
mechanical compressors and also reducing energy costs for compression. 
Vaporized oxygen at a predetermined pressure is introduced into the 
reservoir 10 through open valve 26 and tubing 28 and the oil in the 
reservoir is ignited either by autoignition or by any suitable 
conventional manner such as chemical igniters or heaters. For example, an 
electric heater (not shown) may be positioned in well 12 adjacent the 
perforations establishing communication with the oil-containing reservoir 
10. The heater is an electric heater capable of heating a portion of the 
reservoir immediately adjacent to the production well 12 to a temperature 
sufficient with the oxygen flowing into the well to result in ignition of 
the hydrocarbons in the reservoir 10. The oxygen reacts in the reservoir 
with the hydrocarbons to yield principally gaseous carbon dioxide and 
water in accordance with equation (1) described above. Injection of 
substantially pure oxygen is continued and the resulting combustion front 
30 advances radially through the formation from the well. The heat emitted 
from the in-situ combustion operation lowers the viscosity of the oil and 
the generated carbon dioxide dissolves in the oil also lowering its 
viscosity. 
After the combustion front 30 has advanced a predetermined distance from 
the production well, preferably 5 to 50 feet, injection of oxygen is 
discontinued and the in-situ combustion operation is terminated. 
Thereafter, the valve 26 in tubing 28 is closed and fluids including oil 
are produced through line 32 and opened valve 34. The pressure built up in 
the reservoir 10, particularly the pressure beyond the combustion zone 30 
forces heavy oil reduced in viscosity by heat and dissolved CO.sub.2 into 
the hot burned-over area behind the combustion zone so that the mobilized 
oil passes into production well 12 from which it is produced thru 
production line 32. If desired, after the combustion zone 30 has advanced 
the desired distance from the production well 12 the reservoir 10 is 
allowed to undergo a soak period for a predetermined period of time to 
allow the heat generated to distribute itself and also allow the carbon 
dioxide to more effectively dissolve in the oil thereby lowering its 
viscosity. After the soak period, the well is returned to production. 
Optionally, after in-situ combustion has been established in the reservoir 
10, valves 36 and 38 are opened and water via line 40 is introduced into 
tubing 28 where it is mixed with the oxygen from cryogenic pump 20. The 
water may be periodically injected along with the oxygen. 
If it is desired to reduce the oxygen concentration of the injected gas to 
a predetermined value of at least 75 vol. %, air or an inert gas such as 
nitrogen or carbon dioxide or mixtures thereof is transported via line 42 
through open valve 44, line 40 and open valve 38 into tubing 28 where it 
is comingled with the oxygen from cryogenic pump 20. 
Although the mixture has been described in terms of stimulating a single 
production well, another embodiment of the invention is to conduct the 
in-situ combustion and recovery process in several wells successively. A 
plurality of wells in any selected pattern are operated in the manner 
described one after another. The well pattern may be arranged according to 
any patterns as illustrated in U.S. Pat. No. 3,927,716 to Burdyn et al. In 
this embodiment, while the first well is on in-situ combustion, one (or 
more) of the other wells is prepared for ignition so that it is ready for 
the in-situ combustion phase of the process when the oxidizing fluid is 
cut off from the first well. 
FIG. 2 illustrates one arrangement of wells to which the invention is 
applicable. Central well 46 is surrounded by ring wells 48, 50, 52 and 54. 
Each well penetrates the oil-containing reservoir and is in fluid 
communication with a substantial portion of the reservoir. 
In operation with the well arrangement shown in FIG. 2, an in-situ 
combustion operation is effected in the reservoir surrounding well 46 in 
accordance with the process as previously described and the other wells 
are shut in. After in-situ combustion has been continued for a 
predetermined period of time in well 46, in-situ combustion is terminated 
and well 46 is converted to production. At this point, an in-situ 
combustion operation is effected in one or more of the other wells 48, 50, 
52 and 54 for a predetermined period of time. The various phases of the 
process cycle are operated successively on the various wells in the 
pattern in any selected sequence. The process may be repeated in each well 
for a plurality of cycles until oil production response is unfavorable. 
From the foregoing specification one skilled in the art can readily 
ascertain the essential feature of this invention and without departing 
from the spirit and scope thereof can adopt it to various diverse 
applications.