The Toughening Mechanisms of Tough Cement Slurries for the Underground Gas Storage in North China

As the important engineering of China’s “energy resource supply guarantee”, underground gas storage (UGS) has three functions, which are seasonal peak shaving, emergency response to accidents and national energy strategic reserve. The formations into which the UGSs have been built in north China generally have such characteristics as complex geological conditions, deep buried depths, high bottom hole temperatures, narrow density windows, high fluid loss risks, long intervals that need to be cemented in one job, as well as rigorous requirements for borehole integrity because of long-term sealing of the wellbores, etc. Common cement slurries do not have the required properties to satisfy the requirements of well cementing under these conditions. To solve these problems, a multi-functional active toughening material HFOC was developed through extensive experimental research. The general performance and the toughing mechanisms of tough cement slurries were investigated. Using HFOC, a tough cement slurry was formulated for use in cementing UGS wells, and the well cementing techniques, such as prestressed well cementing, were also optimized for the use of the new cement slurry. Laboratory experimental results show that the tough cement slurry has its density adjustable between 1.88 g/cm³ and 1.92 g/cm³, the thickening time of the cement slurry is adjustable and the cement slurry shows right-angle thickening behavior, the top and bottom density difference of the cement slurry is zero, no free water is observed in the cement slurry, the API filter loss is less than 50 mL, the 7-day elastic modulus is less than 6.48 GPa, and the 80℃/24-hour compressive strength is greater than 14 MPa. The toughening mechanisms of the tough cement slurry are investigated from both physical and chemical aspects. It was found that tetracalcium aluminoferrite (C₄AF) in a cement has positive effects on the toughness and mechanical properties of the set cement; an increase in the content of C₄AF in the cement can better prevent the extension of the micro fractures in the set cement, thereby improving the sealing integrity of the cement sheath. This technology has been successfully applied in cementing the production casing in the UGS drilling in north China, and the quality of the well cementing job has satisfied the requirements of UGS construction. This technology can be used as a technical support and reference for the cementing other UGSs.