Construction and Application of High-Strength Flexible Cement Sheath for Deep and Ultra-deep Shale Gas Cementing

With the gradual development of oil and gas exploration, deep and ultra-deep shale gas has become the main block for shale gas reserve growth and production addition in China. Deep and ultra-deep shale gas is characterized by deep burial depth, high pore pressure and formation fracture pressure, so the production and transformation means in the later stage are more complicated. Deep shale gas cementing puts forward higher requirements on cement slurry system, especially the mechanical properties and long-term sealing ability of cement sheath under high temperature and pressure conditions. This paper studies the cement slurry system which can meet the requirements of staged fracturing and long-term sealing. Through the physical simulation device, the mechanical property analysis and evaluation method of cement sheath under staged fracturing condition is developed. By interphase filling technology, a new type of elastic material is developed. Meanwhile, nano-emulsion filling technology is used to realize void filling and reduce porosity of cement stone and micro-ring gap. Finally, a high-strength flexible cement slurry system for deep and ultra-deep shale gas well cementing is constructed, with a temperature resistance of 200°C, elastic modulus of cement stone less than 6GPa and compressive strength ≥25MPa/48h. It can meet the requirements of 120MPa and 25 cyclic loading staged fracturing. This cement slurry system has been widely applied in shale gas areas, and the durability and sealing ability of cement sheath have been significantly improved, which can meet the needs of large-scale staged fracturing and ensure the efficient development of deep and ultra-deep shale gas wells.