Method of improving conformance in steam floods with steam foaming agents

Abstract

The disclosed invention is a group of steam foaming agents for injection with steam and a non-condensable gas to decrease permeability in steam swept zones and increase oil recovery. The foaming agents have the general formula:RO(R'O).sub.n R"SO.sub.3 Mwhere R is an alkyl radical, branched or linear, or an alkylbenzene, alkyltoluene or alkylxylene group, having from about 8 to about 24 carbon atoms in the alkyl chain, R' is ethyl, propyl or a mixture of ethyl and propyl, n has an average value of about 1 to about 20, R" is ethyl, propyl, hydroxypropyl or butyl and M is an alkali metal or ammonium ion.

Description

This application is related to concurrently filed, copending U.S. patent application Ser. No. 638,918, filed Aug. 8, 1984, and Ser. No. 638,920, filed Aug. 8, 1984, both of which are incorporated herein by reference. The present invention relates to a group of steam foaming agents which when injected with steam and a non-condensable gas will significantly improve vertical conformance.

When an oil reservoir is subjected to steam injection, steam tends to move up in the formation, whereas condensate and oil tends to move down due to the density difference between the fluids. Gradually, a steam override condition develops, in which the injected steam sweeps the upper portion of the formation but leaves the lower portion untouched. Injected steam will tend to follow the path of least resistance from the injection well to a production well. Thus, areas of high permeability will receive more and more of the injected steam which further raises the permeability of such areas. This phenomenon exists to an even larger degree with low injections rates and thick formations. The steam override problem worsens at greater radial distances from the injection well because steam flux decreases with increasing steam zone radius.

Although residual oil saturation in the steam swept region can be as low as 10%, the average residual oil saturation in the formation remains much higher due to poor vertical conformance. Thus it is because of the creation of steam override zones that vertical conformance in steam floods is usually poor.

It has long been the concern of the oil industry to improve the vertical conformance of a steam flood by reducing the permeability of the steam swept zone by various means. The injection of numerous chemicals such as foams, foaming solutions, gelling solutions or plugging or precipitating solutions have been tried. Because of the danger of damaging the reservoir, it is considered important to have a non-permanent means of lowering permeability in the steam override zones. For this reason, certain plugging agents are not deemed acceptable. In order to successfully divert steam and improve vertical conformance, the injected chemical should be (1) stable at high steam temperatures of about 300.degree. to about 600.degree. F., (2) effective in reducing permeability in steam swept zones, (3) non-damaging to the oil reservoir and (4) economical.

The literature is replete with references to various foaming agents which are employed to lower permeability in steam swept zones. The foaming agents of the prior art require the injection of a non-condensable gas to generate the foam in conjunction with the injection of steam and the foaming agent. U.S. Pat. Nos. 3,366,175 and 3,376,924 disclose the injection of a steam foam in a hydrocarbon reservoir at the interface between the hydrocarbons and the gas cap to aid in recovery. U.S. Pat. Nos. 3,410,344 and 3,994,345 disclose the use of a steam foaming agent selected from the generic groups of polyethoxyalkanols and alkylaryl sulfonates to reduce permeability in steam channels. The use of similar surfactants such as sodium lauryl sulfoacetate and alkyl polyethylene oxide sulfate are disclosed as foaming agents in carbon dioxide foams in U.S. Pat. Nos. 4,088,190 and 4,113,011, respectively. U.S. Pat. No. 4,018,278 discloses the use of sulfonated, ethoxylated alcohols or alkylphenols in surfactant flooding solutions without the use of steam.

Several trademarked foaming agents have been field tested by petroleum companies in steam floods. These include such trademarked chemicals as Stepanflo 30 sold by Stepan Chemical Co., Suntech IV sold by Sun Oil, Thermophoam BWD sold by Farbest Co. and COR-180 sold by Chemical Oil Recovery Co. U.S. Pat. No. 4,086,964 discloses the use of lignin sulfonates for a foaming agent and Pat. No. 4,393,937 discloses the use of alpha olefin sulfonates as a steam foaming agent. See also United Kingdom Pat. No. 2,095,309 for a disclosure of alpha olefin sulfonate foaming agents.

