ABSTRACT

Salt cavern gas storages have the problems of shallow burial, low temperature, high requirements for low temperature early strength performance, salt resistance performance of cement slurry and sealing integrity under alternating load of cement sheath. A new cement slurry system was formed by optimizing the key additives such as salt-resistant early strength fluid loss additive, strength enhancing agent and gas self-healing agent, called low temperature early strength toughness self-healing cement slurry system. The performance evaluation experiment shows that the cement slurry system has the advantages of rapid development of low temperature strength (strength development at 52 °C for 6 hours), good comprehensive performance (meet the requirements of field cementing construction), excellent mechanical properties (compressive strength 33.27 MPa and Young's modulus 5.65 GPa for 7 days), and good sealing integrity of cement sheath under alternating load conditions (no damage occurred after 50 cycles under the condition of confining pressure of 28 MPa, alternating internal pressure of 4∼24 MPa and alternating temperature of 50∼65 °C), which can ensure the long-term sealing integrity of salt cavern gas storage.

INTRODUCTION

Sealing performance of salt cavern cavity and wellbore is the key to long-term efficient operation of salt cavern gas storages (Yuan, 2008). Cementing quality is the most important factor to ensure wellbore sealing.

The salt layer of salt cavern gas storage is shallow and always with low formation temperature (maximum 60 °C). The highest temperature in the well is generally about 50 °C during normal cementing operations, and it will be lower in winter. For saltwater cement slurry system, has the problems such as poor stability at low temperature, difficult to control fluid loss, slow development of compressive strength, strong thixotropy, poor rheology and so on. These will make cementing quality difficult to guarantee (Qin, 2015; Zhu, 2006). At the same time, salt cavern gas storage has large gas injection and production, and the wellbore is under alternating stress state. During the service life of salt cavern gas storages, alternating stress must be considered to ensure the safe operation of gas storage for at least 30 years (Ding, 2010; Li, 2012; Li, 2010). Therefore, to ensure the long-term safe operation of gas storages, the stability of cement sheath strength and long-term sealing performance must be guaranteed.

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