Removal of useless and submicrometer-sized solids from drilling fluid, which exert significant effects on drilling performance, is a crucial part of sustainable and eco-friendly circulation in drilling operations. However, current solid-control methods for drilling-fluid reuse and recirculation, such as electronic-adsorption and chemical-flocculation methods, are associated with high cost and low efficiency and/or pollution of drilling fluid. In this study, a novel method using ultrasonic waves has been proposed to remove unwanted submicrometer-sized solids from polysulfonate drilling fluids. The results show that the suspension stability, viscosity, and particle size can all be significantly reduced, while the solid-separation ratio of the drilling fluid can be greatly enhanced by ultrasonic-wave treatment. The parameters of ultrasonic waves are optimized to be power of 3 kW, treating time of 30 minutes, treating frequency of 20 kHz, and ventilation (i.e., air) for 5 minutes in a laboratory scale. The scanning electron microscope (SEM) analysis shows that solid particles exhibit more obvious crystal morphology after ultrasonic-wave treatment, indicating that the breaking of gel-structure of drilling fluid due to the cavitation, mechanical, and heat effects of ultrasonic waves is the main mechanism for decreasing the suspension stability. Thus, the proposed ultrasonic-assisted technique has a high potential for removing undesirable solids from drilling fluid and fulfilling its recirculation in a cost-effective and environmentally friendly manner. This new technology has been successively applied to 12 wells, and good results were obtained.

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