Disclosures of laboratory and field tests of Stepanflo are contained in SPE Paper No. 10774 entitled "The Laboratory Development and Field Testing of Steam/Noncondensible Gas Foams for Mobility Control in Heavy Oil Recovery" by Richard E. Dilgren et al. presented at the 1982 California Regional Meeting of the SPE held in San Francisco Mar. 25-26, 1982 and the Journal of Petroleum Technology, July 1982, page 1535 et seq. The same Journal of Petroleum Technology also discusses tests conducted on Thermophoam BWD. Additional information on tests of Thermophoam BWD are also disclosed in Department of Energy Publications DOE/SF-10761-1, -2 and -3.

Tests of the COR-180 foaming agent of Chemical Oil Recovery Co. are disclosed in SPE Paper No. 11806 entitled "Improvement in Sweep Efficiencies in Thermal Oil-Recovery Projects Through The Application of In-Situ Foams" by R. L. Eson, presented at the International Symposium on Oil Field and Geothermal Chemistry in Denver, June 1-3, 1983 and Department of Energy Reports Nos. DOE/SF/10762-1, -2 and -3.

SUMMARY OF THE INVENTION

The present invention pertains to novel steam foaming agents for injection with steam and a non-condensable gas to decrease permeability in steam swept zones and ultimately, increase oil recovery with steam flooding. The novel foaming agents have the general formula:

RO(R'O).sub.n R"SO.sub.3.sup.- M.sup.+, where R is an alkyl radical, branched or linear, or an alkylbenzene, alkyltoluene or alkylxylene group, having from about 8 to about 24 carbon atoms in the alkyl chain, R' is ethyl, propyl or a mixture of ethyl and propyl, n has an average value of about 1 to about 20, R" is ethyl, propyl, hydroxypropyl or butyl and M is an alkali metal or ammonium ion.

DETAILED DESCRIPTION

The steam foaming agents for the present invention are highly effective in reducing permeability of steam swept zones. These novel foaming agents have an affinity for formation areas of high permeability and low oil saturation. When set up in such areas, they substantially reduce the permeability of the steam swept zones, forcing steam into other, unswept areas of the formation.

The novel steam foaming agents of the present invention are represented by the general chemical formula:

RO(R'O).sub.n R"SO.sub.3.sup.- M.sup.+, where R is an alkyl radical, branched or linear, or an alkylbenzene, alkyltoluene or alkylxylene group having from about 8 to about 24 carbon atoms, preferably about 12 to about 20 carbon atoms in the alkyl chain, R' is ethyl, propyl or a mixture of ethyl and propyl, preferably ethyl, n has an average value of about 1 to about 20, preferably about 2 to about 5, R" is ethyl, propyl, hydroxypropyl or butyl and M is an alkali metal or ammonium cation. The preferred alkali metal ions are sodium, lithium and potassium. It should be noted that n is an average value and that the invention compounds will normally have varying degrees of ethoxylation.

These novel foaming agents are stable at the high temperatures encountered in steam floods (300.degree. to 600.degree. F.). They are not permanent and do not damage the reservoir.

The four most preferred foaming agents are four alcohols herein labeled Agents 2, 3, 4, and 5 which correspond to the example numbers. Although the preferred agents illustrated below have linear saturated alkyl radicals for the R substituent, it should be noted that the saturated alkyl radical may also be branched. The chemical formulas of the most preferred foaming agents are: ##STR1##

The mixture of steam, non-condensable gas and foaming agent can bring about a substantial reduction in permeability when injected into the high permeability areas of the formation at almost any time. Vertical conformance will be significantly improved whether the steam foaming agent and gas is injected into the formation at the very beginning of steam injection, before steam breakthrough at the production well or after steam breakthrough. The most preferred injection times occur shortly after steam injection has begun and a short time before steam breakthrough will occur at the production wells. In the first instance, the injection of the steam foaming agent and gas near the beginning of steam injection will help prevent narrow steam channels from being formed and extended through to the production wells. The injection of the foaming agent prior to steam breakthrough will postpone the time of steam breakthrough and spread the steam over a wider area near the production wells.

The invention foaming agents will also work quite well if steam breakthrough occurred in the past and low oil saturation steam override zones exist. But in such a situation, the foaming agent must be injected in larger concentrations and greater quantities to reduce permeability in a frequently swept area. Generally, the mixture of steam, gas and foaming agent must be injected into the reservoir at a higher pressure than the previous injection of steam so that the foam will move a sufficient distance into the high permeability areas. However, the injection pressure must be less than the reservoir fracturing pressure or damage to the formation will occur.

