Lithologic Reservoirs ›› 2026, Vol. 38 ›› Issue (5): 170-178.doi: 10.12108/yxyqc.20260516

• PETROLEUM ENGINEERING AND OIL & GAS FIELD DEVELOPMENT • Previous Articles     Next Articles

Experiment on the fracability of coal rocks based on CT scanning and 3D reconstruction

ZENG Fanhui1(), YANG Weixin1,2, GUO Jianchun1, ZHANG Yu1, ZHANG Ran3   

  1. 1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation·Southwest Petroleum University, Chengdu 610500， China
    2 Sinopec Exploration Company, Chengdu 610500， China
    3 School of Mechanical Engineering, Xihua University, Chengdu 611743， China
  • Received:2025-09-17 Revised:2025-11-15 Online:2026-09-01 Published:2026-09-04

RichHTML

13

PDF (PC)

16

Abstract:

In response to the problem that conventional fracability evaluation methods for coalbed methane reservoirs are difficult to balance fracture morphology and flow connectivity, fracturing comparison experiments were conducted using a true triaxial hydraulic fracturing system on four groups of raw coal cubic specimens with dimensions of 200 mm×200 mm×200 mm under varying conditions of fracturing fluid, in-situ stress difference, and pumping rate. Combined with high-resolution CT scanning and 3D reconstruction technology, the fracture surface area was quantitatively obtained and the 3D fractal dimension was calculated. Fracability was defined as the total fracture surface area per unit modified reservoir volume, and effects of in-situ stress conditions and fracturing fluid type on fracture development characteristics were systematically analyzed. Research results show that: （1） With low viscosity and strong diffusivity, liquid CO2 can effectively increase pore pressure and reduce breakdown pressure, thus significantly improve the fracture complexity. Compared with clear water, liquid CO2 as a fracturing fluid reduces the initiation pressure by 13.8% and increases the fracture surface area by 50.1%. （2） Fracture complexity is highly positively correlated with the 3D fractal dimension, which can be used as an effective quantitative index to characterize reservoir fracability. （3） Under the same experimental conditions, when the horizontal principal stress difference decreases from 5.00 MPa to 3.00 MPa, reservoir fracability increases by 14.7%.When the pumping rate increases from 30 mL/min to 60 mL/min, reservoir fracability increases by 11.3%. Compared with water fracturing, liquid CO2 fracturing improves reservoir fracability by 50.7%.

Key words: coal rock, true triaxial fracturing experiment, CO2 fracturing, fracture propagation, 3D reconstruction, fractal dimension, fracture surface area

CLC Number: 

  • TE371

Fig. 1

Development of natural fractures in coal rocks at 600 m depth of No. 3 coal seam of Lower Permian Shanxi Formation in well TS-X of Datong Coal Mine, Jincheng, Shanxi province"

Fig. 2

Core samples for true triaxial fracturing experiment of coal seams from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

Table 1

Test results of natural fracture density and mechanical parameters of coal rocks from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

煤样 天然裂缝密度/（条·m-2） 杨氏模量/MPa 泊松比
1# 45 5 563.58 0.29
2# 47 4 570.56 0.31
3# 42 4 860.51 0.34
4# 56 2 920.32 0.41

Fig. 3

Drilling simulated wellbore (a) and sealing glue （b） of coal rocks from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

Fig. 4

Equipments of true triaxial fracturing simulation experiment"

Table 2

Parameter settings for true triaxial fracturing simulation experiment of coal rocks from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

煤样 压裂
介质		 最小水平
主应力/MPa		 最大水平
主应力/MPa		 垂向
应力/MPa		 泵注速率/(mL·min-1)
1# 液态CO2 7.00 12.00 9.00 30
2# 液态CO2 7.00 10.00 9.00 60
3# 液态CO2 7.00 10.00 9.00 30
4# 清水 7.00 10.00 9.00 60

Fig. 5

Pumping curve during fracturing of true triaxial fracturing simulation experiment on coal samples from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

Fig. 6

2D fracture plane features after true triaxial fracturing simulation experiment of coal rocks from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

