A plug for well drilling process provided with mandrel formed from degradable material

A plug for well drilling process provided with:

• (a1) a mandrel formed from a degradable material having a tensile strength of at least 50 MPa at a temperature of 60° C., the mandrel having a thickness reduction of less than 5 mm after immersion for one hour in water at a temperature of 66° C. and having a thickness reduction of at least 10 mm after immersion for 24 hours in water at a temperature of at least 149° C.;
• (b) a pair of rings placed on an outer peripheral surface existing in the orthogonal to an axial direction of the mandrel, at least one of the rings being formed from a degradable material;
• and
• (c) at least one diameter-expandable circular rubber member placed at a position between the pair of rings on the outer peripheral surface existing in the orthogonal to the axial direction of the mandrel.

A plug for well drilling process provided with:

• (a2) a mandrel formed from a degradable material having a tensile strength of at least 50 MPa at a temperature of 60° C.;
• (b) a pair of rings placed on an outer peripheral surface existing in the orthogonal to an axial direction of the mandrel, at least one of the rings being formed from a degradable material;
• and
• (c) at least one diameter-expandable circular rubber member placed at a position between the pair of rings on the outer peripheral surface existing in the orthogonal to the axial direction of the mandrel;
• a reduction rate in a mass of the degradable material after immersion for 72 hours in water at a temperature of 150° C. with respect to a mass prior to immersion being from 5 to 100%.

A plug for well drilling process provided with:

• (a3) a mandrel formed from a degradable material having a shearing stress of at least 30 MPa at a temperature of 66° C.;
• (b) a pair of rings placed on an outer peripheral surface existing in the orthogonal to an axial direction of the mandrel, at least one of the rings being formed from a degradable material;
• and
• (c) at least one diameter-expandable circular rubber member placed at a position between the pair of rings on the outer peripheral surface existing in the orthogonal to the axial direction of the mandrel;
• a reduction rate in a mass of the degradable material after immersion for 72 hours in water at a temperature of 150° C. with respect to a mass prior to immersion being from 5 to 100%.