The foaming agent and gas may be injected into the formation without the concurrent injection of steam, provided that steam is injected into the formation prior to and after the injection of the foaming agent and gas. But preferably, the steam is coinjected with the foaming agent and gas.

The foaming agent, non-condensable gas and steam should be injected in a mixture such that the foaming agent comprises about 0.01% to about 5% preferably about 0.02% to about 1.5% by weight of the steam (cold water equivalent). The foaming mixture contains about 0.01 to about 5, preferably about 0.01 to about 1.5 thousand standard cubic feet of a non-condensable gas per barrel of steam (cold water equivalent). It is contemplated that the injected steam range from about 20% to about 90% quality. A higher concentration of foaming agent is generally necessary if the region has been swept by steam for a considerable time. But individual tests should be run to tailor the concentration of the foaming agent in steam as the increased effectiveness of the foaming agent per increased concentration of foaming agent quickly reaches a point of diminishing returns. Furthermore, other surfactants may also be included in the steam and foaming agent mixture to increase oil recovery providing they do not substantially inhibit the foam.

In general, the non-condensable gas used in the foam mixture of the present invention can comprise substantially any gas which (a) undergoes little or no condensation at the temperatures and pressures at which the foam mixture is subjected, and (b) is substantially inert to and compatible with the foaming agent and other components of that mixture. Such a gas is preferably nitrogen but can comprise other substantially inert gases, such as air, ethane, methane, flue gas, fuel gas, or the like.

Two conditions will be prevalent in a steam override zone, especially in a well developed override zone. The steam flux in the override zone will be high relative to other portions of the reservoir because the vast majority of the steam will be passing through the override zone. In addition, the residual oil saturation in the override zone will be relatively low due to continuous steam flooding.

One of the most desirable attributes of a good steam foaming agent is that it will work best in a low oil saturation area and at a high steam flux. The novel foaming agents of the present invention offer these substantial advantages with their affinity for the areas of low oil saturation and high steam flux, as is indicated in the Examples, particularly Examples 23-26.

No chemical or physical deterioration has been detected in the foaming agents used in the formation at steam injection temperatures. Additionally, no problems have been encountered with thermal or hydrolytic stability of the agents. The foams tested have continued to be effective up to three days but since the foaming agent will be eventually produced, it is generally necessary to continue injecting foam into the high permeability areas. Cooling problems also fail to affect foam stability. This is because the foam will go preferentially into the high permeability areas of the steam override which are very hot. The cool areas of the formation are those areas of low permeability which the foam will avoid.

The following examples will further illustrate the novel steam foaming agents of the present invention. These examples are given by way of illustration and not as limitations on the scope of the invention. Thus, it should be understood that the composition and concentration of the foaming agents may be varied to achieve similar results within the scope of the invention.

EXAMPLES 1-12

Multiple runs were made in a foam testing apparatus to determine the effectiveness of the foaming agents of the present invention and several well known steam foaming agents when injected with steam in the absence of a non-condensable gas. A 90 cm linear cell with an inside diameter of 3.4 cm was packed with sand, oil and water to a porosity of 0.4 and an oil saturation of 0.2. The sand pack was steam flooded at 4 ml per minute of cold water equivalent until no more oil was produced. The pressure drop across the cell length with the steam injection only was measured and determined to be approximately 10 psig in Example 1. A high concentration of 6% active foaming agent (corresponding to about a 1% in situ concentration in the aqueous phase in the cell) was then injected with steam at approximately 0.5 ml per minute. The pressure drop was recorded continuously. When the pressure drop stabilized, nitrogen was injected at 8.4 ml/min. The nitrogen injection rate was then increased to higher levels when the pressure drop restabilized.

Examples 2 and 3 show pressure drops across the cell of some 12 to 18 times the pressure drop experienced with injection of steam alone without the injection of any nitrogen. The effectiveness of these two foaming agents, Agents 2 and 3 increased even more with the injection of 8.4 and 16.8 ml/min of nitrogen. Along with the foaming agents of Examples 4 and 5, Agents 4 and 5, these foaming agents were more than twice as effective as the other well known agents tested. Another foaming agent of the present invention, Agent 6 of Example 6 performed as well as the tested prior art foaming agents.

