ABSTRACT

Aiming at the cementing problems of high oil and gas activity and high gas channeling risk in complex high pressure natural gas wells, an ultra-high density cement slurry system (2.40-2.60 g/cm3) was designed. To ensure that the system has a good foundation, based on the principle of particle size distribution, supplemented by close packing design, the optimal ratio of dry mixed materials is finally determined. After that, the gas self-healing agent was added into the system to enhance the mechanical sealing integrity of the cement sheath in the later stage. This agent can produce enough volume expansion in contact with oil and gas and has cementation effect on the interface to ensure the plugging effect on the micro-cracks of cement sheath. Combining the above performance advantages, an ultra-high density natural gas self-healing cement slurry system was finally formed. In the performance evaluation experiment and field application, it is proved that this system overcomes the problems of instability, poor fluidity, high consistency, and weak anti--channeling performance of the traditional high-density cement slurry system. The comprehensive performance meets the long-term sealing integrity of high-pressure natural gas wells, provides guarantee for the safety and reliability of construction, and provides powerful technical support for improving the service life of oil and gas wells.

INTRODUCTION

The Sichuan basin is rich in conventional and unconventional natural gas resources. It is the most potential basin for natural gas exploration and development in China. Its resources are close to 50 % of China 's total natural gas resources. At present, the proportion of deep and ultra-deep wells in this area is getting higher and higher, and the depth of some natural gas wells has reached 8000 m level. With the increase of well depth, the influence of complex geological conditions and the limitation of well structure, there are many problems in the process of drilling and cementing, such as the long bare hole section (1500∼2000 m); very narrow safe density window (0∼0.12 g/cm3); active oil and gas, high reservoir pressure, high risk of gas channeling. Due to the existence of these problems, the high-performance requirements of cement slurry are necessary: not only have ultra-high density to pressure stabilize the stratum, but also have excellent rheological properties to ensure displacement efficiency, more to have a strong ability to prevent gas channeling.

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