Page 346 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 322

[ ] Practical Wellsite Operations
3.1.4

The differences in the kill graphs are illustrated in Figures 3-1 and
3-2 for a 2000 m MD, 1000 m TVD well, mud gradient = 0.495, Pdp =
200 psi, and the well is kicked off to horizontal at around 850 MD.
It can clearly be seen that if the conventional kill graph is followed,
the BHP will rise by 90 psi at 50% of Phase 1 strokes before falling back
to the balanced BHP at the end of Phase 1.
Calculations Note: These calculations are incorporated in the
spreadsheet wellcalc.xls available from the web site at
http://www.drillers.com.
To calculate Phase 1, balanced method graph for a horizontal or
high-angle well, first establish Pc1 and Pc2 in the conventional man-
ner. Divide the measured bit depth into 10 approximately equal seg-
ments and calculate the increase in circulating pressure drop at each
depth. The depths do not have to be at exactly equal spacing but may
be at convenient depths where both MD and TVD are already known,
such as survey stations. The intermediate circulating pressure (Pci) at
each depth will be:
MDstation x(Pc2 - Pc1)
Pci = Pc1 +
MDbit

Now for the measured depth at the end of each segment, take the
survey depth or calculate the TVD from the directional plot and work
out how much extra pressure will be required to balance formation
pressure (Ps) over the mixed mud hydrostatic. The extra surface pres-
sure Ps will be

Ps = Pf - ((TVDwell - TVDstation) x (ρ2 - ρ1))

For each station, add together Pci and Ps to get the total Phase 1
balanced BHP circulating pressure. If circulation is stopped at any
point, the closed-in surface pressure will equal Ps for that depth (see
Table 3-1).
The easiest way to make these calculations is with a spreadsheet.
The one included as a sample (wellcalc.xls) assumes 10 equal spaced
MD and TVD stations, but allows manual overwriting of MD and TVD
entries and plots the pressures and Phase 1 graph automatically.
In many cases, a driller’s method kill may be preferred. If a bal-
anced kill confers no clear advantages of reduced open-hole pressures

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