Sulfomethylated Lignosulfonates as Additives in Oil Recovery Processes Involving Chemical Recovery Agents

1. Field of the Invention

This invention relates to the recovery of oil from subterranean formations by chemical flooding methods.

2. Description of the Prior Art

Petroleum is frequently recovered from subterranean formations or reservoirs by permitting the natural energy of the reservoir to push the petroleum up through wells to the surface of the earth. These processes are referred to as primary recovery methods since they use the natural energy of the reservoir. However, a large amount of oil, generally in the range of 65-90% or more, is left in the subterranean formation at the conclusion of the primary recovery program. When the natural reservoir energy is unable to produce more petroleum, it is a common practice to resort to some form of supplemental recovery technique in order to recover additional petroleum left in the subterranean formation. These supplemental operations are normally referred to as secondary recovery operations. If this supplemental recovery operation is the second in a series of such operations, it will be referred to as a tertiary recovery operation. However, the terminology is unimportant for the purposes of this application and relates only to the sequence in which they are carried out.

The most widely used supplemental recovery technique beacuse of its ease of implementation and low capital outlay is water flooding through injection wells drilled into the subterranean formation. In a water flooding operation, the injected fluid displaces oil through the formation to be produced from the injection well. A major disadvantage to water flooding, however, is its relatively poor displacement efficiency largely due to the fact that water and oil are immiscible at reservoir conditions and high interfacial tension exists between the flood water and the oil. For this reason, after a water flood, a large portion of the oil is still left unrecovered in the reservoir.

It has been recognized by those skilled in the art that a solution effecting a reduction in this interfacial tension between water and oil would provide a much more efficient recovery mechanism. Therefore, the inclusion of a surface active agent or surfactant in the flood water was recognized as an acceptable technique for promoting displacement efficiency of the reservoir oil by the water. For example, U.S. Pat. No. 3,468,377 discloses the use of petroleum sulfonates in water flooding operations and U.S. Pat. No. 3,553,130 discloses the use of ethylene oxide adducts of alkyl phenols for the same purpose. The use in water flooding operations of water soluble surface active alkaline earth resistant polyglycol ethers is disclosed in U.S. Pat. No. 2,333,381. Other specialized surfactants, as will be discussed later, have been discovered to have special properties useful in water flooding operations such as a tolerance for high salinity and calcium, and/or magnesium ion concentrations often found in reservoir waters.

However, field operations employing surfactants and surface active agents in injected fluid have not always been entirely satisfactory due to the fact that these materials are often adsorbed by the formation rock to a relatively high degree, resulting in an ever declining concentration of the materials as they progress through the reservoir. Therefore, large concentrations of surface active materials have heretofore been necessary to maintain a sufficient concentration at the oil-water interface. Due to this, many proposed flooding operations involving surface active materials have been uneconomical.

Another serious problem for any recovery technique involving the driving of oil with a fluid is premature breakthrough of the injection fluid. This premature breakthrough indicates that the reservoir has not been adequately swept of oil. The problem is often described in terms of sweep efficiency as distinguished from the displacement efficiency described above. Displacement efficiency involves a microscopic pore by pore efficiency by which water displaces oil, whereas sweep efficiency is related to the growth portion of the reservoir which is swept and unswept by the injected fluid. A major cause of poor sweep efficiency is associated with the fact that the injected fluid generally has a lower viscosity than the displaced fluid (petroleum). Thus, the injected fluid has a higher mobility and tends to finger through the oil, prematurely breaking through to the production well.

One solution to this high mobility problem is to increase the viscosity of the driving fluid. A way to do this is to add polymeric organic materials to a driving water which has the effect of increasing the viscosity of the water, thereby increasing the sweep efficiency of the supplemental recovery process. U.S. Pat. No. 3,039,529 and U.S. Pat. No. 3,282,337 teach the use of aqueous polyacrylamide solutions to increase the viscosity of the injected fluid thereby promoting increase sweep efficiency. Polysaccharides as taught in U.S. Pat. No. 3,581,824 have been used for the same purpose. These polymers are quite expensive and any polymer lost to adsorption on the reservoir matrix adds substantially to the cost since additional polymer is required to maintain a given viscosity.

The above described problems have been recognized by those skilled in the art of oil recovery and certain sacrificial compounds have been added to pretreat the formation in order to decrease the adsorption of subsequently injected surfactants and/or polymers. For example, U.S. Pat. No. 3,424,054 discloses the use of aqueous solutions of pyridine; U.S. Pat. No. 3,469,630 discloses the use of sodium carbonate and inorganic polyphosphates, and U.S. Pat. No. 3,437,141 discloses the use of soluble carbonates, inorganic polyphosphates and sodium borate in combination with saline solution of a surfactant having both a high and a low molecular weight component. These materials have not been completely satisfactory from a standpoint of performance and economics however.

