Mechanisms and technological parameter optimization of near-wellbore laser thermal fracturing for deep coal seams
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摘要:
中国深部煤层气资源丰富，是煤层气进一步开发的重要领域，但深部煤层气地质条件复杂，具有低孔、超低渗特征。在钻井过程中，钻井液进入储层易造成近井污染，常规水力压裂技术趋于在最大水平主应力方向造缝，全井眼的解堵困难。激光热裂技术具有短时间破裂岩石、同时通过机械设备调控能自由改变激光照射角度，形成径向裂缝、解决近井污染等优势。使用ABAQUS有限元软件，建立激光热裂煤层模型，探讨激光热裂机理及激光工艺参数的影响。分析裂缝长度与数量的变化规律，优选出解决现场近井污染区域的最佳激光参数。结果表明：(1) 激光照射热裂煤层是使煤层表面存在温差而产生热应力导致煤层破裂。(2) 裂缝数量与激光功率、激光照射煤层的时间呈正相关，激光功率由400 W增大到1 000 W时，裂缝数量由10条增加到37条；激光功率600 W时，照射时间由1 s增至15 s，裂缝数量由24条增至36条；裂缝数量与激光频率呈负相关，随着激光照射煤层距离增大先增大后减小，照射距离为10 cm时产生裂缝数量最多。(3) 裂缝长度与激光功率、照射煤层时间以及激光频率呈正相关，与照射煤层距离呈负相关，其中激光照射时间影响最明显，照射时间1 s时裂缝长度为1.52 mm，照射时间增加到5 s时裂缝长度激增为57.6 mm。以陕西韩城深部取心样品为例，激光热裂深部煤层2 m范围内的近井污染最佳激光功率为20 kW，最佳激光照射时间为2 280 s。相较于水力压裂，激光热裂煤层能形成更加复杂的裂缝，但形成的裂缝长度较小，实际应用中，建议将水力压裂技术与激光热裂技术相结合，以实现解堵和增渗的目的。
Abstract:China boasts abundant deep coalbed methane (CBM) resources, which play a significant role in further CBM production. However, deep coal seams exhibit low porosities and ultra-low permeabilities due to intricate geological conditions. In the drilling process, drilling fluids enter the reservoirs, prone to cause near-wellbore contamination. Although conventional hydraulic fracturing technology tends to create fractures in the direction of the maximum horizontal principal stress, it is challenging for this technology to achieve blockage removing throughout the whole borehole. Laser thermal fracturing technology can break rocks in a short time. Furthermore, it allows for the laser irradiation angle to change freely by regulating mechanical equipment, thus forming radial fractures and reducing near-wellbore contamination. Using the ABAQUS finite element software, this study, establishing a model of laser thermal fracturing of coal seams, explored the fracturing mechanisms and the influence of laser technological parameters. Through analyses of the variation patterns of fracture length and number, this study determined the optimal laser parameters targeting near-wellbore contamination areas. Key findings include: (1) Laser thermal fracturing can cause thermal stress on the surfaces of coal seams through temperature differences, ultimately fracturing coal seams. (2) There was a positive correlation between the fracture number and the laser power and irradiation time. Specifically, the number increased from 10 to 37 as laser power expanded from 400 to 1 000 W. At a laser power of 600 W, the number increases from 24 to 36 as the irradiation time prolonged from 1 to 15 s. In contrast, the fracture number negatively correlated with the laser frequency. With an increase in the laser irradiation distance, the fracture number increased initially and then decreased, peaking at an irradiation distance of 10 cm. (3) The fracture length positively correlated the laser power, irradiation time, and laser frequency but negatively correlated the laser irradiation distance. Among these factors, the laser irradiation time produced the most significant influence on the fracture length, which soared from 1.52 to 57.6 mm as the irradiation time increased from 1 to 5 s. For instance, for samples collected from Hancheng, Shaanxi Province through deep coring, the optimal laser power and irradiation time of laser thermal fracturing for the near-wellbore contamination area of deep coal seams extending within 2 m were 20 kW and 2 280 s, respectively. Compared to hydraulic fracturing, laser thermal fracturing can form more complex but shorter fractures. In practical application, the approach combining hydraulic fracturing with laser thermal fracturing is recommended for blockage removing and permeability enhancement.
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图 2 激光功率600 W的花岗岩实验与模型形貌
Fig. 2 Experimental and model morphology of granite under laser power of 600 W
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图 3 激光功率600 W 时缝长随激光照射时间变化趋势
Fig. 3 Fracture length varying with laser irradiation time under laser power of 600 W
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图 4 不同激光功率煤层发生裂纹扩展的应力云图
Fig. 4 Contour maps showing stress during fracture propagation in coal seams under different laser power
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图 5 不同激光功率下煤层裂缝数量分布
Fig. 5 Fracture number distribution of coal seams under different laser power
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图 6 不同激光功率下裂纹扩展长度变化趋势
Fig. 6 Variations of fracture lengths under different laser power
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图 7 不同激光照射时间下煤层裂缝数量分布
Fig. 7 Fracture number distribution of coal seams under different laser irradiation times
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图 8 不同激光照射时间下裂纹扩展长度变化趋势
Fig. 8 Variation trends of fracture length under different laser irradiation times
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图 9 不同激光照射距离下煤层裂缝数量分布
Fig. 9 Fracture number distribution of coal seams under different laser irradiation distances
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图 10 不同激光照射距离下裂纹扩展长度变化趋势
Fig. 10 Variation trends of fracture length under different laser irradiation distances
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图 11 不同激光频率下煤层裂缝数量分布
Fig. 11 Fracture number distribution of coal seams under different laser frequencies
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图 12 不同激光频率下裂纹扩展长度变化趋势
Fig. 12 Variation trends of fracture length under different laser frequencies
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图 13 激光参数与裂缝长度、裂缝数量的相关性
Fig. 13 Correlations of laser parameters with fracture length and number
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图 15 ZTR-1000岩石力学参数测试设备
Fig. 15 ZTR-1000 rock mechanical parameter experimental system
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图 16 不同激光功率下煤层裂缝数量分布
Fig. 16 Fracture number distribution of coal seams under different laser power
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图 17 不同激光功率下裂纹扩展长度变化趋势
Fig. 17 Variation trends of fracture length under different laser power
表 1 激光照射参数
Table 1 Laser irradiation parameters
模拟参数 激光功率/W 照射时间/s 照射距离/cm 激光频率/Hz 数值 400 1 6 10 600 5 8 100 800 10 10 1 000 1 000 15 12 10 000 
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