Productivity Prediction Of Fractured Well And Propagation Law Of Hydraulic Frctures In Thin Inter-bedded Low Permeability Reservoirs

Thin inter-bedded low permeability reservoir is thin layer, sand shale inter-bedded and serious longitudinal heterogeneous. The hydraulic fracture patterns in such reservoir are complex. During development process in this reservoir, employing degree is low, flood effectiveness is poor, and oil well production is low. If fluid flowing law of fractured wells and fracture propagation rule for this reservoir are unclear, the goal of effective exploitation can not achieve. Therefore, hydraulic fracture propagation law and coupling seepage flow theory were studied in this paper.The hydraulic fracturing simulation experiments of open hole horizontal well were carried out at triaxial stress state. Based on the characteristics of deformation damage of core, the rock failure criterion for thin inter-bedded formation was built. Combining with the wellbore stress field, breakdown pressure equations of different completion methods and different well type were inferred. A three dimensional constitutive model of thin inter-bedded formation was proposed for thin layers of sand and mud with different mechanical properties. The equivalent elastic constants were inferred. Introducing the Dundurs parameters, based on Griffith fracture criterion and fracture criteria for an interface crack, non-planar fracture propagation criterion in thin inter-bedded were established. The complex extending behavior of hydraulic fractures were analyzed, including extending and slippage along formation interface and extending in the other side of interface, when the hydraulic fractures meet the formation interface. The non-planar fracture propagation rules were revealed.Based on equivalent seepage resistance law and flow field division method, coupled seepage mathematical model for fractured vertical well in thin inter-bedded reservoir was proposed, taking into account longitudinal heterogeneity and non-planar fracture character and the non-Darcy seepage conditions. The coupled seepage mathematical models for multi-fractured horizontal wells were proposed, considering perforation well, open hole well and flow field interference between adjacent two fractures. On the basis of above work and water flooding theory in porous media, productivity equations for different well units with different wells were proposed in thin inter-bedded reservoir, taking into account the non-Darcy seepage conditions and various fracture morphology. Based on those models, the parameters of fracture were optimized in different reservoir condition.The results show that, when azimuthal angle of horizontal well is greater than60degrees, the two wings of fracture are symmetrical in hydraulic fracturing simulation experiments. The hole drift angle and azimuth angle and horizontal stress difference are smaller, the fracture pressures are more. When stress difference of the minimal horizontal main stress between adjacent two layers is greater than5MPa, and thickness of interlayer between two sand bodies is greater than6m, the two sand bodies can not be linked by fracture in thin inter-bedded reservoir. The smaller thickness ratio of sand layer and thin inter-bed is, the more serious longitudinal heterogeneity is, and the serious non planarity of fracture is, then the strong influence on fractured well production is. If the start-up pressure gradient is greater than0.01MPa/m, the influence on fractured well production is stronger. When permeability is between1mD and10mD, the optimal flow conductivity of fracture is between10and20D·cm. When permeability is greater than10mD, the optimal flow conductivity of fracture is between20and25D·cm. Those models are applied in oil fields and provide a necessary theoretical foundation for making sure reasonable production in thin inter-bedded reservoir.