Selection and Application of Toughening Agent Used in Cementing Gas Storage Well

A toughness-enhanced cement slurry was formulated to cement gas storage wells which have special requirements for the toughness of set cement. A surface modified rubber powder (particle size between 0.15 mm and 0.18 mm) and a polypropylene fiber were used in the cement slurry as an elastic enhancer and a toughening agent, respectively. The optimum concentrations of the elastic enhancer and the toughening agent were determined to be 3.0% and 0.1%, respectively, through laboratory experiments. PVA latex was used as a filter loss reducer to minimize the brittleness of the set cement, improving its impact resistance and antibreaking performance, and satisfying the needs of cement sheath for long-term airtightness under frequent high-pressure injection and production. Laboratory experiments were done to evaluate the toughness-enhanced set cement for its elastic modulus, anti-breaking strength, compressive strength, bonding strength, permeability, and the airtightness of the cement sheath under stress. Compared with the set cement formed with base cement slurry, the elastic modulus of the toughness-enhanced cement slurry was reduced by 43%, the anti-breaking strength was increased by 118%, and the permeability of the set cement was less than 0.05 mD. These data indicated that the toughness-enhanced cement slurry was able to satisfy the needs for airtightness of the cement sheath under 35 MPa and 35℃ temperature difference, suitable for cementing gas storage wells.