U.S. Pat. No. 3,384,171 to Parker discloses that unmodified lignosulfonates may be used as a preflush followed by a surfactant solution. While this method provides an improvement over using surfactant alone, my invention provides an improvement in oil recovery over the process of Parker. My copending application Ser. No. 715,957 filed Aug. 19, 1976 claims unmodified lignosulfonates as sacrificial agents. The present invention provides an improvement over that process.

The invention is a process of producing petroleum from subterranean formation having an injection well and a production well in communication therewith. The process comprises injecting into the formation via the injection well an aqueous solution of sulfomethylated lignosulfonate salts in admixture with a chemical oil recovery agent, for example, surfactant, polymer and/or a micellar dispersion. It is the usual practice to then inject a fluid such as water to sweep the chemical components through the reservoir to the production well, thereby displacing oil from the subterranean formation to the surface of the earth.

A sacrificial material is injected by the process of this invention through an injection means comprising one or more injection wells into a subterranean petroleum-containing formation to preferably occupy or cover all potential adsorption sites of the rock within the subterranean formation thereby reducing the extent of adsorption of the more expensive chemical oil recovery agent injected therewith. A sacrificial agent performs best when it exhibits high adsorption on active sites of rock surfaces, and thus diminishes surfactant and/or polymer adsorption. Chemical compounds of polyelectrolytic nature have the proper physico chemical and structural requirements to behave as successful sacrificial agents. The functional group on the sacrificial agent molecules enhances adsorption either by hydrogen bonding or electrostatic attraction to active sites on the rock surfaces.

A satisfactory sacrificial material has at least three important characteristics. First, it should be less expensive than the surfactant on a cost effectiveness basis since it is to be sacrificed or adsorbed by the formation, probably not to be recovered. Next, it must be adsorbed readily by the subterranean formation matrix. Preferably the sacrificial material should be adsorbed more readily than the chemical oil recovery agent to be used in the process. The third important characteristic of a sacrificial agent is that the presence of such adsorbed sacrificial material should retard or eliminate adsorption of the surfactant and/or polymer chemical recovery material on the adsorption sites of the formation rock. By adsorption sites of the formation rock it is meant those parts of the surfaces of the pores of the formation rock capable of adsorbing a chemical compound from a solution on contact.

The sacrificial material may not have a detrimental effect on the recovery efficiency of the chemical flooding operation. Additional oil is usually recovered only if the sacrificial material is followed by or is admixed with a surfactant and/or a polymer chemical recovery agent which will effectively increase the amount of oil displaced from the subterranean formation. When the surfactant is chosen as the chemical recovery agent it should be injected in admixture with the sacrificial agent for best results and ahead of the following flooding water thereby achieving the desired interfacial tension reduction between the injected fluid and the displaced fluid with minimal loss of surfactant on the formation matrix. The surfactant may be present in a hydrocarbon solvent or in an aqueous solution or in a combination thereof. Any anionic, nonionic and/or cationic type of surfactant known in the art may be used in the practice of this invention. Some types of surfactants were mentioned previously. In addition, surfactants disclosed and claimed in the following U.S. Patents, for example, are particularly useful since they have been found to be capable of performing in reservoirs having both high salinities and high hardness levels: U.S. Pat. Nos. 3,858,656; 3,811,505; 3,811,504 and 3,811,507.

Likewise, the amount of surfactant which must be employed in the practice of any chemical flood is generally known in the art and is to be found in published literature. However, the slug size of surfactant generally will range from about 0.01 to 1 pore volumes of an aqueous surfactant solution having dissolved therein from about 0.01 to about 10.0 percent by weight of the surfactant itself.

In my invention the sacrificial agent is preferrably injected in admixture with the surfactant slug into the petroleum formation. This surfactant/sacrificial agent mixture may or may not be preceded by a slug of sacrificial material in aqueous solution only. It has been found that this technique is superior to the preflush method of injecting a slug of sacrificial material followed by a slug of surfactant solution without sacrificial material. However, the preflush method is superior to using no sacrificial material at all.

In any of these embodiments and others which are obvious to those skilled in the art, the surfactant containing slug may be followed by a material to taper the viscosity before drive water is injected. This technique known well to those skilled in the art prevents the water from fingering into the more viscous surfactant containing slug.

