## 3 Results and Discussions
### Filtration Properties of Water-Based Drilling Fluid with SDCap  
Figure 11 shows the results of API and HTHP filtration tests of different WBDFs. In API filtration tests, the filtrate volume was 6.6 mL for WBDF-1, while that of WBDF-3 was 5.7 mL, indicating that the filtration property of WBDF was improved by using 0.45% SDCap instead of 0.3% FA-367. The filtrate volume was reduced from 3.7 mL to 3.5 mL when 0.25% PHPA was replaced by 0.375% SDCap (WBDF-2 and SBDF-4). The filtrate volume of the optimized WBDF-5 was 4.8 mL. In general, the API filtrate volume lower than 5 mL could meet the requirement of drilling operations. In HTHP filtration tests, the filtrate volumes were apparently larger than that in API filtration tests for the high temperature and high pressure influence the quality of the filter cake and promote the invasion of drilling fluid filtrates into the wellbore. The HTHP filtrate volume of WBDF-1 was up to 24.2 mL. The WBDF-3 still had lower HTHP filtrate volume than WBDF-1, indicating that the addition of SDCap also improved the filtration property of WBDF under HTHP condition. The HTHP filtrate volumes of WBDF-2 and WBDF-4 were very similar. The HTHP filtrate volume of WBDF-5 was 15.8 mL, which was the smallest among these five WBDFs.  
In order to further investigate the effect of SDCap on the filtration property of WBDF, the micro-structure of the filter cake which was formed by drilling fluid on the surface of filter paper in the API filtration tests was investigated using SEM analysis, as shown in Figure 12. In order to eliminate the influence of other additives on the characterization of the structure, 4% bentonite suspension containing only encapsulator was investigated. The filtrate volume in the API filtration tests was also measured. The SEM photos clearly showed the micro-structures of the filter cakes formed by different test fluids. For 4% bentonite suspension without encapsulator, it could not form a fine filter cake, and the surface of the filter cake was very coarse. There were lots of pores and fractures in the filter cake, as shown in Figure 12a,b. Therefore, the filtrates of the drilling fluid easily invade inside the rock formation in wellbore, resulting in hydration swelling and dispersion of shales. The filtrate volume of the API filtration tests could confirm the analysis for all the test fluid was leaking and the filtrate volume was large than 200 mL, indicating the very poor quality of the filter cake. Therefore, filtrate reducers have to be used. The filter cake formed by 4% bentonite suspension with 0.3% PHPA was better than that formed by bentonite suspension, but it was still not satisfactory, as shown in Figure 12c,d. Owing to the absorption of PHPA on clay particles in the filter cake, the filter cake was tighter with smoother surface than that formed by 4% bentonite suspension without encapsulator. However, there were still lots of pores and fractures, and the flocculation could be observed which would result in uneven surface of filter cake. The filtrate volume was up to 64.0 mL. In comparison, a tight filter cake with smooth surface was formed by 4% bentonite suspension containing 0.3% SDCap, as shown in Figure 12e,f, and the filter volume was 29.6 mL. Very tight filter cake surface was observed,  
Figure 11: Filtration volumes with different WBDFs.  
even in the SEM photo with 5000-fold magnification. The molecular chain was short and the adsorption capacity was strong, so clay particles with adsorbed SDCap molecules were evenly dispersed on the surface of wellbore without apparent flocculation, forming tight filter cake. Therefore, the addition of SDCap improved the filtration property of WBDF, thus mitigating the wellbore instability problems caused by the invasion of drilling fluid filtrates.