Method for preventing degradation and deterioration of vulcanized rubber material

A method of preventing degradation of vulcanized rubber materials used in the operation of oil and gas wells comprising forming said rubber material from a sulfur vulcanized composition comprising a copolymer rubber having in the polymer chain (I) 10 to 60% by weight of a unit from an unsaturated nitrile, (II) not more than 10% by weight of a unit from a conjugated diene, and (III) 30 to 90% by weight of a unit from an ethylenically unsaturated monomer other than the unsaturated nitrile and/or a unit resulting from the hydrogenation of a unit from a conjugated diene, said rubber being vulcanized with a sulfur vulcanization system.

BACKGROUND OF THE INVENTION 
This invention relates to a rubber material vulcanized with a sulfur 
vulcanization system, which has excellent oil resistance and antidegrading 
properties and is particularly adapted for use in contact with casing head 
gas and/or crude oil. 
Today, more than 75% of world energy supply relies on crude oils and 
natural gases recovered from wells. In spite of the estimated future 
increase of energy consumption, however, it has become difficult to 
discover new oil or gas wells. Efforts have therefore been made to 
discover new wells, and attempts have been made at the same time to 
increase the yield of crude oils or natural gases from the wells now in 
operation. 
To recover crude oils or natural gases, drilling of wells is first 
performed. To increase the drilling efficiency at this time, various 
improved drilling oils, drilling fluids, etc. are used. Then, an iron pipe 
called a casing pipe is inserted from above the ground into a bare hole 
with a diameter of 20 cm to about 60 cm formed in the stratum by drilling. 
The well is completed by using cement, rubber materials, etc. so as to 
enable oil recovery. Crude oil is recovered from the completed well by 
spontaneous flowing, pumping, etc. 
However, rocks, mud and sand from the drilled stratum, and the drilling oil 
or fluid used during the drilling operation deposit and build up near the 
well, and this may markedly hamper recovery of crude oil. In such a case, 
measures are taken to increase oil producing ability by, for example, 
adding a liquid obtained by mixing kerosene, light oil or diesel oil with 
an acid, an alkali, chlorine or various surface-active agents to the well 
in advance to dissolve or disperse the sediment, etc. and discharging them 
out of the well. 
When the crude oil has difficulty of flowing into the well because of 
asphalt or wax components deposited near the well as a result of long-term 
operation, or because of the high viscosity of the crude oil in the oil 
reservoir, it is the practice to force steam into the well to dissolve the 
deposits or decrease the viscosity of the crude oil. 
The ratio of recovery of crude oil from the oil reservoir is very low if 
the flow of the crude oil into the well is effected only by natural 
energy. Thus, in order to increase the yield of oil, various treatments 
have been performed. For example, water is forced into the oil reservoir 
to increase the pressure of the oil reservoir, or steam is added to 
increase the temperature of the oil reservoir. Or CO.sub.2 or LPG is 
forced into the crude oil to reduce its viscosity, or the surface tension 
between oil and another material is reduced. As a result, the inside of 
the well has very high pressures and temperatures (usually at least 
150.degree. C.). Crude oils or natural gases recovered from the wells 
contain large amounts of corrosive CO.sub.2 and sulfur compounds such as 
H.sub.2 S. 
Accordingly, an environment with which rubber materials used for well 
completion make contact is very severe. 
An acrylonitrile/butadiene copolymer rubber (to be referred to as NBR) has 
previously been used in large amounts as rubber materials for wells (oil 
and gas wells). Rubber materials composed of NBR, however, become useless 
within a short period of time under severe high-temperature, high-pressure 
environmental conditions by the action of crude oil, hydrocarbon gases, 
H.sub.2 S, CO.sub.2, acids, alkalies, etc. Fluoro-rubber proposed as a 
material intended for removing the defect of NBR has low mechanical 
properties such as tensile strength and tear strength, and does not have 
sufficient resistance to the action of the aforesaid substances. For this 
reason, it has been desired to develop new rubber materials. 
Rubber materials used in wells are required not only to have high 
mechanical strength and resistance to crude oils, but also have resistance 
to additive materials used in various treatments performed to increase the 
yield of oil, H.sub.2 S, CO.sub.2, or a mixture of these, and resistance 
to heat. If they lack a balance of mechanical strength, resistance to the 
aforesaid substances and heat resistance, they do not function as rubber 
materials. Ideally, such rubber materials can be used until the well dries 
up. In practice, however, they have to be exchanged every time they become 
degraded. Since this requires extra work for exchanging and for 
interruption of production owing to exchanging, it gives rise to an 
economic problem. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a rubber material which has a 
well-balanced combination of mechanical strength, resistance to the 
various substances described above and heat resistance and is particularly 
adapted for use in contact with crude oils or gases in oil and gas wells. 
