Plug, retaining member, and method for well completion using plug

A retaining member having an annular shape, used for a plug configured to plug a wellbore, wherein

• the retaining member is configured to be used in the plug in such a manner as to be adjacent to an elastic member having an annular shape and being attached to an outer circumferential surface of a tubular member having a hollow structure, the annular elastic member being deformable by an exerted pressure,
• the retaining member includes:
• an annular inner portion configured to be in contact with the outer circumferential surface of the tubular member and is configured to be spaced from a slip;
• and
• an annular outer portion configured not to be in contact with the tubular member before and after deformation of the elastic member, the annular outer portion having an inner diameter that is equivalent to or larger than that of the annular inner portion and being adjacent to the annular inner portion,
• the annular outer portion includes a face configured to face the elastic member,
• the annular inner portion includes a face that is in contact with the annular outer portion and that is inclined so as to be increasingly distant from the annular elastic member with increasing proximity to an outer periphery of the plug, and
• one of edges of an inner circumferential surface of the annular inner portion that comes in contact with the tubular member extends toward the elastic member, and
• wherein the annular inner portion has first surface in contact with the annular outer portion and the elastic member, and
• wherein the first surface of the annular inner portion has a varying slope with respect to the tubular member, including:
• a first slope at a first side adjacent to the tubular member;
• and
• a second slope extending from the first slope, the second slope being greater than the first slope.