TABLE I______________________________________ Pressure Drop Across Cell (psig) at N.sub.2 Flow Rates Of 0 ml/ 8.4 ml/ 16.8 ml/ 25.2 ml/Ex. Foaming Agent min min min min______________________________________1 Steam Only 102 Agent 2 120 312 3553 Agent 3 185 250 3604 Agent 4 10 260 2805 Agent 5 2956 Agent 6 1297 Agent 7 10 15 16 1268 Stepanflo 30 10 25 25 409 Stepanflo 1390 10 134 17810 Thermophoam BWD 10 64 69 14411 Bioterge AS-40 10 13012 Suntech IV 10 141 139 143______________________________________ Stepanflo 30 and 1390 trademarked alpha olefin sulfonates sold by Stepan Chemical Co. Thermophoam BWD a trademarked alpha olefin sulfonate sold by Farbest Co. Bioterge AS40 a trademarked alpha olefin sulfonate sold by Stepan Chemical Co. Suntech IV a trademarked sulfonate sold by Sun Oil Co.

To determine a lower limit on the nitrogen flow rate, it was felt that a minimum increase in pressure drop over the cell to assure effective operation would be about ten times the pressure drop with injection of steam only. Hence, the lower limit of nitrogen flow is the flow rate that would cause the minimum desired 100 psig pressure drop across the cell. The results of Table I were linearly interpolated to obtain the nitrogen flow rate that would yield a 100 psig pressure drop with a 6% (corresponding to about a 1% in situ concentration) injection of foaming agent. It should be noted that these are only rough estimates and that the behavior of these foaming agents is not entirely linear.

TABLE II______________________________________ Lower Limit N.sub.2 Flow RateExample Foaming Agent ml/min. MSCF/bbl Steam______________________________________2 Agent 2 0 03 Agent 3 0 04 Agent 4 3.0 0.187 Agent 7 23.2 1.388 Stepanflo 30 40.0 2.389 Stepanflo 1390 6.1 0.3610 Thermophoam BWD 20.3 1.2111 Bioterge AS-40 6.3 0.3712 Suntech IV 5.8 0.35______________________________________

EXAMPLES 13-20

Further experiments were conducted with the foam testing apparatus to transform the pressure drop figures into more readily identifiable numbers of percent oil recovery. The procedure of Examples 1-12 was followed and the foam mixture was injected with 6% (corresponding to about a 1% in situ concentration) foaming agent and 16.8 ml/min of nitrogen. The three agents of the present invention that were tested in Examples 18-20 offered commanding oil recovery efficiency advantages over the foaming agents of the prior art.

TABLE III______________________________________Example Foaming Agent .DELTA.P (psig) % Oil Recovery______________________________________13 Thermophoam BWD 69 28.314 Siponate 301-10 32 31.015 Stepanflo 20 19 40.216 Igepal CA 720 11 4.117 COR-180 12 24.318 Agent 4 280 83.719 Agent 3 360 76.320 Agent 2 335 56.7______________________________________ Siponate 30110 a trademarked alpha olefin sulfonate sold by Alcolac Co. Stepanflo 20 a trademarked alpha olefin sulfonate sold by Stepan Chemica Co. Igepal CA 720 a trademarked alkyl phenoxy polyoxyethylene ethanol sold b GAF Corp. COR180 trademarked oxyethylene sulfates sold by Chemical Oil Recovery Co

EXAMPLES 21-24

To determine the minimum effective chemical concentration, additional tests were performed with varying concentrations of foaming Agent 3. Table IV shows the results of these tests. Assuming a minimum 100 psi pressure drop, Table IV can be interpolated to yield a minimum Agent 3 concentration of about 0.26% barrel of chemical per barrel of steam. It should be noted that no substantial improvements in foam efficiency were noted with Agent 3 concentrations over about 0.4% per barrel of steam.

TABLE IV______________________________________Foaming Agent Concentration, %.sup.+ Average .DELTA.pEx. In Cell (Injected) kPa (psi)______________________________________21 1.2* (6) 1076 (156)22 0.6 (3) 1048 (152)23 0.4 (2) 1034 (150)24 0.2 (1) 552 (80)______________________________________ *Assume average steam quality 0.50. .sup.+bbl of 100% active foaming agent per bbl steam.

