Method for scale removal in a wellbore

Alkaline earth metal scales, especially barium sulfate scale is removed from a well (the wellbore and the adjacent formation) by contacting the scale with a scale-removing solvent and thereafter sparging the solvent with air or insert gas to dissolve the scale. The sparging aids in achieving a flushing action by the solvent relative to the scale thereby enabling the solvent to more effectively dissolve the scale in the well. Temperature and caustic compatible surfactuants (anionic and non-ionic) may be used in the solvent to generate a column of foam which increases the extent of cleaning oil from the scale surfaces, enabling the solvent to more effectively remove the scale.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to prior co-pending application Ser. No. 
07/332,147, now U.S. Pat. No. 5,093,020, filed Apr. 3, 1989, of J. M. Paul 
and R. L. Morris; Ser. No. 07/369,897, filed Jun. 22, 1989, now U.S. Pat. 
No. 4,980,077 granted Dec. 25, 1990, of J. M. Paul and R. L. Morris; Ser. 
No. 07/431,114, now U.S. Pat. No. 5,084,105, filed Nov. 3, 1989, of J. M. 
Paul and R. L. Morris; Ser. No. 07/484,970, now abandoned filed Feb. 24, 
1990, of J. M. Paul and R. L. Morris; and Ser. No. 07/612,728, now U.S. 
Pat. No. 5,087,371 filed Nov. 14, 1990, of J. M. Paul and R. L. Morris. 
FIELD OF THE INVENTION 
This invention relates to a method of removing barium sulfate and other 
sulfate scale deposits from a well (the wellbore and the adjacent 
formation) by injecting a scale-removing solvent into the well and 
thereafter sparging the solvent with air or an inert gas to agitate the 
solvent thereby enabling the scale solvent to more effectively dissolve 
the scale deposits. 
BACKGROUND OF THE INVENTION 
Many waters contain alkaline earth metal cations, such as barium, 
strontium, calcium, magnesium, and anions,such as sulfate, bicarbonate, 
carbonate, phosphate and fluoride. When combinations of these anions and 
cations are present in concentrations which exceed the solubility product 
of the various species which may be formed, precipitates form until the 
respective solubility products are no longer exceeded. For example, when 
the concentrations of the barium and sulfate ions exceed the solubility 
product of barium sulfate, a solid phase of barium sulfate will form as a 
precipitate. Solubility products are exceeded for various reasons, such as 
evaporation of the water phase, change the pH, pressure or temperature and 
the introduction of additional ions which can form insoluble compounds 
with the ions already present in the solution. 
As these reaction products precipitate on the surfaces of the 
water-carrying or water-containing system, they form adherent deposits or 
scale. Scale may prevent effective heat transfer, interfere with fluid 
flow, facilitate corrosive processes, or harbor bacteria. Scale is an 
expense problem in any industrial water system, in production systems for 
oil and gas, in pulp and paper mill systems, and in other systems, causing 
delays and shutdowns for cleaning and removal. 
In the co-pending applications reference above, there is disclosed a method 
for removing barium sulfate and other sulfate scales by a solvent 
comprising a combination of a chelating agent comprising a catalyst or 
synergist comprising polyaminopolycarboxylic acid such as EDTA or DTPA 
together with anions of (1) a monocarboxylic acid such as acetic acid, 
hydroxyacetic acid, mercaptoacetic acid or salicylic acid; (2) oxalates; 
or (3) thiosulfates. The scale is removed under alkaline conditions, 
preferably at pH values of at least 10, usually 10-14, with best results 
being achieved at about pH 12. When the solvent becomes saturated with 
scale metal cations, the spent solvent is disposed of by re-injection into 
the subsurface formation or regenerated. 
It is common practice when using scale dissolvers to pump a slug or volume 
of solvent into the well and leave it to stand static for a long period of 
time. This is not effective from an economic stand point because the well 
must be shut in for long periods of time, resulting in loss of production. 
Soaking is also not an efficient method from a reaction rate stand point. 
Also, circulating scale solvent into the well bore is too costly, due to 
the large volume of solvent needed to fill the tubing and allow 
circulation. 
This invention provides an effective method for removing scale from a well 
by contacting the scale with a scale-removing solvent and sparging the 
solvent with air or inert gas to achieve a flushing action by the solvent 
relative to the scale thereby enabling the solvent to more effectively 
dissolve the scale in the well. 
SUMMARY OF THE INVENTION 
A method for removing alkaline earth sulfate scale from a well (the 
wellbore and the adjacent formation) comprising contacting the scale with 
a scale-removing solvent and sparging the solvent with air or an inert gas 
to dissolve the scale.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, a subterranean formation is 
penetrated by a wellbore, including a casing in fluid communication 
therewith by means of well casing perforations in the zone of the 
formation. Production occurs from the formation by the flow of fluids 
including oil, gas and water through the perforations into the wellbore 
with the fluids then being recovered. The production of fluids from the 
well can be inhibited by the formation of scale which plugs or partially 
plugs perforations in the casing of the wellbore, tubing inside the 
casing, downhole equipment such as pumps and safety valves or the 
formation near the well. 
