## 3 Results and Discussions  
### Characterization of SDCap  
The infrared spectrum results are shown in Figure 2. The -NH\({}_{2}\) stretch vibration absorption peak and characteristic absorption peak appeared at 3416.3 cm\({}^{-1}\) and 622.4 cm\({}^{-1}\), respectively, while the C=O characteristic absorption peak in amide groups appeared at 1665.9 cm\({}^{-1}\), indicating that the AM chain segments existed in the product. The -CO absorption peak appeared at 1724.4 cm\({}^{-1}\), while the -CH-N\({}^{+}\)(CH\({}_{3}\))\({}_{3}\) methylene bending vibration absorption peak and quaternary ammonium group absorption peak appeared at 1449.7 cm\({}^{-1}\) and 950.7 cm\({}^{-1}\) respectively, indicating that the DMC chain segments existed in the product. The -SO\({}_{3}\)- absorption peak and vibration absorption peak appeared at 1192.2 cm\({}^{-1}\) and 1042.5 cm\({}^{-1}\), respectively, indicating that the product contained AMPS chain segments. The methyl and methylene asymmetric absorption peak appeared at 2942.9 cm\({}^{-1}\). No C=C absorption peak was observed within 1635-1620 cm\({}^{-1}\), indicating that the three monomers were sufficiently copolymerized. The analysis was further confirmed in Figure 3, which showed the nuclear magnetic resonance spectrum (NMR) of SDCap. There was no absorption peak in the range of 6-7 ppm, indicating that there was no C=C in the product and the three monomers were completely polymerized. The structure was characterized by NMR analysis, as shown in Figure 3.  
The results of gel permeation chromatography test showed that the number-average molecular weight and weight-average molecular weight of SDCap were about 260,000 and 380,000, respectively. The molecular weight was much lower than that of the conventional encapsulator (over 2,000,000). Rheological Properties of Water-Based Drilling Fluids with SDCap under Deepwater Drilling Temperature  
WBDFs are multicomponent systems, so each component has to be compatible with others, otherwise the WBDF system would not be stable during drilling operations [38]. In order to evaluate the properties of SDCap in drilling fluid under field application conditions, two typical WBDFs were selected as the basic test fluids. WBDF-1 was a typical glycol/polymer WBDF, and WBDF-2 was a typical KCl/PHPA WBDF. Both WBDFs have been widely used in onshore and offshore drilling [13; 39], and the formulas are shown in Table 1. In consideration of gas hydrate inhibition in deepwater drilling operations, at least a mass fraction of 10% NaCl was added in the WBDFs. Therefore, good salt resistance was also required for SDCap. It is noticed that the encapsulators in WBDF-1 and WBDF-2 were FA-367 and PHPA, respectively. FA-367 was a widely used zwitterionic encapsulator, and PHPA was the representative of conventional encapsulator. In order to compare the properties of SDCap with FA-367 and PHPA, the 0.3% FA-367 in WBDF-1 was replaced by 0.45% SDCap (WBDF-3), and the 0.25% PHPA in WBDF-2 was replaced by 0.375% SDCap (WBDF-4). The concentration of SDCap in WBDF-3 and WBDF-4 was 1.5 times of that of FA-367 and PHPA because the molecular weight  
Figure 3: Analysis of SDCap nuclear magnetic resonance spectrum. Figure 2: Analysis of SDCap Infrared spectrum. of SDCap was far smaller than that of FA-367 and PHPA, and higher concentration of SDCap was required to maintain the desired encapsulation property. WBDF-5 was a WBDF containing SDCap optimized in our laboratory, and it was expected to have more satisfactory low-temperature rheological property as well as other properties. The rheological properties of these five WBDFs are shown in Figure 4.  
Figure 4: Rheological properties of WBDFs at the temperature range 4–75 \({}^{\circ}\)C: (**a**) changes in AV (apparent viscosity) as a function of temperature; (**b**) changes in PV (plastic viscosity) as a function of temperature; and (**c**) changes in YP (yield point) as a function of temperature. The results showed that, for all tested WBDFs, the rheological parameters obviously increased as the temperature reduced.