Objective The drilling wastewater in a block of Changqing Oilfield is large in volume,complex in composition,and high in Ca2+and other multivalent ions,which makes it challenging to meet the requirements of fracking fluid blending performance after conventional treatment.Methods Based on the above difficult process characteristics,a new type of calcium removal process was proposed by modifying chitosan with multifunctional groups to chelate Ca2+,and its effect on calcium removal,calcium removal mechanism,applicable conditions(temperature,pH),environmental impact(mud production)and back-mixing performance were evaluated.Results The results showed that chitosan with a high degree of deacetylation was used as raw material,potassium persulfate was used as initiator to graft acrylic acid,and chitosan:potassium persulfate:acrylic acid=1.0:1.2:25.0 was reacted under the conditions of temperature of 70℃for 2 h to produce highly water-soluble multifunctional chelating agent,and the Ca2+could be chelated and removed to less than 500 mg/L by using 600 mg/L SZ-1.SZ-1 could be applied under the conditions of 10-40℃,pH=6-14,with low sludge production(0.016 g/L)and suitable viscosity of base fluid(30.9～31.2 mPa∙s),which satisfied the requirements of base fluid viscosity range(10-40 mPa∙s)in SY/T 7627-2021 Technical requirements of water-based fracturing fluid.Conclusion It is expected to provide a new solution for the treatment of drilling wastewater with high calcium content in oil and gas fields,with its green and clean color,excellent performance,and simple preparation.
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