The present invention removes the scale deposits from the well (the 
wellbore and the adjacent formation) with a scale-removing solvent 
comprising a chelating agent and a catalyst or synergist to speed up the 
dissolution of the scale as disclosed in U.S. Pat. No. 4,980,077 which 
issued on Dec. 25, 1990 to J. M. Paul and R. L. Morris. This patent is 
hereby incorporated by reference. The pH of the solvent is maintained at 
pH values of about 8.0 to about 14.0, preferably at about 11 to 13, 
preferably about 12, with the addition of potassium hydroxide (caustic 
potash). Suitable chelating agents comprise polyaminopolycarboxylic acid 
such as EDTA or DTPA. The chelant may be added to the solvent in the acid 
form or, alternatively, as a salt of the acid, preferably the potassium 
salt. In any event the alkaline conditions used in the scale removal 
process will convert the free acid to the salt. 
The concentration of the chelant in the solvent should normally be at least 
0.1M in order to achieve acceptable degree of scale removal. The catalyst 
or synergist comprise anions of at least one monocarboxylic acid such as 
mercaptoacetic acid, aminoacetic acid and hydroxyacetic acid; oxalates, 
and/or thiosulfates. 
The concentration of the catalyst or synergist in the aqueous solvent will 
be of a similar order: thus, the amount of the oxalate anion in the 
solvent should normally be at least 0.1M in order to achieve a perceptible 
increase in the efficiency of the scale removal, and concentrations from 
about 0.3M up to about 0.6M will give good results. The scale solvent may 
also contain caustic compatible anion and non-ionic surfactants. Suitable 
surfactants are selected from the group consisting of sodium tetradecyl 
sulfate sold under the trade designation "NIAPROOF NAS4" by Niacet 
Corporation, sodium 2-ethylhexyl sulfate sold under the trade designation 
"NIAPROOF NASO8" by Niacet Corporation and ethyoxy sulfates sold under the 
trade designation "NEODOL 25-3S" by Shell Chemical Company. The 
concentration of the surfactants in the aqueous solvent will be sufficient 
to generate a stable foam. 
In the FIGURE, an oil-containing formation 10 is situated below the earth's 
surface 12 covered by overburden 14 and supported by strata 16. 
Formation 10 is penetrated by a production well 18 equipped with casing 20 
and is provided with perforations 22 in the productive interval 23 of the 
formation. The bottom of the casing 20 is sealed by means of casing shoe 
24. The production well 18 is equipped with tubing 26 which extends 
downwardly from the top of the well through packer 2 which insures that 
oil produced from the formation enters the production tubing 26. Tubing 26 
terminates adjacent the lower perforations 22. During production of fluid 
including oil from the formation, fluid enters the casing 20 through 
perforations 22 and the produced fluid is conducted to the earth's surface 
12 through tubing 26 and finally is conveyed to a suitable hydrocarbon 
recovery facility. During production of fluids, alkaline earth metal 
scales' especially barium sulfate scale, deposits in the perforations 22 
which plugs or partially plugs the perforations thereby interfering with 
the flow of production fluids. 
Once scale deposits result in the loss of production fluids from the well 
18, the production well is shut in. Thereafter, a slug or predetermined 
volume of scale-removing solvent is injected via tubing 26 into zone 30 of 
the well 18 containing scale deposits located downhole below packer 28 and 
adjacent the productive interval 23 The amount of scale solvent injected 
into zone 30 will depend upon the amount of scale deposits in the casing 
and downhole equipment. Prior to being injected into the well 18, the 
solvent may be heated to a temperature between about 25.degree. C. to 
about 100.degree. C., although the prevailing downhole temperature may 
make pre-heating unnecessary. Thereafter, the scale solvent is sparged 
with air or an inert gas by injecting air or an inert gas through small 
diameter coiled tubing 32 which extends into the scale solvent in zone 30. 
The injected raw or inert gas bubbles through the scale-removing solvent 
creating a flushing action by the solvent relative to the scale thereby 
enabling the solvent to more effectively dissolve the scale which has 
fouled the down hole tubular equipment, e.g., piping, casing, etc., and 
passage ways adjacent the formation. Sparging is continued and the solvent 
is kept in contact with the surfaces and perforations 22 adjacent the 
formation in zone 30 that ar covered with scale for a period of time 
sufficient to dissolve the scale. The surfactants (anionic and non-ionic) 
in the solvent generate a column of foam which increases the extent of 
cleaning the perforations and downhole equipment. Additionally, the 
surfactants clean oil from the scale surfaces, enabling the solvent to 
more effectively remove the scale. After remaining in contact with the 
equipment in zone 30 for the desired time the solvent containing the 
dissolved scale is produced to the surface and the well may then be 
returned to productive use. This procedure can be repeated as often as 
required to remove scale from the equipment in the well and adjacent the 
formation.