Oil Based Drilling Fluids with a Density of 2.04 g/cm³ and Different Oil/Water Ratios at 200°C–232°C

Oil-based drilling fluids with a density of 2.04 g/cm³ and oil/water ratios of 85:15 and 90:10 were prepared using emulsifying and wetting agents, high-temperature stabilizer with certain viscosity-reducing effect, organophilic clay, and fluid loss control additives produced by Unitech Chemicals (Kaiping) Ltd.

The fluids were hot rolled at 200°C or 232°C, respectively. Rheological properties, electrical stability, and high-temperature high-pressure fluid loss were measured before and after hot rolling.

Drill cuttings contamination and 4% brine contamination tests were also conducted on the oil-based drilling fluids to simulate field conditions such as drill cuttings contamination or water invasion, in order to meet the technical requirements of high-temperature and high-pressure drilling operations.

1. Performance of Oil-Based Drilling Fluid Resistant to 200°C

1.1. Oil-Based Drilling Fluid Formulation

The formulation is shown in Table 1-1.

Table 1-1 Formulation of 2.04 g/cm³ Oil-Based Drilling Fluid with an Oil/Water Ratio of 85:15 

Mud Preparation Procedure：

a) Weigh 3# white oil and UT-MUL (HT) into a high-speed mixing cup, and mix at 11,000 r/min for 5 minutes.

b) Reduce the speed to 6,000 r/min. While stirring, slowly add UT-Gel (HT) and CaO. After the addition is completed, increase the speed to 11,000 r/min and mix at high speed for 10 minutes.

c) At 11,000 r/min, slowly add CaCl₂ Solution (25%). After the addition is completed, continue high-speed mixing for 20 minutes.

d) Stop mixing, remove the high-speed mixing cup, add UT-L3 and Barite, and mix at 11,000 r/min for 30 minutes.

e) After the oil-based mud is prepared, heat it to 50°C, then measure the electrical stability ES and rheological properties.

1.2. Performance of Oil-Based Drilling Fluid

    The oil-based drilling fluid was prepared according to the formulation in Table 1-1. After hot rolling at 200°C for 16 hours, the performance of the oil-based drilling fluid was tested at 50°C.

After the performance test was completed, 30 g RevDust was added for contamination. The fluid was then hot rolled for a second time at 200°C for 16 hours, and the drilling fluid performance before and after hot rolling was tested. The results are shown in Table 1-2.

 Table 1-2 Performance of 2.04 g/cm³ Oil-Based Drilling Fluid with an Oil/Water Ratio of 85:15 Before and After Hot Rolling at 200°C

The test results in Table 1-2 show that the drilling fluid with an oil/water ratio of 85:15 and a density of 2.04 g/cm³ maintained good rheological properties at 50°C after hot rolling and aging at 200°C for 16 hours. It also showed high electrical stability and low HTHP fluid loss.

After adding 30 g of RevDust for contamination and hot rolling at 200°C for 16 hours, the drilling fluid still maintained relatively good rheological performance, indicating that this oil-based drilling fluid has strong resistance to drill cuttings contamination.

The oil-based drilling fluid was prepared according to the formulation in Table 1-1. After hot rolling at 200°C for 16 hours, the performance of the oil-based drilling fluid was tested at 50°C. After the performance test was completed, 20 g of 4% sodium chloride brine was added for contamination. The fluid was then hot rolled for a second time at 200°C for 16 hours, and the drilling fluid performance before and after hot rolling was tested. The results are shown in Table 1-3.

Table 1-3 Performance of 2.04 g/cm³ Oil-Based Drilling Fluid with an Oil/Water Ratio of 85:15 Before and After Hot Rolling at 200°C

Table 1-3 shows that the 2.04 g/cm³ OBM with an 85:15 oil/water ratio maintained good rheology after aging at 200°C for 16 h. After contamination with 20 g of 4% NaCl brine and a second 200°C × 16 h hot roll, the fluid showed some thickening, but its overall performance still met field drilling requirements. To further improve performance, 4 g of UT-STAB (HT) was added, followed by a third 200°C × 16 h hot roll. The results are shown in Table 1-4.

Table 1-4 Performance Before and After Hot Rolling Aging at 200°C for 16 h

Table 1-4 shows that after adding 4 g of UT-STAB (HT) and hot rolling at 200°C for 16 h for the third time, the rheology of the oil-based drilling fluid improved. This indicates that this high-temperature, high-density oil-based drilling fluid system can meet field HTHP drilling requirements.

