煤层顶板水平井穿层压裂适应性数值模拟

Numerical simulation of adaptability of horizontal well layer-penetrating fracturing in the roof of coal seam

中国煤层气资源丰富,但煤层物性差、开发难度大,压裂是实现煤层气有效动用的关键技术之一。由于煤层强度低、易破碎、易坍塌,不利于后续压裂改造,煤层顶板水平井压裂技术为软煤层煤层气开发提供了新的思路。然而,目前适合上述压裂工艺的有利地质条件不明晰,水平井离煤层顶面距离、簇间距等参数的确定尚缺乏理论依据。为此,基于平面三维多裂缝扩展模型,同时考虑层间垂向断裂韧性及滤失非均质性,建立了煤层顶板水平井分段多簇压裂裂缝扩展数学模型。研究结果表明:①煤层为相对低地应力时,压裂裂缝沟通煤层的效果好,压裂效果较为理想,而煤层为相对高地应力时,压裂效果则较差;②水平井与煤层顶面距离对穿层压裂效果有重要影响,特别是在“低—高—低”地应力组合剖面下,为获得理想的压裂效果,建议水平井与煤层顶面距离在4 m以内;③地应力组合剖面是压裂效果的关键控制因素,煤层断裂韧性、滤失系数为次要控制因素。结论认为,该研究成果为我国煤层气有效动用和效益开发提供了理论支撑。

In China,there are abundant coalbed methane resources,but coal seams are poor in physical properties and their development difficulty is high.Fracturing is one of the key technologies to produce coalbed methane effectively.Coal seams are of low strength and tend to break and collapse,which is not beneficial to the subsequent fracturing stimulation.The horizontal well fracturing technology in the roof of coal seam provides a new idea for the development of coalbed methane.At present,however,the geological conditions suitable for the horizontal well fracturing technology in the roof of coal seam are not determined,and there is no theoretical basis for determining the distance between the horizontal well and the top surface of coal seam,the cluster spacing and other parameters.To solve these problems,this paper establishes the mathematical model of fracture propagation induced by the multi-stage,multi-cluster fracturing of horizontal well in the roof of coal seam based on the planar 3D multi-fracture propagation model,combined with interlayer vertical fracture toughness and leak-off heterogeneity.And the following research results are obtained.First,when the in-situ stress of a coal seam is relatively low,the coal seam is communicated well through hydraulic fractures and the fracturing effect is better.When the in-situ stress of a coal seam is relatively high,the fracturing effect is worse.Second,the distance between the horizontal well and the top surface of coal seam has a great influence on the layer-penetrating fracturing effect,and especially in the“low-high-low”in-situ stress combination profile.To achieve an ideal fracturing effect,the distance between the horizontal well and the top surface of the coal seam is recommended to be within 4m.Third,the in-situ stress combination profile is the key control factor of fracturing effect,while the fracture toughness and leak-off coefficient of coal seam are the secondary control factors.In conclusion,the research results provide a theoretical support for the effective production and benefit development of coalbed methane in China.