In a specific embodiment of this invention, a sacrificial material comprising sulfomethylated lignosulfonate salts, is injected via the suitable injection means, i.e. through one or more injection wells completed in the subterranean hydrocarbon formation, in admixture with a surfactant solution. By injecting the sacrificial material and surfactant together oil recovery is maximized. The oil recovery is greater than preflushing the formation with sacrificial agent and followed with surfactant solution. Another embodiment of my invention is the use of modified lignosulfonates in conjunction with an emulsion of water, hydrocarbon and surfactant, i.e. a micellar dispersion. The same parameters as discussed above for simple aqueous surfactant solutions would apply to micellar dispersions. Micellar dispersions are known in the art. See, for example, U.S. Pat. No. 3,536,136 incorporated here by reference.

The sacrificial agents useful in the process of my invention are sulfomethylated lignosulfonate salts. Lignosulfonates are anionic polyelectrolytes soluble in water and tolerate hard water (polyvalent ions, e.g. calcium and magnesium). They are also thermally stable in formations where the temperature is high. Lignosulfonates are macromolecules built up by complex condensation of phenyl propane units. The sulfonate groups are attached to the aliphatic side chain, mainly to alpha carbon. Lignosulfonates are water soluble with molecular weights ranging from several thousand to around 50,000 or more. They are economically attractive since being by-products of the pulping industry, they are plentiful and cost less than either the surfactants or the polymers used in enhanced oil recovery methods. The polyelectrolyte lignosulfonates with strongly ionized sulfonate groups are negatively charged species and have a tendency to adsorb on solid surfaces thereby imparting a negative charge to them. The rock surfaces of a reservoir treated with lignosulfonate will be inert towards the anionic surfactants in the flood water and therefore loss of surfactants to the rock surfaces will be kept to a minimum. The same phenomenon will occur with the polymer thickened drive fluid.

Lignin is second only to cellulose as the principal constituent in wood. Generally, lignin is a complex phenolic polyether containing many different functional groups including carboxyls, carbonyls, and alcoholic and phenolic hydroxyls. Lignins and their derivatives are described in Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Vol. 12, beginning at page 362. This publication describes two very broad classes of lignin derivatives: sulfite lignins and alkali lignins.

The difference in the lignins exists because of the method of extraction of lignin material from woody materials. Sulfonated alkali lignins are readily available commercially from various sources including but not limited to West Virginia Pulp and Paper Company under the trade name REAX. Their general method of preparation is described in the Encyclopedia of Chemical Technology referred to above. Briefly, sulfonated alkali lignins are prepared by cooking woodchips with a 10% solution of a mixture of sodium hydroxide with about 20 mole percent of sodium sulfide. The lignin with wood is modified into a sodium compound often termed sodium lignate or alkali lignin which is very soluble in the strongly alkaline solution. These alkali lignins are removed from solution by lowering the pH which precipitates out the alkali lignins. These unsulfonated alkali lignins are sold under various tradenames including INDULIN. These alkali lignins are used to prepare the sulfonated derivatives. Methods of sulfonation are known by those skilled in the art. One typical method involves treating the alkali lignins with a solution of alkali sulfites at elevated temperature and pressure. The degree of sulfonation may be controlled to provide a variety of sulfonated alkali lignins.

The other main type of lignin derivatives are called sulfite lignins or sulfite lignosulfonates. Sulfite lignins are generally made by cooking woodchips under pressure in a solution of sulfurous acid and calcium, magnesium, sodium or ammonium bisulfite. This process converts insoluble lignins to soluble lignosulfonic acid. The lignosulfonic acids or calcium, magnesium, sodium or ammonium salts of the lignosulfonic acids are available under various tradenames including MARASPERSE, LIGNOSITE, ORZAN, TORANIL, and RAYFLO.

The broad term lignosulfonates used herein refers to both sulfonated alkali lignins and sulfite lignosulfonates (sulfite lignins). These are distinct types of compounds as explained above. Since the alkali lignins require sulfonation after extraction of the material from woody products it is proper to call them sulfonated alkali lignins. Likewise since sulfite lignins emerge from the extraction process already sulfonated it is proper to refer to this class of materials as sulfite lignins or sulfite lignosulfonates.

My invention is the use of sulfonated alkali lignins and sulfite lignosulfonates, each modified by sulfomethylation. Lignosulfonates having degrees of sulfonation from about 2.0 to saturation are acceptable as starting materials for the sulfomethylated compounds of my invention. Cations which are acceptable include Na.sup.+, K.sup.+, NH.sub.4.sup.+, Ca.sup.++, and Mg.sup.++. The degree of sulfonation is the weight percentage of sulfonate (SO.sub.3.sup.-) compared to the total molecular weight.

