紧邻碎软煤层顶板水平井分段穿层压裂裂缝延展机理

Fracture propagation mechanism of staged through-layer fracturing for horizontal well in roof adjacent to broken-soft coal seams

紧邻碎软煤层顶板分段压裂水平井是碎软低渗煤层煤层气高效开发的有效技术,裂缝从煤层顶板穿层扩展沟通煤层为煤层气渗流提供通道是该技术应用的关键。针对煤层顶板水平井分段穿层压裂裂缝延展过程,开展真三轴煤岩组合体水力压裂物理模拟试验,应用声发射监测技术对裂缝扩展的动态过程进行监测,研究不同排量条件下的裂缝延伸形态;分析了水力压裂裂缝附近诱导应力场分布,采用扩展有限元方法研究分段穿层压裂的缝间干扰情况,对比了不同压裂段间距条件下,先压裂缝对后压裂缝的延伸形态和起裂压力的影响;最后结合工程试验以及微地震监测数据,对裂缝的空间形态进行了分析。研究结果表明:(1)在合理设置施工排量和水平井距离的条件下,裂缝可从高应力顶板扩展进入低应力煤层中,顶板内形成的有效支撑裂缝能够为煤层气进入井筒提供通道;(2)压裂液注入排量越大,裂缝跨界面穿层扩展深度越大,并且当裂缝穿层扩展时,裂缝在顶板内的高度和长度均大于煤层;(3)先压裂缝会在地层中产生诱导应力,诱导应力的大小与缝内净压力、裂缝高度以及距裂缝的距离等因素有关;(4)较小的分段压裂段间距会导致后压裂段起裂压力升高、裂缝发生偏转,为实现造长缝的目的,应降低缝间应力干扰作用,研究目标区的最佳段间距为2~3倍的裂缝高度;(5)工程试验取得了良好的产气效果,微地震监测结果证明裂缝能够实现穿层扩展沟通下部煤层。

Multi-stage fracturing horizontal well in coal seam roof has proven to be an effective technique to extract coalbed methane(CBM)from broken soft coal seams.Fracture propagating through the coal-roof interface and providing flow channels for gas is the key to the successful application of this novel technique.In this paper,the fracture propagation process for staged through-layer fracturing horizontal wells in coal seam roof is studied.True tri-axial hydraulic fracturing physical simulation experiments are carried out using coal-roof rock samples.The dynamic fracture propagation process is monitored by using acoustic emission(AE)monitoring technology.Experiments under different injection rates are conducted to study the fracture propagation process.The induced stress are analyzed around hydraulic fracture.Then,the interference of adjacent fracture is studied using the extended finite element method(XFEM).Fracture extension morphology and initiation pressure are compared for different fracture spacing.Finally,combined with field test and micro-seismic monitoring results,fracture extension pattern is analyzed.Results show that:(1)Hydraulic fracture could propagate into the coal seam from roof where minimum horizontal stress is relatively larger with reasonable design of horizontal well distance and pump rate.Effective propped fracture formed in roof can provide flow channel for CBM to enter the wellbore.(2)The greater the injection rate of the fracturing fluid,the greater the depth of the fracture across the interface.When the fracture penetrates through the layer,the height and length of the fracture in the roof are larger than that in the coal seam.(3)Induced stress are formed around hydraulic fracture in the formation,and the magnitude of the induced stress is related to the net pressure in the fracture,the height of the fracture and the distance from the fracture.(4)Smaller staged fracture spacing will increase fracture initiation pressure of the post-fracture and cause the fracture to deflect.In order to achieve long fractures,the stress interference between adjacent fractures should be reduced.The optimal spacing for the target area is 2-3 times of the fracture height.(5)Good gas production performance has been achieved in the pilot test in Luling coalmine.Micro-seismic monitoring results prove that hydraulic fracture can propagate into the underlying coal seam.