Page 73 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 49

1.4.4
Casing Design [ ]

be plotted on the depth-pressure graph. Unless there were dramatic
changes in formation stresses or formation depths between the offset
and planned well locations, these should relate fairly well to the new
well. This may not apply in areas of rapid geological change with dis-
tance. In general pore pressure trends and fracture gradient trends fol-
low similar paths on the graph.
Other drilling records that show ROP trends such as background
gas levels, increasing mud chlorides (from formation water), and well-
bore caving, do not give an accurate pore pressure. The indications
may be that the mud in use (with ECD applied) does not overbalance
the pore pressure by much and so it may possibly be said that the pore
pressure is likely to be a maximum of mud hydrostatic + ECD.
Remember, however, that you may not be able to tell where in the open
hole the cavings or background gas are being generated. Also, remem-
ber that some background gas will be liberated in the cuttings as gas
bearing formations are drilled and that a low-permeable, gas-bearing
formation may not produce much gas even if the pore pressure is some-
what higher than the mud pressure. Connection gas and trip gas give
reasonable indications that the formation is just overbalanced by the
mud hydrostatic; the small and temporary drop in hydrostatic due to
upward pipe movement has allowed a bit of gas into the well.
It is possible to measure shale bulk density from well cuttings.
Where the bulk density does not increase with depth in accordance
with the normal compaction trend for the area, then overpressures may
be indicated. The procedure to measure the bulk density is as follows:

Wash the sample of shale and ensure no cavings are present (these
will come from another formation and would therefore affect the
result)
Add shale to the cup of a mud balance until the balance reads 0.433
psi/ft (fresh water) with the top placed on
Top up with fresh water and take the balance reading, W psi/ft
Calculate the shale density as follows:

SGshale = . 0 433
. 0 866 − W

Wireline logs can provide excellent indicators of pressure transi-
tion zones. As previously noted, the normal trends with increasing
depth are for porosity and resistivity to decrease and sonic transit times

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