Stimulation of Oil and Gas Wells With Phosphate Ester Surfactants

1. Field of the Invention

This invention relates to oil and gas production well stimulation where said wells are low-producing or have ceased to produce by primary and secondary recovery means.

2. Description of the Prior Art

Certain phosphate esters have been used in conjunction with water-flood methods of driving oil toward producing wells as a means of secondary recovery of hydrocarbons. The phosphate esters are added to an injection well and perform various functions such as scale inhibition, and the formation of a slug for driving the oil in the formation toward the producing well. The phosphate ester can be injected into the wall either as an aqueous solution or as a soluble oil micro-emulsion. Alkyl and aralkyl polyoxyalkylene phosphates are disclosed as useful surfactants in water-flood secondary recovery processes. Such processes are disclosed in U.S. Pat. Nos. 3,435,898, 3,596,715, and 3,480,083.

Producing oil and gas wells have long been treated to stimulate production thereof utilizing a method termed "acidizing" in which an emulsion of an aqueous mineral acid either alone or in combination with various surfactants, corrosion inhibiting agents, and hydrocarbon oils is added to a producer well. Presumably, such treatments tend to remove deposits from the area of the subterranean oil or gas formation immediately adjacent to the production well bore, thus increasing the permeability of the formation and allowing residual oil or gas to be recovered through the well bore. Another object of such "acidizing" treatment of oil or gas producer wells is the removal of water from the interstices of the formation by the use of a composition which materially lowers the interfacial forces between the water and the oil or gas. Various surface-active agents have been recommended for this use.

Usually, the surface-active agent is injected into the production well in combination with crude oil or other hydrocarbon solvent followed by the injection of additional crude oil to move the treating fluid into the formation. U.S. Pat. Nos. 3,467,194 and 3,470,958 provide specifically for the treatment of a cude oil production well utilizing an oil-external micellar dispersion or a water-external micellar dispersion followed by the injection of a hydrocarbon to move the micellar dispersion outwardly from the well bore into the formation. There is disclosed in U.S. Pat. No. 3,620,303 a method of treating a production well with an aqueous solution of an anionic orthophosphate ester surfactant. The aqueous surfactant solution is retained in the formation at least about 24 hours and thereafter the solubilized hydrocarbon is displaced toward the producer well by fluid drive means utilizing a second well bore which is in fluid contact with the underground oil formation. It is theorized that the surfactant solution forms a soluble oil micro-emulsion with the formation hydrocarbons.

There is no indication in any of the prior art references that the particular phosphate ester surfactants disclosed and claimed herein would be useful in returning to production oil and gas wells which are low-producing or have ceased to produce. The method of the invention is particularly useful in the stimulation of oil and gas wells which have failed to respond to acidizing treatment of the producing well including the use of various acids with various surfactants.

There is disclosed a method of increasing production or returning to production oil and gas wells by the treatment of a production well comprising adding an aqueous solution of an oxyalkylated phosphate ester surfactant and thereafter adding a hydrocarbon to drive the aqueous surfactant solution into the formation a distance of about 3 to about 20 feet from the well bore. The aqueous surfactant solution is allowed to remain in the formation for an effective period generally not less than 24 hours and preferably 24 hours to 1 month. Thereafter, the oil or gas is recovered using pump and/or conventional fluid drive means wherein water is injected into an injection well in communication with said oil or gas subterranean formation.

The oxyalkylated phosphate ester which is an active ingredient in the process disclosed herein for increasing production or returning nonproducing oil and gas wells to productivity is an alkyl or aralkyl polyoxyalkylene phosphate ester surfactant. The surfactant can be used in the free-acid form or as the alkali metal or ammonium salt. The phosphate esters of the invention have the formulas: ##STR1## wherein R.sub.1 represents an alkyl radical having 10 to 18 carbon atoms, for instance, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, uneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, and heptacosyl radicals. R.sub.2 represents an alkyl of about 5 to 27 carbon atoms, e.g., pentyl, heptyl, hexyl, etc., higher alkyls of the same value as R.sub.1, cycloalkyl, e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, methylcyclopentyl, methylcyclohexyl, methylcyclooctyl, methylcyclobutyl, 1,1-dimethylcyclopropyl, ethylcyclopropyl, ethylcyclobutyl, 1-ethyl-2-methylcyclopropyl, 1,1,2-trimethylcyclopropyl, 1,2,3-trimethylcyclopropyl, 1,1-dimethylcyclopentyl, 1,2-dimethylcyclopentyl, 1,3-dimethylcyclopentyl, 1,2-dimethylcyclohexyl, 1,3-dimethylcyclohexyl, 1,4-dimethylcyclohexyl, ethylcyclohexyl, ethylmethylcyclopentyl, 1,1,2-trimethylcyclopentyl, cyclononyl, propylcyclohexyl, 1,2,4-trimethylcyclohexyl, 1,3,5-trimethylcyclohexyl, 1-isopropyl-4-methylcycohexyl, 1,2,4,5-tetramethylcyclohexyl, and radicals derived from mineral oils containing alkyl, cycloalkyl and mixed alkylcycloalkyl radicals having from about 12 to 27 carbon atoms. R.sub.3 and R.sub.4 represent either hydrogen, alkyl of from about 1 to 22 carbon atoms, e.g., methyl ethyl, propyl, butyl, and the higher alkyls defined by R.sub.1 and cycloalkyls defined by R.sub.2 or radicals derived from mineral oils. A represents the residue of ethylene oxide, ethylene oxide and tetrahydrofuran, or mixed lower alkylene oxides selected from the group consisting of ethylene oxide, propylene oxide, and butylene oxide alone or including tetrahydrofuran, wherein the total molecular weight of said ester is about 500 to about 1500, preferably about 600 to about 1200, and wherein A can be heteric or block in molecular configuration. The degree of oxyalkylation is represented by n, x and y are 1 or 2, the sum of x and y is 3, and z is an integer of 0 to 5; X is hydrogen or a monovalent cation selected from the group consisting of at least one of an alkali metal and ammonium. In the formation of the polyhydroxyl oxyalkylene polymer, at least one alkylene oxide or tetrahydrofuran is utilized. Preferably at least 2 alkylene oxides or tetrahydrofuran and one alkylene oxide are utilized. In the formation of said phosphate ester surfactants, alkyl or aralkyl alcohols can be reacted with alkylene oxides in accordance with well known prior art procedures.

