Pore structure and Mechanical Property Change of different rocks under nitrogen freezing

To better understand the liquid nitrogen freezing effect on different rocks, shale, coal, sandstone and carbonate were chosen to investigate the pore structure and mechanical properties damage before and after liquid nitrogen freezing. Experimental study shows that an increase in the number and volume of pores for all rocks after freezing. However, the influence of nitrogen freezing on carbonate was not obvious because of intense mineral compaction and poor connectivity of pores. There was also an obvious reduction in Young's modulus and unconfined compressive strength for coal. Especially, the average decrease ratio of Young's modulus was approximately 0.43 and the average decrease ratio of unconfined compressive strength was 0.63 after and before freezing for all coal samples, which reflected strong freezing damage capability of liquid nitrogen. The mechanical properties also clearly decreased after liquid nitrogen treatment for sandstone samples. Unpronounced changes in shale, and carbonate correlate with tight mineral compaction. These conclusions reflect the characteristics variations of pore structure and mechanical properties were influenced directly by the type of rock during liquid nitrogen freezing. Also, it can provide some insights for avoid wellbore instability during hydraulic fracturing process with liquid nitrogen. © 2017 American Rock Mechanics Association (ARMA). All rights reserved