## 3 Results and Discussions
### Characterization of SDCap
Particularly, when the temperature was reduced from 25 to 4 \({}^{\circ}\)C, the increase in rheological parameter was more apparently, which was indicated by the more sharp slope of the rheology-temperature curve from 25 to 4 \({}^{\circ}\)C. This low temperature interval is just the essential difference between deepwater drilling and onshore or offshore drilling, and it causes significant thickening of the drilling fluids. Therefore, we have to pay special attention to the rheological properties at 25-4 \({}^{\circ}\)C.The ratios of rheological parameters at 4 \({}^{\circ}\)C and 25 \({}^{\circ}\)C were used to describe the thickening of the drilling fluid at a low temperature. The AV represents the total viscosity of the drilling fluid and is the sum of PV and YP. PV represents the viscosity resulting from internal friction, while YP represents the viscosity generated by formations of grid structures among clay particles and high molecular polymers. For WBDF-1, the AV at 4 \({}^{\circ}\)C was 52 mPa\(\cdot\)s. The AV(4 \({}^{\circ}\)C)/AV(25 \({}^{\circ}\)C) ratio was 1.55; PV(4 \({}^{\circ}\)C)/PV(25 \({}^{\circ}\)C) ratio was 1.57; and the YP(4 \({}^{\circ}\)C)/YP(25 \({}^{\circ}\)C) ratio was 1.52. When 0.3% FA-367 was replaced by 0.45% SDCap, the AV of the drilling fluid (WBDF-3) was reduced by 22.1% (from 52 mPa\(\cdot\)s to 40.5 mPa\(\cdot\)s). For WBDF-3, the AV(4 \({}^{\circ}\)C)/AV(25 \({}^{\circ}\)C) ratio was 1.45; PV(4 \({}^{\circ}\)C)/PV(25 \({}^{\circ}\)C) ratio was 1.45; and the YP(4 \({}^{\circ}\)C)/YP(25 \({}^{\circ}\)C) ratio was 1.44. All ratios of rheological parameters at 4 \({}^{\circ}\)C and 25 \({}^{\circ}\)C were reduced, indicating the thickening of the drilling fluid at a low temperature was mitigated using SDCap instead of FA-367. For WBDF-2, the AV at 4 \({}^{\circ}\)C was up to 83 mPa\(\cdot\)s. The AV(4 \({}^{\circ}\)C)/AV(25 \({}^{\circ}\)C) ratio was 1.69; PV(4 \({}^{\circ}\)C)/PV(25 \({}^{\circ}\)C) ratio was 1.71; and the YP(4 \({}^{\circ}\)C)/YP(25 \({}^{\circ}\)C) ratio was 1.64. When 0.25% PHPA was replaced by 0.375% SDCap, the AV of the drilling fluid (WBDF-4) was reduced by 31.3% (from 83 mPa\(\cdot\)s to 57 mPa\(\cdot\)s). For WBDF-4, the AV(4 \({}^{\circ}\)C)/AV(25 \({}^{\circ}\)C) ratio was 1.54; PV(4 \({}^{\circ}\)C)/PV(25 \({}^{\circ}\)C) ratio was 1.56; and the YP(4 \({}^{\circ}\)C)/YP(25 \({}^{\circ}\)C) ratio was 1.50. Therefore, using SDCap instead of PHPA also mitigated the thickening effect of the drilling fluid at a low temperature. For WBDF-5 which was expected to be a drilling fluid with better rheological property, the AV, PV and YP at 4 \({}^{\circ}\)C were 32 mPa\(\cdot\)s, 22 mPa\(\cdot\)s, and 10 Pa, respectively, and were all lower than that of other four WBDFs. The AV(4 \({}^{\circ}\)C)/AV(25 \({}^{\circ}\)C) ratio was 1.33; PV(4 \({}^{\circ}\)C)/PV(25 \({}^{\circ}\)C) ratio was 1.38; and the YP(4 \({}^{\circ}\)C)/YP(25 \({}^{\circ}\)C) ratio was 1.25. In comparison, the AVs of the two conventional WBDFs (WBDF-1 and WBDF-2) were 52 mPa\(\cdot\)s and 83 mPa\(\cdot\)s. The AV of WBDF-5 was only 61% and 39% of that of the conventional WBDFs, respectively. The AV(4 \({}^{\circ}\)C)/AV(25 \({}^{\circ}\)C) ratios of the two conventional WBDFs were 1.55 and 1.69, respectively; the PV(4 \({}^{\circ}\)C)/PV(25 \({}^{\circ}\)C) ratios were 1.57 and 1.71, respectively; and the YP(4 \({}^{\circ}\)C)/YP(25 \({}^{\circ}\)C) ratios were 1.52 and 1.64, respectively. Therefore, all the ratios of rheological parameters at 4 \({}^{\circ}\)C and 25 \({}^{\circ}\)C for WBDF-5 were much lower than that of the conventional WBDFs, indicating this optimized WBDF had better low-temperature rheology. Moreover, the curves of AV, PV and YP with temperature in Figure 4 were more stable in the temperature range 4-75 \({}^{\circ}\)C, indicating