Mud weight

Residual stresses in the formation, resulting from regional tectonic forces may cause the borehole to collapse or deform resulting in stuck pipe. Prevention sometimes high mud weights may help delay deformation of the bore hole. 

When drilling through normally pressured formations, the mud weight in the well is controlled to maintain a pressure greater than the formation pressure to prevent the influx of formation fluid. Atypical overbalance would be in the order of 200 psi. A larger overbalance would encourage excessive loss of mud Into the formation, slow down... 

Similarly, when drilling into an underpressured formation, the mud weight must be reduced to avoid excessive losses into the formation. Again, it may be necessary to set a casing before drilling into underpressures. 

When drilling through normally pressured formations, the mud weight in the well is usually controlled to maintain a pressure greater than the formation pressure to prevent... 

Similarly, when drilling into an underpressured formation, the mud weight must be reduced to avoid excessive losses into the formation. If the rate of loss is greater than the rate at which mud can be made up, then the level of fluid in the wellbore will drop and there is a risk of influx from the normally pressured overlying formations. Again, it may be necessary to set a casing before drilling into underpressures. 

Specific Weight. Mud weight of oil muds is measured with a mud balance. The result obtained has the same significance as in water-base mud. 

Example. Find the volume fraction of brine, the low gravity solids content, the adjusted mud weight, and the oil-to-water ratio from the test data below (use Figure 4-107). 

Mud weight (specific weight) = 15.7 Ib/gal Volume % water (retort) = 20%... 

Step 5. After having found the adjusted mud weight, proceed horizontally from that point to the right to determine the volume percent of solids occupied by the basic emulsifier package. The volume percent of suspended solids is 100% less the sum of the volume-percent oil, the true volume-percent brine (Step 2), and the volume-percent emulsifier solids. 

Find the adjusted mud weight value, extend that point downward until it meets the volume-percent suspended solids line. Proceed horizontally to find the ppg of low gravity solids. 

Mud Weight ppg Plastic Viscosity cP Yield Point Ibs/IOOft Oil-Water Ratio Electrical Stability... 

Shut in well. Record drill pipe and casing pressure. Circulate out gas or water influx and separate on surface. Calculate mud weight necessary to balance formation pressure. Kill the well. 

Keep mud weight as low as possible. Maintain minimum flow resistance of mud. Consider air drilling or foam drilling. 

Control mud weight to counter balance pore pressure. Keep fluid loss as low as possible. Keep viscosity and gel strength low to prevent swabbing. 

A tricone roller rock bit is furnished with three nozzles with the diameters of 32 od -il in. Calculate the bit pressure drop if the mud weight is 10 Ib/gal and flowrate is 300 gpm. 

Turbine Motor, 6V4-in. Outside Diameter, Circulation Rate 400 gpm. Mud Weight 10 Ib/gal... 

The pressure drop Ap (psi) through a given turbine motor design is usually obtained empirically. Once this value is known for a circulation flowrate and mud weight, the pressure drop for other circulation flowrates and mud weights can be estimated. 

If the above performance parameters for a turbine motor design are known for a given circulation flowrate and mud weight (denoted as 1), the performance parameters for the new circulation flowrate and mud weight (denoted as 2) can be found by the following relationships ... 

Table 4-111 gives the performance characteristics for various circulation flowrates for the 318-stage, 6-J-in. outside diameter turbine motor described briefly in Table 4-109. Figure 4-194 shows the performance of the 318-stage turbine motor at a circulation flowrate of 400 gal/min and mud weight of 10 Ib/gal.

Figure 4-194. Turbine motor, 6%-in. outside diameter, three motor sections, 318 stages, 400 gal/min, 10-lb/gal mud weight. (Courtesy Eastman-Chnstensen.)...

Using the basic performance data given in Table 4-110 for the 6-f--in. outside diameter turbine motor with 212 stages, determine the stall torque, maximum horsepower and pressure drop for this motor if only one motor section with 106 stages were to be used for a deviation control operation. Assume the same circulation flow rate of 400 gal/min, but a mud weight of 14 Ib/gal is to be used. 

Stall Torque. From Table 4-110 the stall torque for the turbine motor with 212 stages will be twice the torque value at optimum speed. Thus the stall torque for 10 Ib/gal mud weight flow is... 

Maximum Horsepower. From Table 4-110 the maximum horsepower for the turbine motor with 212 stages is 217. From Equation 4-143 it can be seen that the maximum power is proportional to the stall torque and the runaway speed. Since the circulation flowrate is the same, the runaway speed is the same for this case. Thus, the maximum horsepower will be proportional to the stall torque. The maximum power will be (for the circulation flowrate of 400 gal/ min and mud weight of 10 Ib/gal)... 

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