Influence of Water–Oil Saturation on the Fracture Process Zone: A Modified Dugdale–Barenblatt Model

Yuanxun Nie, Guangqing Zhang, Yuekun Xing and Shiyuan Li
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Yuanxun Nie: Department of Engineering Mechanics, China University of Petroleum (Beijing), Beijing 102248, China
Guangqing Zhang: Department of Engineering Mechanics, China University of Petroleum (Beijing), Beijing 102248, China
Yuekun Xing: Department of Engineering Mechanics, China University of Petroleum (Beijing), Beijing 102248, China
Shiyuan Li: Department of Engineering Mechanics, China University of Petroleum (Beijing), Beijing 102248, China

Energies, 2018, vol. 11, issue 11, 1-14

Abstract: The wetting and nonwetting fluid saturations in porous reservoirs always change during long-term injection and production. The fracture process zone (FPZ) is a prominent feature in the rock fracture process. If the FPZ properties are influenced by pore fluids, the process of hydraulic fracturing will change greatly. The existing models do not consider the role of pore fluid when characterizing the FPZ. In this paper, a modified Dugdale–Barenblatt (D–B) model with capillary pressure is proposed. The model reflects the fact that the FPZ length decreases nonlinearly with the increase in capillary pressure, and it reveals the mechanism of capillary pressure on the equivalent fracture cohesion in the FPZ, which affects the FPZ length. Three-point bending tests were carried out on sandstone under various fluid saturations through digital image correlation (DIC), acoustic emission (AE), and scanning electron microscope (SEM). It was found that the FPZ length of the water–oil-saturated samples was 30–50% smaller than that of water-saturated/oil-saturated samples due to the capillary pressure effect, and the modified D–B model was well consistent with the experiments. The AE behaviors of different saturated samples were not the same: The cumulative AE signals changed abruptly at 90% of the peak load for the water–oil-saturated samples and at 50% of the peak load for water-saturated samples. This demonstrated that the effect of capillary pressure was more obvious than the weakening effect of microstructural damages. The significant influence of capillary pressure on FPZ requires continuous recognition in hydraulic fracturing design.

Keywords: capillary pressure; fracture process zone length; modified Dugdale–Barenblatt model; digital image correlation (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2018
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