Crude unmodified lignosulfonates may be made with ether softwoods or hardwoods. Although having basically the same functional groups the crude unmodified softwood lignosulfonates have more sulfonate and hydroxyl groups than the crude unmodified hardwood lignosulfonates. Thus, in general, crude unmodified softwood lignosulfonates have better hard water (Ca.sup.++, Mg.sup.++) tolerance than the hardwood form. However, sulfomethylated hardwood lignosulfonates are much improved in hard water tolerance due to the additional sulfonate groups present in the finished product.

Sulfomethylation of lignosulfonates resulting in sulfomethylated lignosulfonates may take place by various routes known to those skilled in the art. My invention is not limited to the use of products made by any specific route. Any route which adds the methylene sulfonate radical is acceptable. In one route, formaldehyde and a bisulfite salt is reacted with crude unmodified lignosulfonates under basic conditions to yield a product with additional sulfonate groups attached to the lignosulfonate through methylene groups. For example, sodium, potassium, calcium or magnesium bisulfites are useful. It is believed that these methylene sulfonate groups attach predominantly at the ortho sites of aryl groups in the lignosulfonate although some methylene sulfonate groups attach at the meta site.

The quantity of sulfomethylated lignosulfonates to be injected into the subterranean hydrocarbon formation may be any amount up to and including an amount sufficient to occupy substantially all of the active sites of the formation matrix. If less than the maximum amount is used, there will be a corresponding increase in the adsorption of surfactant from injection solution onto the formation matrix although the amount of increase will not be as great as in the case where the formation is completely free of sulfomethylated lignosulfonate salts. At a maximum, only the amount of sulfomethylated lignosulfonate salts needed to completely occupy the active sites on the formation matrix is needed. The detriment resulting from using excess sulfomethylated lignosulfonate salts would be an increase in the cost of operating the oil recovery program.

The amount of sulfomethylated lignosulfonate salts needed in the process of the invention depends on the particular formation, the area or pattern to be swept and other formation characteristics. Those skilled in the art can determine the exact quantity needed to afford the desired amount of protection.

Generally it has been found that the amount of sulfomethylated lignosulfonate in the surfactant slug will be effective in amounts of from about 0.01 to about 10.0 percent by weight of the total surfactant solution (including the aqueous portion). Total sulfomethylated lignosulfonate will be effective at the above concentrations in amounts ranging from 0.01 to 1.0 pore volumes of the aqueous solution of surfactant-sacrificial agent or only sacrificial agent solution.

The effectiveness of this invention for reducing the adsorption of surfactant or polymer on formation rock and chemical flooding operations is demonstrated by the following examples which are presented by way of illustration and are not intended as limiting the spirit and scope of the invention as defined in the claims.

The following Examples demonstrate the performance of modified lignosulfonates in oil recovery from crushed limestone packs or sand packs in the laboratory.

The starting crude lignosulfonates were either hardwood of softwood products. The calcium salt was converted to sodium salt to evaluate the effect of this modification step. Lignosulfonates were modified by performing reactions such as carboxylation by oxidation, carboxylation by chloroacetic acid, sulfomethylation and carboxylation by reaction with carbon dioxide. Each reaction yields products with different functional groups. The types of functional groups obtained by each reaction are additive, and those affect both the tolerance of lignosulfonates to brine and the affinity of lignosulfonates to adsorb on surfaces of interest. The choice of an adsorbent to be used in the evaluation of sacrificial agents is important. For accurate evaluation, one must utilize adsorbents which allow high surfactant adsorption.

To compare the effectiveness of various lignosulfonates as sacrificial agents the single surfactant system sulfonated four mole ethylene oxide adduct of dodecyl phenol was used. Shake bottle tests were performed to evaluate the mitigation of surfactant adsorption to precipitated CaCO.sub.3 at 77 kg/m.sup.3 TDS and 43.degree. C. Adsorption values of Adduct D-40 CS* with or without lignosulfonates present are tabulated below:

Results indicate that the softwood lignosulfonate derivatives exhibit greater mitigation of surfactant adsorption than the hardwood lignosulfonate derivatives when CaCO.sub.3 is used as adsorbent. Conversion from Ca salt to Na salt has a moderate beneficial effect on the effectiveness of the product. Improved tertiary oil recovery efficiency achieved when lignosulfonates are incorporated into surfactant systems may not be solely due to a sacrificial agent effect. In addition to mitigation of surfactant adsorption on pore surfaces, other mechanisms may also contribute to an improved oil recovery efficiency of a surfactant system.

