New Understanding of Non-Uniform Extension Mechanism of Horizontal Fractures in Shallow Hypertonic Reservoirs and Research on Main Controlling Factors

Abstract The Changyuan block of Daqing oilfield in China has a shallow reservoir depth (<1000m), high permeability (>100mD), is in the late stage of development, and has a high moisture content (>95%). Currently, fracturing has now been used to reduce water content and enhance production. Microseismic monitoring indicated that the fracturing produced non-uniformly expanding horizontal fractures. Current studies provide little information on the mechanism and main controlling factors of non-uniform extension of such horizontal fractures. In this paper, we proposed the expansion pattern index to characterize the horizontal fracture expansion, and the influence factor to characterize the influence size of each factor. Based on FracMan and other software, we investigated the mechanism of influencing horizontal fracture formation and non-uniform expansion mechanism, and then clarified the horizontal fracture expansion law and the main controlling factors through the changes of different geological factors and engineering factors. The study results show that horizontal fractures will be formed when the horizontal maximum principal stress > horizontal minimum principal stress ≥ vertical stress in the reservoir. The extension direction of horizontal fractures will be along the direction of large fracture initiation stress difference (Fracture initiation stress difference is the difference between horizontal minimum principal stress and vertical stress). Horizontal fracture extension is mainly affected by the net pressure of construction, plugging distance, pumping scale, the fracture initiation stress difference and the non-homogeneity of the fracture initiation stress difference. The oil production after fracturing is mainly affected by the net pressure, oil saturation, pumping scale, plugging distance and formation pressure. With the increase of pumping scale, the horizontal fracture inflow capacity increases, the length-short axis ratio increases, and the cumulative oil production increases; with the increase of fracture initiation stress difference non-homogeneity, the horizontal fracture inflow capacity increases, the length-short axis ratio of fracture area increases, and the cumulative oil production increases. The microseismic monitoring results of two wells show that the horizontal fracture extension correlates well with the microseismic response, which verifies the correctness of the results of this study. In this study, we found for the first time the law of non-uniform extension of horizontal fractures in shallow hypertonic reservoirs, which provides a reference for the design of the later block development program and the formulation of reservoir reforming process.