This paper describes techniques for improving the percentage of wellbore volume that is circulated before cementing. The effects of drilling fluid filtercake deposits, aging of filtercake, mechanical scraping of the filtercake, and annular velocity on the displacement efficiency are discussed. Four different water-based drilling fluids (potassium acetate, potassium chloride, gypsum, and fresh water bentonite) used by Shell Canada in their day-to-day operations were investigated in this study.
A large-scale test apparatus was used for this study. The test apparatus consisted of a steel casing run inside a permeable core to realistically simulate a wellbore. The test procedure simulated conditions such as circulation, drill string tripping, drilling fluid conditioning, and cementing. A computerized tracer technique was developed to continually monitor the percentage of wellbore volume that was circulated. A continuous history of the wellbore was recorded when parameters such as annular velocity, drilling fluid rheology, and fluid loss of the drilling fluid were varied. Drilling fluid filtercake buildup and erosion were monitored throughout the test. After cementing, the test model was removed from the test well and cut into sections, and displacement efficiency was measured. The agreement between the measured displacement efficiency and calculated percentage circulatable hole volume verified the test method. The effects of modifying parameters such as drilling fluid properties, annular velocity, and mechanical scraping of filtercake on cement displacement efficiency are presented. Recommendations on which drilling fluids are most desirable for cement displacement efficiency are included.