Completion and workover fluids

A completion and workover fluid is described, containing a soluble salt, a hydroxyethyl cellulose and at least one compound selected from 2-mercaptobenzimidazole compounds, 2-mercaptobenzothiazole compounds, 2-mercaptothiazoline and 2-thioimidazolidone.

FIELD OF THE INVENTION 
This invention relates to completion and workover fluids. More 
particularly, this invention relates to improved completion and workover 
fluids which enable sand or other deposits in the bottom of a well to be 
smoothly cleaned up to the ground surface by the prevention of a lowering 
of their rheological properties under high temperature conditions. 
BACKGROUND OF THE INVENTION 
Progress in drilling techniques in recent years has been remarkable, and 
the developments of improved techniques for controlling drilling fluids 
applied to subterranean formations has permitted maintaining a high 
specific gravity under high temperature conditions. Thus, it becomes 
possible to drill deeper rock formations smoothly. 
After completion of an well, oil production can be carried out, but 
formation damages as described below often occur. More particularly, if 
drilling mud and solution filtrated from a cement slurry penetrate into 
reservoirs which contain clay, the clay is swollen and pluggs pores to 
result in a lowering of permeability; or, when reservoirs of sandstone 
have not been fully concreted, sand flows into the well with the oil and 
gas, thus decreasing the productive capacity. As countermeasures to 
resolve such damages, various workovers can be carried out. In these cases 
the use of a clear brine as completion and workover fluids, without solids 
such as bentonite and barite, have been recommended. That is, the clear 
brines heretofore in use typically contain soluble salts used in an amount 
sufficient to obtain a desired specific gravity, hydroxyethyl cellulose as 
viscosifier, which is not substantially affected by the soluble salts, and 
if desired, various additional materials. 
However, the temperature in the well tends to rise as the well becomes 
deeper, and a lowering of rheological properties of the completion and 
workover fluids results in the problem that the completion and workover 
fluids can not fully display their capacity for cleaning the sand or other 
deposits in the well up to the ground surface. Thus, it has been found 
that the necessary working time becomes extended and the resulting sand or 
other deposits can not be completely cleaned out. 
Accordingly, only when a method that is able to prevent a lowering of 
rheological properties of completion and workover fluids heretofore in use 
can be found, it become possible to fully enjoy the beneficial properties 
of such fluids. 
SUMMARY OF THE INVENTION 
Based on extensive investigations, it has now been found that completion 
and workover fluids which do not undergo adverse lowering of rheological 
properties as described above can be prepared by the incorporation of 
certain compounds. 
More particularly, the present invention is directed to a completion and 
workover fluid containing at least one soluble salt selected from the 
group consisting of calcium chloride, calcium bromide and zinc bromide, a 
hydroxyethyl cellulose, and at least one compound selected from the group 
consisting of 2-mercaptobenzimidazole compounds, 2-mercaptobenzothiazole 
compounds, 2-mercaptothiazoline and 2-thioimidazolidone and its object is 
to resolve the problems described above. 
As will be seen from the examples described below, the completion and 
workover fluids of the present invention are superior to conventional 
clear brines, in that lowering of the rheological properties according to 
the present invention is remarkably small, and thus it is possible to 
fully enjoy their beneficial properties. 
DETAILED DESCRIPTION OF THE INVENTION 
The completion and workover fluids of the present invention contain a 
soluble salt, a hydroxyethyl cellulose and at least one compound selected 
from 2-mercaptobenzimidazole compounds, 2-mercaptobenzothiazole compounds, 
2-mercaptothiazoline and 2-thioimidazolidone. 
Of the selected compounds that can be contained in the completion and 
workover fluids of the present invention, preferred 
2-mercaptobenzimidazole compounds are represented by the formula (I) 
##STR1## 
wherein each of R.sub.1, R.sub.2, R.sub.3, and R.sub.4 represents a 
hydrogen atom or a lower alkyl group, specifically, a methyl or ethyl 
group, and X represents a hydrogen atom, an alkali metal or ammonium. Of 
these compounds, 2-mercaptobenzimidazole (wherein R.sub.1 through R.sub.4 
=H and X=H), methyl-2-mercaptobenzimidazoles (wherein at least one of 
R.sub.1 through R.sub.4 is a methyl group, and X=H) and these sodium salts 
(X=Na) are preferred. 
Further preferred 2-mercaptobenzothiazole compounds that can be contained 
in the clear brine completion and workover fluid of the present invention 
are represented by the formula (II) 
##STR2## 
wherein each of R.sub.1, R.sub.2, R.sub.3, and R.sub.4 represents a 
hydrogen atom or a lower alkyl group, specifically, methyl or ethyl group, 
and X represents a hydrogen atom, an alkali metal or ammonium. Of these 
compounds, 2-mercaptobenzothiazole (wherein R.sub.1 through R.sub.4 =H, 
and X=H), methyl-2-mercaptobenzothiazoles (wherein at least one of R.sub.1 
through R.sub.4 is a methyl group, and X=H) and these sodium salts (X=Na) 
are preferred. 
The concentration of the at least one compound selected from 
2-mercaptobenzimidazole compounds, 2-mercaptobenzothiazole compounds, 
2-mercaptothiazoline and 2-thioimidazolidone contained in the completion 
and workover fluids of the present invention is preferably in the range of 
from about 0.001% to about 0.2% by weight, and more preferably from about 
0.005% to about 0.1% by weight, based on the total weight of the fluid. 
Other components and their amounts contained in the clear brine completion 
and workover fluids heretofore in use, which are the objects of intended 
improvements according to the present invention, are as follows. The 
hydroxyethyl cellulose has a mean molar substitution of 1.5 or more, and a 
viscosity of 2,000 cp or more at 1% by weight (Brookfield Viscometer, No. 
3 Spindle, 30 RPM, 25.degree. C.), and its concentration in said fluids is 
in the range of about 0.2% to about 2% by weight. Also, the soluble salt 
is at least one selected from calcium chloride, calcium bromide, and zinc 
bromide, and its sort and concentration can be determined in consideration 
of a specific gravity required (pH of 1.01 or more, preferably 1.16 to 
2.30), working temperature and crystallized temperature. The concentration 
of the soluble salt in said fluids is generally in the range of about 
0.001% to about 70% by weight. Furthermore, as other components, 
defoamers, anticorrosives, pH control agents, fillers, and suspension 
agents can be employed. 
A completion and workover fluid of the present invention can be stored at a 
high temperature of about 80.degree. C. or more, preferably at about 
80.degree. to about 150.degree. C., and their rheological properties, 
i.e., their apparent viscosities (AV), plastic viscosities (PV) and yield 
points (YP) are high as compared with those of the fluids heretofore in 
use. They are superior on their capacities of cleaning the sand or other 
deposits in the well up to the surface of the ground. 
The present invention is described below by means of the examples. However, 
since the reproducibilities of workovers in real well are difficult, the 
completion and workover fluids were stored in a Roller Oven at a high 
temperature and their rheological properties above were measured using a 
Fann VG meter Model 35 (at 30.degree. C.). The completion and workover 
fluids of the present inventions were compared with those heretofore in 
use. 
In the examples below all expressions of "parts" are parts by weight, 
unless otherwise indicated.