Modified lignosulfonates exhibit high sacrificial agent activity when exposed to calcium carbonate surfaces in shake bottle tests. Four different reactions have been employed to obtain derivatives of lignosulfonates.

(A) Oxidation reaction

(B) Chloroacetic acid reaction

(C) Sulfomethylation reaction, and

(D) Carbon dioxide reaction.

Modified lignosulfonates with several types of functional groups can be prepared by employing any one of the above reactions or combinations of these reactions. The availability of modified lignosulfonates with different types of functional groups is desirable because the more functional groups lignosulfonates have, the greater tolerance to higher salinity and hardness brines. The presence of different types of functional groups (both polar and nonpolar) on lignosulfonates should contribute to the attractive forces on substrates and enhance the sacrificial agent activity of these compounds.

The surfactant system (as in Example I) (with or without lignosulfonates) was used to determine relative sacrificial agent effectiveness of lignosulfonates on precipitated calcium carbonate. Shake bottle tests were performed to evaluate surfactant adsorption mitigation at 77 kg/m.sup.3 TDS and 43.degree. C. Several adsorption values of Adduct D-40CS with or without lignosulfonates present are tabulated below:

It is observed that the oxidized and carboxylated lignosulfonates are more effective sacrificial agents here than the sulfomethylated and carboxylated ones. This is true for both the hardwood and softwood derivatives. It is to be noted that the softwood modified lignosulfonates exhibit greater surfactant adsorption inhibition on CaCO.sub.3 than the hardwood modified lignosulfonates. This is probably due to the fact that the softwood degree of sulfonation is higher than the hardwood degree of sulfonation.

Since various structural forms of modified lignosulfonates are available, it is possible to prepare modified lignosulfonates which should exhibit high surfactant adsorption mitigation for a given surfactant system on a substrate of interest such as limestone, sandstone and others.

Derivatives of lignosulfonates exhibit varying degrees of sacrificial agent activity when calcium carbonate is used as substrate. Oxidized and carboxylated lignosulfonates have been tested for surfactant adsorption mitigation. In this investigation sandstone (Dog Lake EE sand) was used to determine how structural differences of modified lignosulfonates affect their performance as sacrificial agents on sandstone.

The surfactant (as in Examples I and II) was used in surfactant systems where the salinity was maintained at 77 kg/m.sup.3 TDS and the temperature at 43.degree. C. Quantities of 50 g of sandstone were exposed to 50 cc surfactant solutions to achieve large enough surfactant concentration changes for accurate adsorption value determinations. Bottles containing the mixtures were shaken for 24 hours and then they were equilibrated for 3 days prior to centrifugation, which was done to effectively separate the adsorbent from the equilibrium solutions. Several adsorption values of systems composed of 10 kg/m.sup.3 (1%, w/v) surfactant and 10 kg/m.sup.3 (1%, w/v) lignosulfonate are tabulated below:

The adsorption tests indicated that both the oxidized and the oxidized and carboxylated lignosulfonates have greater effectiveness as sacrificial agents on sandstone substrates than the unmodified lignosulfonate. Results also indicate that both the oxidized and the oxidized and carboxylated hardwood lignosulfonates are more effective than the corresponding softwood lignosulfonates as sacrificial agents where sandstone is used as adsorbent. For CaCO.sub.3 as adsorbent oxidized or oxidized and carboxylated softwood lignosulfonates were more effective sacrificial agents than oxidized or oxidized and carboxylated hardwood lignosulfonates.

The differences in performance between the hardwood and softwood derivatives can possibly be explained on the basis of their chemical structures. Softwood lignosulfonates are predominantly of the guaiacyl structure, whereas hardwood lignosulfonates have both the guaiacyl and the syringyl structures. The major feature of the oxidation reaction is the conversion of methoxyl groups (inactive) to phenolic groups (active). Thus, oxidized hardwood lignosulfonates have more phenolic groups because the syringyl structure has twice as many methoxyl groups than the guaiacyl structure. Hydrogen bonding between the phenolic groups of the oxidized lignosulfonates and the oxygens of the sandstone surfaces is a probable mechanism contributing to the enhanced adsorption of oxidized lignosulfonates on sandstone. It should be emphasized that electrostatic attraction between the anionic functional groups (sulfonates and carboxylates) on the modified lignosulfonates and sandstone surfaces also contributes to the adsorption of these lignosulfonates.

The following examples (IV-VIII) demonstrate the performance of modified lignosulfonates in oil recovery from crushed limestone packs or sand packs in the laboratory. In those tests the packs were saturated with oil and then water flooded to a residual oil saturation. Then the surfactant solutions were injected followed by polymer solutions.