A Practical Method of Predicting Chemical Scale Formation in Well Completions

摘要

Scale formation in downhole tubular-flow passages can cause partial to complete plugging that will affect production or injection rates adversely. In an intelligent well completion in which the interval control valve (ICV) positions must be changed in order to control flow rate; the completion will become ineffective, if plugging of clearances prevents valve actuation. To mitigate these problems, a method to predict the potential rate of scale formation under realistic conditions has been developed. This paper describes this method, which allows prediction of tool performance under scale-forming conditions for downhole applications. This semi-empirical method uses chemical data and flow fields generated by computational fluid dynamics (CFD) models for downhole tools. Chemical data are obtained from laboratory tests on coupons using brines matching the chemistry of connate fluids. Tests in a high-pressure, corrosion-resistant vessel over a range of high pressures (100 to 10,000 psi) and high temperatures (75 to 150°C) to simulate downhole well conditions have been conducted. Two test sets each with fluid at rest and where an impeller generates low velocity in the reaction vessel were conducted, ranging from 4 hours to 4 days with scaling rates determined from coupon weight gain. Concentrations in the range of 50% to 125% of the typical connate fluid concentration were used. The laboratory test data are used with velocity field data to develop an artificial-intelligence-based mathematical model to determine scale formation rates. The model can be applied to any tool geometry as long as the operating conditions are within allowable limits of the model. The model also provides some insight into the mechanism of scale formation. To verify accuracy, scale formation in a 4.5-inch interval control valve was predicted at high-pressure, high-temperature conditions at a low flow rate. Laboratory tests on the valve matched the model predictions reasonably well, enabling Petrobras to design a better completion and fluid-handling system for a pre-salt well.