Fig. 7

3D CT images of fractures before and after true triaxial fracturing simulation experiment of coal samples from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

Fig. 8

3D fracture models after true triaxial fracturing simulation experiment of coal samples from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

Table 3

Fracture surface area per unit modified reservoir volume and fractal dimension after fracturing of true triaxial fracturing simulation experiment of coal samples from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

煤样 压裂
介质		 压裂前裂缝
面积/104 mm2		 压裂后裂缝
面积/104 mm2		 裂缝面积增量/104 mm2 单位改造体积内裂缝的
总表面积/（10-2 mm2·mm-3）		 裂缝二维
分形维数		 裂缝三维
分形维数
1# 液态CO2 0.007 2 13.58 13.57 1.70 1.091 2.087
2# 液态CO2 1.450 0 17.32 15.87 2.17 1.127 2.212
3# 液态CO2 0.007 6 15.64 15.63 1.95 1.152 2.163
4# 清水 2.080 0 11.54 9.46 1.44 1.052 2.058

Fig. 9

Workflow diagram of 3D box-counting method"

Fig. 10

Schematic diagram of 3D reconstructed fractures by box-counting method after fracturing of true triaxial fracturing simulation experiment of coal samples from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

Fig. 11

3D fractal dimension of fractures after true triaxial fracturing simulation experiment of coal samples from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

Fig. 12

Fitting relationship between fracture fractal dimension and total fracture surface area per unit modified reservoir volume after simulation experiment of coal samples from Lower Permian Shanxi Formation in Datong Coal Mine, Jincheng, Shanxi province"

