# Effect of Amphiphilic Polymer/Nano-Silica Composite on Shale Stability for Water-Based Muds
## 1 Introduction  
Drilling fluids are considered as circulating fluids in the drilling operations, which provide multiple functions. Their major functions consist of suspending/transporting drill cuttings, cooling and lubricating the drilling bits, maintaining borehole stability and reducing the formation damage [1; 2; 3]. Drilling fluids are usually grouped into three types: water-based muds (WBMs), oil-based muds, and synthetic-based muds. Due to their lower cost and lesser impact on environment, WBMs have become the most widely used drilling fluids in the world [4; 5]. In recent years, the development of shale gas as an unconventional oil and gas resource has gained an increase in attention [6; 7; 8; 9; 10; 11; 12]. The technology of drilling fluids for the long well section of a horizontal well is one of the key technologies used to drill shale gas formations. Since the shaleformations develop nano-scale pores and fractures and have a strong water-sensitivity characteristic, shale instability in the drilling process is prone to occur. This restricts the process of shale gas exploration and development [13; 14]. Figure 1 shows the general sketch of the shale instability problems caused by WBMs. Due to a large number of nano-scale pores and fractures developed in the shale formations, they are characterized by extremely low permeability and low porosity [15; 16; 17]. In the drilling operations, pressure transmission in WBMs will result in an increase in the pore pressure of shale formations, and thus cause wellbore instability. The best approach for solving this problem is to plug the nano-scale pores and fractures developed in the shale formations [18; 19; 20]. However, conventional agents for WBMs are too large to play an effective role of plugging. Therefore, seeking for plugging agents with a suitable size to achieve better plugging capacity is essential. In the last few years, the interest in using nanotechnology in the oil and gas industry has been increasing [21; 22; 23]. Various kinds of nanomaterials have been introduced into WBMs as promising shale stabilizers [24; 25; 26]. The inorganic nanoparticles such as unmodified nanosilica were first introduced into WBMs as plugging agents. It was found that nanosilica could seal the shale pores and improve the filtration loss performance of WBMs [27; 28]. Graphene oxide has also been investigated as another form of rigid nanoparticles, and it showed good plugging performance in WBMs [29]. However, inorganic nanoparticles are prone to aggregation, and thus the plugging ability can be weakened. Polymeric nanoparticles (such as polymer latex) possessing better dispersing performance in WBMs have also been introduced as plugging agents. The studies showed that the polymeric nanoparticles could decrease shale permeability and effectively retard pressure transmission [30; 31]. However, polymer latex is usually preserved in the emulsion state and sensitive to the environmental change, which restrict its application to oilfields. Recently, polymer-inorganic nanocomposites have attracted more and more attention in the drilling fluids industry [32; 33; 34; 35]. The benefits of nanocomposites to be applied in WBMs include excellent dispersing performance, decrease in filtration volume, and improvement of rheological properties [36; 37]. Therefore, the polymer-inorganic nanocomposites may be an effective way to solve shale instability problems. Polyacrylamide-grafted-polyethylene glycol/silica nanocomposite was reported as a potential additive for WBMs, which demonstrated good shale hydration inhibition performance [28]. But the plugging capacity was not considered in their work. Poly(AM-AMPS)/nano-SiO\({}_{2}\) composite was introduced into WBMs as a fluid loss agent [26]. Research showed that the composite could plug the nanopores of shale, but the shale hydration inhibition performance was not further evaluated. Figure 1: General sketch of the shale instability problems caused by WBMs (water-based muds).