The phosphorus acid reactants with which the polyhydroxy oxyalkylene compounds are reacted in the formation of the phosphate esters of the invention can be selected from the group consisting of at least one of phosphorus pentoxide, and polyphosphoric acid. Where the polyhydroxy oxyalkylated compounds are derived from mixtures of lower alkylene oxides or tetrahydrofuran, it is preferred that the compounds be mixtures of ethylene oxide and propylene oxide and that where block or heteric copolymers of these alkylene oxides are formed, it is preferred that the proportion of ethylene oxide be about 10 to about 90 percent by weight and the proportion of propylene oxide be about 90 percent to about 10 percent by weight. These surfactants can be utilized in the form of mono-, di- and mixed mono- and diphosphate esters. Where the surfactants are utilized in the free acid form, the monoesters are characterized by the formulas above in which X is hydrogen.

The aqueous medium which is utilized to form the solution of the phosphate ester surfactant can be soft, brackish, or a brine. Preferably, the water is soft but it can contain small amounts of salts which are compatible with the ions in the subterranean oil or gas formation being treated.

The phosphate ester disclosed has been found superior to the surfactants disclosed in the prior art as useful in oil well stimulation and thus the disclosed phosphate ester surfactant can be utilized as the sole surfactant in preparing the aqueous solution useful in treating the oil or gas production well. Generally, the concentration of said surfactant in the aqueous medium is about 5 to about 50 percent by weight, preferably about 10 to about 20 percent by weight, and most preferably about 12 to about 18 percent by weight. The amount of aqueous surfactant solution utilized in the treatment of an oil or gas well will, of course, vary with the vertical feet of oil-bearing formation around the producing well bore. Generally, there is injected into the oil- or gas-bearing subterranean formation, about 0.5 to about 10 barrels of the aqueous surfactant solution per vertical foot of oil- or gas-bearing formation. There is thereafter added to the well sufficient hydrocarbon such as crude oil into the well bore to displace said aqueous surfactant solution out into the formation. Generally, the aqueous surfactant solution is retained in the formation not less than 24 hours and preferably 24 hours to 1 month. Thereafter, the aqueous solution of said phosphate ester and said hydrocarbon are displaced from the formation by conventional fluid drive means by use of an injection well in fluid communication with the treated production well. The desired oil or gas is thereby produced.

In addition to crude oil having generally the viscosity of the oil-bearing formation of the oil well to be treated, various hydrocarbon solvents are also useful to displace the aqueous solution of surfactant out into the reservoir. Such hydrocarbon solvents as the low molecular weight, generally liquid hydrocarbons boiling below the gasoline range, such as the lower alkanes including butane, propane, pentane, hexane and heptane, as well as natural gasoline, petroleum naphtha and kerosene or mixtures of these hydrocarbons, are useful. Both sweet and sour crude oil is useful as a hydrocarbon to displace the aqueous surfactant solution out into the subterranean reservoir of oil or gas.

The following examples illustrate the various aspects of the invention but are not intended to limit its scope. When not otherwise specified throughout this specification and claims, temperatures are given in degrees centigrade and parts, percentages, and proportions are by weight.

About 4 months after a Texas oil well having 17 feet of perforation, had stopped producing oil, a 15 percent aqueous solution of a mixed mono- and diphosphate ester of a block copolymer of 85 percent by weight ethylene oxide and 15 percent by weight propylene oxide initiated with an aliphatic alcohol having 10 to 12 carbon atoms and a molecular wight of about 900 was pumped into the well and displaced with approximately 25 barrels of crude oil from the well. The well was allowed to stand idle for about 72 hours after which it was found that a pressure head of 60 pounds per square inch was built up. The pressure was insufficient to cause the well to flow so that pumping was started to recover oil. The well produced 37 barrels of oil per day for about 2 weeks. Production then declined to 25 barrels of oil per day where it remained for the rest of the month. Previous to the treatment utilizing the process of the invention, this well had been acidized several times. Each acid treatment failed to bring the desired results in restored production of the well.

A gas well in Southeast Texax, which had been producing 40,000 cubic feet per day, was treated in accordance with the process of the invention utilizing the phosphate ester of Example 1 in the amount of 1 barrel per vertical foot of perforation. The aqueous phosphate ester solution was displaced from the well into the formation with water rather than a hydrocarbon oil and the well was closed for a period of 72 hours. At the end of this time, the well was opened up and production occurred at the rate of 1 million cubic feet of gas per day. This production rate was maintained for over 3 weeks.

While this invention has been described with reference to certain specific embodiments, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of the invention and it will be understood that it is intended to cover all changes and modifications of the invention disclosed herein for the purpsoes of illustration which do not constitute departures from the spirit and scope of the invention.