Laboratory scale inhibitor performance tests have been widely accepted to screen products and determine the minimum effective concentrations (MEC) of selected products for field treatment. The dynamic tube-blocking test is the most common lab testing method used to obtain an inhibitor MEC number because the testing temperature, oxygen level, pH, and pressure can be well controlled to duplicate the field scaling conditions. A new benchtop testing method, the Kinetic Turbidity Test (KTT), has drawn increased attention for scale inhibitor evaluation and MEC determination under some scaling circumstances due to its ability to monitor the formation and growth of mineral scales continuously throughout the test duration.
In west Texas, a well being evaluated had failed three times within 1.5 years because of the formation of scale on the ESP pump, even though it had an aggressive squeeze treatment with the inhibitor residuals of 20 ppm and above. Both lab dynamic tube-blocking tests and KTT results show that 5 ppm of the selected scale inhibitor would be sufficient to prevent the formation of scales for a general produced water sample. However, under the worst scaling scenario according to historical produced water data, KTT results indicate that more than 30 ppm of scale inhibitor is required to achieve an effective scale treatment, whereas the dynamic tube-blocking test MEC is still well below 20 ppm. This paper is intended to give some insight on the field treatment failure analysis by comparing the difference between the MEC obtained from different testing methods at various scaling conditions. The other possible reasons that may cause treatment failure, such as uneven scale inhibitor distribution at different producing zones and inhibitor residual samples contamination, are also included in this study.
The formation of mineral scales is one of the most problematic threats to the oil and gas operations which can lead to loss of production, increased lifting costs and assets deterioration.1 Mineral scales can precipitate at any locations within an oil and gas production system and create blockage in perforations, production tubulars, pumps, and surface equipment. The formation of scale deposits can be attributed to the mixing of incompatible waters from different production zones or physical and chemical condition changes associated with produced water transporting from reservoir to wellhead and further to processing facilities.2,3 The most efficient method to control scale deposition is the use of scale inhibitors that can catalytically prevent the precipitation of mineral scales. These chemical scale inhibitors are able to prevent the formation of scale at a treatment dosage that is typically hundreds to thousands times less than the concentration required for acid or chelate treatment, and thus are referred to as "threshold" inhibitors. For a given produced water, the threshold concentration required to prevent the formation of scale is one of the most important terms used to guide the design and deployment of scale inhibitor treatment programs, and it is normally referred to as the minimum effective concentrations (MEC) in the oil and gas industry.