Drill pipe elongation will occur because the higher bottom hole temperature.
Rule of Thumb
Pipe will elongate approximately 0.83 inc per 100 ft of length, per 100 degree F increase in Temperature.
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When cement is in transition period (forming the bond), you will lose some hydrostatic pressure because cement becomes solid phase, therefore only water in the cement will provide hydrostatic pressure. In many cases happened, there is no issue while performing the cement job, however; once the cement is set after period of time, there is a casing pressure indicating that there is hydrocarbon in the annulus.
This example will demonstrate you why the well is in underbalance condition while waiting on cement.
Well information: Previous 9-5/8” casing shoe at 4000’ ft.
The vertical well (8.5” hole) is drilled to TD at 10,000ft with 12.1 ppg mud and the pay zone is at 9,800’ TVD with reservoir pressure of 11.6 ppg. The cement is planned to cover 3,500 ft in the annulus above the casing shoe. Water used to mix cement is 8.3 ppg weight.
Another equation to determine equivalent circulating density is to use yield point.
This formula below is used to calculate the ECD and it is good for mud weight more than to 13.0 ppg
Where:
ECD is equivalent circulating density in ppg.
MW is mud weight in ppg.
Hold ID is inside diameter of hole in inch.
Pipe OD is outside diameter of pipe in inch.
YP is mud yield point.
YP can be calculated by these following equations.
YP = Reading at 300 rpm – PV
PV = Reading at 600 rpm – Reading at 300 rpm
AV is annular velocity in ft/min
AV can be determined by the following equation.
AV in ft/min = (24.5 x Q) ÷ (Dh2 – Dp2)
where
Q = flow rate in gpm
Dh = inside diameter of casing or hole size in inch
Dp = outside diameter of drill pipe, drill collars, or tubing in inch
Another way to determine equivalent circulating density (ECD) is to use yield point.
This formula below is used to calculate the ECD and it is good for mud weight less than or equal to 13.0 ppg.
Where:
ECD is equivalent circulating density in ppg
MW is mud weight in ppg
Hold ID is inside diameter of hole in inch
Pipe OD is outside diameter of pipe in inch
YP is mud yield point
YP can be calculated by these following equations
YP = Reading at 300 rpm – PV
PV = Reading at 600 rpm – Reading at 300 rpm
