1、 Item No. 24008 NACE International Publication 1D191 (2003 Edition) This Technical Committee Report has been prepared by NACE International Specific Technology Group 31* on Oil and Gas ProductionCorrosion and Scale Inhibition A Review of Screening Tests for Gypsum Scale Removal Chemicals March 2003,
2、 NACE International This NACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products, p
3、rocesses, or procedures not included in this report. Nothing contained in this NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protect
4、ing anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumstances
5、may negate the usefulness of this report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determinin
6、g their applicability in relation to this report prior to its use. This NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this report. Users of th
7、is NACE report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this report. C
8、AUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE reports may recei
9、ve current information on all NACE International publications by contacting the NACE Membership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281/228-6200). Foreword Hydrocarbon production is often accompanied by the production of a brine. Minerals may precipit
10、ate from a brine and become a deposit within the production system. Often the deposit has an adverse effect on production and is removed. Although chemical-based methods have been used to remove gypsum (CaSO42H2O) deposits for many years, the industry has not established a standard test method by wh
11、ich to evaluate the gypsum scale removers. As a consequence, performance tests on a remover or collection of removers yield widely differing absolute and relative results depending on the test procedure used. Responding to an expressed need for a standard test method for the evaluation of chemical-b
12、ased gypsum removers, NACE Unit Committee T-1D on Control of Oilfield Corrosion by Chemical Treatment formed Task Group T-1D-32 in 1987. The task groups first objective was to publish a technical committee report. The focus would be laboratory tests currently in use for screening gypsum remover chem
13、icals. A survey form (see Appendix A), developed to acquire the necessary data for this report, was sent to members of T-1D. This report is intended as a resource for individuals who have an interest in mineral scale control or who seek an understanding of how to remove gypsum deposits from oil and
14、gas production systems. This NACE technical committee report was originally published in 1991 by Task Group T-1D-32 on Laboratory Tests for Screening Gypsum Scale Removers. It was reaffirmed in 2003 by Specific Technology Group (STG) 31 on Oil and Gas ProductionCorrosion and Scale Inhibition. This r
15、eport is issued by NACE International under the auspices of STG 31. _ *Chairman Mike Cushner, Dow Chemical, S. Charleston, WV. NACE International 2 General Scale deposition has been occurring in oil and gas production and injection wells and surface equipment for many years. The scale deposits can b
16、e located both downhole and in surface equipment. Minerals carried in oilfield-produced brine are the source of these deposits. Although there are many factors that affect scale precipitation, the primary causes are temperature changes, a reduction of pressure on the produced brine, or the mixing of
17、 waters having incompatible ions in solution. The composition of mineral scales differs with the environment. The most common oilfield scale deposits are calcium carbonate, calcium sulfate, barium sulfate, and strontium sulfate. Various iron compounds, such as iron carbonate, iron sulfide, and iron
18、oxide may also form as the corrosion products generated by carbon dioxide, hydrogen sulfide, or oxygen dissolved in the water.1As various solids precipitate from solution and deposit onto the surfaces of tubularswithin the perforations or on the reservoir facethe flow of fluids is reduced. The resul
19、ting loss in gas and/or oil production has a negative economic impact. Therefore, an effective scale inhibition program is usually maintained, because preventing scale from forming is usually more economical than removing it after formation and deposition. If scale deposits do develop, they usually
20、are removed either chemically or mechanically. A summary of the various methods and equipment used for removing scale mechanically has been presented,2and there continues to be further development in this area.2,3Further discussion is beyond the scope of this report. Chemical scale removal can inclu
21、de the use of mineral acids to remove acid-soluble scales such as calcite (calcium carbonate), perhaps the predominant form of scale encountered in oilfield production systems, and various iron-based scales. Sequestrants or chelating agents are sometimes used alone or in combination with acid. The m
22、ost commonly used chelating compounds, salts of ethylene-diaminetetraacetic acid (EDTA), have been used to remove calcite scale deposits in several oil production systems.4-7Excellent summaries of developments in this area have been presented.2,8Given the serious impact that gypsum scale formation c
23、an have on hydrocarbon production, it is not surprising that producers and service companies devote considerable effort to developing and marketing effective treating chemicals. The performance of these products can be verified most effectively after an actual field trial. However, field testing can
24、 be very difficult and time consuming, especially when evaluating many chemicals. As a result, most testing of the effectiveness and performance of gypsum removers takes place initially in the laboratory. Although most laboratory tests cannot exactly duplicate field conditions, the advantage of such
25、 tests is to provide the user with a comparison of one products performance against that of another under standard laboratory conditions. The topic that is specifically addressed in this report is the use of screening tests to evaluate chemical scale-removal agentsmost notably alkaline solutions and
26、/or various organic acids and sequestrantsto remove acid-insoluble scales, specifically gypsum (calcium sulfate dihydrate). Common scale-removal chemicals include sodium hydroxide, sodium carbonate/bicarbonate, ammonium carbonate/bicarbonate, EDTA salts, nitrilotriacetic acid (NTA) salts, hydroxyace
27、tic acid salts, and gluconic acid salts. Additives such as surfactants are sometimes added to these chemicals to improve the penetration rate of the chemical treating solution into the scale deposits.2Numerous proprietary products have been formulated specifically for the removal of gypsum deposits.
