Screening Tests for Evaluating the Effectiveness of Gypsum Scale Removers
|Publication Date:||1 June 2018|
The two test methods described in this standard are static laboratory screening tests designed to measure the ability of chemicals to remove gypsum scale deposits. The first method is a single-stage dissolver process in which the test agent dissolves the gypsum solid. The second method is a two-stage process where the test agent converts the gypsum to an acid-soluble salt, and this salt is dissolved using an HCl solution. Both methods should be performed, and should use the same original form of gypsum to make economic and performance decision on which method and chemicals are most appropriate for each field situation. There are two types of scale-removal chemicals: dissolvers and converters. Scale dissolvers, generally chelating or sequestering agents, can affect the dissolution and removal of gypsum scale in one step. Converters, such as those formulations based on sodium (or potassium) glycolate and sodium (or ammonium) carbonate (or bicarbonate), are used to alter or convert the calcium sulfate to another compound which is then removed by dissolution with a dilute mineral acid (typically hydrochloric acid). Test methods for screening both gypsum scale dissolvers and converters are described. The purpose of this test method is to provide an approach for users to compare 1) their scale dissolvers against EDTA, a standard dissolver chemical and 2) their scale converter against potassium glycolate, a standard converter chemical.
These test methods are recommended only for ranking the performance of different scale-removal chemicals under the laboratory conditions set by these test methods. Results for gypsum cannot be extrapolated to other scales, such as barite or iron sulfide.
Many factors such as reaction kinetics, fluid velocity, temperatures and pressures, scale surface area, water chemistry and scale composition can significantly affect scale removal under field conditions. Detailed consideration of these parameters is outside the scope of this standard. However, field conditions, field brine composition, and others noted above should be considered at some point in the evaluation prior to final selection of a scale remover for field use.
Tests can be conducted using varying amounts of gypsum to obtain a better comparison of scale removers under the laboratory conditions set by these test methods. The actual ratio of scale remover to gypsum required for a field application may be different from that established by these test methods.
This standard lists the necessary apparatus, reagents, and procedures for conducting screening tests of both gypsum dissolvers and converters.
A reference scale dissolver, tetrasodium ethylene-diaminetetr