2. Performance of Oil-Based Drilling Fluid Resistant to 232°C

2.1. Oil-Based Drilling Fluid Formulation

The formulation is shown in Table 2-1.

Table 2-1 Formulation of 2.04 g/cm³ Oil-Based Drilling Fluid with an Oil/Water Ratio of 90:10

Mud Preparation Procedure：

a) Weigh 3# white oil, UT-MUL (HT), and UT-STAB (HT) into a high-speed mixing cup, and mix at 11,000 r/min for 5 minutes.

b) Reduce the speed to 6,000 r/min. While stirring, slowly add UT-Gel (HT) and CaO. After the addition is completed, increase the speed to 11,000 r/min and mix at high speed for 10 minutes.

c) At 11,000 r/min, slowly add CaCl₂ Solution (25%). After the addition is completed, mix at high speed for 20 minutes.

d) Stop stirring, remove the high-speed mixing cup, add UT-L3 and Barite, and mix at 11,000 r/min for 30 minutes.

e) After the oil-based mud is prepared, heat it to 50°C, then measure the electrical stability ES and rheological properties.

2.2. Performance of Oil-Based Drilling Fluid

The oil-based drilling fluid was prepared according to the formulation in Table 2-1. After hot rolling at 232°C for 16 h, the fluid performance was tested at 50°C. After the test, 30 g RevDust was added for contamination. The fluid was then hot rolled at 232°C for 16 h, and the drilling fluid performance before and after hot rolling was tested. The results are shown in Table 2-2.

Table 2-2 Performance of 2.04 g/cm³ Oil-Based Drilling Fluid with an Oil/Water Ratio of 90:10 Before and After Hot Rolling at 232°C

The test results show that the drilling fluid with an oil/water ratio of 90:10 and a density of 2.04 g/cm³ maintained good rheological performance at 50°C after hot rolling and aging at 232°C for 16 h. It also showed high electrical stability and low HTHP fluid loss. After contamination with 30 g RevDust and hot rolling at 232°C for 16 h, the drilling fluid rheology remained stable, indicating strong resistance to drill cuttings contamination.

The oil-based drilling fluid was prepared according to the formulation in Table 2-1. After hot rolling at 232°C for 16 h, the fluid performance was tested at 50°C. After the test, 20 g of 4% sodium chloride brine was added for contamination. The fluid was then hot rolled for a second time at 232°C for 16 h, and the drilling fluid performance before and after hot rolling was tested. The results are shown in Table 2-3.

Table 2-3 Performance of 2.04 g/cm³ Oil-Based Drilling Fluid with an Oil/Water Ratio of 90:10 Before and After Hot Rolling at 232°C

Table 2-3 shows that the 2.04 g/cm³ OBM with a 90:10 oil/water ratio maintained good rheology after aging at 232°C for 16 h. After contamination with 20 g of 4% NaCl brine and a second 232°C × 16 h hot roll, the fluid showed some thickening, but its overall performance still met field drilling requirements. To further improve performance, 4 g of UT-STAB (HT) was added, followed by a third 232°C × 16 h hot roll. The results are shown in Table 2-4.

Table 2-4 Performance Before and After 16 h Hot Rolling at 232°C

Table 2-4 shows that after adding 4 g of UT-STAB (HT) and hot rolling at 232°C for 16 h for the third time, the HTHP fluid loss increased slightly, but the rheology of the oil-based drilling fluid improved. This indicates that this high-temperature, high-density oil-based drilling fluid system can meet field HTHP drilling requirements.

3. Conclusion

（1）This high-temperature, high-density oil-based drilling fluid system can meet HTHP drilling requirements at 232°C and at a density above 2.0 g/cm³.

（2）When preparing oil-based drilling fluids at temperatures not exceeding 200°C, it is not necessary to add the high-temperature stabilizer UT-STAB (HT); only the high-temperature emulsifying and wetting agent UT-MUL (HT) is required. When the temperature exceeds 200°C, such as 232°C, both UT-STAB (HT) and UT-MUL (HT) are required. If the apparent viscosity or yield point of the drilling fluid is too high, adding UT-STAB (HT) can reduce the viscosity and yield point after high-temperature aging, indicating that UT-STAB (HT) has a certain viscosity-reducing  effect.