28、 Case histories involving the field application of gypsum scale removers have been reported in the literature.2,9-15The development of new chemicals and processes specifically designed for removing gypsum scale deposits is described in some recent patents.16-20Performance of Chemical-Based Gypsum Re
29、movers Chemical RemoversConverters and Dissolvers. A thorough review of the chemistry essential to understanding various scale dissolution processes has been presented by a number of authors.2,9,11,12This report contains a brief discussion of the mechanisms of the scale removal process. Simply state
30、d, the removal of gypsum scale is accomplished conventionally by one of two basic methods. The insoluble calcium sulfate scale can be removed by what is commonly termed a scale “converter” or by use of a chelating chemical, often termed a “dissolver.” Converters. A number of alkaline scale-removal a
31、gents have been widely used to remove gypsum deposits. One method involves contacting the calcium sulfate scale with a strong solution of potassium or sodium hydroxide. These converter solutions alter (or “convert”) the calcium sulfate to calcium hydroxide.2Other converters, such as sodium (or ammon
32、ium) carbonate or bicarbonate, are used to convert the calcium sulfate scale to calcium carbonate. The remaining solids (calcium sulfate or calcium carbonate) usually are removed by dissolution with a mineral acid, typically 15% hydrochloric acid, as shown in Reactions (1) through (4): 2NaOH + CaSO4
33、2H2O Ca(OH)2+ Na2SO4+ 2H2O (1) Na2CO3+ CaSO42H2O CaCO3+ Na2SO4+ 2H2O (2) NACE International 3 Ca(OH)2+ 2HCl CaCl2+ 2H2O (3) CaCO3+ 2HCl CaCl2+ CO2+ H2O (4) Dissolvers. A second conventional method for removing gypsum deposits involves the use of chelating or sequestering agents. The most common chem
34、icals in this category include the salts of EDTA and NTA. The EDTA, for example, may react with the solid calcium sulfate and form a soluble Ca (EDTA) complex, as shown in Reaction (5): Na4EDTA + CaSO42H2O Na2CaEDTA + Na2SO4+ 2H2O (5) Under appropriate treatment conditions the sulfate ion liberated
35、in the process enters and remains solvated in the treatment fluid. The chelant may effect the complete dissolution and removal of the calcium sulfate scale deposit in one step. Test Procedures Currently in Use General Test Considerations The basic gypsum scale-remover screening test generally involv
36、es the following steps: gypsum sample selection, pretreatment of gypsum with the remover, contact of gypsum with the remover, and separation and quantification of residual solids to determine remover efficiency. Those who have developed or modified gypsum remover tests have had many choices to make.
37、 These choices typically begin with the source of the gypsum test samples and end with how the test results are reported. The differing responses to the many decisions made when establishing a test procedure account, in part, for the wide variability in laboratory-to-laboratory test results for the
38、same treatment chemical. Factors normally considered include: (a) In the selection of the gypsum samples: source: laboratory reagent or field sample continued availability of the samples the mineralogical purity and size of a field-collected gypsum sample (b) In conducting the test: pretreatment of
39、hydrocarbon-covered scale deposit number of replicate samples method for addressing differences in surface area of deposits the appropriate volume (remover) to mass (gypsum) ratio dilution of the remover test temperature, pressure, and duration post-treatment procedures, for example, acid treatment
40、and drying and weighing of solids (c) In reporting the results: how the numerical results are expressed. It is not surprising, in light of the many considerations that relate to the testing of gypsum removers, that the procedures in use vary widely. The survey responses summarized in the following s
41、ections represent consensus on some of these issues and extreme disagreement on others.Survey Responses Gypsum Sample Selection Source and Availability Ten of the 20 respondents used field-collected gypsum samples for their testing. Five respondents indicated either reagent crystals or field samples
42、 are used in their companies procedures. One respondent used only reagent gypsum. Another described the use of a specially prepared gypsum briquette. Three of the respondents did not indicate the source of their samples. There is a benefit in having a large amount of gypsum of the same size and mine
43、ralogical purity. This material can be used as a standard for future tests. An informal survey of task group members indicated that some of the members maintain a supply of gypsum for just this purpose while others choose to standardize the test on the basis of a specific treatment chemical. Some ma
44、ke no attempt to standardize data from tests run at different times. Size and Mineralogical Purity of Gypsum Samples Ten of the 20 responses did not indicate the mesh size of their gypsum samples. The remaining 10 specified crystal sizes varying from very fine 16 or 18 mesh (about 1 mm 0.04 in.) to
45、greater than 6.4 mm (0.25 in.). With the exception of the “same company” responses, there was no agreement on a desirable gypsum particle size or the closely related total surface area of gypsum. The use of reagent gypsum crystals in these tests ensures chemical purity. However, field-collected samp
46、les are more representative of the actual scale deposits to be removed. An informal survey of task group members found that some of those who did use field-collected samples did little to document the mineralogical purity of those test samples. Also, because the available surface area for a given ma
47、ss of reagent crystals is much larger than for gypsum deposits normally encountered in the oil field, reagent gypsum NACE International 4 crystals may react faster and/or more completely than field-collected gypsum samples. Conducting the Test Pretreatment When field samples were cleaned prior to te
48、sting, a xylene wash was commonly used. This wash was followed by an isopropanol or acetone rinse and a brief air or low-temperature oven drying. Thirteen of the respondents either chose not to address this question on the survey, used reagent gypsum crystals, or did not clean their field-collected
49、gypsum samples. The cleaning of field samples affords optimum contact between the scale removers and the gypsum pieces. Testing with clean samples ensures that removers can be ranked according to their scale-removal efficiency. However, in field practice, the surface of gypsum deposits may or may not be freed of organic materials prior to gypsum remover use. Number of Replicate Tests The general tendency among respondents was to run replicate tests if sufficient gypsum and remover solution were available. Surface Area/Volume-
