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ABSTRACT: Shale reservoirs are often associated with hydraulic fracturing, which involves interactions of invading fluids and shale minerals. Formation damage caused by injection impairment is a well-known challenge in the exploration and development of unconventional reservoirs. In this study, the impact of loading history and high salinity water (HSW) on the permeability of fractured shale has been investigated via laboratory studies. The fractured shale sample was loaded and unloaded under different effective stresses with nitrogen gas injection, resulting in a decrease in fracture permeability due to inelastic compaction. Then, the shale sample was stepwise loaded with HSW injection and further decrease in its permeability was registered. This phenomenon became more expressed after extending the imbibition duration to more than ten days, suggesting that shale swelling was caused by fluid-shale interactions. Furthermore, sudden drops in the differential pressure were observed during the process, implying fracture generation. This study shows that the chemistry mismatch between treating fluids and shale can lead to permeability impairment, which can have negative impacts on well injectivity in field applications. 1. INTRODUCTION Unconventional resource extraction methods are of great interest to the petroleum industry due to their potential in supplying abundant resources to the market. It has been estimated that oil production from tight oil reservoirs can exceed 12 million barrels per day (Aloulou and Cook, 2017). To effectively extract oil and gas resources in deeper, tighter, and more depleted conditions, hydraulic fracturing is necessary to improve well productivity (Xu et al., 2016). Shale formation in the vicinity of wellbores can experience cyclic loading during drilling, completion, and production activities. Therefore, it is crucial to gain a better understanding on how shale fracture permeability responds to repeated stress-relief process over time. Applying loading-unloading cycles under various effective normal stress conditions is a conventional method used to examine the response of shale fracture permeability and its deformation. Three factors influence this permeability changes: changes in fracture roughness, changes in the contact area, and changes in fracture aperture (Zimmerman and Bodvarsson, 1996; Witherspoon et al., 1980). The main stages of the permeability change can be described as decrease in the permeability with the increase in the axial stress and partial recovery of the permeability after the stress relaxation. Experiments conducted on fractured shale samples by Gutierrez et al. (2000) demonstrated that fracture permeability after loading was always lower than before it. Hashemi and Zoback (2021) stated that the irreversible fracture permeability is likely caused by fine-grain displacement and compliant elements such as clays and total organic carbon within shale.
Bai et al. (Sun,) studied this question.