Research On Plugging System Suitable For Permanently Abandoned Wells At Sea

Permanently abandoned wells refer to oil,gas,and water wells that cannot continue to be used due to various reasons and are approved as permanently abandoned oil,gas,and water wells by relevant departments.As more and more oil and gas wells reach or approach production life,it is becoming more and more important to seal them for completeness.Especially with the improvement of environmental protection awareness,the requirements for offshore oilfields are gradually strict.In addition to ensuring that oil and gas do not leak during the production stage,permanent plugging operations are required after they reach the production life to prevent the occurrence of residual oil and gas underground after the shutdown.Leakage,causing environmental damage.In addition,the cost of offshore operations is high,and leakage not only means a huge cost for remedial operations,but also causes significant damage to the reputation of the operator.Therefore,it is of great significance to study the permanent plugging of abandoned offshore wells.The blocked layers of permanently abandoned wells are mainly inside casing and nonpermeable formations.The common features are: the solidification of cement slurry in the well is no longer a groundwater environment,and the hydration reaction of cement is basically not affected by external water.Carried out under airtight conditions,the shrinkage of the cement stone,which is originally hydrated and shrinkable,will be more serious,resulting in the formation of microannular gaps between the cement and the casing and the formation,the cement stone cementation quality is reduced,and the sealing performance is greatly weakened.Aiming at the special environment of permanently abandoned offshore wells,this article has developed a new type of composite shrinkage reducing agent suitable for oil well cement.The agent is composed of a compound surfactant and a water-absorbing resin as the main functional materials.The combined effect of the shrinkage force and the downhole pressure replenishing effect of the water-absorbing resin achieves the shrinkage effect of the cement stone.Using Jiahua G-grade cement at a water-cement ratio of 0.44,the types and amounts of surfactants and water-absorbing resins are optimized.The composite shrinkage reducing agent developed can control the linear shrinkage rate of cement at 90 ℃ within 0.1025‰ while maintaining The high compressive strength of the cement stone and the long-term(56 d)high cementation strength of the simulated casing have greatly improved the quality of the packing.In addition,the dispersion modification method and use method of nanomaterials were studied,which effectively improved the dispersion effect of nanomaterials;the research optimized the dispersion-modified nano-calcined kaolin with enhanced permeability reduction,which reduced the permeability of cement stone by 44%.The compressive strength of cement stone has been increased by 14%,which effectively improves the corrosion resistance of cement stone to seawater and carbon dioxide and is more suitable for the requirements of cement stone corrosion resistance when plugging abandoned offshore wells.Researched and constructed an oil well cement slurry system suitable for the plugging of abandoned offshore wells.The performance of the system meets the needs of engineering operations.It has excellent strength performance,stability performance,excellent rheology,corrosion resistance and high air tightness.The study of the plugging system provides an effective guarantee for the long-term completeness of the permanent abandonment well.