New Understanding of Non-Uniform Propagation Mechanism of Horizontal Fractures in Shallow High-Permeability Reservoirs and Research on Main Controlling Factors

The Changyuan block of Daqing oilfield in China has a shallow reservoir burial depth (<1000m), high permeability (>100mD), is in the late stage of development, and has a high water content (>95%). Hydraulic fracturing produced non-uniformly propagating horizontal fractures. At present, the mechanism and controlling factors of non-uniform propagation of horizontal fractures are not well understood. In this paper, based on the geological numerical model of Changyuan block, the non-uniform propagation of horizontal fracture is studied. The main conclusions are as follows: the horizontal fracture will expand in the direction of large fracture initiation stress difference (minimum horizontal principal stress - vertical stress); the main controlling factors for the non-uniform propagation of horizontal fracture are construction net pressure, temporary plugging distance, fracture initiation stress difference and its non-uniformity; with the increase of the fracture initiation stress difference non-uniformity, horizontal fractures increased flow conductivity, and the cumulative oil production volume will be increased. The micro-seismic monitoring results show that the non-uniform propagation law of horizontal fracture is in line with the actual situation. This is of great significance for the redevelopment of the shallow layer of the old oilfield. 1. INTRODUCTION At present, conventional shallow hyperpermeable reservoirs have entered the late stage of development, with high water content and very low oil recovery, and the remaining oil is mainly distributed in the reservoir with poor physical properties or imperfect injection and extraction zones. In order to realize the secondary development of high water content wells, it is necessary to carry out secondary utilization of residual oil. Hydraulic fracturing can be used to increase the mobility in the areas with poor physical properties and improve the injection and extraction relationship (Jintang Wang, 2019; Yanxin Lv, 2022; Chen B, 2022). Through experimental research and theoretical analysis, it is considered that the formation and propagation of hydraulic fractures are affected by many factors (Hou B, 2014; Li Q, 2015). In the early years, Warpinski (1987) pointed out that the horizontal stress difference, the relative angle relationship between hydraulic fractures and natural fractures, and the pumping pressure during fracturing would affect the formation and propagation of fractures based on the Mohr-Coulomb criterion. More laboratory experiments show that the propagation of hydraulic fractures is affected by the distribution of natural fractures in the reservoir, the original in-situ stress and the type of reservoir rock (Detournay E. 2016; Jia Y, 2016; Altammar, M. J., 2019). The shear strength of natural fractures and the magnitude of in-situ stress are related to the passage or cessation of hydraulic fractures (Blanton TL., 1982; Olson, J. E., 2012; Lei Q, 2022). At the same time, the mineral composition, Young ‘s modulus and tensile strength of reservoir rocks have an important influence on the morphology and effect of hydraulic fractures (Rickman, R., 2008; Zou, 2016; Huang, L., 2022). Numerical simulation is also an important method to study hydraulic fracturing. Finite element method (FEM) (Guo T, 2015; Ni, T, 2020), extended finite element method (XFEM) (Paul B, 2018; Zhou, Y., 2021), particle flow discrete element method (PFC) (Zheng H, 2020), finite-discrete element method (FDEM) (Shan Wu, 2022) and other methods are widely used to study the propagation of reservoir fractures (Songze Liao, 2024). Hu et al. (2021) conducted a two-dimensional XFEM simulation based on the Mohr-Coulomb yield criterion. The simulation results show that the influence of plastic deformation on fracture propagation cannot be ignored in the formation with large stress difference and friction angle. Zeng et al. simulated the fracture propagation process using XFEM based on Drucker-Prager criterion (Zeng Q, 2019). At present, Mohr-Coulomb criterion and Drucker-Prager criterion are widely used in elastoplastic rock constitutive models. The main difference between the two models is that the Drucker-Prager model considers hydrostatic pressure more comprehensively (Liu W, 2021).