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 because 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 affecting 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. 2,233,381. 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 or petroleum. Thus, the injected fluid has a higher mobility and tends to finger through the oil thus prematurely breaking through to the production well.

The solution to this high mobility problem is to increase the viscosity of the driving fluid. One 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 increased 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. 3,414,054 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 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 unexpected improvement in oil recovery over the process of Parker.

U.S. Pat. No. 3,700,031 refers to lignosulfonate as a sacrificial agent for alkylaryl sulfonate.

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 lignosulfonate salts in admixture with a chemical oil recovery agent, for example, surfactant, polymer and/or a micellar dispersion thereby displacing oil from the subterranean formation to the surface of the earth. The surfactant solutions useful herein are alkyl benzene alkoxylated sulfonates and/or sulfates, sulfonated or sulfated alkoxylated alkyl surfactants, mixtures of petroleum sulfonates and alkyl benzene alkoxylated sulfonates or sulfates. Also useful are mixtures of petroleum sulfonates and alkoxylated alkyl sulfonates or sulfate or, mixtures of petroleum sulfonates and alkoxylated organic alcohols or organic alcohols and petroleum sulfonates alone. 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 therebehind. A sacrificial agent performs best when it exhibits high adsorption on active sites of rock surfaces, and thus diminishes surfactant and/or polymer adosrption. Chemical compounds of polyelectrolytic nature have the proper physico chemical and structural requirements to behave as successful sacrificial agents. The functional groups on the sacrificial agent molecules enhance 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 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 site of the formation rock. By adsorption sites of the formation rock 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.

Typical examples of suitable polymers include polyacrylamides having from about 0 to about 75% of the amide groups hydrolyzed to carboxylate groups. It is preferred to have from about 0 to 30% of the amide group hydrolyzed to carboxylate groups. Especially preferred is a polyacrylamide of greater than 6 million molecular weight and approximately 5% of the amide groups hydrolyzed. Polysaccharides are also polymers useful in my invention. For example, a commercially available material is Kelzan MF (a xanthan gum produced by the action of the bacterium Xanthomonas campestris). Modifications of polysaccharides to increase their ionic character will make them even more useful in my invention.

Another type of polymer useful in my invention are water soluble starch derivatives containing carboxyl sulfonate or sulfate groups in the form of sodium or ammonium salts. Other useful polymers include but are not limited to soluble cellulose derivatives, polyvinyl alcohol, polyvinyl pyrrolidone, poly(acrylic acid), poly(ethylene oxide), polyethyleneimine and colloidal silica. This listing is not exhaustive. Various other polymers may be used without departing from the scope and spirit of my invention.

The amount of polymeric material 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 polymer generally will range from about 0.1 to 2 pore volumes of an aqueous polymer solution having dissolved therein from about 0.01 to about 0.5 percent by weight of the polymer itself.

In my invention, the sacrificial agent is injected ahead of or in admixture with the polymeric material. A surfactant may be mixed with the sacrificial agent and/or the polymer.

The surfactants useful in my invention comprise an aqueous solution of alkyl benzene alkoxylated sulfonates or sulfates having the following general formulae: ##STR1## where R.sub.1 and R.sub.2 are hydrogen or alkyl with at least one being an alkyl group of from 6 to 20 carbon atoms,

x is a number from 1 to 10,

M.sup.+ is a cation selected from the group consisting of sodium, potassium, lithium, and ammonium,

R.sub.3 is either --CH.sub.3 or hydrogen.

Other surfactants useful in my invention comprise an aqueous solution of sulfonated or sulfated alkoxylated alkyl surfactants having one of the following general formulae: ##STR2## where

R.sub.1 is an alkyl group of from 8 to 22 carbon atoms,

R.sub.2 is either CH.sub.3 of hydrogen, and

x is a number from 1 to 10.

Other useful surfactants comprise an aqueous solution of a mixture of petroleum sulfonates and solubilizers or formulas I-IV above.

Also, petroleum sulfonate or a mixture of petroleum sulfonate and a water soluble organic alcohol or a mixture of petroleum sulfonate and an organic alcohol which has been alkoxylated and which displays amphiphilic properties are useful.

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.

One embodiment of my invention is the use of an aqueous solution of modified lignosulfonate in a slug ahead of or in admixture with an aqueous solution of polymer used to drive oil from an injection well to a production well.

In another embodiment of this invention, a sacrificial material comprising 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 followed by a solution of polymer. By injecting the sacrificial material and surfactant together followed by polymer oil recovery is maximized.

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 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 about 50,000 or more. They are economically attractive since being byproducts 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 lignosulfonates 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 Kirth-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 80's. 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.

Modified sulfonated alkali lignins and sulfite lignosulfonates, such as those with ring sulfomethylation, oxidation, ethoxylation, formaldehyde condensation, phenolation, and/or carboxylation are also useful as sacrificial agents. Lignosulfonates having degrees of sulfonation from about 2.0 to saturation are acceptable for 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.

The quantity of sacrificial lignosulfonate materials 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 chemical recovery agent 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 sacrificial lignosulfonate salts. At a maximum, only the amount of sulfonate salts needed to completely occupy the active sites on the formation is needed. The detriment resulting from using excess lignosulfonate salt would be an increase in the cost of operating the oil recovery program.

The amount of sacrificial 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.

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.

(When tradenames are used, their chemical composition is defined at Table X following). Various lignosulfonates and derivates were tested to determine whether they will improve the efficiency of oil recovery of a surfactant system from a pack.

