Mechanical characteristics and fracture propagation mechanisms of the Gulong shale
), Abstract
The Gulong shale oil represents China’s first attempt at large-scale exploration and exploitation of the oil contained in shale sequences without intercalations. Clarifying the rock mechanical characteristics and fracture propagation mechanisms of the Gulong shale is vital for guiding the selection of landing zones and fracturing design and engineering parameter optimization. In this study, the mineral distribution, thin section observation and rock mechanics tests are performed to clarify the Gulong shale as “fine layered” texture in mechanics and reveal the fracture propagation mechanisms under the control of multiple geological and engineering factors. It is shown that the Gulong shale is characterized by high clay mineral content (Avg. 46.6 %), strong plasticity, a foliation intensity of up to 1000~3000 stripes per meter and strong mechanical anisotropy. Unlike the brittle fracturing of conventional shale, the typical rock samples from Gulong exhibit high-frequency fluctuation in mechanical property, with a fluctuation frequency of 3.33 times per cm for a compressive strength greater than 20 MPa. The fracturing process is observed as a steady gradual process with a slow post-peak stress decline, and along a random path in a zigzagged shape. Meanwhile, in the case of high-density foliation fractures, the hydraulic fractures in the Gulong shale are of complex morphology, with their height and length being significantly constrained. The limited vertical and horizontal extension of hydraulic fractures has been a major constraint for the effective stimulation of the Gulong shale oil reservoir. It is thereby suggested that the hydraulic stimulation of the Gulong shale oil reservoir should follow the principle of controlling near-wellbore fracture branching and further extending distal fracture networks, while placing the fracturing treatment under more effective control to suppress the development of near-wellbore fractures and boost the extension of main fractures to sufficiently expand the stimulated reservoir volume.
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References
ZHI Dongming, LI Jianzhong, ZHANG Wei, et al. Exploration breakthrough and its significance of Jingjingzigou Formation in Shuangji tectonic zone of Jimsar Sag in Junggar Basin[J]. Acta Petrolei Sinica, 2022, 43(10): 1383-1394.
FU Jinhua, LI Shixiang, NIU Xiaobing, et al. Geological characteristics and exploration of shale oil in Chang 7 Member of Triassic Yanchang Formation, Ordos Basin, NW China[J]. Petroleum Exploration and Development, 2020, 47(5): 870-883.
ZHOU Lihong, HE Haiqing, GUO Xujie, et al. Main factors controlling the medium-mature shale oil enrichment and exploration breakthrough in the Paleogene Lower E3s1L in Qikou Sag, Bohai Bay Basin[J]. Oil & Gas Geology, 2022, 43(5): 1073-1086.
LI Guoxin, ZHU Rukai, ZHANG Yongshu, et al. Geological characteristics, evaluation criteria and discovery significance of Paleogene Yingxiongling shale oil in Qaidam Basin, NW China[J]. Petroleum Exploration and Development, 2022, 49(1): 18-31.
LIU He, KUANG Lichun, LI Guoxin, et al. Considerations and suggestions on optimizing completion methods of continental shale oil in China[J]. Acta Petrolei Sinica, 2020, 41(4): 489-496.
SUN Longde, LIU He, HE Wenyuan, et al. An analysis of major scientific problems and research paths of Gulong shale oil in Daqing Oilfield, NE China[J]. Petroleum Exploration and Development, 2021, 48(3): 453-463.
PANG Yanming, WANG Yongzhuo, WANG Rui, et al. Production test analysis and productivity prediction of horizontal wells in Gulong shale oil reservoirs, Songliao Basin[J]. Petroleum Geology & Oilfield Development in Daqing, 2020, 39(3): 137-146.
LIU Chenglin, LIU Xinju, ZHANG Hongjun, et al. Application of an integrated geology-reservoir engineering approach to shale oil development in Ansai area, Ordos Basin[J]. Oil & Gas Geology, 2022, 43(5): 1238-1248.
