Numerical simulation on the effect of drilling engineering factors on coal fines output in coalbed methane horizontal wells: a case study of Liulin block
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摘要: 煤粉产出是制约煤层气开发的关键因素。影响煤粉产出的因素包括静态和动态因素，钻井工程是动态因素中影响煤粉产出的重要因素之一。以柳林煤层气区块为例，从井壁稳定性角度出发，运用ANSYS软件建立了二维、三维模型，分别模拟了不同钻井方位、钻井液密度、井眼半径、井斜情况下井壁的受力和变形，分析了煤层气水平井钻井工程因素对煤粉产出的影响。研究表明，井眼大小和井斜对煤粉产出的影响较大，而井眼方位和钻井液密度对煤粉产出的影响较小；井眼方位沿最大水平地应力方向时，井周等效应力差值最大，应力集中最明显；钻井液密度过大会导致井壁拉伸破坏；水平井钻进过程中，井眼半径越大、井斜越大，越易产生煤粉。为了减少钻井因素导致的煤粉产生，建议研究区水平井钻进时，井眼方位选择沿最小水平地应力方向，水平段选用清水钻进，水平段井眼倾角尽量与煤层倾角保持一致。研究成果为煤粉管控措施提供参考。Abstract: The coal fines output is the key factor restricting the development of coalbed methane.Factors affecting coal fines output include static factors and dynamic factors.Drilling engineering is one of the important dynamic factors affecting coal fines output.Taking Liulin coalbed methane block as an example, this study established 2D and 3D models by ANSYS software from the perspective of the wellbore stability, and simulated the force and deformation of the borehole wall under different drilling orientations, drilling fluid density, wellbore size, and wellbore inclination.The influence of factors in drilling engineering on the coal fines production was analyzed for the coalbed methane horizontal well.Results show that borehole size and inclination on coal fines production exert greater effect than borehole orientation and drilling fluid density.When the borehole orientation is along the direction of the maximum horizontal in-situ stress, it demonstrates the biggest difference in the equivalent stress around the borehole, and the most obvious stress concentration.Excessive density drilling fluid will lead to tensile failure of the wellbore.In the process of horizontal well drilling, the larger the radius of the wellbore and the larger the inclination of the well, the easier it is to generate coal fines.In order to reduce the generation of coal fines caused by drilling factors, it is suggested that when drilling horizontal wells in the study area, the borehole orientation should be along the direction of the minimum horizontal in-situ stress, the horizontal section should be drilled with clear water, and the inclination of the horizontal section should be consistent with the inclination of the coal seam.The research results provide a theoretical basis for the control measures of coal fines.
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图 4 井周等效应力、位移随井周角的变化曲线
Figure 4. Variation curves of the equivalent stress and displacement around the well with the angle around the well circumference
图 5 井眼沿最大、最小及中间水平地应力方向(从左到右)的等效应力和位移云图
Figure 5. Equivalent stress and displacement contours of the borehole along the maximum, minimum, and intermediate horizontal in-situ stress directions (from left to right)
图 6 等效塑性应变随井周角变化曲线
Figure 6. Variation curve of equivalent plastic strain with the angle of the well circumference
图 7 井斜85°、87.5°、90°等效应力和位移云图
Figure 7. Equivalent stress and displacement nephogram with well inclination of 85°, 87.5° and 90°
图 8 等效应力、位移随节点距井眼中心距离变化曲线
Figure 8. Curves of equivalent stress and displacement with distance between node and borehole center
表 1 4号煤层试验井结果数据
Table 1. Results from test wells
试验井编号 试验点深度/m 最小水平地应力/MPa 最大水平地应力/MPa ED1 479.32 8.82 12.93 ED2 862.83 12.11 17.7 EP2 1 013.59 14.87 21.57 EP3 710.07 12.48 17.51 EP5 564.8 8.19 13.08 
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