Oil well pipe connection structure and oil well pipe

An oil well pipe coupling structure comprising:

• a first oil well pipe having an external thread portion on an outer circumferential surface;
• and
• a second oil well pipe that has an internal thread portion that is screwed to the external thread portion on an inner circumferential surface and can be connected to the first oil well pipe,
• wherein the external thread portion is made up of a first external thread portion having a diameter of a predetermined dimension and a substantially tapered second external thread portion that is disposed on one end portion side with respect to the first external thread portion and gradually contracts radially toward one end side,
• the internal thread portion is made up of a first internal thread portion that is disposed on an opening end side and has a diameter of a predetermined dimension and a substantially tapered second internal thread portion that gradually contracts radially from the first internal thread portion side toward an inner side,
• the first external thread portion is screwed to the first internal thread portion, the second external thread portion is screwed to the second internal thread portion, and the first oil well pipe and the second oil well pipe are coupled together;
• wherein the external thread portion has a third external thread portion that is disposed on the one end side with respect to the second external thread portion and has a diameter of a predetermined dimension,
• the internal thread portion has a third internal thread portion that is disposed on the inner side with respect to the second internal thread portion and has a diameter of a predetermined dimension, and
• the third external thread portion can be screwed to the third internal thread portion, wherein the external thread portion has a first circular region including the first external thread portion,
• a second circular region including the second external thread portion, and a third circular region including the third external thread portion that are virtually divided in a radial direction,
• an area of the first circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the first oil well pipe,
• an area of the second circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the first oil well pipe,
• an area of the third circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the first oil well pipe,
• the internal thread portion has a fourth circular region including the first internal thread portion, a fifth circular region including the second internal thread portion, and a sixth circular region including the third internal thread portion that are virtually divided in a radial direction,
• an area of the fourth circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the second oil well pipe,
• an area of the fifth circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the second oil well pipe, and
• an area of the sixth circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the second oil well pipe.