The present inventors have made extensive investigations in order to 
achieve this object. These investigations have led to the surprising 
discovery that a rubber material having a very good balance of the above 
properties can be obtained by vulcanizing an unsaturated 
nitrile/conjugated diene copolymer rubber composed of specified 
proportions of (I) an unsaturated nitrile unit, (II) a conjugated diene 
unit and (III) a saturated unit, or a partially hydrogenated product 
thereof, with a sulfur vulcanization system. 
The rubber material in accordance with this invention is used in contact 
with crude oils from oil or gas wells, natural gases called casing head 
gas, corrosive gases such as sulfur compounds (e.g., H.sub.2 S and 
SO.sub.2), steam and CO.sub.2, mixtures of these gases, and various 
additives used in well treatment. It is an excellent rubber material 
having a well-balanced combination of mechanical strength, heat 
resistance, resistance to corrosive gases of an oil well, crude oil 
resistance, and resistance to treating additives. 
The characteristic feature of the invention lies in the fact that the 
specified rubber material described in detail hereinbelow is vulcanized 
with a sulfur vulcanization system. If an organic peroxide is used as a 
vulcanizer, a rubber material having the aforesaid properties in good 
balance cannot be obtained. 
As stated hereinabove, mineral oils such as kerosene, light oil and diesel 
oil, with or without acids such as hydrochloric acid, acetic acid, 
sulfuric acid or nitric acid for dissolving reservoir rocks, are usually 
added during treatment of wells. There are also added alkalies such as 
sodium hydroxide and potassium hydroxide in order to neutralize the 
carboxyl and sulfo groups in the crude oil; metal chlorides such as sodium 
chloride and calcium chloride and other metal compounds to adjust 
viscosity; surface active agents such as sodium alkyl-arylsulfonates, 
fatty acids, and sulfates of alcohols to improve oil penetration; and 
acetamide, aliphatic amines and quaternary ammonium salts to prevent 
corrosion of the iron pipes. Various other materials are added. These 
materials, either individually or synergistically, affect the rubber 
materials used. 
The entire rubber material of this invention or that surface of the rubber 
material of this invention which is to make contact with the aforesaid 
crude oil, casing head gas, etc. is formed of the unsaturated 
nitrile-conjugated diene copolymer specified in the present invention. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The unsaturated nitrile-conjugated diene polymer rubber used in this 
invention is a copolymer rubber which has in the polymer chain (I) a unit 
from an unsaturated nitrile, (II) a unit from a conjugated diene and (III) 
a unit from an ethylenically unsaturated monomer other than the 
unsaturated nitrile and/or a unit resulting from the hydrogenation of a 
unit from a conjugated diene. In the rubber, the content of the unit (I) 
from the unsaturated nitrile is 10 to 60% by weight. If it is less than 
10% by weight, the oil resistance of the rubber is inferior. If it exceeds 
60% by weight, the cold resistance of the rubber is reduced. The preferred 
content of the unit (I) is 20 to 50% by weight. The content of the unit 
(II) from the conjugated diene in the rubber is not more than 10% by 
weight. If it exceeds 10% by weight, the rubber has inferior resistance to 
degradation under the action of heat, corrosive gases or treating 
additives. The preferred content of the unit (II) is 0.5 to 8% by weight, 
especially 1 to 6% by weight. If it is less than 0.5% by weight, sulfur 
vulcanization is difficult, and the properties of the resulting rubber 
material are not entirely satisfactory. The content of the unit (II) may 
be 0% by weight when it is desired to obtain a rubber containing a monomer 
other than the conjugated diene as a crosslinking unit. The content of the 
unit (III) from an ethylenically unsaturated monomer other than the 
unsaturated nitrile and/or a unit resulting from the hydrogenation of a 
unit from a conjugated diene is 30 to 90% by weight. 
The copolymer rubbers used in this invention include (1) a polymer rubber 
obtained by partially hydrogenating the conjugated diene unit of an 
unsaturated nitrile/conjugated diene copolymer rubber, (2) a polymer 
rubber obtained by partially hydrogenating the conjugated diene unit of an 
unsaturated nitrile/conjugated diene/ethylenically unsaturated monomer 
copolymer rubber, (3) an unsaturated nitrile/conjugated 
diene/ethylenically unsaturated monomer copolymer rubber, (4) an 
unsaturated nitrile/ethylenically unsaturated monomer copolymer rubber, 
etc. 
Monomers for the production of the copolymer rubbers used in this invention 
are unsaturated nitriles such as acrylonitrile and methacrylonitrile; 
conjugated dienes such as butadiene, isoprene and 1,3-pentadiene; 
ethylenically unsaturated monomers including unsaturated carboxylic acids 
such as acrylic acid, methacrylic acid, itaconic acid and maleic acid; 
alkyl esters of carboxylic acids, such as methyl acrylate, 2-ethylhexyl 
acrylate and octyl acrylate; alkoxyalkyl acrylates such as methoxyethyl 
acrylate, ethoxyethyl acrylate, and methoxyethoxyethyl acrylate; allyl 
glycidyl ether and vinyl chloroacetate; ethylene; butene-1; and 
isobutylene. In the production of an unsaturated nitrile/ethylenically 
unsaturated monomer copolymer rubber, a part of the unsaturated monomer 
may be replaced by a non-conjugated diene such as vinyl norbornene, 
dicyclopentadiene or 1,3-hexadiene. 
