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Received Jul 2, 2017; Revised Aug 19, 2017; Accepted Sep 13, 2017
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1. Introduction
Drilling fluid is typical water-based or oil-based depending on the needs of field operations [1, 2]. Compared to water-based drilling fluids, oil-based drilling fluids can provide excellent lubrication, stabilize water-sensitive clays, reduce leak-off, and form thinner filter cakes; besides, they can be applied in deep reservoirs with characteristics of high pressure and extreme temperature [2–5].
Among different types of oil-based drilling fluids, water-in-oil emulsion (i.e., inverted emulsion) is mainly used in the field because of its outstanding properties [3, 6]. However, depending on their droplet sizes, emulsions therein can invade reservoir rock and plug pores/pore throats, resulting in diminished hydrocarbon production [7–9]; moreover, surfactants which are added for generating emulsions can be adsorbed on rock surface and alter the wettability, which may also cause formation damage [7, 10]. Besides the emulsions, suspended solids in the inverted emulsion, which is added for hindering leak-off, may also plug pores/pore throats; and this can make the formation damage even worse [8, 11]. Furthermore, drilling fluid may also affect the quality of cementation [12, 13] or hydrocarbon production through multiphase flow [14]. Therefore, it is imperative to evaluate the formation damage due to drilling fluid and explore the corresponding solutions.
Once the drilling process is accomplished, hydraulic fracturing is commonly used to stimulate the reservoir; this process can also cause formation damage. During the fracturing, a large volume of proppants is carried into the reservoir by the fracturing fluid, and this is aimed at creating a complex and highly conductive fracture network [15, 16]. Gel is typically used as the fracturing fluid to maximize the proppant-carrying capacity because of its excellent viscosity and elasticity [17–22]. However, gel residuals can block fractures and pores at fracture faces, thus impeding the flow of hydrocarbon [23–25]. Besides gel residuals, water can imbibe rock matrix and cause phase trapping, which reduces hydrocarbon permeability due to multiphase flow [26–29].
Formation damage due to drilling and fracturing fluids is likely different in the low-permeability sandstone with well-developed natural fractures. For such...