For comparison purposes, Table V has been compiled to present the foaming agent concentration and nitrogen amounts used in recent field tests of foaming agents.

TABLE V______________________________________ bbl Agent MSCF N.sub.2Foaming Agent Field bbl Steam bbl Steam______________________________________Suntech IV Kern River 0.714% 0.078COR-180 Witmer B2-3 0.104% 0Thermophoam San Ardo 0.090% 0.06BWDThermophoam Midway-Sunset 0.060% 0.014BWDStepanflo 30 Kern-River Mecca 0.500% 0.0207______________________________________

It is apparent that these field tests were conducted with extremely low concentrations of foaming agent and relatively small quantities of injected nitrogen per barrel of steam. Although it is probable that larger quantities of foaming agent and nitrogen could have been more effective, the economics of field tests, even on a small scale, require the use of the smallest quantities practicable. The economics become even more critical for large, field-wide applications.

It should be remembered that the foaming agents of the present invention performed substantially better than the agents of the prior art at higher concentrations in laboratory tests. These novel foaming agents would probably be employed at concentrations similar to those of Table V in field tests.

EXAMPLES 25-28

Examples 25 and 26 were run in a cell two meters in length with an inside diameter of 11.4 cm. The cell was packed with sand and water to a porosity of 0.4 and an oil saturation of 0. The sandpack was steam flooded to bring the cell up to steam temperature at a steam flux of 0.096 ft/min. A solution of 6% of Agent 3 was injected at 0.1 ml/min until a steady state was reached. The injection of Agent 3 was continued with the injection of 4.2 ml/min. of nitrogen and a steady state pressure drop of 14.0 psig was measured across the cell length. The steam flux rate was then increased to 0.192 ft/min. which gave a pressure differential of 150 psig over the length of the cell, indicating that the foam offered little resistance to a low steam flow rate, but offered substantial resistance to a high steam flux.

______________________________________Ex. 25 0.096 ft/min. steam flux gave .DELTA.P of 14.0 psig.Ex. 26 0.192 ft/min. steam flux gave .DELTA.P of 150 psig.______________________________________

Examples 27 and 28 were run in the 90 cm sandpack of Examples 1-12 with a porosity of 0.4. Two runs were made with a steam flux of 1.8 ft/min., 6% of foaming agent Agent 3 with a flow rate of 0.5 ml/min. and a nitrogen injection rate of 8.4 ml/min. The cell of Example 27 had an initial oil saturation of 20% and an average oil saturation of 15% when steady state was reached with a .DELTA.P of 250 psig. The second cell of Example 28 contained no oil and had a .DELTA.P of 390 psig during flooding.

______________________________________Ex. 27 S.sub.o of 15% had a .DELTA.P of 250 psig.Ex. 28 S.sub.o of 0% had a .DELTA.P of 390 psig.______________________________________

From the above examples, it can be seen that the novel foaming agents of the present invention offer substantially increased resistance under the conditions of relatively higher steam flow rates and lower oil saturation that exist in steam override zones.

EXAMPLES 29-31

Tests were also conducted in a dual pack apparatus prepared as per Examples 1-12, wherein two 90 cm linear sandpacks were connected in parallel so that the mixture of steam and foaming agent would have an equal opportunity to travel into and through each of the two sandpacks. One of the cells was a high oil saturation pack having an oil saturation of approximately 35% and the second cell was a low oil saturation sandpack having an oil saturation of about 20% to simulate a steam override zone.

When steam was injected, the vast majority of steam passed through the low oil saturation pack having higher permeability. But as indicated in Table VI, when the steam was injected with the foaming agent Agent 2 at a concentration of about 6%; most of the injected steam went through the low permeability high oil saturation sandpack.

TABLE VI______________________________________ % Of Injected Steam Through Each Cell Low Oil High OilExample Saturation Saturation______________________________________29 Steam Injection Only 71.7% 28.3%30 Steam and Agent 2 41.5% 58.5%31 Steam and Agent 3 15.4% 84.6%______________________________________

Many other variations and modifications may be made in the concepts described above by those skilled in the art without departing from the concepts of the present invention. Accordingly, it should be clearly understood that the concepts disclosed in the description are illustrative only and are not intended as limitations on the scope of the invention.