LU Cong, LU Yunxiao, GUO Jianchun, et al. Stability of the formation interface under the impact of hydraulic fracture propagation in the vicinity of the formation interface[J]. Petroleum Science, 2020, 17(4): 1101-1118.
DONG Jingnan, YUAN Guangjie, WANG Xiangyang, et al. Experimental study of multi-timescale crack blunting in hydraulic fracture[J]. Petroleum Science, 2021, 18(1): 234-244.
ZHAO Xianzheng, JIN Fengming, LIU Xuewei, et al. Numerical study of fracture dynamics in different shale fabric facies by integrating machine learning and 3-D lattice method: A case from Cangdong Sag, Bohai Bay Basin, China[J]. Journal of Petroleum Science and Engineering, 2022, 218: 110861.
WANG Yuhua, LIANG Jiangping, ZHANG Jinyou, et al. Resource potential and exploration direction of Gulong shale oil in Songliao Basin[J]. Petroleum Geology & Oilfield Development in Daqing, 2020, 39(3): 20-34.
ZHAO Peng, XIE Lingzhi, FAN Zhichao, et al. Mutual interference of layer plane and natural fracture in the failure behavior of shale and the mechanism investigation[J]. Petroleum Science, 2021, 18(2): 618-640.
LABANI M M, REZAEE R. The importance of geochemical parameters and shale composition on rock mechanical properties of gas shale reservoirs: A case study from the Kockatea Shale and Carynginia Formation from the Perth Basin, western Australia[J]. Rock Mechanics and Rock Engineering, 2015, 48(3): 1249-1257.
LI L C, TANG C A, WANG S Y, et al. A coupled thermohydrologic-mechanical damage model and associated application in a stability analysis on a rock pillar[J]. Tunnelling and Underground Space Technology, 2013, 34: 38-53.
LI Tianjiao, LI Lianchong, Tang Chun’an, et al. A coupled hydraulic-mechanical-damage geotechnical model for simulation of fracture propagation in geological media during hydraulic fracturing[J]. Journal of Petroleum Science and Engineering, 2019, 173: 1390-1416.
JIANG Tingxue, BIAN Xiaobing, SU Yuan, et al. A new method for evaluating shale fracability index and its application[J]. Petroleum Drilling Techniques, 2014, 42(5): 16-20.
ZHAO Jinzhou, XU Wenjun, LI Yongming, et al. A new method for fracability evaluation of shale-gas reservoirs[J]. Natural Gas Geoscience, 2015, 26(6): 1165-1172.
ZHAO Jinzhou, YIN Qing, LI Yongming. Key scientific issues of hydraulic fracturing in Chinese shale gas reservoir[J]. SCIENTIA SINICA Physica, Mechanica & Astronomica, 2017, 47(11): 15-28.
JIAO Fangzheng. FSV estimation and its application to development of shale oil via volume fracturing in the Ordos Basin[J]. Oil & Gas Geology, 2021, 42(5): 1181-1188.
WU Qi, XU Yun, WANG Xiaoquan, et al. Volume fracturing technology of unconventional reservoirs: Connotation, optimization design and implementation[J]. Petroleum Exploration and Development, 2012, 39(3): 352-358.
LIU Zhanli, WANG Tao, GAO Yue, et al. The key mechanical problems on hydraulic fracture in shale[J]. Chinese Journal of Solid Mechanics, 2016, 37(1): 34-49.
SHI Can, LIN Botao. Principles and influencing factors for shale formations[J]. Petroleum Science Bulletin, 2021, 6(1): 92-113.
CAI Meng, TANG Pengfei, WEI Xu, et al. Optimization of composite volume fracturing technology for Gulong shale oil[J]. Petroleum Geology & Oilfield Development in Daqing, 2022, 41(3): 156-164.
YAN Honglin, DUAN Yanqing, FAN Lihua, et al. Fracturing operation diagnosis and process control technology of the shale oil reservoirs in Gulong Sag, Songliao Basin[J]. Petroleum Geology & Oilfield Development in Daqing, 2020, 39(3): 176-184.
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