--> Factors Influencing the Development of Diagenetic Shrinkage Fracture of Shale in Marine-Continental Transitional Facies

AAPG ACE 2018

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Factors Influencing the Development of Diagenetic Shrinkage Fracture of Shale in Marine-Continental Transitional Facies

Abstract

As an important type of (non-structural) fracture, diagenetic shrinkage fracture of shale (DSFS) plays an important role in the reservoir, seepage and especially fracturing production of shale oil and gas. However, the study on it is relatively unsubstantial. According to the similarity of desiccation crack of argillaceous sediment and DSFS in the cause of formation, morphology and development law, 4 groups and 19 experiments on the desiccation crack of shale core powder in the marine-continental transitional facies were designed. Combined qualitative analysis with quantitative research, Four factors, including shale thickness, water salinity, clay mineral content and heating temperature, were conducted analogy study on the DSFS. The study showed that the greater the thickness, the later the time of fracture initiation (TFI), and the higher the critical water ratio (CWR) of fracture initiation; and that the greater the average fracture length, width and fracture ratio, the smaller the area density of fractures. Influenced by the strong diagenesis and low content of montmorillonite in shale, the water salinity has smaller relation with the DSFS. Samples with different water salinities have similar TFI and CWR. As water salinity increases, the width, length, and fracture ratio of DSFS tend to decrease, and area density (the number of fractures) increases, but their decrease or increase degree are minor. The high content of clay mineral in transitional facies shale exerts a tremendous influence on DSFS. In addition, higher clay mineral content causes later TFI and greater CWR of shale; the fracture length and fracture ratio increase, and the width increases sharply, but the area density decreases. With higher the temperature, there is a decreased tendency of the TFI, length, width and fracture ratio, but the CWR and area density are increased. On the whole, the influence of clay mineral content on the DSFS is largest, followed thickness and temperature, and water salinity impacts least. The DSFS under different influence factors all have four development stages: formation of main-fracture, formation of sub-fracture, stabilization stage and fixed stage. Other influence factors of DSFS need be further studied.