ABSTRACT
Permeability hysteresis, as permeability value discrepancy during unloading/loading path, is a product of stress change in the porous media. This may impact the transport mechanisms and it would be more significant in unconventional reservoirs which are tight and heterogeneous. In this study we considered the effect of pore size distribution (PSD) on permeability hysteresis path during pressurizing/depressurizing for two Bakken rock samples using digital rock physics (DRP). The results showed that for core samples with variable pore sizes, the hysteresis effect can be higher compared to monotone pore sizes. Also, the lab measurements showed that for constant value of net stress (σ=Pconfining- α Ppore), permeability amount can decrease with increasing pore pressure depending on type of PSD. Furthermore, the effect of confining pressure is more significant than pore pressure, especially on higher values of net stresses. In general, increasing net stress (due to increasing confining-pressure or decreasing pore-pressure) can increase the separation of measured permeabilities during loading/ unloading process and this is more paramount in heterogeneous size media. The characterization of Bakken core samples presented in this work (i.e. PSD, permeability hysteresis) can enhance our knowledge for studying the potential of different EOR method in unconventional reservoirs.
Increasing the knowledge of permeability hysteresis during loading/unloading pressure, can help to understand the role of the different stresses, pore structures and fractures on rock hydrocarbon production (Elhaj et al., 2018). Many studies focused on the hysteresis phenomenon during loading and unloading net stress versus cyclic fracture and matrix permeability for different formations (Kranzz et al., 1979; Bernabe, 1987; Selvadurai, 2015).
In 2010, Dong et al. concluded that compaction in both sandstone and shale formations was not reversible during permeability hysteresis. Also, he showed that the rock type of investigation is one of the significant parameters that can affect this phenomenon. However, they did not see any impact of rock types on porosity hysteresis. Later, (Teklu et al., 2016), studied on different parameters, such as temperature, net stress, pore pressure, cyclic matrix and fracture in tight reservoirs. They showed that as net stress increases permeability decreases, while permeability has direct relation with temperature changes. Also, they investigated on cyclic hysteresis for both matrix and fracture and concluded that each time of injection can create new micro cracks which can improve hydraulic fracturing technique. In the same year (Ma and Zoback ,2016), presented the effect of pressure loading/unloading on ultra-sonic velocities and geo-mechanics properties and considered their dependency on confining and pore pressure. They suggested the use of simple effective stress law is not accurate and can induce large errors when Pc and Pp are high. In 2017, Civan theoretically illustrated the effect of pore elasticity on hysteresis of permeability and porosity, which is more accurate than common empirical correlations. Besides, he indicated porosity and permeability effective stress dependency can be analyzed successfully using the kinetics-based phenomenological models (Civan, 2017).