[1] 邹才能, 杨智, 黄士鹏, 等. 煤系天然气的资源类型,形成分布与发展前景[J]. 石油勘探与开发, 2019, 46(3):433-442.
ZOU Caineng, YANG Zhi, HUANG Shipeng, et al. Resource types,formation,distribution and prospects of coal-measure gas[J]. Petroleum Exploration and Development, 2019, 46(3):433-442.
[2] 王青, 田冲, 罗超, 等. 四川盆地遂宁—合江地区二叠系龙潭组煤岩气储层特征及勘探前景[J]. 岩性油气藏, 2025, 37(4):26-37.
WANG Qing, TIAN Chong, LUO Chao, et al. Characteristics and exploration prospects of coal-rock gas reservoirs in Per-mian Longtan Formation in Suining-Hejiang area,Sichuan Basin[J]. Lithologic Reservoirs, 2025, 37(4):26-37.
[3] ZENG Fanhui, PENG Fan, GUO Jianchun, et al. Gas transport study in the confined microfractures of coal reservoirs[J]. Journal of Natural Gas Science and Engineering, 2019, 68:102920.
[4] 卞晓冰, 侯磊, 蒋廷学, 等. 深层页岩裂缝形态影响因素[J]. 岩性油气藏, 2019, 31(6):161-168.
BIAN Xiaobing, HOU Lei, JIANG Tingxue, et al. Influencing factors of fracture geometry in deep shale gas wells[J]. Lithologic Reservoirs, 2019, 31(6):161-168.
[5] ZENG Fanhui, GONG Gaobin, ZHANG Yu, et al. Fracability evaluation of shale reservoirs considering rock brittleness,fracture toughness,and hydraulic fracturing-induced effects[J]. Geoenergy Science and Engineering, 2023, 229:1-14.
[6] CHONG K K, GRIESER W V, PASSMAN A, et al. A completions guide book to shale-play development:A review of successful approaches towards shale-play stimulation in the last two decades[R]. Calgary, Society of Petroleum Engineers Canadian Unconventional Resources and International Petroleum Conference, 2010.
[7] LI Jianhua, XIE Heping, LU Jun, et al. New permeability model of deep coal rock considering the structure and 3D stress compression-induced anisotropy[J]. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2022, 8(6):204.
[8] 彭文春, 米洪刚, 徐立富, 等. 神府区块深部煤储层可压性评价方法及应用[J]. 煤炭科学技术, 2025, 53(3):238-247.
PENG Wenchun, MI Honggang, XU Lifu, et al. Fracability evaluation and classification of deep coal reservoirs in the Shenfu block[J]. Coal Science and Technology, 2025, 53(3):238-247.
[9] 李倩, 李童, 蔡益栋, 等. 煤层气储层水力裂缝扩展特征与控因研究进展[J]. 煤炭学报, 2023, 48(12):4443-4460.
LI Qian, LI Tong, CAI Yidong, et al. Research progress on hydraulic fracture characteristics and controlling factors of coalbed methane reservoirs[J]. Journal of China Coal Society, 2023, 48(12):4443-4460.
[10] CHEN Lipeng, WANG Lei, LIU Huaiqian, et al. Study on the micro-fracture-structure and permeability behavior of coal under the action of CO2 based on micro-CT[J]. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2025, 11(1):25.
[11] 孟召平, 刘翠丽, 纪懿明. 煤层气/页岩气开发地质条件及其对比分析[J]. 煤炭学报, 2013, 38(5):728-736.
MENG Zhaoping, LIU Cuili, JI Yiming. Geological conditions of coalbed methane and shale gas exploitation and their comparison analysis[J]. Journal of China Coal Society, 2013, 38(5):728-736.
[12] 翟文宝, 李军, 周英操, 等. 基于测井资料的页岩储层可压裂性评价新方法[J]. 岩性油气藏, 2018, 30(3):112-123.
ZHAI Wenbao, LI Jun, ZHOU Yingcao, et al. New evaluation method of shale reservoir fracability based on logging data[J]. Lithologic Reservoirs, 2018, 30(3):112-123.
[13] YUAN Junliang, ZHOU Jianliang, LIU Shujie, et al. An improved fracability-evaluation method for shale reservoirs based on new fracture toughness-prediction models[J]. SPE Journal, 2017, 22(5):1704-1713.
[14] JIN Xiaochun, SHAH S N, ROEGIERS J C, et al. An integrated petrophysics and geomechanics approach for fracability evaluation in shale reservoirs[J]. SPE Journal, 2015, 20(3):518-526.
[15] 艾林, 周明顺, 张杰, 等. 基于煤岩脆性指数的煤体结构测井定量判识[J]. 岩性油气藏, 2017, 29(2):139-144.
AI Lin, ZHOU Mingshun, ZHANG Jie, et al. Quantitative identification of coal structure based on coal rock brittleness index by logging data[J]. Lithologic Reservoirs, 2017, 29(2):139-144.
