Mechanisms of Cold Shock during Coalbed Fracturing Assisted with Cryogenic Gases

Zhang Liang; Luo Jiong; Cui Guodong; Fan Zhikun; Ren Shaoran; Zhang Jianguang; Yang Yong; Che Hang

doi:10.3799/dqkx.2016.055

Abstract：The common mechanisms of coalbed fracturing assisted with cryogenic gases,such as liquid nitrogen,have been rela-tively clear,but the impact of low-temperature characteristics of cryogenic gases on coalbed physical properties and its mecha-nism of improving the fracturing performance have not yet attracted wide attentions,and lack of theoretical understanding.The mechanisms of cold shock of cryogenic gases to the coal rock are summarized first.Then based on the freezing condition of for-mation water and the thermal properties of cryogenic gases,the cold shock performance of liquid nitrogen to the coal rock was verified by laboratory experiments,and the behavoirs of the cooled coalbed around the wellbore after liquid nitrogen injection were predicted by numerical simulation method.Then the application potential of cold shock was analyzed.The results show that injecting cryogenic gases,such as liquid nitrogen,during coalbed fracturing,can cause a cold shock to the coal rock around wellbore,which will induce the coal matrix shrinking and the formation water freezing.A large number of new microcracks will generate in the coal rock.The mechanical strength of the coal rock will decrease by 10%-30%.These phenomena are favorable for the ice temporary blocking and the improvement of coalbed fracturing.Taking the coalbed of layer 3 in the Fanzhuang block in the Qinshui basin as example,when a amount of liquid nitrogen with a volume of 30 - 120 m3 is injected underground,a good cooling and shock performance in the coalbed will be obtained within 3-5 m around wellbore.The porosity of the nearby coalbed will increase about 1.5 times and the permeability will increase 4 times.A stretching and squeezing damage may also oc-cur in the coal rock around wellbore.The cold shock of cryogenic gases and its induced ice temporary blocking can be used for the improvement of hydraulic fracturing techniques in coalbed methane and shale gas wells.