Claims

1. A process for recovering hydrocarbons from an underground hydrocarbon formation penetrated by at least one injection well and at least one production well producing a water cut of at 90%, which comprises:

injecting steam into an injection well;

injecting into the injection well a mixture of steam, about 0.01 to about 5 thousand standard cubic feet of a non-condensable gas per barrel of steam in the injected mixture and about 0.01% to about 5% by weight of a foaming agent based upon the weight of the steam in the injected mixture,

said foaming agent represented by the formula,

RO(R'O).sub.n R"SO.sub.3.sup.- M.sup.+,

where R is an alkyl radical, branched or linear, or an alkylbenzene, alkyltoluene or alkylxylene group having from about 8 to about 24 carbon atoms in the alkyl chain, R' is ethyl, propyl or a mixture of ethyl and propyl, n has an average value of about 1 to about 20, R" is ethyl, propyl, hydroxypropyl or butyl, and M.sup.+ is an alkali metal or ammonium ion;

injecting steam into said injection well; and

recovering hydrocarbons and other fluids from a well.

2. The process of claim 1 for recovering hydrocarbons, wherein said mixture is injected into a steam override zone.

3. The process of claim 1 for recovering hydrocarbons, wherein R is an alkyl radical having about 16 to about 18 carbon atoms, R' is ethyl, n has an average value of about 4, R" is propyl and M.sup.+ is sodium.

4. The process of claim 1 for recovering hydrocarbons, wherein R is an alkyl radical having about 16 to about 20 carbon atoms, R' is ethyl, n has an average value of about 3, R" is propyl and M.sup.+ is sodium.

5. The process of claim 1 for recovering hydrocarbons, wherein R is an alkyl radical having about 12 to about 14 carbon atoms, R' is ethyl, n has an average value of about 2 to about 4, R" is propyl and M.sup.+ is sodium.

6. The process of claim 1 for recovering hydrocarbons, wherein the concentration of said foaming agent in steam is increased as the steam injection rate is increased.

7. The process of claim 1 for recovering hydrocarbons, wherein the non-condensable gas is selected from the group consisting of nitrogen, carbon dioxide, air, methane, ethane and flue gas.

8. The process of claim 1 for recovering hydrocarbons, wherein said mixture is injected into an injection well near the beginning of steam injection.

9. The process of claim 1 for recovering hydrocarbons, wherein said mixture is injected into an injection well immediately prior to steam breakthrough at a production well.

10. The process of claim 1 for recovering hydrocarbons, wherein said mixture is injected into an injection well after steam breakthrough at a production well.

11. The process of claim 1 for recovering hydrocarbons, wherein the non-condensable gas is injected at a rate of about 0.01 to about 1.5 thousand standard cubic feet per barrel of steam in the injected mixture.

12. A process for recovering hydrocarbons from an underground hydrocarbon formation penetrated by at least one injection well and at least one production well producing a water cut of at 90%, which comprises:

injecting into an injection well a mixture of steam, about 0.01 to about 5 thousand standard cubic feet of a non-condensable gas per barrel of steam in the injected mixture and about 0.01% to about 5% by weight of a foaming agent based upon the weight of the steam in the injected mixture,

said foaming agent represented by the formula,

RO(R'O).sub.n R"SO.sub.3.sup.- M.sup.+,

where R is an alkyl radical, branched or linear, or an alkylbenzene, alkyltoluene or alkylxylene group having from about 8 to about 24 carbon atoms in the alkyl chain, R' is ethyl, propyl or a mixture of ethyl and propyl, n has an average value of about 1 to about 20, R" is ethyl, propyl, hydroxypropyl or butyl, and M.sup.+ is an alkali metal or ammonium ion;

injecting steam into said injection well; and

recovering hydrocarbons and other fluids from a well.

13. A process for recovering hydrocarbons from an underground hydrocarbon formation penetrated by at least one injection well and at least one production well, which comprises:

injecting steam into an injection well;

injecting into said injection well a mixture of steam, about 0.01 to about 1.5 thousand standard cubic feet of nitrogen per barrel of steam in the injected mixture and about 0.02% to about 1.5% by weight of a foaming agent based upon the weight of the steam in the injected mixture,

said foaming agent represented by the formula ##STR2## injecting steam into said injection well; and recovering hydrocarbons and other fluids from a well.