Propagation Mechanism Of Through Strata Fractured Cracks In Multi-layered Reservoir

Lin Xing-Shenfu area is located in the eastern margin of the Ordos Basin.The blocks are characterized by multi-layer superimposed development of the coal measure gas,and all kinds of unconventional gas reservoirs are vertically distributed.When the hard brittle sandstone and plastic coal were developed together,it is hard to predict hydraulic fracture morphology because of stress difference between layers,rock mechanical properties,interfacial bonding,hole deviation and azimuth,fracturing parameters and other factors.It is a challenge to design rational reservoir stimulation scheme.In this paper,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,fracturing operation parameters which affected the expansion of the hydraulic fracture.In order to qualitatively characterize the influence factors of interlayer cementation strength,the further physical simulation experiment of layered media has been developed.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 is 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 can easily dissipate hydraulic energy,which makes hydraulic fractures difficult to penetrate formation.Although the gel had a better ability to generate fracture than the active water,the gel is damage to reservoir.So it is necessary to consider select the fracturing fluid on the basis of the field requirement.In order to study the effect of perforation parameters on hydraulic fracture,according to the rock mechanics parameters of indoor experiment and logging data,the paper calculated mechanical parameters and stress of reservoir which were used to establish 3D lattice method hydraulic fracture extension model.The model study the influence of perforation parameters on hydraulic crack propagation.The simulation results show that perforation in the upper part of the coal seam is conducive to the expansion of hydraulic fractures.With the increase of the perforation length,the effective reconstruction area of the crack is reduced.The indirect fracturing can realize hydraulic fracture through multilayer,which is beneficial to the comprehensive development of the layer group.On the basis of physical simulation experiments and numerical results,the paper put forward a mechanism for determining fracture propagation,especially in multi-layers combined with tight gas and coal bed gas.The study provided theoretical guidance for improving the fracturing effect in the field production.