Specific examples of the copolymer rubber used in this invention include 
hydrogenation products of a butadiene/acrylonitrile copolymer rubber, an 
isoprene/acrylonitrile copolymer rubber, and a 
butadiene/isoprene/acrylonitrile copolymer rubber; a butadiene/methyl 
acrylate/acrylonitrile copolymer rubber and a butadiene/acrylic 
acid/acrylonitrile copolymer rubber, and hydrogenation products of these 
copolymer rubbers; a butadiene/ethylene/acrylonitrile copolymer rubber; a 
butyl acrylate/ethoxyethyl acrylate/vinyl chloroacetate/acrylonitrile 
copolymer rubber; and a butyl acrylate/ethoxyethyl acrylate/vinyl 
norbornene/acrylonitrile copolymer rubber. 
The rubber material of this invention should be one obtained by vulcanizing 
with a sulfur vulcanization system the aforesaid copolymer rubber or a 
blend of it with an amount of another rubber which does not affect the 
essence of the present invention. Vulcanization with an organic peroxide 
cannot achieve the object of this invention. The sulfur vulcanization 
system in this invention is a combination of a vulcanizer, i.e. sulfur 
and/or a sulfur-donating compound, with various vulcanization aids and 
vulcanization accelerators. 
Examples of the sulfur donating compound are thiuram-type compounds such as 
tetramethylthiuram disulfide, tetraethylthiuram disulfide and 
dipentamethylenethiuram tetrasulfide; morpholine-type compounds such as 
morpholine disulfide and 2-(4-morpholinodithio)benzothiazole; and 
compounds which yield active free sulfur under the action of heat, such as 
selenium dimethyldithiocarbamate, selenium diethyldithiocarbamate, 
alkylphenol disulfides and aliphatic polysulfide polymers. The 
vulcanization aids and accelerators may be those which provide a 
sufficient vulcanized state when used in combination with sulfur and/or 
the sulfur donating compound. Those which are usually employed in the 
rubber industry can be used in this invention without limitation. Examples 
of the vulcanization aids are metal oxides such as zinc oxide and 
magnesium oxide, stearic acid, oleic acid, and zinc stearate. Examples of 
the vulcanization accelerators are guanidine-type accelerators such as 
diphenylguanidine; thiazole-type accelerators such as 
mercaptobenzothiazole and dibenzothiazyl disulfide; sulfenamide-type 
accelerators such as N-cyclohexyl-2-benzothiazyl sulfenamide and 
N,N'-dicyclohexyl-2-benzothiazylsulfenamide; thiuram-type accelerators 
such as tetramethylthiuram monosulfide and tetramethylthiuram disulfide; 
and dithioacid salt-type accelerators such as dizinc 
dimethyldithiocarbamate. 
The rubber material of this invention may, as desired, contain reinforcing 
agents, fillers, plasticizers, antioxidants, and other compounding agents 
normally used in the rubber industry. 
The rubber used in this invention is mixed with the sulfur vulcanization 
system and other compounding ingredients and kneaded by an ordinary mixer 
to form a rubber stock. The rubber stock is molded, either as such or 
after inserting a metallic reinforcing layer or a fiber cord reinforcing 
layer therein, into a desired shape such as a sheet, hose, tube or belt by 
ordinary molding machines, and then subjected to vulcanization means such 
as press vulcanization or can vulcanization. Thus, the rubber material of 
this invention is produced. 
In order to achieve the object of this invention, the rubber material of 
this invention should be in an optimum vulcanized state in which its 
tensile strength is nearly at a maximum and its initial tensile strength 
in a heat aging test does not markedly increase as a result of secondary 
vulcanization proceeding during use. There is no restriction on the 
vulcanization conditions, such as the amounts of the vulcanizer and the 
vulcanization accelerator, the vulcanization temperature and the 
vulcanization time, for providing an optimum vulcanized state. When the 
optimum vulcanized state cannot be obtained under ordinary vulcanizing 
conditions, secondary vulcanization may be performed to obtain it. 
Specifically, the rubber material of this invention may, for example, be 
various packers for use in obtaining impressions in wells, cables for oil 
pumps, sealing materials such as O-rings and B.O.P. (Blowout-Preventor), 
hoses, belts, and diaphragms. It can be in the form of any articles which 
are used in contact with casing head gas and/or crude oils and/or treating 
additives. 
The following examples illustrate the present invention more specifically.