120

8 12 16 20 24 Circulation Time, Hours

Figure 9-45. Temperature trace for various depths in a simulated well. (From Raymond.58 Copyright 1969 by SPE-AIME.)

bentonite mud and a low-viscosity oil mud (85/15 oil/water ratio). Both muds were loaded to three different densities (approximately 11, 14, and 18 lb/ga! — ! .32, 1.68, and 2.16 SG), but changes in density appeared to be unaffected by the initial density.

Hoberock et al5Sb calculated the pressure at the bottom of the hole from the sum of incremental changes in density with depth. They used a material balance to account for the differences in compressibility between oil and water. Solids were assumed incompressible. Calculations were made to show the effect of total depth, temperature gradient, circulation rate, and mud formulation. Table 9-7 shows some typical results. The pressure difference shown in the last column is the approximate difference between the bottom-hole pressure as calculated from the mud density at the surface and the actual bottom-hole pressure. The values shown are for a water base mud (77% water, no oil). Results for an oil base mud (70% oil, 7% water) were not substantially different from the water base mud.

PRESSURE. M psig

Figure 9-46. Effect of temperature and pressure on the density of oil and water base muds. (From McMordie. Copyright 1982 SPE-AIME.)

PRESSURE. M psig

Figure 9-46. Effect of temperature and pressure on the density of oil and water base muds. (From McMordie. Copyright 1982 SPE-AIME.)

Table 9-7

Approximate Difference in Bottom Hole Pressure Due to Changes in Mud Density (After Hoberockm)
0 0

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