# Studies of Fracture Damage Caused by the Proppant Embedment Phenomenon in Shale Rock
## 1 Introduction  
Hydraulic fracturing is one of the main methods for stimulating unconventional hydrocarbon reservoirs. In the case of shale formations, which are characterized by increased content of clay minerals, for intensification treatments to be effective, numerous fractures and cracks should be created [1; 2; 3; 4; 5; 6; 7], as shown in Figure 1[2; 8; 9; 10]. The producing formation is fractured using hydraulic pressure, and then proppants are pumped into the fractures with a fracturing fluid [11]. The industry has been making use of slickwater, where the proppant transport is governed by the high velocity of the injected water, unlike polymer-based fluids for which the transporting mechanism is based on viscosity [12]. The literature [12; 13; 14] has reported a significant use of hybrid technologies that combine slickwater and polymer fluids. In hydraulic fracturing, energized fluids are also used (fluids with one compressible component such as nitrogen or carbon dioxide) [15]. The use of a gas component helps to reduce the hydrostatic pressure. It also supports wellbore and fracture clean up. Polymer-based fluids are still the most commonly used type of fracturing fluids [12]. The material used for proppants can range from natural sand grains called frac sand and resin-coated sand to high-strength ceramic materials and resin-coated ceramic materials [11]. The typical propgant sizes in shale reservoirs hydraulic fracturing are generally between 30 and 50 mesh (from 0.300 mm to 0.600 m) and between 40 and 70 mesh (from 0.212 mm to 0.420 mm). Numerous fractures and cracks created in shale formations are characterized by low width and large values of the stimulated reservoir volume (SRV). It must also be pointed out that a proper selection of the propgant should ensure high conductivity of the whole generated system of fractures distributing the propgant to the furthest parts of the fractures [11; 16; 17]. Apart from the way by which the propgant is transported and placed in the fractures, the phenomena presented in Figure 2 have a significant influence on the effective packaging of fractures with the propgant [10; 17]. This occurs after the treatment, when compressive stress closes the fracture on the propgant. The propgant embedment phenomenon presented in this model causes the decrease of the width \(W_{f}\) of the created fracture (Figure 3) [7; 10; 18; 19; 20; 21] and an increase in the damage of the fracture face, which results in a decrease of its permeability and conductivity [22; 23; 24; 25]. Figure 1: Visualization of numerous fractures and microcracks that allow an absorbed gas to be release from the shale rock. Figure 2: The phenomena influencing the effective packaging of a fracture and the achievement of high conductivity. For many years, a number of laboratory tests, imaging tests, and mathematical modeling of the embedment phenomenon have been performed. They are constantly being modernized as the capabilities of hardware and software increase. In a research project [23], the fracture surface strength and fracture conductivity for American shale reservoirs (Barnett, Haynesville, Marcellus) were studied, and the kneading depth of small proppant grains into the fracture face was determined [23; 26]. In the mentioned study, long-term conductivity of the dry and pre-saturated fracture was determined. A study of proppant embedment in shales and its effect on hydraulic fracture conductivity is present also in the literature [4]. It described the relations between rock mineralogy, mechanical properties, fluid composition, and proppant embedment. Initial results showed a close correlation between the amount of proppant embedment at a given stress and the rock stiffness, which is affected by its mineral content, mainly, the amount and type of clay minerals. These correlations are used to predict the amount of conductivity loss due to proppant embedment in different unconventional reservoirs [4].