砂煤互层水力裂缝穿层扩展机理

临兴地区含煤岩系地层纵向互层发育煤层、致密砂岩层产层组,当砂煤薄层组合开发时,受界面胶结性质、地应力差、压裂选层、煤层厚度、施工参数等因素的影响,难以判断裂缝是否垂向穿层。为了分析砂煤互层水力压裂裂缝穿层扩展规律,通过选取砂煤天然露头岩样进行交错组合,利用大尺寸真三轴水力压裂系统开展了砂煤产层组组合的物理模拟实验,研究了压裂选层、层间胶结强度、煤层厚度、胶液和活性水对水力裂缝穿层扩展的影响。实验结果表明:在煤岩中起裂时,受煤岩层理和自身天然裂缝的影响,裂缝在扩展过程中,容易发生转向,使得裂缝形态复杂多样,不利于裂缝穿层;在砂岩层中起裂时,砂岩中水力裂缝形态简单,形成水力主缝,容易穿层扩展;当界面胶结强度较弱时,裂缝扩展至砂煤界面,容易发生转向,沿界面扩展,形成水平裂缝;煤层越厚越容易耗散水力能量,使得水力裂缝难以穿层扩展;胶液的造缝能力优于活性水,但其对储层伤害较重,压裂液类型的选取需根据现场压裂所需进行甄选。

Propagation mechanism of hydraulic fracture in sand coal interbedding

The strata of Linxing Area is composed of many layers of coal seam and tight sandstone.When multi-layers with coal seam and tight sandstone are developed together,it is hard to predict hydraulic fracture morphology because of interfacial property,stress difference,fractured layer,thickness of coal seam,construction parameters and other factors.In order to analyze the hydraulic fracture propagation mechanism in sand coal interbedding,the natural sandstone and coal outcrop rocks were mutually combined to carry out hydraulic fracturing simulation experiments with a true tri-axial pressure machine,and a study was conducted on the natural fracture,different stratum,interface bonding strength,thickness of coal seam and the type of fracturing fluid affected the expansion of hydraulic fracture.The experiment result showed natural fracture and bedding in coal rock would change the propagation direction of hydraulic fracture and cause complex fracture morphology,which was bad for hydraulic fracture to propagate across the interface.However,when the target fractured formation was sandstone,the hydraulic fracture morphology was simple and easier to connect interlayers.When hydraulic fracture approached the weak bonding interface between layers,hydraulic fracture could turn to propagate along the interface.Thick coal seams could easily dissipate hydraulic energy,which made hydraulic fractures difficult to penetrate formation.Although the gel had a better ability to generate fracture than the active water,the gel was damage to reservoir.So it is necessary to consider the selection of fracturing fluid according to the field requirement.