Laboratory Investigation of Cryogenic Fracturing Effect of Hot Dry Rock Wellbores Under Triaxial-Confining Stresses

Hot dry rock (HDR) contains abundant thermal energy, which can be extracted through fracturing and used for electricity generation. Due to its deep depth, high-temperature and high-pressure conditions, it is difficult to initiate fractures for conventional hydraulic fracturing technology. This paper study the advantage of cryogenic fracturing on the HDR. We conducted a series of laboratory experiments on rock samples with different length of open-hole under triaxial-confining stresses (10 MPa). The nitrogen fracturing wellbores of high temperature (100–300 ℃) granites were treated with LN (liquid nitrogen) and NO-LN and retained with 20 mm and 30 mm open-hole to form four control groups. The fracturing results show that LN cryogenic stimulation is more effective in reducing the HDR initiation pressure. When the open-hole is 20 mm, the breakdown pressure of samples with LN decreases by 13.9%–18.7% compared with untreated samples. When the open-hole changed from 20 mm to 30 mm, the breakdown pressure of samples with NO-LN are reduced by 6.7%–15%. The samples with a 30 mm open-hole inside the wellbore formed complex fracture patterns after the nitrogen fracturing. This could be attributed to the longer open-hole, the more complex micro-fractures on the surface and the force of the direction parallel to the cross section is significantly increased.