Research on Key Materials and Systems of Ultra High Temperature Cement Slurry for Ultra-Deep Wells Cementing

Abstract With the continuous deepening of oil and gas exploration and development, the number of ultra-deep and ultra-high temperature wells is gradually increasing in China. The key materials and systems of cementing slurry are crucial for ensuring the safe construction of wells, effective sealing of cement sheath, and safe production of oil and gas. However, the complex working conditions such as ultra-high temperature pose serious challenges to the materials and systems of cement slurry. Focusing on the technical difficulties of ultra-high temperature cement slurry for deep and ultra-deep wells cementing, the cementing retarder, fluid loss additive, stabilizer, and set cement strength retrogression inhibitor with the temperature resistance of 240 ° C were developed through the technical methods, such as optimizing polymer molecular structure, surface modification of powder materials, and synergistic effects between materials. The novel cement slurry system with the temperature resistance of 240 ° C was formed on basis of the above-mentioned four key additives and their supporting materials. Its comprehensive performance was evaluated. The cement slurry system has the resistance to bottom hole circulating temperature of 240 ° C. After curing at 240 ° C, the settling stability of cement slurry can be controlled within 0.05 g/cm3, the API fluid loss is less than 50 mL, and its thickening time can be adjusted arbitrarily within 200 to 600 min, which is linearly related to the test temperature and retarder dosage. The compressive strength of set cement is 45.8 MPa and 55.2 MPa cured at 240 ° C for 2 d and 28 d, respectively. The comprehensive performance of the cement slurry system is excellent, solving a series of problems such as slurry instability, non-adjustable thickening time, uncontrollable water loss, cement stone cracking, and strength decline under ultra-high temperatures. It can meet the requirements of the ultra-high temperature cementing operations and long-term sealing of cement sheath for complex deep and ultra-deep wells. The key materials and systems of ultra-high temperature cement slurry have been successfully applied for more than 20 wells in China, including the highest temperature well of Qiantan 1 with 213 ° C in Dagang Oilfield and the deepest vertical well Pengshen 6 in Asia with 9026 m. The novelty of this paper is in the ability to solve the problems of high-temperature performance failure of cementing additives and the instability of slurry system. Meanwhile, it can provide the technical guidance for the polymer structural design and synthesis of key additives, and the formation of ultra-high temperature cement slurry. It plays an important role in the safe and efficient exploration and development of deep and ultra-deep oil and gas.