[16] LI Yuwei, LONG Min, ZUO Lihua, et al. Brittleness evaluation of coal based on statistical damage and energy evolution theory[J]. Journal of Petroleum Science and Engineering, 2019, 172:753-763.
[17] JARVIE D M, HILL R J, RUBLE T E, et al. Unconventional shale-gas systems:The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment[J]. AAPG Bulletin, 2007, 91(4):475-499.
[18] 袁俊亮, 邓金根, 张定宇, 等. 页岩气储层可压裂性评价技术[J]. 石油学报, 2013, 34(3):523-527.
YUAN Junliang, DENG Jingen, ZHANG Dingyu, et al. Fracability evaluation of shale-gas reservoirs[J]. Acta Petrolei Sinica, 2013, 34(3):523-527.
[19] 王登科, 曾凡超, 王建国, 等. 显微工业CT的受载煤样裂隙动态演化特征与分形规律研究[J]. 岩石力学与工程学报, 2020, 39(6):1165-1174.
WANG Dengke, ZENG Fanchao, WANG Jianguo, et al. Dynamic evolution characteristics and fractal law of loaded coal fractures by micro industrial CT[J]. Chinese Journal of Rock Mechanics and Engineering, 2020, 39(6):1165-1174.
[20] SUI Lili, JU Yang, YANG Yongming, et al. A quantification method for shale fracability based on analytic hierarchy process[J]. Energy, 2016, 115:637-645.
[21] SUI Lili, YU Jian, CANG Dingbang, et al. The fractal description model of rock fracture networks characterization[J]. Chaos,Solitons & Fractals, 2019, 129:71-76.
[22] WANG Fei, WU Xiang, DUAN Chaowei, et al. Quantitative characterization and fracturability evaluation of shale fractures based on 3D CT reconstruction[J]. Progress in Geophysics, 2023, 38(5):2147-2159.
[23] 蒋长宝, 付银兰, 王光淇. 水力压裂煤裂隙网络表征与造缝性能评估试验研究[J]. 煤炭科学技术, 2023, 51(6):62-71.
JIANG Changbao, FU Yinlan, WANG Guangqi. Experimental study on characterization hydraulic fracturing coal fracture network and evolution of fracture forming performance[J]. Coal Science and Technology, 2023, 51(6):62-71.
[24] CIPOLLA C L, WARPINSKI N R, MAYERHOFER M J, et al. The relationship between fracture complexity,reservoir properties,and fracture-treatment design[J]. SPE Production & Operations, 2010, 25(4):438-452.
[25] 侯冰, 陈勉, 李志猛, 等. 页岩储集层水力裂缝网络扩展规模评价方法[J]. 石油勘探与开发, 2014, 41(6):763-768.
HOU Bing, CHEN Mian, LI Zhimeng, et al. Propagation area evaluation of hydraulic fracture networks in shale gas reservoirs[J]. Petroleum Exploration and Development, 2014, 41(6):763-768.
[26] 刘向君, 王小军, 赵保伟, 等. 砂砾岩储集层水力压裂裂缝扩展规律与可压性评价[J]. 新疆石油地质, 2023, 44(2):169-177.
LIU Xiangjun, WANG Xiaojun, ZHAO Baowei, et al. Propagation of hydraulic fractures and fracability evaluation of sandy conglomerate reservoirs[J]. Xinjiang Petroleum geology, 2023, 44(2):169-177.
[27] 丁勇, 马新星, 叶亮, 等. CO2破岩机理及压裂工艺技术研究[J]. 岩性油气藏, 2018, 30(6):151-159.
DING Yong, MA Xinxing, YE Liang, et al. Rock breaking mechanism of CO2 and fracturing technology[J]. Lithologic Reservoirs, 2018, 30(6):151-159.
[28] 寇淑清, 杨宏宇, 高岩, 等. 裂解连杆断裂结合面缺损面积定量描述与分析[J]. 吉林大学学报(工学版), 2013, 43(6):1541-1545.
KOU Shuqing, YANG Hongyu, GAO Yan, et al. Quantitative description and analysis of defect area of fracture surface of splitting connecting rod[J]. Journal of Jilin University (Engineering and Technology Edition), 2013, 43(6):1541-1545.
[29] 徐祖新. 基于CT扫描图像的页岩储层非均质性研究[J]. 岩性油气藏, 2014, 26(6):46-49.
XU Zuxin. Heterogeneity of shale reservoirs based on CT images[J]. Lithologic Reservoirs, 2014, 26(6):46-49
[30] WU Mingyang, WANG Wensong, SHI Di, et al. Improved box-counting methods to directly estimate the fractal dimension of a rough surface[J]. Measurement, 2021, 177:109303.
[1] HUANG Li, ZHAO Longmei, CHEN Dong, ZHAO Haoyang, ZHANG Yixin, SHI Shi, ZHANG Wen, JI Liang. Characteristics and exploration potential of coal rock gas reservoirs in Carboniferous-Permian in Yichuan area, Ordos Basin [J]. Lithologic Reservoirs, 2026, 38(5): 40-50.
