Stimulation of oil and gas wells with phosphate ester surfactants

Crude oil and gas production wells, which no longer produce oil or gas utilizing conventional primary and secondary means of recovery, can be returned to production by treatment comprising addition of an aqueous solution of certain phosphate ester surfactants followed by successive treatment with a hydrocarbon. Increased production also can be obtained in low-producing oil and gas wells by similar treatment with certain phosphate ester surfactants. Subsequent to treatment of a producer well, the phosphate ester surfactant is forced into the formation utilizing a hydrocarbon and said surfactant and hydrocarbon mixture is allowed to remain in the producing well, for an effective period of time ranging from not less than 24 hours to one month. Thereafter, pumping and/or conventional fluid drive means are then utilized to recover oil or gas from the subterranean oil or gas formation. The amount of aqueous surfactant solution and hydrocarbon utilized are sufficent to permeate the oil or gas subterranian formation in the area immediately adjacent to the producing well bore and up to a radius therefrom of about 20 feet.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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.

EXAMPLE 1 
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. 
EXAMPLE 2 
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.