According to the SPE E&P glossary, high temperature is where the undisturbed bottom hole temperature (at prospective reservoir depth or total depth) is greater than 300°F or 150°C. As for high pressure, that definition is met when the maximum anticipated pore pressure of the porous formation to be drilled exceeds a hydrostatic gradient of 0.8 psi/ft, or the well requiring pressure control equipment has a rated working pressure in excess of 10,000 psi or 69 MPa.
Halliburton has further defined HP/HT wells as follows:
>300°F (150°C) - 350°F (175°C)
>10,000 psi (69 MPa) - 15,000 psi (103 MPa)
>350°F (175°C) - 400°F (200°C)
>15,000 psi (103 MPa) - 20,000 psi (138 MPa)
>400°F (200°C) and above
>20,000 psi (138 MPa) and above
As tools and technologies for HP/HT wells become conventional, the temperatures and pressures considered high, extreme or ultra will continue to rise.