[2] ZHAO Sen, CHEN Sheng, LI Xinyu, DU Wenhui, WANG Xiujiao, YANG Hao, LI Yandong, DAI Chunmeng. Intelligent seismic prediction method for porosity of coal rock gas reservoirs based on hybrid random forest: A case study of Carboniferous Benxi Formation 8# coal in northern Mizhi area, Ordos Basin [J]. Lithologic Reservoirs, 2026, 38(5): 159-169.
[3] ZHANG Mengbo, PENG Jiankang, CUI Xiaojie, ZHANG Dong, NI Na, LONG Shengfang, WEI Penghui. Seismic prediction technology for coal rock gas reservoir of Carboniferous Benxi Formation in northern Mizhi area, Ordos Basin [J]. Lithologic Reservoirs, 2026, 38(1): 162-171.
[4] ZHANG Yanjun, LIU Zhengjun, XU Hao, HE Wenjie, LIU Yaru, XING Liang, ZHOU Desheng, WANG Zhen. Research progress on retention effects of pre-CO2 fracturing fluid of shale oil reservoirs [J]. Lithologic Reservoirs, 2026, 38(1): 180-190.
[5] SU Shuai, QU Hongjun, YIN Hu, ZHANG Leigang, YANG Xiaofeng. Fractal characteristics of pore throat structure and their influence on reservoir physical properties of tight sandstone reservoir: A case study of Triassic Chang 8 member in Fuxian area, Ordos Basin [J]. Lithologic Reservoirs, 2025, 37(6): 88-98.
[6] TANG Shukai, GUO Tiankui, WANG Haiyang, CHEN Ming. Numerical simulation of fracture propagation law of in-fracture temporary plugging and diverting fracturing in tight reservoirs [J]. Lithologic Reservoirs, 2024, 36(4): 169-177.
[7] ZHANG Fengqi, LI Yinong, LUO Julan, REN Xiaofeng, ZHANG Lanxin, ZHANG Jieyu. Microscopic pore structure characteristics of shale of Ordovician Wulalike Formation in western Ordos Basin [J]. Lithologic Reservoirs, 2022, 34(5): 50-62.
[8] LIANG Zhikai, LI Zhuo, LI Lianxia, JIANG Zhenxue, LIU Dongdong, GAO Fenglin, LIU Xiaoqing, XIAO Lei, YANG Youdong. Relationship between multifractal characteristics of pore size and lithofacies of shale of Shahezi Formation in Changling fault depression，Songliao Basin [J]. Lithologic Reservoirs, 2020, 32(6): 22-35.
[9] WANG Xiangzeng, SUN Xiao, LUO Pan, MU Jingfu. Progress and application of CO2 fracturing technology for unconventional oil and gas [J]. Lithologic Reservoirs, 2019, 31(2): 1-7.
[10] DING Yong, MA Xinxing, YE Liang, XIAO Yuanxiang, ZHANG Yanming, GU Yonghong, MA Chaoxing. Rock breaking mechanism of CO2 and fracturing technology [J]. Lithologic Reservoirs, 2018, 30(6): 151-159.
[11] CAO Taotao, DENG Mo, LIU Hu, SONG Zhiguang, CAO Qinggu, HUANG Yanran. Influences of soluble organic matter on reservoir properties of shale [J]. Lithologic Reservoirs, 2018, 30(3): 43-51.
[12] ZHU Hanqing, JIA Ailin, WEI Yunsheng, JIA Chengye, JIN Yiqiu, YUAN He. Nanopore structure characteristics of shale based on Ar adsorption [J]. Lithologic Reservoirs, 2018, 30(2): 77-84.
[13] CHEN Jukai, ZHU Yanming, CUI Zhaobang, ZHANG Chuanghui. Pore structure and fractal characteristics of Longmaxi shale in southern Sichuan Basin [J]. Lithologic Reservoirs, 2018, 30(1): 55-62.
[14] CHEN Mingqiang, WANG Ning, ZHANG Yang, REN Long. Productivity prediction method of shallow reservoir inWeibei Oilfield [J]. Lithologic Reservoirs, 2017, 29(5): 134-139.
[15] GE Xiaobo, LI Jijun, LU Shuangfang, CHEN Fangwen, YANG Dexiang, WANG Quan. Fractal characteristics of tight sandstone reservoir using mercury intrusion capillary pressure:a case of tight sandstone reservoir in Jizhong Depression [J]. Lithologic Reservoirs, 2017, 29(5): 106-112.
Viewed
Full text


Abstract

Cited
  Shared   
  Discussed   
No Suggested Reading articles found!
TRENDMD:

Copyright © 2018 Lithologic Reservoirs

Support by Beijing Magtech support@magtech.com.cn