Crushed Slaughter cores were used in limestone packs to study the ability of lignosulfonates to improve the efficiency of surfactant systems to recovery tertiary oil from packs. The packs had water permeabilities of about 17 darcies and porosities about 0.5. The floods were performed under similar conditions using an injection rate equivalent to a frontal advance of about 15 ft/day.

The packs were initially saturated with field water (Mallet water, 70,000 ppm Total Dissolved Solids) and with Slaughter crude oil. They were then water flooded with Mallet water to a residual oil saturation. The surfactant slug was then injected followed by a solution of polymer. After polymer injection water flooding was resumed until completion of the flood. The results of the tests are summarized in Tables I, II, III, IV and V. Table X gives a chemical description of materials used. In the discussion, the tradenames are used for convenience.

Different types of lignosulfonates were incorporated in a dual surfactant system (0.4% Sulframin 1240+0.6% Sulfonated Surfonic N-40) at 1% concentration (Table I) and the systems were injected in crushed limestone packs. Tertiary oil recovery of Slaughter crude was determined at 77.degree. F. The water composition of simulated Mallet produced water used is given in Table V. The surfactant system was injected as a slug (25% P.V.) and was followed by a slug (50% P.V.) of 0.1% Kelzan MF polymer solution which was driven through by simulated Mallet produced water.

The lignosulfonates listed in Table I contributed to additional tertiary oil recovery, which indicated that they can be utilized as sacrificial agents in chemical recovery systems. The selection of the best candidate among the lignosulfonates described in Table VI will also depend on the salinity and hardness of water encountered in a particular formation, since these lignosulfonates have different salt tolerance.

Three lignosulfonates were tested as sacrificial agents in chemical recovery floods, where the salinity and hardness of brine used were very high (Table II). The composition of simulated Bob Slaughter Block connate water used is given in Table VII. Tertiary oil recovery was determined at 77.degree. F. The single surfactant system (1% Alipal CO-436+1% lignosulfonate) was injected as a slug (20% P.V.) in a crushed limestone pack saturated with Slaughter crude, and was followed by a slug (50% P.V.) of 0.1% Kelzan MF polymer solution which was driven through by simulated Bob Slaughter Block connate water. All three lignosulfonates contributed to additional tertiary oil recovery.

Two types of sulfonated alkali lignins were tested in chemical recovery systems in crushed limestone packs saturated with Slaughter crude at Mallet water salinity, and their performance has been tabulated in Table III.

The sulfonated alkali lignins were incorporated in a single surfactant system [1% sodium dodecyl phenol polyoxyethylene (4 moles) sulfonate] at 1% concentration, and the systems were injected in crushed limestone packs. The tertiary oil recovery of Slaughter crude was determined at 77.degree. F. The surfactant system was injected as a slug (24% P.V.) and was followed by a slug (50% P.V.) of 0.1% Kelzan MF polymer solution, which was driven through by simulated Mallet produced water. Both sulfonated alkali lignins contributed to additional tertiary oil recovery, which indicated that they can be utilized as sacrificial agents equally well as sulfite lignosulfonates in chemical recovery systems. The selection of the proper lignin sulfonate as sacrificial agent will depend on the salinity and hardness of water encountered in a particular formation.

The mineral nature and characteristics of rock surfaces in a porous medium play a major role in tertiary oil recovery process when surfactant flooding is employed. In Table IV, clayey sandstone packs were used to determine tertiary oil recovery by a chemical recovery system. The flooding process applied and the conditions in the packs were the same as those given in Table I. It was observed that both the flood containing no lignosulfonate and the ones containing lignosulfonates recovered less tertiary oil than the corresponding ones performed in crushed limestone packs. However, as before, the presence of lignosulfonate sacrificial agents increased oil recovery. The presence of clays contributed to lower tertiary oil recovery.

Table V demonstrates the improvement in oil recovery obtained by flooding with a mixture of lignosulfonates and surfactants in an aqueous solution over surfactant solutions alone as well as a preflush of lignosulfonate followed by surfactant each with polymer injection.

In Case 1, a surfactant solution was injected into a previously water flooded limestone pack. This surfactant solution was followed by a polymer and the polymer was followed by brine water. The tertiary oil recovery by this method was 41 percent. The oil cut, that is, the largest fraction of oil produced with the water, was 22 percent. In Case 2, a solution of lignosulfonates was used as a preflush and was followed by a solution of the same surfactant as used in Case 1. This surfactant was followed by the same polymer used in Case 1 and the polymer was followed by water as in Case 1. In Case 2, the tertiary oil recovery was 55 percent and the oil cut was 27 percent.

In Case 3, the surfactant and lignosulfonate were mixed together and injected as a mixture. This mixture was followed by the same polymer used in the previously cases and the polymer was followec by water as in the previous cases. The tertiary oil recovery for Case 3 was 79 percent and the oil cut was 30 percent. Thus, the unexpected improvement of my invention is demonstrated.

All parameters were controlled as closely as possible to be constant throughout each flood. The crushed limestone packs were made to be as similar as possible and, of course, the same fluids were used from case to case where applicable.

Table VI demonstrates the effect of Lignosite 458 on the oil recovery capability of a surfactant system comprising two petroleum sulfonates and a solubilizer, which is nonyl phenol ethoxylated with 6 moles of ethylene oxide and sulfonated to the sodium salt followed by polymer. It is noted that the tertiary recovery when the lignosulfonate sacrificial agent is used is 76.9% whereas without the sacrificial agent the tertiary is 62.1%.

Table VII shows the results of a series of comparative runs wherein various surfactants followed by polymer were used in floods in laboratory cores. It is noted that in every case the addition of the lignosulfonate sacrificial agent improved the oil recovery.