Drill pipe pressure

If the drill string is thought to be a thin wall cylinder with closed ends, then the drill pipe pressure produces the axial stress and tangential stress given by the following formulas ... 

Find the tensile load capacity of 5-in., nominal weight 19.5-lb/ft, steel grade E, premium class drill pipe exposed to internal drill pipe pressure = 3,000 psi and rotary torque T = 15,000 ft-lb. 

Note that the drill pipe pressure is a sum of all the pressure losses in the circulating system. 

Prior to the drilling of the well all geological data, including the seismic was thoroughly reviewed and analyzed. From the data at hand there was no reason to expect an influx of fluid during the second core run in the Prairie Evaporite Formation. Not only is it unusual to encounter fluids in the middle of the salt bed, the pressures encountered in this well are abnormally high for the potash exploration industry. The zero casing and drill pipe pressure was achieved when the fluid column was at 2000 kg/m3. This equates to a bottom-hole pressure of approximately 30 MPa which is hydrostatically over-pressured by about 15 MPa. 

Pressure records show that, at about 2101 hours on April 20, a kick began and drill-pipe pressure began to increase, rising by about 100 psi over 7 minutes. This showed itself as a gently rising trend on a screen which followed various parameters - it would hardly have been noticeable unless someone was looking very closely. 

Between 2108 and 2114 hours, the drill-pipe pressure increased again, although this was unnoticed. 

At 2130 the pumps were shut off to investigate an unexpected pressure difference between the drill-pipe and the kill line. Drill-pipe pressure increased by 550 pounds per square inch over the next five and a half minutes - while the kill line pressure remained significantly lower. (Remember, low pressure in the kill line had been taken as the reason for confirming the negative pressure test was satisfactory. With hindsight, therefore, it looks like the kill line was blocked.) This was now clear evidence of a kick, but no further action was taken. 

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. 

The torsion, tension, collapse and internal pressure resistance for new, premium class 2 and class 3 drill pipe are specified in Tables 4-80, 4-81, 4-82 and 4-83, respectively. 

The yielding of pipe does not occur provided that the equivalent stress is less than the yield strength of the drill pipe. For practical calculations, the equivalent stress is taken to be equal to the minimum yield strength of the pipe as specified by API. It must be remembered that the stresses being considered in Equation 4-54 are the effective stresses that exist beyond any isotropic stresses caused by hydrostatic pressure of the drilling fluid. 

New Drill Pipe—Torsional, Tensile, Collapse and Internal Pressure Data [30]... 

During DST operations, the drill pipe may be affected by a combined effect of collapse pressure and tensile load. For such a case,... 

From Table 4-84, the collapse pressure resistance in uniaxial state of stress, P - 6,010 psi. Reduced wall thickness for class 2 drill pipe = (0.65)(0.337) = 0.219 in. Reduced D for class 2 drill pipe = 3.826 + (2)(0.219) = 4.264 in. Reduced cross-sectional area of class 2 drill pipe equals ... 

To minimize pressure losses, the ratio of drill pipe outside diameter to borehole diameter, whenever possible, should be about 0.6. 

Typically, water based muds are considered to be incompressible or slightly compressible. For the flow in drill pipe or drill collars, the acceleration component (AP J of the total pressure drop is negligible, and Equation 4-104 can be reduced to... 

The wellbore, drill string and drilling fluid data from the previous example are used. Casing depth is 4,000 ft. Assuming a drill pipe length of 5,000 ft and a drill collar length of 500 ft, find the friction pressure losses. 

Bleed-Off Line. This line bleeds off pressure within the standpipe, rotary base, kelly and the drill pipe to the depth of the top float valve. The bleed-off line allows air (or gas) under pressure to be fed directly to the blooey line. 

Bottomhole Assemblies. In general, the drill pipe, drill collars and, in particular, bottomhole assemblies for air (or gas) drilling operations are the same as those in mud drilling. However, because the penetration rate is much greater in air (or gas) drilling operations due to the lack of confining pressure on the bit cutting surface, care must be taken to control unwanted deviation of the borehole. Thus, for air (or gas) drilled boreholes, a packed-hole or stiff bottom-hole assembly is recommended. 

Knowing the bottomhole pressure, the number of bit orifice openings and the inside diameter of these openings, the pressure inside the drill pipe just above the bit and the surface injection pressure can be found. 

Compute the pressure drop in the drill pipe while circulating. 

Differential pressure sticking occurs when the drill pipe becomes imbedded in the mud wall cake opposite a permeable zone. The difference between the hydrostatic pressure in the drill pipe and the formation pressure holds the pipe in place, resulting in a sticking pipe. Differential sticking may be prevented, and a stuck drill bit may be freed, using an oil-mud or an oil- or water-based surfactant composition. 

Basically, the drill stem test involves attaching a tubing assembly to the end of the drill pipe, isolating the lest zone with rubber packers, and perforating the zone. The tool is then opened so that the fluids or gas in die formation can How up the drill pipe for metering at the surface. During this process, extensive pressure measurements are taken, which can help to indicate the extent of the reservoir and the rate at which the hydrocarbons could be recovered. Prior to describing how a well is finally completed (if the hole is not dry ), it is in order to describe the forces utilized to transfer the oil from the reservoir to the surface. 

Drill Pipe. Iron oxide pigmented coatings are used to protect oil well drill pipe. In this application, phenolics are required for their resistance to abrasion, acids, hydrocarbons, and water at high temperatures and pressures, as encountered in drilling operations. 

A typical flow rate of 2 m3/min in the drill pipe (diameter = 0.1 m) gives a fluid velocity of 4.2 m/s, which causes fully turbulent flow in most drilling fluids. The calculation of the pressure drop down the length of the drill pipe is made somewhat complex by the flow being turbulent and the non-Newtonian rheology of the drilling fluid. These calculations are, however, important as about 30% of the total frictional pressure losses occur in the drill pipe. 

In turbulent flow the frictional pressure drop down the drill pipe must be calculated from equations that have been determined empirically. The commonest method in drilling fluid hydraulics (91-93) is to use a friction factor/, the so-called Fanning friction factor, defined by the ratio of the wall shear stress rw to the kinetic energy per unit volume of the flowing fluid 0.5 pvm2 (94)... 

Annulus. From the drill pipe, the drilling fluid is jetted out of the bit nozzles and into the annulus. The jets are designed to remove drilled cuttings away from the drill bit. The pressure drop across the nozzles accounts for about 60% of the total frictional pressures losses during the circulation of the drilling fluid. About 90% of the pressure drop across the nozzles is due to the turbulence at the outlet, with only about 10% due to the flow through the nozzles. 

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