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Scale formation during oil and gas production may cause formation damage, pump failure, blockage of tubing or annulus, and deposits in surface lines and equipment that can result in shutdowns or loss of production. Therefore, the prevention of scale is of considerable interest and concern to the oil and gas industry. Control of scale, particularly halite in high salinity Bakken brines (total dissolved solids > 250,000 mg/L), is challenging on two fronts: 1) laboratory evaluation of scale products by conventional dynamic tube-blocking tests or static bottle tests are not optimal for halite scale, and 2) halite formation and high tendencies for carbonate and sulfate scales characterize high-salinity brines, which are difficult to manage with conventional scale control strategies. This paper describes a novel laboratory testing procedure to better evaluate halite/scale inhibition and how to apply this method. Conventional testing is also included to provide a more robust strategy for effective scale control in the high-salinity Bakken brines. Currently, there is not an industry-recognized method for laboratory testing of halite due to difficulty controlling the amount of halite precipitation and reproducibility during the test. However, a test method was devised utilizing an ultraviolet-visible (UV-Vis) spectrophotometer to monitor dynamic scale formation during product evaluation. This kinetic turbidity test (KTT) uses the information on scale formation kinetics to differentiate various product performances with reproducible testing results. Halite inhibitors and combination products (halite inhibitor + common scale inhibitors) were screened with this method for potential use in field applications. The combination of products exhibited good scale inhibition performance for both halite and other common scales. The challenges of halite and other scales in Bakken brines cause typical scale inhibitors to be ineffective. Diluting brines with freshwater is the common field treatment to lower scaling ion concentrations and wash out salt deposits. However, this approach is restricted in the Bakken because of the limited fresh water availability, poor fresh water quality (i.e., high bicarbonates and high pH), and remote locations that can impede freshwater treatment. Therefore, more effective treatments are essential for economic oil and gas production.
Scale formation during oil and gas production may cause formation damage, pump failure, blockage of tubing or annulus, and deposits in surface lines and equipment that can result in shutdowns or loss of production. Therefore, the prevention of scale is of considerable interest and concern to the oil and gas industry. Control of scale, particularly halite in high salinity Bakken brines (total dissolved solids > 250,000 mg/L), is challenging on two fronts: 1) laboratory evaluation of scale products by conventional dynamic tube-blocking tests or static bottle tests are not optimal for halite scale, and 2) halite formation and high tendencies for carbonate and sulfate scales characterize high-salinity brines, which are difficult to manage with conventional scale control strategies. This paper describes a novel laboratory testing procedure to better evaluate halite/scale inhibition and how to apply this method. Conventional testing is also included to provide a more robust strategy for effective scale control in the high-salinity Bakken brines. Currently, there is not an industry-recognized method for laboratory testing of halite due to difficulty controlling the amount of halite precipitation and reproducibility during the test. However, a test method was devised utilizing an ultraviolet-visible (UV-Vis) spectrophotometer to monitor dynamic scale formation during product evaluation. This kinetic turbidity test (KTT) uses the information on scale formation kinetics to differentiate various product performances with reproducible testing results. Halite inhibitors and combination products (halite inhibitor + common scale inhibitors) were screened with this method for potential use in field applications. The combination of products exhibited good scale inhibition performance for both halite and other common scales. The challenges of halite and other scales in Bakken brines cause typical scale inhibitors to be ineffective. Diluting brines with freshwater is the common field treatment to lower scaling ion concentrations and wash out salt deposits. However, this approach is restricted in the Bakken because of the limited fresh water availability, poor fresh water quality (i.e., high bicarbonates and high pH), and remote locations that can impede freshwater treatment. Therefore, more effective treatments are essential for economic oil and gas production.
This paper describes a laboratory and modelling study into halite (sodium chloride, rock salt) deposition in mature gas production wells. Halite deposition can result in significant production and integrity issues, and mitigation measures are primarily based upon injection of low-salinity wash water, often coupled with careful control of production parameters (such as well drawdown), for which scale-prediction models are used. Laboratory investigation of halite scaling and performance of halite inhibitors typically uses the mixing of incompatible brines or cooling a saturated solution, either in bottle tests or using a dynamic scale rig. While these methods allow precipitation kinetics and inhibitor performance to be examined, they are far from ideal as they require significant modification of the brine chemistry and deposition conditions. In this paper, we describe a novel approach to laboratory investigation of halite deposition that much more closely mimics the evaporative scaling mechanism mostly widely experienced in the field, and can be performed using produced-water compositions and conditions that are much more representative of the field. It is based upon a dynamic, flowing system where field-representative formation water is co-injected with gas at an appropriate level of under-saturation in water content to that expected under production conditions. Electrolyte prediction software was used to model the halite scaling tendency in the experiments, and very good correlation was found between the prediction of supersaturation and the onset of precipitation and deposition. This agreement implies that the scale-prediction model is accurate for halite scaling via this mechanism, and adds much confidence to the use of this tool for optimizing production parameters to minimize the effects of halite scaling in the field. The work also confirms earlier reports that the critical scaling tendency for halite – the value at which significant precipitation and deposition occurs in the field – is very close to unity for the conditions tested in this work. The new laboratory method was also used to generate calcium carbonate scaling by the mechanism – primarily transfer of CO2 to the gas phase – that is by far the predominant one found in the field; this led to observation of a mixed halite/calcium carbonate deposit.
Halite (NaCl) scale is a non-conventional scale, which happens due to the temperature or pressure drop, or water evaporation at extremely high TDS environment (TDS up to 350,000 mg/L), such as deepwater field, shale formations, gas and gas condenstate fields. Compared to other conventional mineral scales, like barite or calcite, there is no standardized experimental procedure to screen and evaluate halite scale inhibitors. Because of the extremely high solubility of NaCl, it is very challenging but important to accurately control and calculate the halite saturation index (SI) or saturation ratio (SR) experimentally and theoretically. The Brine Chemistry Consortium has been continuously working on the development and optimization of the thermodynamic model, ScaleSoftPitzer (SSP), for the the prediction of the solubility and scaling risks of various minerals in mixed electrolytes over a wide range of temperature and pressure based on Pitzer theory. Recently the new SSP2017 version which can predict the halite SI within SI error of 0.01 from 25 to 90 °C with up to 1.0 m Ca2+ (~35,000 mg/L). In this study, two static bottle testing methods have been developed by taking the advantage of the powerful calculation ability of SSP2017 to fast and accurately screen and evaluate the inhibition efficiency of different chemicals. The Method One is a temperature-driven and monitoring method and the Method Two is a brine-mixing method of mixing a naturally saturated NaCl solution, a highly concentrated chloride salt solution and an inhibitor solution. By using these two novel methods, over 40 different chemicals are screened and evaluated, including small molecules and polymers in the category of polyacrylate, polyaspartate, polysulfonate, carboxylate sulfonate copolymer, phosphonate polymer, etc. From the experimental results, five carboxylate sulfonate copolymers (Inh #H14, #H23, #H26, #H30, #H40) have better inhibition efficiency. The synergistic effect of Inh #H40 and Fe(CN)64– is also investigated, and 130 mg/L of Inh #H40 and 20 mg/L of Fe(CN)64– can effectively inhibit the halite scale at simulated field condition of SI = 0.09 (SR = 1.23, TDS ~ 353,800 mg/L) and 0.6 m Ca2+ at 70 °C. In summary, two fast and accurate methods have been developed for the screening and evaluation of halite scale inhibitors, and with the usage of effective halite scale inhibitors, the costs of halite scale control can be significantly reduced both in freshwater usage for dilution and high-salinity produced water treatment.
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