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.
Highlights
As global energy provided by conventional reservoirs has become more difficult, the development of oil and gas in unconventional reservoirs is increasingly necessary [1]
Fracture initiation and propagation are closely related to the nonlinear elastic response that occurs in the fracture process zone (FPZ) [3]
FPZ length length is is discussed discussed based based on the measurement of the length according toto and thethe combined damage datadata from the acoustic emission (AE)
Summary
As global energy provided by conventional reservoirs has become more difficult, the development of oil and gas in unconventional reservoirs is increasingly necessary [1]. The wetting and nonwetting fluid saturations in reservoirs always change as a result of long-term injection and production [2]. The fractures produced by hydraulic fracturing and the existing fractures in depleted reservoirs are affected by the water–oil saturation. Fracture initiation and propagation are closely related to the nonlinear elastic response that occurs in the fracture process zone (FPZ) [3]. Investigating the effect of water–oil saturation on the FPZ is of great importance for production improvement. The effect of pore fluids on mechanical properties needs to be taken into consideration during the rock fracture process. Many scholars have performed a series of experiments to research the influence of water content on the mechanical properties of rocks.
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