Although the hydraulic fracturing treatment can improve the conductivity of shale reservoirs, the low recovery rate of the fracturing fluid may cause potential environmental and production issues. For an accurate investigation of these issues, an appropriate model of the water imbibition in shales is required. However, the hydraulic parameters related to water imbibition in shales are hard to be measured due to their tiny pores. In this study, an effective method is proposed to estimate the water imbibition volume. The nuclear magnetic resonance method is applied to obtain the related parameters including the capillary curve, the intrinsic and relative permeability of the shale, which can significantly cut down the time and cost needed to get these data. This model is validated by water imbibition experiments. In addition, we compare two empirical equations used to calculate intrinsic permeability in the NMR method and calibrate the corresponding parameter a for shale, which is poorly investigated in literature. Finally, we suggest that the capillary force dominates the early stage of water imbibition process in unsaturated shales, and the late period may be influenced more by other mechanisms such as the osmosis and the surface hydration.

This paper addresses evaluating the evolution of stress inside the casing-cement sheath-formation system during the cement injection, setting, completion, and production stages of hydrocarbon recovery. This full-life-cycle analysis of cement sheath integrity gives rise to assessment of potential failure mode (i.e., tensile mode, shear mode, and microannulus) in different stages, and the prevention measures can be proposed accordingly. Considering the loading history, two regimes should be distinguished. Before the accomplishment of cementation, as the cement slurry can merely withstand its hydraulic pressure, the in situ stress and the wellbore pressure are withstood by the rock and the casing, respectively. Once the cementation process is completed, the stress increment (e.g., hydraulic fracturing pressure) is withstood by the casing-cement sheath-formation system. The autogenous shrinkage of cement adversely affects the resistance of the system to all types of failure, whereas a moderate swelling of cement is favorable to the cement sheath integrity. In addition, the cement sheath integrity is strongly influenced by the depth: the failure is encountered more easily at the shallow layer. Both the hydraulic fracturing pressure in the completion stage and the increase in casing temperature in the production stage may lead to tensile circumferential stress, and the hydraulic fracturing is the most critical stage for the integrity of cement sheath.

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