How Barium Sulfate Is Formed: An Interpretation

Abstract

Basic thermodynamic relations are used to predict the locations and rates of BaSO4 scale formation under field conditions. When kinetics are considered, the prediction method is much improved. However, the inclusion of hydrodynamic factors in the predictive model makes it possible to explain the full range of BaSO4 scale formation in the possible to explain the full range of BaSO4 scale formation in the oilfield. Introduction Barium sulfate (BaSO4) scale is one of the most annoying and costly problems encountered in oilfield operations. The problem is far from being solved. The literature contains a number of excellent publications on the physical and chemical properties of BaSO4. publications on the physical and chemical properties of BaSO4. The works of Templeton, Uchameyshvili et al., Weintritt and Cowan, and Gates and Caraway provide some of the fundamentals necessary to understand provide some of the fundamentals necessary to understand the occurrence of BaSO4 in actual field operations. However, the literature does not provide sufficient information to understand and treat the problem under all conditions. Additional information is required before a precise mechanism of BaSO4 deposition can be developed to predict reliably the occurrence and effective treatment of this type of scale. This paper describes an improved method for predicting BaSO4 precipitation under various sets of oilfield predicting BaSO4 precipitation under various sets of oilfield conditions. Published data of the chemical and physical characteristics of BaSO4 are combined with observations from many oil and gas fields. BaSO4 Scaling Tendencies Are Determined by Water Analyses It is common practice in the oil field to "determine" the BaSO4 scaling tendency of an oilfield brine in either of two ways. 1. The scale is physically encountered; that is, a sample of the solid scale material is seen and analyzed. (The big disadvantage of this "method" is that all preventive measures are too late. The damage has preventive measures are too late. The damage has occurred, and expensive remedial procedures may never restore the original productivity of a well). 2. Water samples are analyzed for their constituents, particularly for their Ba++ and SO4 -- ion contents. If particularly for their Ba++ and SO4 -- ion contents. If the product of the Ba++ and SO4 -- ion concentrations comes close to the solubility product of BaSO4, a "strong" scaling tendency is assumed. (For reasons outlined later, this method often fails. Frequently, this method will indicate no scaling tendency even though the well may be heavily scaled up. This method considers only thermodynamic factors leading to scale formation (such as temperature, pressure, and salt concentrations) and, in most instances, pressure, and salt concentrations) and, in most instances, many of these factors are greatly simplified or ignored because the water is analyzed under a set of thermodynamic conditions that may not occur anywhere in the field where scale formation can be expected. Other factors that might possibly cause formation of scale are entirely ignored by this method.) Water analyses are the most important aid in determining the scaling tendency of water containing Ba++ and SO4 -- ions. However, the interpretation of the analytical data has to be performed more carefully than is commonly done in the field. In addition, all the important thermodynamic conditions and their changes within the particular field have to be evaluated. JPT P. 1515