碎软低渗煤层顶板水平井分段压裂煤层气高效抽采模式

摘要

碎软低渗煤层的煤层气高效抽采一直是制约我国煤层气产业化发展和煤矿瓦斯灾害防治的技术瓶颈.以安徽淮北矿区芦岭煤矿8号碎软低渗煤层为研究对象,通过开展现场调研、分析测试、理论分析、水力压裂物理模拟和数值模拟等工作,提出了碎软低渗煤层的煤层气顶板岩层水平井分段压裂高效抽采模式,揭示了该模式下水力压裂裂缝的扩展延伸规律及控制机理,构建了该模式实施的主要工艺流程.研究结果表明:顶板岩层相对脆性、裂缝扩展压力较高,碎软煤层相对塑性、裂缝扩展压力低.在顶板岩层水平井进行套管射孔和水力压裂,顶板岩层中产生的压裂裂缝,在垂向上向下扩展伸延并穿入碎软煤层;同时在水平方向上也快速扩展延伸,由此产生的牵引作用撕裂下部碎软煤层形成较长的压裂裂缝.数值模拟结果显示,在给定的压裂施工参数条件下,顶板岩层中压裂在碎软煤层中形成的压裂裂缝长度,是直接在碎软煤层中压裂形成的压裂裂缝长度的6.7倍.碎软煤层和顶板岩层中形成的这些压裂裂缝在后续加砂压裂过程中被充填,成为煤层气从下部煤层向顶板岩层水平井运移的导流通道.显然,采用这种抽采模式,碎软低渗煤层可以获得良好的压裂改造效果.研究成果应用于淮北矿区芦岭煤矿煤层气顶板岩层水平井抽采示范工程,取得了很好的产气效果,水平井单井曾连续3,6,12个月平均日产气量分别为10358,9039,7921 m3,截至2017-11-16,已累计产气500万m3,日产气量仍在3200 m3以上,创造了我国碎软低渗煤层的煤层气水平井气产量的新记录.%Coalbed methane high efficiency production from broken soft and low permeable coal seams has been being a technical bottleneck of restricting coalbed methane industrialization development and coal mine gas control for many years.This paper has investigated the No.8 broken soft and low permeable coal seam in Lulling coal mine of Huaibei mining area in Anhui Province,China.Site investigation,sampling and testing,theoretical analysis,hydraulic fracture physical and numerical simulation have been performed.As a result,a new model of coalbed methane high efficiency production from broken soft and low permeable coal seam by roof strata-in horizontal well and staged hydraulic fracture has been presented,the hydraulic fracture extension and control mechanism under this model have been revealed,and the main technological processes of the model implementation have been given.The results show that the roof strata are relatively brittle and have high propagation pressures,while the broken soft coal seam is relatively plastic and has a low propagation pressure.When case perforation and hydraulic fracture are performed in horizontal well of the roof strata,the hydraulic fractures in roof strata,on the vertical,extend downward and penetrate the broken soft coal seam.At the same time,the hydraulic fractures also get rapid horizontal extension,and the traction engendered by this rapid horizontal extension rips the coal seam to form long hydraulic fractures in the coal seam.Numerical simulation results show that the hydraulic fracture length formed in the broken soft and low permeable coal seam when fracturing in roof strata is long as 6.7 times as that when fracturing directly in the coal seam.Those hydraulic fractures formed in the roof strata and the broken soft coal seam are filled with sand proppant in follow-up adding-sand fracturing,and come into being a coalbed methane migration pathway from the underlying coal seam to the roof strata-in horizontal well.It can be seen that broken soft coal seam can get a good fracturing stimulation effect in this new model.The results of this study are applied to the coalbed methane development demonstration project by roof strata-in horizontal well and staged hydraulic fracture in Luling coal mine of Huaibei nining area in Anhui province,China and have obtained a good gas production effect.The average daily gas production rate of the single coalbed methane horizontal well was respectively 10358 m3,9039 m3,7921 m3 in three,six and twelve consecutive months.As of November,16,2017,this horizontal well already produced 5 million cubic meters of coalbed methane gas,and its daily gas production rate was still over 3200 m3.This has created a new record of gas production of a single coalbed methane horizontal well from broken soft and low permeable coal seam in China.